10th International Workshop on Radiation of High Temperature Gases for Space Missions

9 Sept 2024, 00:30 → 12 Sept 2024, 23:59 Europe/London

Oxford e-Research Centre (University Oxford)

Louis Walpot (ESA/TEC-MPA)

The workshop is organized by the Working Group “Radiation of High Temperature Gas” (RHTG) managed by ESA, through the ESA Technology Directorate. The local organization for this event is managed by the University of Oxford.

The Workshop is devoted to promoting a dialogue on the state of the art and recent advances for simulation/modelling and experimental techniques of hypersonic radiating gas flows for the determination of radiative heat fluxes encountered during atmospheric entry. The workshop provides the opportunity to explore related areas of research to face the challenges of future space flight.

• Non-equilibrium chemical kinetics
• Hypersonic flows
• Plasma radiative emission and absorption
• Experimental facilities and experimental techniques
• Experimental and numerical modelling improvements of radiative heat transfer: refinement, verification, validation and comparison for space object re-entry simulation tools
• Test cases proposed for validation

 

Please submit your abstract online under the heading  "Call for abstracts".

The prelimibary timetable is online, it is still subject to change. Please check the online timetable for any change.

To attend the RHTG-10 workshop, you are required to register first on this web-portal and pay the conference organisation fees via the following link. 

Link for the registration fee payment

The payment is required by the 5th of August. After this deadline, the conference charge is increased to £300.

 

 

 

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rhtg-10_template-v1.docx

Privacy Policy RHTG-10_PrivacyPolicy.pdf (Protected)

Slides Lunch Places.pptx (Protected)

Monday 9 September

08:30 Coffee -

Opening

Radiation modeling and simulation

1 Thermochemical Characterisation of Shock Heated Flows

As a vehicle re-enters the Earth’s atmosphere, it will be travelling at hypersonic speeds through the quiescent atmospheric gas for the majority of its journey. Consequently, a bow shock forms ahead of the vehicle, creating a sudden temperature and pressure increase. The post-shock temperatures are high enough to excite internal energy modes of the gas particles and promote vibrational excitation, dissociation, ionisation and photon emission. These processes occur over a finite time as the particles progress through the shock layer, thus allowing various states of thermochemical non-equilibrium to exist. Correctly characterising this evolution is important for future entry vehicle design. The reach of the non-equilibrium phenomena towards the vehicle surface is important to accurately determine radiative and convective heating to the surface, as well as understanding the onset of communications blackout from the presence of free electrons.
Due to the expense of real flight experiments, ground test facilities have traditionally been used to recreate these high-enthalpy flow environments, so they may be studied. This allows clean radiation data, amongst other parameters, to be captured to inform computational models. One such type of ground test facility is a shock tube, where a normal shock compresses and accelerates a test gas (of controlled pressure and composition) at a speed equivalent to the entry vehicle trajectory point. Windows and ports along such facilities allow radiation to escape and be captured. Most famously, the Electric Arc Shock Tube (EAST) at NASA Ames [1, 2] has captured radiation data since the 1960's to inform a range of proposed planet entry missions. More recently, the Oxford T6 Stalker Tunnel (T6) has been developed [3, 4] as a new high-enthalpy multi-mode impulse facility for aerothermodynamics research of hypersonic flow environments and has also acquired data relevant to Earth [5]. Recent LEO return equilibrium absolute spectral radiance data acquired from both facilities has shown discrepancies against the predictions of NASA’s CEA and NEQAIR codes, as shown by Glenn et al. [6] and Cruden et al. [7].
A new constrained spectral fitting routine has been developed in the Hypersonics Group at the University of Oxford, calling both NASA’s radiative transport code, NEQAIR [8], and Oxford’s in-house thermochemistry and transport property library, OCEAN. These are used in conjunction with optimisation routines in Matlab 2023b’s Optimization Toolbox to spectrally fit the calibrated spatio-spectrally resolved radiance data through the test slug, resulting in temperature and specie number density profiles. This is performed within the framework of a two-temperature model and Boltzmann distributions are assumed. The advantage of the proposed routine over previous spectral fitting methods lies in constraints implemented to conserve total enthalpy and follow appropriate static pressure profiles through the test slug. This limits the myriad of possible fits to those that obey conservation of energy, momentum and mass. This work will show the validation process used to confirm the new methodology produces accurate results, by fitting against artificial data generated by NESS-NEQAIR simulations. Finally, recent results from applying the routine to real EAST and T6 pure nitrogen data is provided, and comparisons made to prior spectral fitting methods used by Tibère-Inglesse et al. [9].

References
1. Cruden, B., Martinez, R., Grinstead, J., and Olejniczak, J., “Simultaneous vacuum-ultraviolet through near-IR absolute radiation measurement with spatiotemporal resolution in an electric arc shock tube,” 41st AIAA Thermophysics Conference, 2009, p. 4240
2. Grinstead, J. H., Wilder, M. C., Reda, D. C., Cornelison, C. J., Cruden, B. A., and Bogdanoff, D. W., “Shock tube and ballistic range facilities at NASA Ames Research Center,” Tech. rep., 2010.
3. Collen, P., “Development of a High-Enthalpy Ground Test Facility for Shock-Layer Radiation,” Ph.D. thesis, Univ. of Oxford, Oxford, UK, 2021.
4. Collen, P., Doherty, L. J., Subiah, S. D., Sopek, T., Jahn, I., Gildfind, D., Penty Geraets, R., Gollan, R., Hambidge, C., Morgan, R., et al., “Development and commissioning of the T6 Stalker Tunnel,” Experiments in Fluids, Vol. 62, No. 11, 2021, pp. 1–24.
5. Collen, P. L., Glenn, A. B., Doherty, L. J., and McGilvray, M., “Absolute Measurements of Air Shock-Layer Radiation in the T6 Aluminium Shock Tube,” Journal of Thermophysics and Heat Transfer, 2023, pp. 1–14.
6. Glenn, A. B., Collen, P. L., and McGilvray, M., “Experimental Non-Equilibrium Radiation Measurements for Low-Earth Orbit Return,” 2021.
7. Cruden, B. A., “Radiative Emission in Incident Air Shocks From 3–7 km/S,” AIAA AVIATION FORUM AND ASCEND 2024, 2024, p. 3653.
8. Whiting, E. E., Park, C., Liu, Y., Arnold, J. O., and Paterson, J. A, "NEQAIR96, Nonequilibrium and Equilibrium Radiative Transport and Spectra Program: User’s Manual," 1996.
9. Tibère-Inglesse, A., and Cruden, B. A., “Analysis of nonequilibrium atomic and molecular nitrogen radiation in pure N2 shockwaves,” Journal of Quantitative Spectroscopy and Radiative Transfer, Vol. 290, 2022, p. 108302.

Speakers: Alex Glenn (Oxford University DPhil Student), Matthew Mcgilvray (University of Oxford)

AG_RHTG_09-09-2024.pptx

State to state and Collisional Radiative Modelling

2 Ab Initio Electronic Structure Calculations of CNN for CN Excitation Studies

Background

Titan's atmosphere is composed mostly of N$_2$ with a small amount of CH$_4$, and so, shock layers around craft entering Titan's atmosphere will contain a variety of molecules formed from H, C, and N atoms, including the cyanogen radical CN. Sensitivity analysis has shown that the radiative heat flux predicted by computational fluid dynamics (CFD) simulations of Titan entry has up to 14\% uncertainty due to the rate coefficients for collisional (de)excitation reactions that control the population of CN in its first and second excited states.[1]

$ \hspace{2cm} \textrm{CN}(\textrm{A} ^2\Pi) + \textrm{M} \longleftrightarrow \textrm{CN}(\textrm{B} ^2\Sigma^+_g) + \textrm{M} \hspace{3cm} \textrm{(1)} $

The red and violet emission bands from CN's first and second excited states, respectively, are known to be large sources of radiative heat flux on capsules entering Titan's atmosphere.[2,3] So, the simulated population of CN in its first and second excited states is very important, but, currently has some inherent uncertainty coming from the data for the rate coefficients for reaction (1).

The goal of the current work is to provide improved rate coefficient data for these reactions from first principles quantum chemistry calculations. The first step of this process is to calculate ab initio Potential Energy Surfaces (PESs) and state couplings that link the ground, first excited, and second excited states of CN. PESs are constructed by calculating single point energies and state couplings by solving the Schrodinger equation under the Born-Oppenheimer approximation. These energies and couplings can then be transformed to a diabatic basis where the state couplings are smoother, and fit with functional forms. Then, nonadiabatic dynamics calculations using the diabatic surfaces and couplings will determine the heavy particle (de)excitation rate coefficients at conditions relevant to shock layers around vehicles entering Titan's atmosphere.

Methodology

Electronic Structure Calculations
We focus first on heavy particle (de)excitation by nitrogen atoms, so our current structure calculations are of CNN. We use the new electronic structure code MPEC to calculate ab initio single point energies using the i$^2$c-MRCI(SD) method.[4] I$^2$c-MRCI(SD) calculations require reference orbitals, which we generated using the Multi Configuration Hartree Fock (MCHF) method to optimize a set of Complete Active Space (CAS) orbitals.

Preliminary calculations of N atoms and CN molecules showed that there are Quintet A'' and Triplet A'' manifolds for CNN that link the $\textrm{CN(X,A,B)+N(}^4\textrm{S}_\textrm{o}\textrm{)}$ states. Reference orbitals were generated for the lowest 14 states of both of these symmetries at a large number of geometries of the CNN system. To generate these orbitals, we used augmented-correlation-consistent-polarized-valence-triple-zeta (aug-ccpvtz) basis sets for each atom, and an active space consisting of the 2s orbital on the carbon atom and all the 2p orbitals for each atom. The 1s orbitals for each atom, and the 2s orbitals for the nitrogen atoms were kept doubly occupied. The i$^2$c-MRCI(SD) calculations for those states corresponding to $\textrm{CN(X,A,B)+N(}^4\textrm{S}_\textrm{o}\textrm{)}$ are currently in progress.

Functional Forms
The I$^2$c-MRCI(SD) energies of CN have already been fit to functions using repulsive, short range, and long range terms: $V(R_{C-N}) = V_{Rep} + V_{SR} + V_{LR}$ as described in Ref. [5].The triatomic energies will be fit in a similar manner, also discussed in Ref. [5].

Dynamics Simulations
Once the i$^2$c-MRCI(SD) energies and state couplings are completed, diabatized, and fit with functional forms, they can be used in nonadiabatic dynamics simulations of CN+N collisions that will use Tully's fewest switches surface hopping method to determine rate coefficients for reaction (1) and others.[6]

Results

The i$^2$c-MRCI(SD) calculations are still in progress, but the MCHF calculations already illustrate important features of the CNN system. While the calculations are not perfectly resolved at every geometry, they show important features of the adiabatic states they represent. There are several avoided crossings at intermediate distances of 3.5-6 a$_0$ for the first three Quintet A'' states at collinear geometries, while for the perpendicular bisector geometries there is only one avoided crossing near 2.4 a$_0$. These features indicate that the heavy particle (de)excitation in reaction (1) is more likely to occur through collinear geometries because the states of interest are more closely coupled there.

Conclusions and Further Work

This work will produce ab initio PESs that describe the CNN complex and allow nonadiabatic dynamics simulations of CN (de)excitation by N atoms. The electronic structure calculations to form the PESs are underway. Reference orbitals have been generated at a large number of triatomic geometries for the symmetries of interest and show multiple avoided crossings at collinear arrangements. This suggests that collisional (de)excitation of CN by N atoms is likely to proceed through these geometries. Once the PESs are completed, thorough analysis will show possible reaction pathways in detail, and nonadiabatic dynamics simulations will determine improved rate coefficients that can be used in CFD simulations to lower uncertainties in radiative heat flux predictions for atmospheric entry to Titan.

References

[1] Thomas K West IV et al. “Uncertainty Analysis of Radiative Heating Predictions for Titan Entry”. In: Journal of Thermophysics and Heat Transfer 30.2 (2016), pp. 438–451.
[2] Aaron M Brandis and Brett A Cruden. “Titan Atmospheric Entry Radiative Heating”. In: 47th AIAA Thermophysics Conference. 2017, p. 4534.
[3] LM Walpot et al. “Convective and Radiative Heat Flux Prediction of Huygens’s Entry on Titan”. In: Journal of Thermophysics and Heat Transfer 20.4 (2006), pp. 663–671.
[4] David W Schwenke. “Introducing MPEC: Massively Parallel Electron Correlation”. In: The Journal of Chemical Physics 158.8 (2023).
[5] Richard L Jaffe, David W Schwenke, and Marco Panesi. “Chapter 3: First Principles Calculation of Heavy Particle Rate Coefficients”. In: Hypersonic Nonequilibrium Flows: Fundamentals and Recent Advances. American Institute of Aeronautics and Astronautics, Inc., 2015.
[6] John C Tully. “Molecular Dynamics with Electronic Transitions”. In: The Journal of Chemical Physics 93.2 (1990), pp. 1061–1071.

Speaker: Eric Geistfeld (NASA)

Geistfeld_RHTG_2024_Slides_V1.pptx

Coffee Break

State to state and Collisional Radiative Modelling

3 State-to-State investigation of hypersonic high-enthalpy nitrogen flows

1 Background of the study
Nowadays, reusable vehicles to access space are very attractive for cost reduction. Many reusable spacecrafts, such as Space Shuttle, have been conceived for both orbital and suborbital services [1]. These vehicles must withstand very high temperatures when they fly at hypersonic speeds in the continuum regime, and an adequate Thermal Protection System (TPS) is needed.

Modeling high enthalpy flows is a challenge requiring an efficient CFD solver coupled with an accurate chemical-physical scheme capable to describe non-equilibrium conditions. The multi-temperature (mT) approach has the advantage to implement a minimal kinetic scheme, describing the vibrational energy of molecules with a temperature Tv, but its accuracy is limited. Many assumptions are needed to extend the rate given for thermal equilibrium system to mT and to determine the effects of chemical reaction on the internal energy. As an alternative, the State-to-State (StS) kinetics, describing the evolution of each internal level as an independent chemical species, obtains this information directly from the distributions. However, the computational load is much larger than mT, limiting the diffusion of StS models.

Over the last few years, these authors have developed a CFD solver of the Navier-Stokes equations, accelerated by GPUs, implementing a vibrationally resolved StS kinetics for the neutral air mixture. They demonstrated [2,3] that this approach to thermochemical non-equilibrium provides better results than the classical mT model [4] due to non-equilibrium vibrational distributions that, departing from the Boltzmann one, affect the global dissociation rates. In general, the shock stand-off distance predicted by the StS model is larger than the one provided by the mT model. This study was limited to freestream conditions with total enthalpy not large enough to generate plasma flows.

Recently, Guo et al. [5] showed analogous results by comparing the shock stand-off distance predicted by the StS and the mT models with values measured in nitrogen hypersonic flows past spheres [6]. Nevertheless, they employed a neutral kinetic model even if the translational temperature downstream of the bow shock was larger than 10000 K and ionization phenomena could be not negligible.

2 Methodology
In this scenario, an effort is ongoing to extend the StS model for neutral air mixture to the ionization regime. A first step in this direction has been made by implementing a kinetic model for weakly ionized nitrogen in order to study a titanium plume produced by a ns-pulsed laser in a nitrogen environment [7].
The model includes the nitrogen StS vibrational kinetics, i.e., vibration-vibration, vibration-translation by molecules and by atoms, and dissociation/recombination by molecules and by atoms.
For charged species the following processes have been considered: vibrationally induced ionization, associative ionization and resonant charge exchange.
Finally, electron impact causes atomic nitrogen ionization, N2 dissociation, ionization and internal transitions.

This kinetic scheme is integrated in a cell-centered finite-volume solver of the Navier-Stokes equations. The solver employs an operator-splitting approach thus solving the frozen fluid dynamic equations first and then updating the solution by integrating the chemical source terms. Convective fluxes are discretized by using the Steger and Warming flux vector splitting with second order accuracy obtained by a MUSCL reconstruction of primitive variables. As regards diffusive terms, the Gauss theorem is employed to compute derivatives at face centers. Finally, a multi-step Runge-Kutta approach is employed for time integration. On the other hand, the stiffness of source terms is managed by a Gauss-Seidel iterative scheme.

3 Results
In this work the in-house solver is employed to investigate hypersonic nitrogen flows past spheres at enthalpies that could induce ionization by considering the experimental setup of Ref. [6]. The results in terms of shock stand-off distance, flow and wall quantities are compared to those provided by experiments [6] and numerical simulations [5].

4 Conclusion
The results represent a first application of a vibrationally-resolved model to nitrogen flows including charged species, to test the StS performances in ionization regimes. This study can be regarded as a first attempt to extend the CFD code capabilities to high-enthalpy Earth atmosphere re-entry in conditions of moon return.

Acknowledgements
G. Pascazio, F. Bonelli and D. Ninni were partially supported by the Italian Ministry of University and Research under the Programme “Department of Excellence” Legge 232/2016 (Grant No. CUP - D93C23000100001).
This work is part of the project NODES which has received funding from the MUR –M4C2 1.5 of PNRR funded by the European Union - NextGenerationEU (Grant agreement no. ECS00000036).
F. Bonelli, G. Pascazio, G. Colonna, A. Laricchiuta were funded by the European Union – Next Generation EU – Piano Nazionale Resistenza e Resilienza (PNRR), Missione 4 Componente C2 Investimento 1.1 – Decreto Direttoriale n. 1409 del 14/09/2022, Bando PRIN 2022 PNRR, CUP Master - D53D23018520001, CUP Politecnico di Bari - D53D23018520001, CUP CNR - B53D23027270001, Hypersonic Entry flow simulator for Access To Space – HEATS.

References
[1] P. Baiocco, "Overview of reusable space systems with a look to technology aspects." Acta Astronautica 189 (2021): 10-25.
[2] G. Colonna, F. Bonelli & G. Pascazio, "Impact of fundamental molecular kinetics on macroscopic properties of high-enthalpy flows: The case of hypersonic atmospheric entry." Physical Review Fluids 4.3 (2019): 033404.
[3] D. Ninni, F. Bonelli, G. Colonna & G. Pascazio, "Unsteady behavior and thermochemical non equilibrium effects in hypersonic double-wedge flows." Acta Astronautica 191 (2022): 178-192.
[4] C. Park, Nonequilibrium Hypersonic Aerothermodynamics (Wiley, New York, 1990).
[5] J. Guo, X. Wang & S. Li, "Investigation of high enthalpy thermochemical nonequilibrium flow over spheres." Physics of Fluids 36.1 (2024).
[6] J. Olejniczak, G. V. Candler, M. J. Wright, I. Leyva & H. G. Hornung, "Experimental and computational study of high enthalpy double-wedge flows." Journal of Thermophysics and Heat Transfer 13.4 (1999): 431-440.
[7] G. Colonna, G. Pascazio & F. Bonelli, "Advanced model for the interaction of a Ti plume produced by a ns-pulsed laser in a nitrogen environment." Spectrochimica Acta Part B: Atomic Spectroscopy 179 (2021): 106120.

Speaker: Francesco Bonelli (Politecnico di Bari)

BONELLI-RHTG-2024-uploaded.pptx

4 Electronic State-to-State Modelling of a Recombining N2/Argon Plasma and Comparisons with Experiments

An electronic state-to-state kinetic model for nitrogen/argon mixtures is obtained by reduction of a state-of-the-art vibronic-specific model. The model is used to study nitrogen recombination. A high-temperature plasma initially at local thermodynamic equilibrium at 6750 K and 1 atm passes through a water-cooled tube that forces rapid cooling and nonequilibrium recombination. Simulations using the electronic-specific model are performed and compared with the measurements. The model reproduces the experimentally observed behaviour. A detailed analysis of the main processes governing the recombination inside the water-cooled tube suggests that the three-body recombination of nitrogen, especially into N2(A), plays a key role in the kinetics of recombination. An accurate prediction of N2(A) population is necessary to correctly predict the other species densities.

Speaker: Ulysse Dubuet (Laboratoire EM2C, CNRS, CentraleSupélec, Université Paris-Saclay)

2024_RHTG10_Dubuet_Presentation.pptx

5 Collisional-Radiative Modeling of Air Plasma Generation at Suborbital Hypersonic Velocities

See attached.

Speaker: Timothy Aiken (University of Colorado)

RHTG Slides.pptx

12:30 Lunch

Radiation modeling and simulation

6 Sensor-based Radiation Signature Computations in Hypersonic Flow using a Reverse Photon Monte Carlo Method

Background of the study

When a vehicle travels through Earth’s atmosphere at hypersonic speeds, the temperature in the shock layer rises significantly. This causes air molecules to dissociate. For higher enthalpies flow regions may experience chemical nonequilibrium, ionization, and thermal nonequilibrium. In addition, transitions between bound-bound, and, for a partially ionized gas, bound-free and free-free states contribute toward thermal radiation. At sufficiently high enthalpies, e.g. for the FIRE II experiment, thermal radiation from the gas phase travelling toward the vehicle makes up a significant portion of the heat load on the vehicle surface. In [1], at the peak heating point of the FIRE II trajectory (1643s), 27% of the total heat flux on the wall was caused by radiation. Thermal radiation travelling outwards to a sensor becomes the radiation signature of the vehicle. This signature can be estimated from surface radiation of the vehicle. To improve this estimation, radiation from the gas phase leaving toward the sensor can be included.
The method of choice for this work to compute gas radiation is the photon Monte Carlo method [2]. A high-order variation of the method was developed by Wu et al. in 2006 [3]. In 2012 a version was presented by Feldick et al. which exploits 2-D axial symmetry to reduce computation time significantly [4]. Karl et al. integrated the method into the DLR TAU code and computed radiation for the FIRE II capsule in 2015 [1]. In 2019 an implementation of the photon Monte Carlo method called StaRad has been applied at the 8th iteration of this workshop to Martian reentry by Bonin et al. [5]. They also computed radiation for the FIRE II 1642s flight point and a generic triple cone configuration [6]. The method has also been integrated into the open-source DSMC software SPARTA to compute the radiation in the flow field of the Huygens probe by Oblapenko et al. in 2023 [7]. Recent work by Swenson et al. from 2024 implemented a voxelized version into DART where they validated the method using a 2-D cylinder [8].
In this work we focus on computing the radiation signature as seen by a sensor. 2-D axisymmetric CFD computations in thermo-chemical nonequilibrium for a generic hypersonic glide vehicle and the FIRE II capsule have already been performed.

Methodology

The first step in the radiation analysis is determining the emission and absorption coefficients. Given the species densities and temperatures from CFD, these spectral properties can be computed e.g. from the plasma radiation database PARADE [9]. The next step is solving the radiative transfer equation [2]. This equation describes propagation of directional radiation intensity in a medium along a line of sight. Unfortunately, solving for the radiative heat flux leads to an integrodifferential equation that is challenging to solve. 1-D approximate methods such as the infinite slab method have been developed.
More accurate and more computationally demanding are 3-D methods that transfer the problem into a set of PDEs. The method of spherical harmonics (P_N) approximates the radiative intensity in a certain direction with a generalized series. P_1 is a well-known version of this method. The P_N method suffers from low accuracy in optically thin media as they are encountered during the initial part of reentry. Another popular 3-D method is the method of discrete ordinates (S_N) which discretizes the range of solid angles with a fixed number N of directions and then solves the radiative transfer equation along each of those lines. Due to the discretization, it can produce unphysical geometrical radiation patterns in the solution.
The photon Monte Carlo method solves the radiative transfer equation by directly simulating it for a statistically large number of photon bundles. Hence, it will produce the exact solution in the limit of infinitely many bundles. This makes the method attractive for reentry applications. By discretizing radiation into photon bundles, radiative energy is not treated as a continuous quantity anymore. This allows high flexibility regarding the spatial and spectral resolution. Ray tracing of these photon bundles is computationally expensive but very suitable for parallel computing as all bundles are independent of each other. Due to statistical sampling, results from this method are noisy. The higher the number of simulated photon bundles, the lower the noise level.
Tallying the photon bundles that are emitted or absorbed by the vehicle wall then gives the radiative surface heat load. Similarly, the radiative heat flux in the gas phase is determined. These results can then be fed back to the CFD tool as a source term in the energy equation to achieve loose coupling.
To determine the radiation signature as seen from a sensor, running the photon Monte Carlo method in reverse is more efficient. A satellite-based sensor can be many kilometres away from the vehicle of interest and thus only a very small portion of the solid angle is of interest. Reverse means the calculation starts at the sensor. Absorbed photon bundles are traced backward to their point of origin. From that origin, the associated energy and wavelength can be determined. This ensures that only photon bundles of interest are simulated.

Results

CFD results are already generated for the 1648s and 1636s flight points of the FIRE II capsule, and also for a generic hypersonic glide vehicle. The CFD tool is the in-house code KEGNI (see e.g. [10] and [11]) which splits the flow field into an inviscid Euler region and a viscid boundary layer. KEGNI is a shock-fitting finite difference method which uses flux vector splitting to discretize spatially with a third-order upwind scheme. To reach a steady-state solution it integrates in artificial time with a second-order low-storage Runge Kutta algorithm.
Special emphasis is placed on the sensor-based radiation signature calculations. The reverse photon Monte Carlo method and gas radiation will be discussed.

References

[1] S. Karl, D. Potter, M. Lambert, K. Hannemann: Computational Analysis of Radiative Heat Loading on Hypervelocity Reentry Vehicles, Journal of Spacecraft and Rockets, vol. 52, No.1, 2015, https://doi.org/10.2514/1.A32791
[2] Modest, M. F., Radiative Heat Transfer, Academic Press, New York, 2003, pp. 295–569.
[3] Y. Wu, M. F. Modest, D.C. Haworth, A high-order photon Monte Carlo method for radiative transfer in direct numerical simulation, Journal of Computational Physics 223 (2007) pages 898-922, https://doi.org/10.1016/j.jcp.2006.10.014
[4] A. M. Feldick, M. F. Modest, A Spectrally Accurate Two-Dimensional Axisymmetric, Tightly Coupled Photon Monte Carlo Radiative Transfer Equation Solver for Hypersonic Entry Flows, Journal of Heat Transfer 134(12): 122701 (2012), https://doi.org/10.1115/1.4007069
[5] J. Bonin, Ch. Mundt: 3d Monte Carlo radiative transport computation for Martian atmospheric entry, 8th Int. workshop on Radiation of high temperature gases for space missions, Madrid, 2019.
[6] J. Bonin, Ch. Mundt: Full 3D Monte Carlo radiative transport for hypersonic entry vehicles, AIAA Journal of Spacecraft and Rockets, vol. 56, pp. 44-52, 2019, https://arc.aiaa.org/doi/10.2514/1.A34179
[7] G. Oblapenko, T. Ecker, S. Fechter, T. Horchler, S. Karl, Development of a photon Monte Carlo radiative heat transfer solver for CFD applications, Aerospace Europe Conference 2023 10th EUCASS - 9th CEAS 2023, DOI: 10.13009/EUCASS2023-423
[8] S. J. Swenson, B. M. Agrow, C. P. Turansky, The voxelized photon Monte Carlo method for hypersonic radiation modeling, Computers & Fluids, Vol. 271 106168 (2024), https://doi.org/10.1016/j.compfluid.2023.106168
[9] A. Smith, J. Beck, M. Fertig, H. Liebhart, and L. Marrafa: Plasma Radiation Database PARADE V3.1, ESA Contract Rept. TR28/96 No. 8, Fluid Gravity Engineering, Ltd., 6 June 2011.
[10] F. Monnoyer, Ch. Mundt, M. Pfitzner: Calculation of the hypersonic viscous flow past reentry vehicles with an Euler-boundary layer coupling method, AIAA-paper 90-417, 1990. https://arc.aiaa.org/doi/pdf/10.2514/6.1990-417
[11] Ch. Mundt: Calculation of hypersonic, viscous non-equilibrium flows around reentry bodies using a coupled Euler/boundary layer method, AIAA-paper 92-2856, 1992. https://arc.aiaa.org/doi/10.2514/6.1992-2856

Speaker: Prof. Christian Mundt (Universität der Bundeswehr München)

2024-09-09 10th ESA Rad Workshop Presentation James Mundt.pdf

2024-09-09 10th ESA Rad Workshop Presentation James Mundt.pptx

2024-09-09 10th ESA Rad Workshop Presentation James Mundt V2.pdf

2024-09-09 10th ESA Rad Workshop Presentation James Mundt V2.pptx

7 Kinetic Modeling and Impact of Methane on Radiative Heating for Ice and Gas Giant Entry

Background: Recent investigations of ice and gas giant entry flows have been motivated by the high priority listing of probe missions to Saturn and Uranus in the 2023-2032 planetary science decadal survey [1]. The atmospheres of these planets are primarily composed of H2 and He, with trace amounts of CH4. The convective and radiative heat loads encountered by these vehicles is controlled by the composition of the gases behind the shock that forms in front of the probe. Therefore, in this work, the chemical-kinetic processes for H2/He/CH4 entry flows are reviewed in three parts: the ro-vibrational relaxation and dissociation of H2, the electronic excitation and ionization of H, and finally the impact of trace CH4.

Ro-vibrational relaxation and dissociation of H2: Behind the shock, the first chemical process to occur is H2 dissociation, which has a direct impact on radiative heating. The reverse process, H2 recombination, has a large impact on convective heating. For several decades, H2 dissociation/recombination has been investigated in lower temperature regimes (300K-5000K) through flow reactors and shock tube experiments, and more recently in higher temperature regimes (>3000K) through computational studies. Unfortunately, limited work has been done to compile the existing data together and extract rates that are accurate across the entire post-shock temperature range of interest. Comparing rates from different sources is complicated by the fact that the dissociation process is coupled to the non-equilibrium relaxation of the ro-vibrational modes of H2. Therefore, in this work, we first quantify the non-thermal and non-Boltzmann effects using a quasi-steady-state (QSS) approximation. Where appropriate, corrections for each effect are applied to the rates reported in the literature. From these updated rates, best fits of the reviewed data are proposed.

Electronic excitation and ionization of H: Behind the shock, the dissociation of H2 is followed by the electronic excitation/ionization of H. A previous experimental study by Cruden and Bogdanoff conducted at the Electric Arc Shock Tube (EAST) facility at NASA Ames revealed that the post-shock ionization/equilibration process occurs slowly, with evidence of non-Boltzmann H state distributions [2]. To reproduce accurately the experimental data, a State-to-State (StS) kinetic model for the excitation/ionization of H is developed [3]. Shock calculations are then performed using a space-marching Euler code, and corresponding radiance values are computed using the radiation code NEQAIR [4]. These calculations are found to reproduce the total integrated radiance observed in the EAST experiments relatively accurately while treating explicitly only 23 species. These results also highlight the importance of accurate heavy particle impact excitation/ionization rates, as the heavy particle impact processes are rate-limiting for the subsequent electron impact processes.

Impact of trace CH4: While ice and gas giant atmospheres are primarily composed of H2/He, recent investigations have suggested that trace CH4 may have a non-negligible impact on radiative heating [2,5-7]. Therefore, CH4 and the subsequent dissociated/ionized species are incorporated into the StS kinetic model. The computed radiance values are then compared to the measured values from the recent EAST test campaign conducted by Cruden and Tibère-Inglesse for H2/He/CH4 mixtures [8].

References:
[1] National Academies of Sciences, E., and Medicine, Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032, The National Academies Press, Washington, DC, 2022. https://doi.org/10.17226/26522.
[2] Cruden, B. A., and Bogdanoff, D. W., “Shock Radiation Tests for Saturn and Uranus Entry Probes,” Journal of Spacecraft and Rockets, Vol. 54, No. 6, 2017, pp. 1246–1257. https://doi.org/10.2514/1.a33891.
[3] Carroll, A. T., Blanquart, G., Brandis, A. M., and Cruden, B. A., “State-Specific Kinetic Modeling for Predictions of Radiative Heating in H2/He Entry Flows,” AIAA Scitech 2024 Forum, American Institute of Aeronautics and Astronautics, 2024. https://doi.org/10.2514/6.2024-1482.
[4] Cruden, B. A., and Brandis, A. M., “Updates to the NEQAIR Radiation Solver,” 6th International Workshop on Radiation of High Temperature Gases in Atmospheric Entry, 2014.
[5] Park, C., “Nonequilibrium Chemistry and Radiation for Neptune Entry,” Journal of Spacecraft and Rockets, Vol. 48, No. 6, 2011, pp. 897–903. https://doi.org/10.2514/1.51810.
[6] Coelho, J., and da Silva, M. L.,“Aerothermodynamic analysis of Neptune ballistic entry and aerocapture flows,” Advances in Space Research, Vol. 71, No. 8, 2023, pp. 3408–3432. https://doi.org/10.1016/j.asr.2022.12.024.
[7] Steer, J., Collen, P. L., Glenn, A. B., Hambidge, C., Doherty, L. J., McGilvray, M., Sopek, T., Loehle, S., and Walpot, L., “Commissioning of Upgrades to T6 to Study Giant Planet Entry,” Journal of Spacecraft and Rockets, 2024, p. 1-18. https://doi.org/10.2514/1.A35893.
[8] Cruden, B. A., and Tibère-Inglesse, A. C., “Impact of Trace CH4 on Shock Layer Radiation in Outer Planet Entry,” AIAA Scitech 2024 Forum, American Institute of Aeronautics and Astronautics, 2024. https://doi.org/10.2514/6.2024-2084.

Speaker: Alex Carroll (California Institute of Technology)

RHTG2024.pdf

8 Aerothermodynamic, material response and radiation modelling for the detection of demising spacecraft during re-entry

In recent times, satellite systems boasting thousands of satellites have been launched into orbit. The relatively short lifespan of these satellites, typically lasting only 3-5 years, has lead to increasing chances of in-orbit collisions and overall space cluttering. As the number of space debris experiencing post-mission uncontrolled re-entry rapidly increases, there is growing concern for the threat of ground impact and collisions with operating spacecraft. The complete and controlled demise of spacecraft that have reached the end of their lifespan has therefore risen to the forefront as a matter of utmost importance in sustainable space exploration. This pressing scenario underlines the crucial need for a specialized, sustained monitoring detection system designed to identify and characterize re-entry events from orbit. The information gathered from these observations, coupled with rigorous processing and analysis, has the potential to greatly improve our comprehension of re-entry phenomena. Moreover, it can contribute to refining re-entry models, facilitating precise predictions regarding the timing and location of spacecraft re-entries and associated demise. The work proposed in this abstract is part of the ESA project ”Detection of IR to UV Re-Entry Signatures from Orbit” that has as main objective to design a wide-band orbital detector system to record hundreds of kilometer long streaks emitted from the Earth’s atmosphere due to destructive re-entries of space debris in the IR, Visual, and UV spectrum. One of the main tasks of the project is the simulation of destructive atmospheric re-entry and associated radiative spectra emitted by the fragments, for the purpose of validating the detector system design. The simulation of destructive atmospheric re-entry events is performed with a multi-fidelity tool that has been developed in the context of previous ESA projects. This multi-disciplinary framework combines low- and high-fidelity aerothermodynamics, thermal analysis, 6DOF dynamics and structural analysis in a modular approach to achieve a favourable trade-off between computational cost and accuracy. The present abstract introduces the most recent developments on the tool that focus on improving the accuracy and robustness of the thermal modelling by coupling to an external material response solver. The solver is a modular analysis platform for multiphase porous reactive materials, but it can be run as a simple Fourier heat transfer code. In the coupling methodology, for each trajectory point, the reentry code provides information on the shape of the fragments, as well as surface convective heating, to the material
response solver. The latter handles the simulation of the heat transfer problem within the solid, with a surface energy balance being solved at the wall boundary of each fragment, i.e., the solid-fluid interface, accounting for convection, conduction and radiation. The improved modelling of the surface temperature distribution of the fragments is then used to calculate the spectral emissions to be detected by the sensor.

Speaker: Catarina Garbacz (University of Strathclyde)

2024-09-Oxford-AerothermodynamicMaterialResponseRadiation(3).pptx

9 A reinvestigation of the C$_2$ Deslandres-d'Azambuja radiative system

Previous studies in the scope of Icy Giants entries and aerocapture have highlighted \cite{Coelho:2023} that small concentrations of \ce{CH4} in the freestream, as well as injection of carbon-containing ablation products in the boundary layer may yield dramatic increases of radiation, owing to the formation of C-containing species that are known to be strongly radiative at the representative post-shock temperatures.

In the scope of our analysis presented in our other conference paper (Radiative Heating for Ice Giant Entries), it is found that radiation from the \ce{C2} Deslandres-d'Azambuja radiative system is dominant, with radiation of the \ce{C2} Swan System a second to this system.

However, neither ground-test experiments, neither other studies in related thermodynamic conditions (radiation emitted from Brown Dwarf stars) show a preponderance of the \ce{C2} Deslandres-d'Azambuja system, compared to the \ce{C2} Swan System, with both Systems having at best euivalent intensities.

We have therefore re-investigated the Einstein coefficients for this system. Our findings will be presented in this work. We will conclude the presentation with a small outlook on a work of James Prescott Joule on Shooting Stars.

Speaker: Mario Lino da Silva (Instituto de Plasmas e Fusão Nuclear - Instituto Superior Tecnico)

C2-deslandres-dazambuja.pdf

Coffee Break

High speed facilities, flight testing and propulsion

10 Optical Emission Spectroscopy Measurements of Nitric Oxide In The Shock Layer Produced In a Hypersonic Shock Tunnel

Background
Hypersonic flows are characterized by complex thermochemical processes that are difficult to simulate. Ultimately, there is a need for experimental data to validate the multiple modeling approaches present in the literature. Optical emission spectroscopy (OES) can be used as a non-intrusive measurement to probe the excited thermo-chemical state of important radiating species, such as nitric oxide (NO). NO readily forms in hypersonic flows and is the dominant radiator in the ultraviolet (UV). In the current study, spatially resolved OES measurements of NO in the UV are taken within the shock layer over a cylinder and wedge geometry in the Sandia Hypersonic Shock Tunnel (HST). These spatially resolved emission spectra are fit using two spectral modeling codes, NASA NEQAIR and the Sandia Spectral Physics Environment for Advanced Remote Sensing (SPEARS). The spectral model fits are used to extract the NO rotational and vibrational temperatures and are compared to direct simulation Monte Carlo (DSMC) simulations of the hypersonic flow-field that utilize the latest thermo-chemical models. The modeling approach is described in more detail in an accompanying abstract submitted by the authors. The OES measurements and comparisons to simulation are made in regions of rapid flow expansion, where thermal non-equilibrium between the NO rotational and vibrational temperature may be induced.

Methodology
OES measurements are made in Sandia’s HST, a free-piston shock tunnel that produces high-enthalpy Mach 8-10 flow. Spatially resolved OES measurements are performed in the shock layer formed around two models placed in the core flow of the HST: a 50.8mm cylinder and 50.8mm square wedge. For the cylinder case, OES measurements are made through the shock layer along the stagnation streamline and at angles 30 and 60 degrees to the horizontal. In the wedge case, measurements are made along a horizontal line 1.59 and 6.59 mm below the tip of the expansion corner. Measurements were performed for two freestream conditions generated in the HST.

Shock layer emission was collected using a 50.8mm diameter lens mounted into the shock tunnel side wall. A 2f imaging setup directly imaged the shock layer onto the slit of a spectrometer mounted onto an angular rotation stage. This allows the vertical extent of the slit to align with the various angles measured in the cylinder geometry. The imaging spectrometer captured a single shot image for each shock tunnel run, where the vertical extent of the image represented a spatial dimension through the shock layer and the horizontal captured a wavelength dimension ranging from 205-265nm. Vertical bins of 30 pixels were taken to determine NO spectra within a given spatial region of the shock layer.

The spectra measured at each location are corrected for relative intensity and fit using two spectra modeling codes: NASA's NEQAIR v15.1 and the Sandia Spectral Physics Environment for Advanced Remote Sensing (SPEARS). Four NO emission bands will radiate in the UV range: the γ, β,δ, and ε bands. Spectral fits performed using NEQAIR utilize all four band systems while SPEARS only includes the first three taken from the ExoMol NO linelist. Spectral fits using both codes assume a multi-Boltzmann distribution for independent rotational and vibrational states. The NEQAIR spectral fits use a non-Boltzmann quasi-steady-state (QSS) solver to solve for the electronic state distribution function while the SPEARS spectral fits assume a Boltzmann distribution.

Results
For each freestream condition and measurement location, OES measurements were repeated a minimum of three times. The vibrational and rotational temperatures determined through spectral modeling fits of the repeated cases were averaged and the variance across each repetition was small, giving confidence in the repeatability of the measurement.

Initial temperature fits of the OES measurements made along the stagnation streamline for the cylinder geometry show the vibrational and rotational NO temperature are in thermal equilibrium and agree with DSMC simulations. At the 30 and 60 degree locations on the cylinder geometry the spectral radiance decreases rapidly as the flow expands and cools. Initial results show a decrease in the fitted rotational and vibrational temperature at larger angles to the horizontal. The fitted rotational temperatures found using both spectral modeling codes agree with DSMC predictions. However, only the vibrational temperature fit found using SPEARS agrees with simulation. Fits performed using NEQAIR predict vibrational temperatures ~2000K higher than the rotational temperature.

The sources of the discrepancy between the fitted vibrational temperatures produced using SPEARS and NEQAIR will be explored and discussed in the full presentation. Further, the data collected on the wedge geometry will be fully processed and the results will be presented for the full presentation.

Conclusion
OES measurements were made of NO produced within the shock layer for a cylinder and wedge geometry in Sandia’s HST facility. The collected emission was fit using Sandia’s SPEARS code and NEQAIR to determine line profiles of the vibrational and rotational temperature of NO. Measurements were collected over multiple areas on each geometry and targeted areas of flow expansion where thermodynamic non-equilibrium may occur. Fitted temperatures are compared to DSMC simulations utilizing the latest thermo-chemical models.

Initial results show the fitted rotational temperature agrees with DSMC predictions while the NEQAIR vibrational temperature fits yield temperatures ~2000K higher than the DSMC predictions and the SPEARS vibrational temperature fits. Potential sources of these disagreements and the measurements on the wedge geometry will be discussed in the full presentation.

Note
The authors are submitting a companion abstract which discusses in detail the simulation and modeling results whereas this abstract describes the experimental effort.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

Speaker: Kyle Daniel (Sandia National Labs)

RHTG_OES.pptx

11 Modeling of Hypersonic Flow Over a Cylinder in a Reflected Shock Tunnel Facility

Background of the study

Optical emission spectroscopy (OES) is a valuable tool to gather information about complex thermochemical processes that take place in hypersonic flowfields. Furthermore, optical emission data plays a vital role in the validation of high-fidelity thermo-chemical models used to numerically simulate these flowfields. This work is part of an ongoing study at the Sandia National Laboratories at Albuquerque, NM which is focused on improving diagnostic techniques for hypersonic flows and the overall understanding of flow physics. The experimental side of the project involves taking spatially-resolved emission measurements within the shock layer for a hypersonic flow over a cylinder in the Sandia Hypersonic Shock Tunnel (HST). The computational side of the project entails the utilization of latest thermo-chemical models to simulate the hypersonic flowfield over the cylinder and compare estimates of flow macroparameters obtained numerically as well from experimental measurements. Computational estimates of the flowfield also help identify flow regions which are favorable and important for making emission measurements.

Methodology

The experimental setup consists of a cylinder made of machined aluminum with a diameter and length of 5.08cm (2”) and 10.16cm (4”) respectively. The cylinder is placed 10.16cm (4”) downstream of the nozzle exit and optical windows on the tube wall provide access to the stagnation, expansion and wake regions of the flow. The experimental campaign spans two test conditions with freestream velocities of 4km/s and 4.5km/s. Banded spectral imaging in the infrared (IR), 5090-5510nm, and ultraviolet (UV), 227-237nm, regions are performed for the stagnation region of the flow. Furthermore, wavelength resolved UV emission measurements are made along the stagnation line as well as along lines which are at angles of 30 degrees and 60 degrees from the stagnation line. For the computational setup, the cylinder is modeled with an isothermal wall approximation at 300K. The computational domain has a size of 16.51 x 5.08cm with the geometry set up to have the stagnation point at x=0cm. The complete set of freestream parameters for the computational domain are obtained based on a nozzle expansion calculation performed at Sandia National Laboratories using the Sandia Parallel Aerodynamics and Reentry Code (SPARC). The results from the SPARC calculations show that the freestream for the cylinder consists of atomic oxygen, diatomic oxygen, diatomic nitrogen and nitric oxide (NO). Furthermore, it is in a state of thermal non-equilibrium with the diatomic species having different vibrational temperatures from each other.

The hypersonic flowfield over the cylinder is computed using a Direct Simulation Monte Carlo (DSMC) solver. The computational domain has about 1billion simulation particles, with at least 250 particles in each collision cell. The DSMC results used in this work are steady state time averaged over at least 100,000 time step samples. The numerical setup for the DSMC calculations consists of a five species air mixture with 19 reactions. A high-fidelity approach that takes vibrational favoring into account is used to model dissociation reactions. A forced harmonic oscillator (FHO) is used to calculate vibrational relaxation rates for diatomic oxygen and diatomic nitrogen and a calibrated Larsen-Borgnakke (LB) model is used to model the vibrational relaxation of NO through collisions with atomic oxygen. The redistribution of energy after a reaction is performed using the LB model. Furthermore, the high fidelity nonequilibrium model utilizes discrete vibrational and rotational energy levels in its computations to represent the behavior of internal modes more accurately.

Results

We note from the DSMC solutions that the shock standoff distance from the cylinder wall along the stagnation line is roughly 8.2mm. The bulk translational temperature peaks around 11,200K and the mixture achieves thermal equilibrium roughly 2.8mm downstream of the shock. A comparison of the DSMC estimates for the bulk translational temperature and bulk vibrational temperature highlights the difference in relaxation times associated with the respective internal energy modes. The peak bulk translational temperature observed in the domain is near the shock front whereas the peak bulk vibrational temperature is at a small distance downstream of the shock. Furthermore, the bulk translational temperature decreases rapidly as the flow expands over the shoulder of the cylinder whereas the drop off in the bulk vibrational temperature is not as quick. It is to be noted, however, that the bulk temperatures are dominated by the respective specie specific temperatures for diatomic nitrogen which has the highest mole fraction in the flow mixture.

Distributions of DSMC estimates for NO mole fractions show that NO populations peak a small distance downstream of the shock in the stagnation region and are then convected downstream into the expansion and wake regions due to high flow speeds. We note significant quantities of NO in the expansion and wake regions with spatial variation in distributions governed solely by flow transport properties. This implies a low degree of chemical reactivity in this region of the flow and that the thermo-chemistry is dominated by the high flow speeds. NO vibrational temperatures peak near the shock front and decrease rapidly as the flow goes through the expansion and wake regions. This behavior, which contrasts with the bulk vibrational temperature, shows that the vibrational relaxation rate for NO is higher than that of diatomic nitrogen. The expansion and wake regions of the flow are characterized by thermal non-equilibrium with the bulk vibrational temperature exceeding the bulk translational temperature by at most 400K near the 60deg line and by 1000-2500K beyond the shoulder of the cylinder. NO vibrational temperatures also exceed bulk translational temperatures by up to 600K in the expansion and wake regions.

Conclusion

The ongoing experimental campaign at the Sandia HST will provide spatially and wavelength resolved measurements of UV emission spectra along the stagnation, 30deg and 60deg lines as well as spectrally banded images of UV and IR emission in the stagnation region. We will generate numerical estimates of the UV emission spectra using the Nonequilibrium Radiative Transport and Spectra Program (NEQAIR), our DSMC results and Collisional-Radiative models for NO electronic states developed by the authors. The numerical estimates of UV emission will be compared to experimental measurements. We will also generate fits of the experimental spectra to obtain estimates of NO rotational temperature and NO vibrational temperature and produce comparisons of them with DSMC results for those quantities.

Note

The authors are submitting another abstract which discusses in detail the experimental set up and results whereas this abstract goes over the simulation and modeling results.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

Speaker: Mr Shubham Thirani (University of Illinois Urbana-Champaign)

SandiaCyl_RHTG2024

12 VACUUM ULTRAVIOLET EMISSION SPECTROSCOPY IN SHOCK TUBE FLOWS

1. INTRODUCTION
   Atmospheric entry leads to extreme heat flux onto the
   flight vehicle’s surface due to the high enthalpy of the surrounding
   flowfield. Currently, thermal protection systems
   are severely oversized which makes vehicles too heavy,
   restricting performance and payload capacity. One large
   source of uncertainty is thermochemical non-equilibrium,
   which has been shown to have a strong effect on surface
   heat flux and shear stress, gas radiation, and flowfield
   characteristics. Improved design of hypersonic vehicles
   and re-entry capsules necessitates more accurate predictive
   capabilities of non-equilibrium flows. Ground testing
   is conducted to generate flows similar to those encountered
   in flight, replicating the essential features of
   non-equilibrium flows. One such canonical experimental
   setup is the shock tube, where a normal shock transiently
   passes through a straight tube [CB17]. Although subject
   to several facility-related artefacts [CMM23, CSMM22],
   shock tubes represent one of the most fundamental fluid
   mechanical processes, which allows the isolation of thermochemical
   non-equilibrium from other aspects of complex
   flowfields. As such, shock tubes provide the opportunity
   of studying fundamentals of thermochemical reactions
   for flight-relevant enthalpies. Previous datasets span
   different facilities, with NASA Ames’ EAST and Oxford
   University’s T6 representing some of the most recent examples
   [GCM22, CB17].
   Both gas radiation and thermochemistry can be readily
   analysed using optical diagnostics. Optical emission
   spectroscopy (OES) collects the emitted light from highly
   energetic particles, which produce the majority of radiative
   heat flux. Data can be used to infer number densities
   of highly excited states, as well as internal excitation
   temperatures of electronic, vibration, and rotation
   modes. In cases of significant self-absorption, OES can
   be utilised to infer the population density of low energy
   states, or even the ground state, i.e. the most abundant
   energy states which make up the majority of the particles
   present [HLF+17]. Transitions suitable for this analysis
   can be found in the VUV spectral region. This region features
   emission lines of atomic oxygen and nitrogen where
   the lower state, i.e. the absorbing state, corresponds to
   metastable and ground states. Furthermore, the VUV is
   the dominant spectral region for radiative heat flux to a reentry
   capsule’s surface. Existing data of emission spectra in the VUV is very sparse due to the experimental complexity
   associated with the respective measurement technique
   [CMGO09, MJM+23, HLF+17], e.g. the optical
   path needs to be either evacuated or flushed with a nonabsorbing
   gas.

2. METHODOLOGY
   Shock tube flows are generated in the Oxford T6 Stalker
   Tunnel in aluminium shock tube mode [GCM22]. The
   strong driver conditions of T6 enable high shock velocities
   while retaining a large tube diameter, providing a
   long optical path for a spectroscopic system. This enables
   a larger signal to noise ratio in the measurement
   of spectroscopic data while minimising exposure times
   in order to reduce the amount of spatial blurring due
   to shock movement. Optical measurements are taken
   through windows set in the shock tube wall, and acquisition
   is triggered to record as it passes this location. A
   number of flush-mounted piezoelectric pressure sensors
   along the length of the facility record the arrival time of
   the shock wave which allows subsequent rebuilding of
   the shock history. Recent work has shown that the shock
   history, and associated hydrodynamic behaviour of the
   flow has a significant influence on the thermodynamic
   state [CSMM22]. Furthermore, hydrodynamic effects
   have been shown to significantly affect the time of flight
   of reacting particles which necessitates a spatial transformation
   to match conditions between flight case and shock
   tube [CMM23]. Flow conditions investigated in this work
   will consist of a set of velocities between 5.5 km.s􀀀1 and
   10 km.s􀀀1 [GCM22]. The current work extends their
   characterisation to include the now accessible VUV spectral
   region. Some of the considered test conditions also
   directly correspond to data collected in the EAST facility,
   allowing cross-facility comparison [CB17].
   The VUV spectroscopic system is designed to collect radiation
   between 116 nm and 900 nm. Even though longer
   wavelengths in the Ultraviolet, Visible and Near Infrared
   are accessible, the system is optimised towards wavelengths
   between 116 nm and 250 nm where the quantum
   efficiency of the detector, blazing angle of the dispersion
   grating are greatest. The detailed functionality of the system
   is presented in [MGC+22], and previous measurements
   in the OPG2 plasma wind tunnel facility are presented in [BCWH24]. Because molecular oxygen and
   water vapour absorb VUV wavelengths in ambient air,
   any acquisition set-up must operate either using oxygenfree
   gas or under a high level of vacuum, the latter being
   the approach of the current work. The collection optics
   system is designed to be contained in vacuum chambers
   fitted to the shock-tube via the test section window. The
   telecentric optical system images light onto the entry slit
   of a spatially resolving VUV spectrograph (McPherson
   207V). An intensified P43 iStar sCMOS camera is connected
   to the spectrograph to record the spectra. The
   whole system is pumped down to a low vacuum level
   (10􀀀3 Pa range) up to the test section’s window using a
   dry scroll pump (Edwards nXDS10i) and a turbopump
   (Edwards nEXT300D) mounted in series.
   Wavelength calibration is performed by fitting easily
   identifiable atomic lines from an IntelliCal mercury (Hg)
   and neon-argon (Ne􀀀Ar) calibration source to their expected
   wavelengths. The fitted wavelength axis is subsequently
   used to obtain the Spectral Instrument Line Shape
   (ILS) by fitting the most intense spectral line measured to
   the square-root of a Voigt profile, the most accurate shape
   function to capture the ILS with an intensified CCD array.
   The experimentally measured full-width half-maximum
   parameters are 0.86 nm and 0.01 nm for Gaussian and
   Lorentzian profiles respectively. Spatial smearing was
   measured using a knife edge placed in front of an integrating
   sphere at the centreline of the test-section. Images
   were recorded at different locations along the field
   of view to obtain edge spread functions (ESF) and resolve
   spatial smearing in space. The LSF is a good indication of
   signal intensity loss and spatial smearing of information,
   and dictates the practical spatial resolution limit of the
   optical system. The experimentally measured full-width
   half-maximum parameters varied from 0.556 to 1.75mm
   for Gaussian and 0.008 and 0.210mm for Lorentzian profiles,
   with averages of 0.8362 and 0.0575 mm.

3. RESULTS
   The full paper will present the data of the currently ongoing
   test campaign and will contain setup, calibration and
   initial post-processing. Calibration is carried out in all
   three dimensions of wavelength, space and absolute spectral
   radiance. This system will allow a cross-comparison
   to measurements taken in NASA’s EAST facility and will
   extend the measured wavelength range of previously investigated
   conditions in T6. Furthermore, the data will
   allow the investigation of chemical non-equilibrium, by
   utilising the characteristic features of emitted radiation
   behind strong shock waves.

Speaker: Maïlys Buquet

09_09_MB_RHTG.pptx

Tuesday 10 September

Coffee Break

Plasma facilities, simulations and diagnostics

13 A Tightly Coupled Numerical Framework for Hypersonic Material Testing in ICP Environments

Background of the study: To prevent damage to a re-entry vehicle, its windward side must be protected by a heat shield made from advanced thermal protection materials (TPMs). In-flight testing of new TPMs is prohibitively expensive, so the extreme heat experienced during hypersonic re-entry is simulated by exposing a TPM sample to a hot plasma jet. Inductively coupled plasma (ICP) facilities are particularly useful for this testing because they generate large, contamination-free plasma volumes without using electrodes, making them a preferred choice for evaluating thermal protection systems. Numerical simulation of ICPs plays an important role in supplementing the experiments in the facility in order to accurately characterize the ablative properties of the TPM. In aerothermodynamics, the primary quantities of interest are the surface heat flux and the time-integrated heat flux (heat load). Conventionally, a decoupled approach using the film coefficient engineering model is used to study the material response, where a fixed boundary condition obtained from CFD calculations is given to the material response code [1]. However, the most effective way to account for all the strongly interacting physics is to combine the fluid flow field and thermal response analysis into a single computational simulation. This requires a fully coupled approach where the flow solver is tightly coupled with the material response solver to accurately predict the ablative characteristics of the material being tested in the plasma facility. Hence, this work aims to develop a strongly coupled framework for accurate prediction of the response of TPM samples under the inductively coupled plasma environment.

Methodology: The dynamics of the gaseous plasmas treated in this work are governed by the species mass, global momentum and energy, and vibronic energy equations which have been implemented in a finite-volume solver HEGEL [2] which uses the PLATO [3] library for the evaluation of plasma-related quantities (e.g. thermodynamic properties, etc.). The electromagnetic field inside ICPs is governed by the set of Maxwell’s equations which are solved in a mixed finite-element solver FLUX [4]. The material response is simulated using CHyPS [5] employing a high-order discontinuous Galerkin (DG) formulation.
The governing equations for the plasma are coupled to those for the electromagnetic field via the Lorentz forces and Joule heating source terms in the momentum and energy equations, respectively. At the same time, the electric and magnetic fields within the plasma are affected by the electrical conductivity of the plasma. As a result, the two datasets (Lorentz forces and Joule heating for HEGEL, and electrical conductivity for FLUX) are to be passed at every fluid time-step to accurately capture the magneto-hydrodynamic phenomena occurring within an ICP. Similarly, HEGEL passes the heat flux, pressure, and species fluxes to CHyPS for the surface to be coupled. CHyPS in return provides the surface temperature and composition along with the blowing rates. The coupling between HEGEL and FLUX takes place in the entire volume of the domain and hence is denoted as volume coupling. The HEGEL-CHyPS coupling occurs only along the surface of the material denoted as surface coupling. The required coupling between the above-mentioned solvers is here realized using preCICE [6], an open-source coupling library for partitioned multi-physics simulations.

Results: Simulations have been conducted for the UIUC Plasmatron X facility under the Local Thermodynamic Equilibrium (LTE) assumption with a circular sample placed in an air plasma jet. The torch has two injectors: central injection consisting of 15 holes which are inclined 15 degrees down and have 24 degrees swirl angle, and sheath gas injection consisting of 72 holes which inject gas straight in the axial direction. For simplification, both injectors are assumed to be continuous annular injectors for 2D axi-symmetric simulations conducted in this work. At the end of the torch, a straight nozzle of length 129.54 mm has been used. Mass flows in the central and the sheath gas injection is kept fixed to be 0.86 g/s and 7.13 g/s respectively. The working gas is air modeled using an 11-species mixture: $\mathcal{S} = \left\{\mathrm{e}^-,\, \mathrm{N}_2, \, \mathrm{O}_2, \, \mathrm{NO},\mathrm{N},\, \mathrm{O}, \, \mathrm{N}_2^+, \, \mathrm{O}_2^+,\, \mathrm{NO}^+,\, \mathrm{N}^+,\, \mathrm{O}^+\right\}$. The operating conditions for the simulation are as follows: pressure 590 Pa, and power 300 kW. The coil frequency in the facility is fixed to be 2.1 MHz. First, an uncoupled ICP simulation (i.e. without the material solver) is performed for the above operating condition which gives a supersonic plasma jet in the chamber. This agrees with the observations from experiments where the torch becomes choked at low-pressure and high-power combinations, giving a supersonic plasma jet in the facility.
Next, a fully coupled simulation is performed where the plasma flowfield is coupled with the material response. The preliminary result presented here considers only heat conduction calculation in the material response code ignoring the pyrolysis and ablation effects to showcase the correct implementation of the coupling framework. The material is assumed to be non-porous with thermal conductivity, density, and specific heat capacity of 0.5W/mK, 180 Kg/m3, and 2000 J/KgK, respectively. For the coupled simulation, the material code CHyPS runs in a time-accurate manner for 10 s. The coupling occurs every 0.01 s i.e. CHyPS provides the surface temperature to the plasma solver HEGEL and gets back the heat flux every 0.01 s. For each coupling window, HEGEL converges to a steady state for the given temperature boundary condition. This two-way coupling ensures that the flow boundary layer in the fluid domain responds to the changes occurring in the material, thereby updating the heat flux in every coupling window. This allows for a more accurate prediction of material response. The plasma and sample temperature contours at different instances of time show the coupled dynamics of the material. At t = 0 s, the material is completely cold (300K) everywhere. As time progresses, the sample temperature increases due to the heat flux being applied by the plasma jet at the sample surface. At t = 10 s, the maximum temperature in the sample reaches around 2400K. The plasma temperature remains unchanged everywhere except for regions very close to the sample surface.

Conclusion: A high-fidelity multi-physics simulation framework for inductively coupled plasma discharges has been presented where a fluid solver, an electromagnetic solver, and a material response solver, have been coupled to provide a flexible formulation for predicting TPS material response under ICP environments. A preliminary plasma-material coupled simulation has been presented for a heat conduction problem indicating the correct implementation of the framework. Future work will include the pyrolysis and ablation effects in the material incorporating the effect of material recession in the coupled simulations via the mesh movement capability of the solvers.

Acknowldegments: The present research is funded by Vannevar Bush Faculty Fellowship OUSD(RE) Grant No: N00014-21-1-295. The work is also supported by the Center for Hypersonics and Entry Systems Studies (CHESS) at UIUC. Computations were performed on Frontera, an HPC resource provided by the Texas Advanced Computing Center (TACC) at The University of Texas at Austin, on allocation CTS20006.

References: [1] Cooper, J., Salazar, G., and Martin, A., “Numerical investigation of film coefficient approximation for chemically reacting boundary-layer flows,” J. Thermophys. Heat Transf., Vol. 37, No. 3, 2023, pp. 644–655.
[2] Munafò, A., Alberti, A., Pantano, C., Freund, J. B., and Panesi, M., “A computational model for nanosecond pulse laser-plasma interactions,” J. Comput. Phys., Vol. 406, 2020, p. 109190.
[3] Munafò, A., and Panesi, M., “Plato: a high-fidelity tool for multi-component plasmas,” AIAA Aviation 2023 Forum, 2023, p. 3490.
[4] Kumar, S., Munafò, A., Le Maout, V., Mansour, N., and Panesi, M., “Self-consistent magneto-hydrodynamic modeling of ICP discharges,” AIAA SCITECH 2022 Forum, 2022, p. 1619.
[5] Chiodi, R. M., Stephani, K. A., Panesi, M., and Bodony, D. J., “CHyPS: A high-order material response solver for ablative thermal protection systems,” AIAA SCITECH 2022 Forum, 2022, p. 1501.
[6] Bungartz, H.-J., Lindner, F., Gatzhammer, B., Mehl, M., Scheufele, K., Shukaev, A., and Uekermann, B., “preCICE–a fully parallel library for multi-physics surface coupling,” Comput. Fluids, Vol. 141, 2016, pp. 250–258.

Speaker: Mr Sanjeev Kumar (University of Illinois at Urbana-Champaign)

Sanjeev_RHTG.pptx

14 Development of a Computational Framework for Hypersonics and Plasma Discharges

The calculation of reactive flows such as those found in hypersonics, combustion, laser-plasma interactions and gas discharges requires a theoretical framework encompassing non-equilibrium thermodynamics, transport, collisional and radiative kinetics as well as gas-surface
interactions and electromagnetics. In many situations the multi-component working fluid may be viewed as a collection of dilute gases. Under these circumstances the most rigorous hydrodynamic description may be obtained based on kinetic theory (e.g., Chapman-Enskog method), though approximate formulations exist and are still in use.

The hypersonics community is currently devoting significant efforts in the development of innovative physics-based models to overcome correlation-based formulations developed mainly
during the 1980’s (e.g., multi-temperature models). In this scenario a simulation framework should be designed by separating as much as possible the physics from the numerics, by
minimizing the dependency of the data structure on the details of the physico-chemical model in use, and by allowing for multi-physics coupling.

The above considerations motivated the development of hegel (High-fidElity tool for maGnEto-gasdynamics simuLa- tions). This tool is written in objected oriented Fortran 2008 and is parallelized based on a domain decomposition approach through the combined use of MPI and petsc. To enable multi-physics simulations, hegel may be coupled
to other solvers using the preCICE open source library. The fluid governing equations are discretized in space based on the cell-centered finite volume method. Time integration is accomplished by means of explicit, implicit or implicit-explicit (IMEX) methods. The calculation of thermodynamic, transport and optical properties as well as source
terms resulting from kinetic processes is delegated to plato (PLAsmas in Thermodynamic nOn-equilibrium), a library capable of treating physico-chemical models of increasing complexity and accuracy, ranging from legacy multi-temperature to grouping and/or state-to-state models. Usage of other tools available in the literature such as eglib, mutation, mutation++ and kappa, is also possible with minimal implementation
efforts. Both hegel and plato are part of the suite of in-house softwares developed at the Center for Hypersonics and Entry Systems Studies (CHESS) at University of Illinois at Urbana-Champaign.
The purpose of this work is to showcase the capabilities of hegel.

Applications will consider simulations of inductively coupled plasma wind tunnels, laser-induced breakdown phenomena and hypersonic flows past blunt bodies.

Speaker: Alessandro Munafo

talk.pdf

movie

• Q_1e7_iso_volume.mp4
• T_iso_volume.mp4
• T_jet_movie.mov
• Xe_movie.mov

15 An overview of the VKI Plasmatron testing envelope through Optical Emission Spectroscopy

Plasma wind tunnels are fundamental assets when designing Thermal Protection Systems (TPS) for re-entry capsules or the demise process of Space Debris (SD) subjected to uncontrolled atmospheric re-entry. In ground, thermal and momentum variables such as enthalpy, pressure and velocity must be accurately measured to reproduce the aerothermochemical environment of the hypersonic flow experienced during the atmospheric re-entry. However, the harsh testing environment due to the extremely high temperatures, makes a direct measure of such quantities extremely challenging.

The von Karman Institute Plasmatron, a leading Inductively Coupled Plasma (ICP) wind tunnel, is extensively used for testing thermal protection systems (TPS) and studying the demise of space debris. The facility can operate in supersonic and subsonic conditions, but only the latter are here considered. Even though re-entry trajectories occur at hypersonic speeds, experiments in subsonic regime are crucial for TPS and space debris studies. These tests replicate the aerothermochemical environment of the stagnation region near the boundary layer, as described by the Local Heat Transfer Simulation (LHTS) methodology. The LHTS guarantees that, under the assumption of Local Thermochemical Equilibrium (LTE) at the boundary layer edge, the chemical non-equilibrium boundary layer, and consequently the total heat flux at the wall, in the ground and flight are the same if the total enthalpy, total pressure and the velocity gradient are matched. The velocity gradient, mostly linked with the geometry of the samples/flying body, is determined through numerical computations. The pressure is assumed constant and equal to the measured static pressure of the chamber ($p_{c}$) being the plasma flow in a low subsonic regime. The enthalpy is estimated using rebuilding procedures or semi-empirical formulation.

In the last few years, experimental campaigns at VKI highlighted OES as a powerful tool to extract thermodynamic quantities such as enthalpy and temperature. Extracting such variables involves several steps and assumptions that will be reviewed in the following. This work aims to extend the application of OES to a broad range of testing conditions covering the majority of VKI Plasmatron envelope, in subsonic regime and considering an air mixture, and consolidate the procedure highlighting some corrections. Results, in terms of temperature profiles, will be compared with those of CFD computations simulating the flow of the facility.

Enthalpy is computed through equilibrium computation, $h=f(T_{OES},p_{s} \approx p_{c})$ with $T_{OES}$ the temperature from the OES. Retrieving $T_{OES}$ involves multiple steps that can be summarized as follows: i) filter the raw data from the experimental noise and background, ii) calibrate the system in absolute intensity, iii) retrieve the local emission ($\varepsilon$) through the Abel inversion transform assuming axisymmetry of the jet and negligible absorption effects. iv) Temperatures are then extracted and compared employing two different procedures. The first approach considers the electromagnetic radiation emitted by an atomic species, and it relies on the
isotropic volumetric emission intensity equation

\begin{equation}
\varepsilon_{u l}=\frac{E_u-E_l}{4 \pi} \mathcal{A}{u l} n_i(p, T) g_u \frac{e^{-E_u /\left(k{\mathrm{B}} T\right)}}{Q_{\mathrm{int}, i}(T)},
\end{equation}

\noindent that can be inverted and solved for the temperature, with the left-hand side evaluated experimentally. In this work, we will consider only the transition of the atomic oxygen at $\approx$ 777nm, with the corresponding temperature $T_{O777}$. Attention is directed towards validating the approach with synthetic spectra. We simulate the local emission of the VKI Plasmatron using the NASA spectral model ($\textit{NEQAIR}$). For a fixed value of $p_c$, the database is generated for a set of temperatures assuming LTE (translational, rotational, vibrational and electronic temperatures are set equal, and the number density of the species of interest follows a Boltzmann distribution), for air mixture of 11 species (e-, N+, O+, NO+, N2+, O2+, N, O, NO, N2, O2). The synthetic database is then convolved with the Instrumental Line Shape (ILS) function providing an estimate of the left-hand side of the equation. For $p_c$ = 50mbar and temperatures ranging from 5500 to 7900K, results show discrepancies between 160 and 260K. The reason lies in the typical ILS function of the set-up approximated by the square root of a Voigt function with a Full Width Half Maximum of 0.53nm. The second approach retrieves the temperature by fitting the experimental spectra with the simulated counterpart. The estimated temperature, $T_{SF_{\lambda_1-\lambda_2}}$, result from an optimization procedure minimizing the residual

\begin{equation}
\mathcal{R}(T_{SF_{\lambda_1-\lambda_2}})=\sum_p \left[\varepsilon^\text{synth}\left(\lambda_p,T\right)-\varepsilon^\text{meas}\left(\lambda_p\right)\right]^2 / \left[\varepsilon^\text{meas}\left(\lambda_p\right)\right]^2,
\end{equation}

\noindent with $\lambda_1$-$\lambda_2$ denoting the boundaries of the spectral range considered, and $\lambda_p$ the discrete wavelength. Two separate ranges are considered : 710-875nm and 340-430nm. The former is dominated by the atomic transition of O and N, the latter by some of the molecular rovibronic bands of CN and N2+ and N2. Although the concentration of CN (CO2 disocciates and atoms of C and N recombines) is in the order of particles per million (ppm), small variations may significantly impact the temperature estimation being CN a strong radiator. Four different databases are considered with different concentrations of CO2: 0, 50, 100 and 410ppm.

An experimental campaign is carried out at four different electrical power of the VKI Plasmatron facility, $\text{P}_{el}$ = 175, 200, 250 and 275kW, and $p_c$ = 50mbar. Plasma emissions are recorded from the lateral side at 335mm and 385mm along the jet-centerline from the torch exit section. Results show an excellent agreement between $T_{O777}$, corrected to take into account the effect of the ILS, and the spectral fitting in 710-875nm, $T_{SF_{710-875}}$, with a maximum discrepancy below 50K. Results in the 340-430nm range show 100ppm being the concentration minimizing the residual in all conditions. Temperature differences between $T_{SF_{340-430}}$ and $T_{O777}$ range from 60 and 130K for the low and mid power and up to 350K at the highest condition, due to possible thermal non-equilibrium effect. Overall, the good agreement between the temperatures validate the assumption of LTE, at least for temperatures lower than 7300K. Results can be therefore compared with numerical simulation of the temperature profile of the in-house CoolFluid-ICP solver that simulates the flow field of the facility under the assumption of LTE and axisymmetric steady flow. Results show that an efficient of $\eta= \frac{\text{P}^{num}_{el}}{\text{P}^{exp}_{el}}$ $\approx$ 44$\%$ exists between the numerical and experimental $\text{P}_{el}$ even though further investigations, both from a numerical and experimental perspective, are necessary. By the time of the conference, the objective is to extend the analysis to multiple testing conditions (e.g, $p_s$ = 15, 30, 75, 100, 200mbar), covering the majority of the VKI Plasmatron envelope in subsonic regime.

Speaker: Enrico Anfuso (Vrije Universiteit Brussel & von Karman Institute)

2024_Anfuso_Enrico_ppt_v5.pptx

16 Spectral Analysis of High-Enthalpy Flows for Earth & Mars Atmospheric Entry Testing

At the Institute of Space Systems (IRS) of the University of Stuttgart three plasma wind tunnels (PWK1, PWK3, PWK4) are operated to experimentally simulate spacecraft entry into a large variety of atmospheres. In this work, the facilities PWK1 and PWK3 are used to study plasma emission relevant for Mars entry and hyperbolic Earth entry, respectively. In both scenarios, radiative heat flux significantly contributes to the overall thermal load on the thermal protection system (TPS) of a spacecraft. Moreover, analyzing the plasma radiation in ground testing facilities allows for non-intrusive flow characterization, aimed at better understanding the prevailing test conditions. This presentation provides an overview of a large variety of optical diagnostics applied to air and carbon dioxide plasma in PWK1 and PWK3, respectively.
Radial profiles of plasma emission are recorded with an Ocean Optics HR4PRO USB spectrometer in order to identify dominantly radiating species and determine excitation temperatures. Moreover, the radiation simulation tool PARADE (Plasma Radiation Database) is used to determine local number densities of plasma species. A spectral fitting algorithm, developed at IRS for air and carbon dioxide plasma, allows for the automated spectral analysis of large OES datasets in the order of hundreds of spectra, acquired in the course of this work.
Besides OES, specific optical diagnostic methods are developed for characterization of the Mars-relevant carbon dioxide plasma. Tunable Diode Laser Absorption Spectroscopy (TDLAS) is applied to measure the translational temperature of atomic oxygen inside the high-enthalpy plasma jet. This laser absorption method also provides radial number density profiles of excited oxygen states.
An optical setup for Fourier-Transform Infrared (FTIR) Spectroscopy is designed and built at PWK3 at the moment. This system shall target CO2 as well as CO at the same time in the plasma jet, providing information about excitation temperatures and number densities. A nitrogen purging system is installed already to decrease the signal absorption by the atmosphere in the optical path as well as the instrument itself.
With regard to the air plasma in PWK1, extensive studies have been carried out on the effect of a magnetic field on highly enthalpic air plasma flows. For this purpose, thousands of emission spectra of the plasma flow in front of an MHD plasma probe were recorded. In addition, a radiative heat flux (HF) insert was designed and mounted in the stagnation point of a MHD plasma probe. A high temperature superconducting solenoid was used to generate the magnetic field within the MHD plasma probe. The radiative HF sensor consists of five thermo-piles, each positioned at a different angle, with the aim of measuring the radiative HF at the stagnation point over a wide solid angle. Preliminary results indicate that the radiative HF at the stagnation point tends to increase with increasing magnetic field strength, a trend also observed in numerical simulations.
The current effort to design and build the optical setup SEMA (Scanning Etalon for MHD Applications), a confocal Fabry-Perot interferometer, is expected to provide further insight into the effects of magnetic fields on the translational temperature changes in the shock and boundary layer of high enthalpy air plasmas. This will be a key element in better understanding the influence of Joule heating in the shock layer on the radiative HF at varying magnetic field strengths.

Speaker: Georg Herdrich (Institute of Space Systems, University of Stuttgart)

2024-09-10-RHTG_Herdrich_et_al_v2.pdf

10:40 Coffee Break

Radiation modeling and simulation

17 Radiative Heating for Ice Giant Entries

We present an analysis for the radiative heating of characteristic spacecraft shapes in representative entry trajectories into the Icy Giants Uranus and Neptune.

The current analysis develops upon the CFD analysis performed by Fluid Gravity Eng. (FGE) which has yielded a complete set of simulations for an axi-symmetric capsule flying at four representative trajectory points during an Uranus entry, and five representative points for a Neptune entry. The flow is also considered to be axi-symmetric (no angle of attack for the capsule), and accounts for the injection of ablation products into the flowfield boundary layer.

Entry velocities range between 17.5 and \SI{19.5}{\kilo\metre\per\second}. Nominally, for an entry at such velocity ranges in a pure \ce{H2}--\ce{He} atmosphere, radiation should be negligible, as attested by the experimental shock-tube results presented by Cruden \textit{et al.} \cite{Cruden:2017} which showed that shocked flows below \SI{25}{\kilo\metre\per\second} are expected to be radiationless. However, recent results by Coelho \cite{Coelho:2023} have highlighted that small concentrations of \ce{CH4} in Neptune (1.5\%) would yield dramatic increases of radiation, owing to the formation of C-containing species that are known to be strongly radiative at the representative post-shock temperatures in the \SI{5000}{\kelvin} range (atomic \ce{C}, and molecular \ce{C2} and \ce{CH}). Further, even in the absence of freestream carbon species, the presence of ablation products in the hot boundary layer might enhance radiation near the wall.

This warrants a detailed analysis of the radiative properties for those flows. This has been carried out considering the flowfields supplied by FGE, and preforming an analysis of these flowfields treating radiative emission/absorption/transfer in an uncoupled fashion. Flowfield conditions over four representative line of sights have been supplied, and radiative properties have been calculated using a specifically tailored radiative database, and deploying a simplified tangent-slab radiative transfer model.\

Speaker: Mario Lino da Silva (Instituto de Plasmas e Fusão Nuclear - Instituto Superior Tecnico)

IGSens-presentation.pdf

18 Characterization of high temperature N2/CH4 emission

Emission spectroscopy measurements of N2/CH4 plasma are presented from 350 to 850 nm. The high-temperature plasma is generated at atmospheric pressure using a 50-kW Inductively Coupled Plasma (ICP) torch. The plasma is found to be close to LTE at ~ 6900 K and is then passed at high velocity through a water-cooled tube. The water-cooled tube induces rapid cooling and fast recombination. Spatially resolved emission measurements are presented at the torch exit (entrance to the water-cooled tube) and at the exit of the water-cooled tube. Several different tube lengths are currently being studied. The spectra are calibrated in absolute intensity and wavelength. The measured spectra are compared to simulations using the SPECAIR line-by-line radiative code, which has been updated using the trihybrid Exomol database. The collected data will be used to obtain the evolution of the rotational temperature and the density of the radiating states.

Speaker: Cyrine Merhaben (EM2C Laboratory, CentraleSupélec)

RHTG_24_09_05.pptx

19 Through-Model Fibre Optic Emission Spectroscopy for Studying Planetary Entry Radiative Heat Flux

Background of the study:

For most re-entry cases, spacecraft experience radiative heating from the hot shock layer flow which envelops the vehicle during planetary entry. Due to the long length scales that are required for the flow to equilibrate at conditions near peak heating, there is often significant uncertainty in the chemical environment in many planetary entry scenarios which creates uncertainty in the radiative heat flux experienced by the vehicle.

To lower these uncertainties, experiments are performed in shock tubes and expansion tubes where generally emission spectroscopy is used to ascertain the flow relaxation in this post-shock flow with distance by looking normal to the flow. In a shock tube, for example, this involves imaging the flow as it passes a set position in the facility. This allows information about the chemical length scales to be ascertained as well as knowledge of the species present in the post-shock flow.

In flight, many planetary entry vehicles are equipped with thermocouples embedded in their heat shields or radiometers looking out into the flow to measure the radiative heat flux. These onboard sensors are vital to further understanding the planetary entry environment, however, they rely on a large amount of post-processing and simulations of the re-entry environment to interpret the data. These data are extremely useful for understanding the radiative heating that a heat shield experiences during planetary entry. However, this data is very rare due to the rarity of planetary entry missions themselves.

An important step which connects the two important scenarios discussed above is to directly measure radiative heat flux to test models being tested in hypersonic impulse wind tunnels such as expansion tubes. As mentioned above, most ground testing experiments measure radiation through the radiative shock along the line-of-sight parallel to the model, which is not exactly representative of the radiative heat flux that a heat shield experiences during the entry. Radiation measurements through the surface of the model are closer to an actual entry scenario. This is not a new technique, but its application is limited and in most cases it relies on measuring total radiative heat flux by placing standard heat flux sensors behind windows. This does not allow any information about where in the wavelength spectrum the radiation is originating from to be captured.

Instead this work aims to validate the even rarer technique of using a fibre optic cable to directly measure radiative heat flux to the surface of a test model and beam it out to a spectrometer external to the facility. While this technique has received limited use in the past, this work aims to fully validate the technique as a way to directly measure radiative heat flux for planetary entry by fully characterising the system used in terms of its transmission and angular field of view and by comparing through-model data to both standard externally measured spectroscopy results and simulations. This will allow the through-model tunnel data to be used as a way to validate predictions of radiative heat flux for planetary entry vehicles from surface mounted radiometer data because the results will be spectrally resolved and will be able to be compared to external emission spectroscopy data captured during the same experiment.

Methodology:

This work involves the placing of a fibre optic cable at the centre of a scaled planetary entry capsule in UQ’s X2 expansion tube which is piped out to a Mini spectrometer (Thorlabs CCS200) which can be triggered to expose during the X2 facility’s test time. The spectrometer has a wavelength region of 200 to 1,000 nm and the designed system allows radiation to be captured from 350 to 1,000 nm. For select validation cases, experiments will also be performed with a flat model which should have a very uniform post-shock environment.

Test conditions have been selected targeting important planetary entry cases for different planets in the solar system to show validation in different environments. These conditions include superorbital Earth re-entry, Saturn entry, and Titan entry, all cases where appreciable flow radiation over the region where the system is sensitive is expected

Results:

Experimental results include bench testing validation of the angular field of view of the fibre which is important for calculating the amount of radiation which would be incident on the fibre in simulations.

Currently experimental delays have delayed our ability to perform all of the tunnel validation experiments which we would like, but preliminary through-model spectroscopy data performed last year for a Stardust entry condition will be presented if new data is not able to be captured before the conference.

Conclusion:

Further understanding radiative heat flux experienced during planetary entry is important for the future of space travel. This work aims to increase that understanding by providing a novel method to perform direct radiative heat flux measurements for planetary entry and by characterising that method so that it can be easily compared to radiating CFD simulations of the re-entry environment. This technique is still being developed by us at UQ, but preliminary results have been presented here.

Speaker: Chris James (The University of Queensland)

James_RHTG_10_through-model-spectroscopy.pdf

12:30 Lunch

Radiation modeling and simulation

20 Electron Number Density Estimation for High-Speed Mars Entry Condition

Background of the study:

With increasing interest in both manned and unmanned missions to Mars, human spaceflight to the red planet is becoming a critical focus. Unlike robotic missions, human missions require significantly shorter transit times between Earth and Mars to minimize astronaut exposure to space hazards. For a 120-day transit to Mars, entry velocities could reach as high as 15 km/s, compared to the current 6-10 km/s range for robotic missions.

Such high-speed entry results in significant kinetic energy that ionizes the shock layer in front of the spacecraft. This ionized layer leads to a complex and intense thermal environment, with a high heat load. Additionally, the high velocities necessitate large deceleration forces for safe landings.

Magnetohydrodynamic (MHD) aerobraking presents a potential solution to mitigate these issues by exploiting the interaction between the ionized shock layer and a magnetic field generated by the entry vehicle. This interaction induces azimuthal currents and produces a Lorentz force. The resulting force, called "MHD drag," opposes the flow, reducing velocity and heat. Another outcome is the increased shock standoff, which reduces velocity and temperature gradients within the shock layer, thereby decreasing convective heat flux.

For MHD studies, electron number density measurements are critical to understanding the ionizing flow interacting with the magnetic field. Since the numerical modeling of magneto-flow interactions is still under development, experimental diagnostics of the shock layer are important for validating simulations.

Methodology:

In this study, experiments were conducted using the X2 expansion tube to simulate high-speed Mars entry conditions at 13 km/s. Emission spectroscopy along the stagnation streamline of a quasi-2D shock layer on a cylindrical model was employed to diagnose the flow. Hydrogen-beta emission from the Balmer series, representing transitions between the 4th and 2nd energy levels, was used to estimate the electron number density within the shock layer.

Hydrogen contamination from the X2 facility contributed to the hydrogen-beta emissions, making it a suitable line for analysis. Various broadening effects impact the spectral line shape, with Stark broadening being the most significant for ionized flows. When the electron number density is relatively low and the shock is optically thin, the hydrogen-beta line exhibits a clear shape that can be analyzed using its full width at half maximum (FWHM), by deconvolving the measured FWHM from the FWHMs from other broadening effects. At high electron number density scenario, Stark broadening becomes so prominent that the H-beta line experiences physical separation into distinct peaks due to the splitting of energy levels in the presence of strong electric fields. The separation between these peaks, can be directly related to the electron number density, which enables electron number density to be estimated based on the separated hydrogen-beta lines.

Results:
Spectroscopic data from the UV to the IR were measured, and UV data have been calibrated and analyzed for electron number density calculation. UV spectra for both coarse and fine grating experiments showed good agreement on the calibrated intensity. Shock standoff was measured to be approximately 1.7 mm. Fine-resolution spectral profiles of the hydrogen-beta line at different positions along the stagnation streamline were processed and analyzed to estimate the electron number density from the Stark splitting.

The experimental electron density profile was compared with numerical simulations using the lmr CFD code, which utilized a two-temperature, 11-species CO2 model to simulate the viscous shock layer. The results showed good agreement between the experimental and simulated electron number densities, although the simulation overestimated the shock standoff distance.

Conclusions:

Electron number density for high-speed Mars entry conditions was successfully estimated by analyzing the Stark splitting of the hydrogen-beta line. The results showed a good quantitative match with numerical simulations. This study provides a reliable method for diagnosing ionizing flows, essential for future MHD research. Future work will involve applying this technique to analyze spectra with and without a magnetic field, enabling a deeper understanding of ionization in shock layers under the influence of MHD effects.

Speaker: Yu Liu (The University of Queensland)

RHTG_2024_Yu.pdf

21 Calibration of Rate Parameters Against Shock Tube Emission Spectroscopy

Predictive models are a cornerstone of NASA’s ability to design missions to outer planets. For hypersonic entry into an atmosphere, chemical rate data, excitation cross sections, and emission probabilities are a necessary precursor for these models to compute the radiative heat flux on a vehicle. This data is inferred from ground based test facilities which attempt to reproduce certain aspects of flight conditions in order to best inform these computational models.
The NASA electric arc shock tube (EAST) facility represents one such data source. EAST generates high velocity shockwaves in various gas compositions. The emission for this shock is imaged using high speed spectroscopic techniques, yielding two-dimensional images of the emitted spectra at various positions around the shock.
Computational models are be employed to make a prediction of the images that EAST generates. Rarely do the predictions match the experiment data at all positions and wavelengths. It is expected that much of this discrepancy is due to heritage chemical data which is outdated. However, because this chemical data has such a non-linear coupling, it has been found that using improved rates can increase modeling error. Furthermore, a rigorous comparison of the model variance to the EAST uncertainties has not been performed.
In order to improve the modelling accuracy, it is necessary to take a more holistic, statistical approach. First, a sensitivity analysis of the computation model is performed, analyzing all chemical rate data. Once the most important parameters are isolated, a Bayesian inference is performed against the EAST data to determine a improved probability distribution for these parameters.
The computational models used in this study are DPLR and NEQAIR. DPLR is a flow solver used at NASA Ames to predict hypersonic flow environments. For this study, DPLR is run in ‘space marching’ mode, a one dimensional technique which calculates the temperature and number density of a fluid behind a shock. This data is passed to NEQAIR, a line-by-line radiative solver which predicts the emitted spectrum and radiative heating from a flow. NEQAIR uses an internal database to find the quasi-steady-state (QSS) excited state populations of each species in the flow. It then constructs each spectral line from all allowed transitions between these states and performs the necessary spectral convolutions to yield a prediction of the EAST facility.
Monte Carlo techniques are using to sample the DPLR and NEQAIR chemical databases and create a database of predictions of EAST’s pure Nitrogen shots. From these, the Sobol indices are calculated for each chemical rate parameters. It is shown that parameters both in DPLR and NEQAIR are responsible for the variance of the model outputs. The largest five are then used in a Monte Carlo Bayesian inference, using an adaptive delayed rejection Markov chain Monte Carlo (DRAM-MCMC) to calculate posterior probability distributions on these parameters.

Speaker: Kaelan Hansson (AMA Inc at NASA Ames)

presentation.pdf

presentation.pptx

ablative-radiative TPS and Meteors

22 Meteorites ablation experiment using free-piston driven expansion tube

This study explores meteorite ablation using a free-piston driven expansion tube and a CW 4 kW fiber laser. Meteors, which are dust and rocks from comets or asteroids entering the Earth's atmosphere at high speeds (11.2 km/s to 72.8 km/s), emit light due to atmospheric compression and heating, occurring at altitudes of 80 km to 120 km. Analyzing their emission spectra can reveal their elemental composition. Ground-based experiments are necessary due to the unpredictable nature of meteors.

Previous simulations used arc-heated and plasma wind tunnels and laser ablation. This study combines an expansion tunnel with a fiber laser to more accurately replicate reentry flow. Basalt, similar to meteorites, was used in experiments with flow speeds up to 8 km/s. Spectral analysis identified strong sodium lines and other elements like magnesium, calcium, chromium, iron, and potassium. This new approach enhances the study of meteorite flight conditions.

Speaker: Dr Kohei Shimamura (Tokyo Metropolitan University)

RadiationGasdynamics.pptx

23 Spectral radiative heat transfer instructions with ablative materials

The study explores the impact of spectral radiative heat flux on material response by developing a coupling scheme between the KATS-MR code and a P1 approximation model for radiation transfer. A band model is created, dividing the spectral domain into small, unequal bands. Verification studies compare the band model’s results with pure conduction and the fully resolved radiative transfer, showing its computational efficiency for simulating material response under spectral radiative heat flux. Simulations using spectral boundary conditions reveal higher internal temperatures and earlier decomposition within the material.

Speaker: Alexandre Martin (University of Kentucky)

RHTG2024_v6.pptx

15:40 Cofffee break

Radiation modeling and simulation

24 Shock Curvature Effects in Shock Tube Data

Shock tube flows can be used to investigate non-equilibrium thermochemistry and radiative processes found in hypersonic flows, commonly through spectroscopy techniques which can be used to infer energy levels and number densities. The flow in a shock tube contains many flow non-uniformities, in particular boundary layer effects. For the purpose of studying the properties of the test slug, the shock itself is usually considered planar. In reality, the growth of the boundary layer produces a curved shock front. Therefore any comparison to a one-dimensional flow simulation, such as a stagnation line solution, to experimental data must account for this physical phenomenon. This work develops a method to approximately account for the influence of shock curvature on observed radiance through use of an analytical shock structure formula and subsequently to numerical data via a convolution function. This is applied to a stagnation line simulation of an air test completed in Oxford’s T6 Shock Tube, and a low pressure Titan entry condition. Over 10% change in peak radiance is observed, in addition to changing the spatial profile of the non-equilibrium radiance. This methodology provides a method to correct for the curvature effects previously ignored but which can have a significant influence on peak radiance, particularly for lower pressure test cases.

Speaker: Justin Clarke (University of Oxford)

RHTG_2024_JC.pptx

High speed facilities, flight testing and propulsion

25 Implementation of Rapidly-Scanned, Infrared Laser Absorption Diagnostics in NASA Electric Arc Shock Tube Experiments

Abstract file attached

Speaker: Prof. Ronald Hanson (Stanford University)

RHTG_Chang.pptx

26 Ultraviolet Laser Absorption Studies of the Vibrational Relaxation and Kinetics of High-Temperature Air

PDF Attached

Speaker: Jesse Streicher (Stanford University)

RHTG_Streicher_final.pptx

Dinner at Cherwell Boathouse: dinner at the Boathouse

Cherwell Punting & BBQ

Location: Cherwell Boathouse Contact | Cherwell Boathouse Oxford
Bardwell Road
Oxford
OX2 6ST

Cherwell Boathouse is located a walk from the city centre so please make sure you allocate adequate time to get to the venue.

Please arrive promptly at 6pm for punting, you will be split into small groups and allocated boats. One person from each group will be responsible for punting. Prior to the punting we strongly recommend you take a look at the following information and short video about punting - How to Punt | Cherwell Boathouse Oxford https://cherwellboathouse.co.uk/punting/how-to-punt/

This will be followed by a BBQ at 7:30pm

Wednesday 11 September

Coffee Break

Plasma facilities, simulations and diagnostics

Convener: Brett Cruden (AMA Inc/NASA Ames)

27 Investigation of Slug Calorimeter Heat Flux Measurements in the Plasmatron X Wind Tunnel

In this work, we examine the use of an in-house developed slug calorimeter in the 350 kW inductively coupled plasma (ICP) wind tunnel at the Plasmatron X facility, developed by the Center for Hypersonics & Entry System Studies (CHESS) at the University of Illinois Urbana-Champaign. Plasmatron X is currently the largest ICP facility in the United States, which allows near-continuous operation, dedicated to aerothermal testing for hypersonic flight and reentry environments. The plasma is generated in a radiofrequency (RF) ICP torch by applying a 2.1 MHz excitation to a three-turns induction coil surrounding a 100 mm diameter ceramic tube. The input plate power ranges from 13.5 kW to 350 kW. Gas is injected into the torch body through central and sheath gas lines, the latter being injected at a 15-degree angle to introduce swirl to the flow. The possible operating gases include air, argon, carbon dioxide, hydrogen, methane, and nitrogen. The RF torch can then be interfaced with different nozzle configurations to get the desired flow field inside the cylindrical stainless steel vacuum chamber (1.2 m internal diameter, 1.8 m length) where the jet is discharged.
The wall heat flux depends on the shape of the sample and the chemical and physical properties of the material, such as surface absoprtivity, catalycity, thermal conductivity, etc. As there is no direct measurement of the wall heat flux on a generic thermal protection material, the duplication of this important effect relies in turn on the matching the flow conditions that generate said flux, primarily the centerline freestream enthalpy of flow. However, enthalpy cannot be directly measured directly either, and its determination must rely on semi-empirical correlations or rebuilding procedures. According to the Local Heat Transfer Simulation (LHTS) methodology, the duplication of the hypersonic flight condition heat flux at stagnation point can be achieved in a ground subsonic facility under the Local Thermochemical Equilibrium (LTE) assumption if the flight total enthalpy, the total pressure, and the velocity gradient are locally matched on the test sample. Key to apply LHTS and determine enthalpy is a reliable experimental value of the stagnation point cold wall heat flux.
A slug calorimeter provides a simple and reliable way to estimate cold wall heat flux by measuring the rate at which a slug of material heats up while subjected to a heat input. It was selected among the different techniques for its simplicity, reliability, and adaptability, allowing for easy adjustments to study various sample shapes. Characterizations of the Plasmatron X cold wall heat fluxes are performed with air, nitrogen, and carbon dioxide plasma, and using two types of nozzles, a straight 100 mm diameter nozzle, and a 21.80 mm throat diameter/86.50 mm exit diameter converging-diverging (CD) nozzle. For the experimental tests of the present work, two types of nozzles were used: a straight nozzle with a 100 mm diameter, and a contoured converging-diverging nozzle with 21.80 mm throat diameter and a 86.50 mm exit diameter. During each test, the rotary arm is equipped with a water-cooled Pitot probe (3.18 mm inner diameter, 9.53 mm outer diameter) to measure the stagnation pressure, and a slug calorimeter probe for the heat flux measurement. The slug calorimeter is mounted on a water-cooled copper holder with an external diameter of 20.32 mm. The water-cooling circuit cools down only the interface between the slug calorimeter and the probe holder.
The slug calorimeter used in this works consists of a cylindrical slug (7.62 mm diameter, 10.16 mm length) of Oxygen-Free High-Conductivity (OFHC) C101 copper held concentrically inside an C101 holder by three equally spaced 316 stainless steel spheres pushed against the slug body by 316 stainless steel set screws. A conical cavity is machined on the back face of the slug in order to connect the two wires of a 30 AWG K-type thermocouple by punching them into the cavity using a C101 copper pin.
The heat flux that is imposed on the front face of the slug by the external environment can be defined in terms of three contributions: convective heat flux, radiative heat flux, and chemical heat flux (also called diffusive heat flux). The radiative term depends on the absoprtivity of the slug material front surface. The chemical term is given mainly by the exothermic recombination reactions of the free species present in the dissociated plasma flow that take place on the material surface. In particular, copper is a material with a high catalycity, and the heat flux on a fully catalytic wall could increase up to 50% with respect to a low catalytic wall. Additionally, this chemical contribution depends on the boundary layer type. In an equilibrium boundary layer, surface catalycity of the sample does not play a role because the particles already recombine beforehand and transfer their energy to the gas mixture, whereas in a frozen boundary layer the recombination time is much longer than the diffusion time through the boundary layer so that all potential reaction partners that enter the boundary layer also reach the front surface of the sample thus increasing the wall heat flux if the surface encourages recombination due to its high catalycity. The electrode-less technology for the plasma discharge used in inductively coupled plasma (ICP) wind tunnel provides a pristine environment which enables the study of gas surface interactions such as the effects of surface catalycity on the wall heat flux.
In order to study the effect of the surface catalycity on the cold wall heat flux of an air plasma, the results of a copper slug are compared with the heat flux measurements of two calorimeters with the same shape but different surface materials. In particular, the copper calorimeters have been coated with a 20 nanometers layer of silver and silicon dioxide using magnetron sputter guns. Silver and silicon dioxide are materials that present respectively one of the higher and lower catalycity, thus providing two boundaries for the flow enthalpy reconstruction from cold wall heat flux measurements.
Slug calorimeters of different shapes are tested at different operating conditions relevant to TPS material testing. In particular, air, nitrogen, and carbon dioxide plasma are examined using a straight nozzle and a converging-diverging nozzle, at different power and pressure conditions. For all tests, a direct proportionality between heat flux and power was observed. In particular, this proportionality assumes a linear dependence for the tests with a straight nozzle configuration. For the CD nozzle configuration, higher powers resulted in increased heat fluxes as well, but it appears that heat flux is more influenced by variations in chamber pressure with respect to the straight nozzle case. The effects of the calorimeter shape and surface material are discussed as well, providing a useful overview of these effects on the heat flux characterization of the Plasmatron X ICP wind tunnel.

Speaker: Massimo Franco (University of Illinois Urbana-Champaign)

2024_09_11_Oxford.pptx

2024_09_11_Oxford.pptx

28 Evaluation of free stream acoustic noise in a shock tunnel using focused laser differential interferometry

Background
The state of the boundary layer has a large role in the investigation of high-speed flow fields for aerospace applications. A turbulent boundary layer causes heat flux on the surface of a hypersonic vehicle to dramatically increase when compared to the laminar case. Performance is also impacted by boundary layer turbulence due to increased skin friction drag, and fluid-structure interactions may impose harsher structural and environmental constraints than those in the laminar flow.
Experimental data on high-speed boundary layer transition and turbulence obtained in shock tubes or shock tunnels is fundamental to advance the current understanding of the underlying phenomena. Many of the widely used low-order computational models have been developed and calibrated for low-speed flows, requiring correction approaches to be expanded into the high-speed domain. Higher-fidelity computations, such as Large Eddy Simulation (LES), still require turbulence models to emulate the flow field at its smallest scales, and Direct Numerical Simulation (DNS) of complex, three-dimensional flow fields is still computationally challenging.
However, when collecting the experimental data for such application, the effects of free stream disturbances inevitably produced by a conventional shock tunnel itself must not be neglected. These disturbances include, for example, acoustic fluctuations radiated from the turbulent boundary layer of the nozzle [1]. Disturbances of acoustic nature are known to be amplified in the boundary layer on the test model and cause transition to occur much earlier than in flight, even though the exact governing mechanisms are still not fully understood.
In order to enable the best use of experimental observations conducted in conventional shock tubes or shock tunnels and support the interpretation of data obtained in different facilities, it is therefore important to quantify the free stream environment, in terms of inherent acoustic disturbances. Among the most widely used techniques to that end are hot-wire anemometry (HWA) and pitot probes. However, HWA faces many challenges for application in short-time impulse facilities, such as a limited bandwidth, excessive total temperature, and environmental conditions that may be compromising for the delicate hot wires. Pitot probes require accounting for damping effects of protective cavities and possible effects of the probe geometry, which is not standardized [2].
In the present work, free stream disturbance measurements in a shock tunnel are presented using the alternative technique of Focused Laser Differential Interferometry (FLDI). FLDI is a non-intrusive technique capable of detecting density fluctuations up to remarkably small scales at high frequency in high-speed flow fields.

Methodology
The experiments are conducted in the High Enthalpy Shock Tunnel Göttingen (HEG) of the German Aerospace Center (DLR). HEG is a free-piston shock tunnel, capable of generating a range of hot free stream conditions equivalent to atmospheric flight at multiple altitudes [3]. Free stream density disturbances are obtained in the present work for free stream unit Reynolds numbers of approximately 1.5e6, 2.2e6, and 2.4e6 1/m, and total enthalpies of 11.9, 9.8, and 3.5 MJ/kg, respectively.
The FLDI setup uses an Oxxius LCX-532S DPSS, 532 nm continuous wavelength laser, expanded to approximately 45 mm diameter and focused on a plane 1920 mm away from the field lens. Sanderson prisms are used to produce the differentiation pair, which are separated by 175 μm. The probing pair is multiplied into a 6-by-1 multi-foci array, by means of a diffractive optical element (DOE). The DOE is positioned in the optical setup such that all 6 FLDI probes maintain parallelism between each other across the test section. Both the differentiation axis and the axis containing the multiple independent probes are oriented in the streamwise direction and coincident to the center axis of the nozzle. Detection is performed by means of Thorlabs DET36A2 photodetectors at 25 MHz, using 25x amplification provided by SRS SR445A DC-350 MHz preamplifiers.
The FLDI data is post-processed to account for the wavelength-dependent sensitivity length, assuming a uniform flow field in the nozzle core region. Density fluctuations are transformed into pressure fluctuations assuming isentropic conditions.

Results
Free stream fluctuations in terms of FLDI phase shift spectra will be shown. In this data, the lower and upper frequency bounds of the FLDI instrument are identified. Convection velocities are calculated by cross-correlating the time signals from the streamwise FLDI probes, which are separated by a known distance. Using the obtained convection velocities, the frequency bounds are converted into the smaller and larger detected disturbance wavelengths.
RMS values of density and pressure fluctuations will be calculated from the post-processed FLDI data within the identified frequency bounds. The RMS of pressure fluctuation will be compared with measurements performed in a previous work using pressure transducers in a wedge probe in HEG [2], for the free stream unit Reynolds number 2.4e6 1/m case. The spectra of the pressure fluctuations for all cases will also be shown, and the effect of Reynolds number and total enthalpy will be analyzed.

Conclusion
This work presents measurements of free stream acoustic disturbances using a multi-foci Focused Laser Differential Interferometer (FLDI) in the free-piston shock tunnel HEG of the German Aerospace Center (DLR).
The ease of implementation of FLDI and its clean view of the disturbance spectrum make it a promising candidate technique for the standardization of free stream disturbance measurements, aiming at improving the interpretation and comparison of boundary layer data across different facilities.

References
[1] L. Duan et al. “Characterization of Freestream Disturbances in Conventional Hypersonic Wind Tunnels”. Journal of Spacecraft and Rockets 56.2 (Mar. 2019), pp. 357–368. DOI: 10.2514/1.a34290.
[2] A. Wagner et al. “Combined free-stream disturbance measurements and receptivity studies in hypersonic wind tunnels by means of a slender wedge probe and direct numerical simulation”. Journal of Fluid Mechanics 842 (Mar. 2018), pp. 495–531. DOI: 10.1017/jfm.2018.132.
[3] Deutsches Zentrum für Luft- und Raumfahrt (DLR). “The High Enthalpy Shock Tunnel Göttingen of the German Aerospace Center (DLR)”. Journal of large-scale research facilities 4, A133 (Oct. 2018). DOI: 10.17815/jlsrf-4-168.

Speaker: Giannino Ponchio Camillo

2024_RHTG_GPC.pdf

29 New plasma technique for bulk viscosity and sound velocity measurements

Background of the study
According to the recent paper of Asokakumar Sreekala et al. [1], the discrepancy between Navier Stokes Fourier-based Computational Fluid Dynamics and aero thermodynamic flight test data becomes noticeable from altitudes above 50 km. These researchers argue that bulk viscosity (BV) is required to complete the Navier-Fourier model and that the effect of vibrational nonequilibrium must be considered to accurately predict the high-temperature flow field of diatomic and polyatomic gases, in line with the results obtained by Klimov et al. two decades earlier [2]. This can be accounted for by methods such as state-to-state chemical kinetic approaches, which can be applied to arbitrary deviations from local thermodynamic equilibrium, but require much more computation [3, 4]. In the classical form of the Navier-Stokes equations, the Stokes hypothesis is applied, which is equivalent to assuming that the BV coefficient is zero. This is used in cases where compressibility is important [5, 6] because the BV is extremely difficult to measure [7]. However, during hypersonic re-entry it can have an influence on the shock wave structure [8]. According to Molevich et al [9], this parameter can even become negative in non-equilibrium media such as vibrationally excited plasma, leading to acoustic amplification [10, 11].
At ground facilities, very high enthalpy levels are achieved with plasma wind tunnels for extended test times [12]. The non-equilibrium vibrational kinetics in hypersonic flows is similar in many respects to that of electrical discharges in gases, but as mentioned in [13], “the main difference is that in the latter case the vibrational quanta are primarily pumped by electrons while during reentry they are pumped by recombination processes...” This difference may however be suppressed in our setup, thanks to the plasma ball formation (PBF), an acoustic plasma confinement mode we have discovered during the development of a pulsed sulfur plasma lamp [14], also observed by a team at UCLA [15]. Indeed, the molecular dissociation is then governed by the pure vibrational mechanisms [14], i.e. by the collisions between vibrationally excited molecules, rather than by electron impacts [16], p. 228. Another similarity between hypersonic plasmas and the plasma in our device is the radiative energy transfer resulting from the relaxation of the excited electronic states of the molecules, resulting in an optical emission spectrum that deviates significantly from Planck's law. In addition, the wall temperature, heat flow and plasma temperature are close to the STS-2 re-entry flight data at an altitude of about 50 km [17]. In a previous ESA project, the physics were established for the design of an experimental PBF device to provide a technique for testing radiation models in non-equilibrium plasma with control of the plasma translational and vibrational temperatures thanks to the power control parameters [18]. It has been pointed out that the PBF can pave the way for the measurement of the BV in addition to the velocity of sound. In the present study we determine how this coefficient can be calculated from the PBF measurements.
Methodology
Hollow spherical resonators are excellent tools for measuring the sound velocity c in the filling, derived from the resonant frequency, and the thermophysical properties that determine this velocity, such as the adiabatic factor γ [19]. The sphere is of particular interest because it has the highest degree of symmetry and the smallest surface-to-volume ratio. The PBF is also particularly interesting because it acts as a well-centred spherical acoustic generator, eliminating the need for acoustic transducers and thus avoiding the technical difficulties associated with mechanical coupling to the shell. Moreover, the PBF takes place in a spherical mode [14], eliminating acoustic losses due to friction against the wall. In addition, there is significant acoustic dispersion due to vibrational relaxation, allowing the study of this effect [14]. The BV can be investigated through its effect on the acoustic pressure, as the so-called dynamic pressure arises proportional to the particle velocity divergence. The spherical resonator is of particular interest because in spherical modes this parameter peaks at the centre. In the fundamental spherical mode, the acoustic pressure can be measured from the oscillation of the light emission [20]. Furthermore, since the experiments show that the quality factor Q is not significantly affected by the amplitude of the light emission modulation, it is possible to use the physical models developed in photo-acoustics to calculate Q, which in our case was measured by means of long power interruption tests [14]. From the formula given in reference [19], p. 103, using simple algebra, we obtain the following expression for the BV coefficient

η_b=(2 ϱ c^2)/(ω Q)-(3 η)/4-(γ-1)/γ λ/C_v
with ϱ the mass density, ω the resonance angular frequency, η the dynamic shear viscosity, λ is the thermal conductivity and C_v is the isochoric heat capacity per unit volume.

Results

Using the data given in [14], we proceed with the numerical application of this equation. The mass density ϱ is calculated to be 1.82 〖 mg/cm〗^3 from the bulb volume of 15.6〖 cm〗^3, and the filling conditions: 28.1 mg of sulphur, solid at room temperature, plus argon gas at low pressure to ignite the lamp by electrical breakdown of the gas.
The parameters C_v, γ and c are assumed to vary between their equilibrium and frozen values at the average temperature in the bulb, 1930 °C, i.e. 469 – 660 J∙K^(-1)∙〖kg〗^(-3), 1.19 - 1.28, and 586 - 608 m/s respectively. In this approach, the Chapman-Enskog theory was applied for single component gas, diatomic sulphur, to determine η and λ as functions of temperatures. At 1930 °C, the values obtained for η_b in 〖 10〗^(-3) Pa∙s are between 14.0 and 15.2, going from the frozen state to equilibrium (cf. Figure 1), i.e. with a magnitude 500 times greater than η.
Figure 1 Bulk viscosity coefficient η_b of the PBF at 1930°C (mainly diatomic sulphur vapour). From left to right, the thermodynamic state varies from fully equilibrium to fully frozen.
Conclusion
The results show that the bulk viscosity of a partially dissociated diatomic vapour can be 500 times greater than its dynamic shear viscosity at high temperatures and under strong vibrational conditions; neglecting it could therefore lead to serious errors in the calculation of hypersonic flows at boundary layers. The experiment was carried out with a mixture of sulphur vapour and argon; it would be worth trying other diatomic or light polyatomic gases such as air and CO2. The PBF phenomenon opens the way to a new technique for measuring bulk viscosity and sound velocity, providing an opportunity to improve low-cost numerical simulations of hypersonic flows.

[1] Asokakumar Sreekala, V., Chourushi, T., Sengupta, B., & Myong, R. S. (2022). Effects of bulk viscosity, vibrational energy, and rarefaction on flow and vorticity fields around simple bodies at hypersonic speeds. In AIAA SCITECH 2022 Forum (p. 1065).
[2] Klimov, A., Bityurin, V., & Serov, Y. (2001). Non-thermal approach in plasma aerodynamics. In 39th Aerospace Sciences Meeting and Exhibit (p. 348).
[3] Colonna, G., Armenise, I., Bruno, D., & Capitelli, M. (2006). Reduction of state-to-state kinetics to macroscopic models in hypersonic flows. Journal of thermophysics and heat transfer, 20(3), 477-486.
[4] Kustova, E., Nagnibeda, E., Oblapenko, G., Savelev, A., & Sharafutdinov, I. (2016). Advanced models for vibrational–chemical coupling in multi-temperature flows. Chemical Physics, 464, 1-13.
[5] Chikitkin, A. V., Rogov, B. V., Tirsky, G. A., & Utyuzhnikov, S. V. (2015). Effect of bulk viscosity in supersonic flow past spacecraft. Applied Numerical Mathematics, 93, 47-60.
[6] Kosuge, S., & Aoki, K. (2018). Shock-wave structure for a polyatomic gas with large bulk viscosity. Physical Review Fluids, 3(2), 023
[7] Taniguchi, S., Arima, T., Ruggeri, T., & Sugiyama, M. (2018, May). Shock wave structure in rarefied polyatomic gases with large relaxation time for the dynamic pressure. In Journal of Physics: Conference Series (Vol. 1035, No. 1, p. 012009). IOP Publishing.
[8] Galkin, V. S., & Rusakov, S. V. (2005). On the theory of bulk viscosity and relaxation pressure. Journal of applied mathematics and mechanics, 69(6), 943-954.
[9] Molevich, N., Galimov, R., Makaryan, V., Zavershinskii, D., & Zavershinskii, I. (2013, June). General nonlinear acoustical equation of relaxing media and its stationary solutions. In Proceedings of Meetings on Acoustics (Vol. 19, No. 1). AIP Publishing.
[10] Molevich, N. (2004). Acoustical properties of nonequilibrium media. In 42nd AIAA Aerospace Sciences Meeting and Exhibit (p. 1020).
[11] Molevich, N., & Riashchikov, D. (2021). Shock wave structures in an isentropically unstable heat-releasing gas. Physics of Fluids, 33(7).
[12] Hermann, T., Löhle, S., Zander, F., & Fasoulas, S. (2017). Measurement of the aerothermodynamic state in a high enthalpy plasma wind-tunnel flow. Journal of Quantitative Spectroscopy and Radiative Transfer, 201, 216-225.
[13] M. Capitelli, C. M. Ferreira, B. F. Gordiets, A. I. Osipov, "Plasma Kinetics in Atmospheric Gases", Springer-Verlag, 2000, p. 3
[14] G. Courret, P. Nikkola, S. Wasterlain, O. Gudozhnik, M. Girardin, J. Braun, S. Gavin, M. Croci, and P. W. Egolf, "On the plasma confinement by acoustic resonance", The European Physical Journal D, 71(8):1–24, 2017
[15] J. P. Koulakis, S. Pree, A. L.F. Thornton, and S. Putterman, "Trapping of plasma enabled by pycnoclinic acoustic force", Physical Review E, 98(4):043103, 2018
[16] M. Capitelli et al., Fundamental Aspects of Plasma, "Chemical Physics, Kinetics", Springer, 2016.
[17] Shinn, J., Moss, J., & Simmonds, A. (1982, June). Viscous-shock-layer heating analysis for the shuttle windward-symmetry plane with surface finite catalytic recombination rates. In 3rd Joint Thermophysics, Fluids, Plasma and Heat Transfer Conference (p. 842).
[18] Courret, G., & Nikkola, P. (2022). Plasma ball formation: an experimental technique to test radiative models in non-equilibrium plasmas. In Proceedings of the 9th International Workshop on Radiation of High Temperature Gases for Space Missions, 12-16 septembre 2022, Santa Maria, Portugal.
[19] Bailey, R. T., Bernegger, S., Bicanic, D., Bijnen, F., Blom, C. W. P. M., Cruickshank, F. R., ... & Zuidberg, B. (2012). Photoacoustic, photothermal and photochemical processes in gases (Vol. 46). Springer Science & Business Media.
[20] Courret, G., Nikkola, P., Croci, M., & Egolf, P. W. (2020). Investigation of a molecular plasma from its acoustic response. IEEE Transactions on Plasma Science, 49(1), 276-284.

Speaker: Gilles COURRET (HEIG-VD)

RHTG_10th_Courret.pdf

RHTG_10th_Courret.pptx

30 Investigation of Radiative Heating in Hydrogen-Helium Plasma Wind Tunnel Flows

In the plasma wind tunnel facilities PWK1/2 and PWK4 of the Institute of Space Systems, the high-enthalpy flow field is generated using electric arc-jet generators. A sample positioned downstream of the nozzle is thus exposed to a combination of radiative, convective, and chemical heating from the plasma flow, but potentially also from radiation originating from the generator. Assessing the amount of heating due to each of those contributors is vital for thermal protection system development. For hydrogen-helium plasma flows, the amount of radiative heating experienced by a probe in a PWT is unknown. This work will provide first experimental data measured with a radiometer incorporated into a water-cooled copper probe.

Speaker: David Steuer (HEFDiG, Institute of Space Systems, University of Stuttgart)

DSteuer_Investigation_of Radiative_Heating_in_Hydrogen_Helium_Plasma_Wind_Tunnel_Flows_UPLOADVERSION.pdf

DSteuer_Investigation_of Radiative_Heating_in_Hydrogen_Helium_Plasma_Wind_Tunnel_Flows_UPLOADVERSION.pptx

10:40 Coffee Break

EDL instrumentation

31 DRAGONFLY ENTRY AEROSCIENCES MEASUREMENTS (DrEAM) SUITE SCIENCE OBJECTIVES

Dragonfly Entry Aerosciences Measurements (DrEAM), will provide key aerothermodynamic data and performance analysis for Dragonfly’s forebody and backshell Thermal Protection System (TPS). Titan's atmosphere predominantly consists of nitrogen (~ 98% by mole) with small amounts of methane (~ 2% by mole). CN is a strong radiator and is found in nonequilibrium concentrations for Titan entry, the modeling of which has proven to be a difficult task. The DrEAM instrumentation suite will significantly advance the state-of-the-art not only by documenting the environment and performance of Dragonfly’s entry system but also by making key measurements in Titan’s atmosphere for the first time, thus providing new benchmark data applicable to entry science more generally.

Speaker: Dr Aaron Brandis (NASA Ames Research Center)

DrEAM-2024-RHTG-ScienceObjectives.pptx

32 Characterization and calaibration and calibration of the COSSTA radiative heating measurements

In collaboration with NASA Ames and Langley, the German Aerospace Center (DLR) is advancing the development of a specialized instrumentation suite for NASA’s Dragonfly mission to Titan. This suite, known as the Dragonfly Entry Aerosciences Measurements (DrEAM), is designed to provide critical aerothermodynamic data and performance analysis for the Thermal Protection System (TPS) of the Dragonfly spacecraft. Entry into Titan's atmosphere, which is predominantly composed of nitrogen (N2, ~98% by volume) with minor amounts of methane (CH4, ~2% by volume) and trace gases, will result in the presence of CN species. This compound plays a crucial role with respect to radiative heat loads on the backshell of the Dragonfly capsule. It exists in nonequilibrium concentrations during Titan entry, making accurate modeling extremely difficult.

The suite will integrate the COmbined Sensor System for Titan Atmosphere (COSSTA) provided by DLR, which is based on the sensor configuration used in the COMbined Aerothermal and Radiometer Sensor (COMARS) suite [1] previously flown on the Schiaparelli mission. COSSTA consists of three sensor clusters, each with a specific arrangement of instruments to maximize science return within our data constraints:

COSSTA 1 includes two narrowband radiometers and one total heat flux sensor;

COSSTA 2 features two narrowband radiometers and two pressure sensors; and

COSSTA 3 is equipped with two narrowband radiometers, one total heat flux sensor, and one pressure transducer.

The COSSTA radiometers are based on thermopile technology and equipped with CN-red and CN-violet filters, essential for quantitatively assessing the contributions from the main bands of CN on radiative heating on the vehicle's aftbody. In addition to the six narrowband radiometers, a broadband radiometer will be positioned near the capsule’s shoulder to measure total radiative heating.

Radiative heating is particularly crucial for the backshell, where the heat load predominantly depends on shock layer radiation rather than convection, underlining the importance of understanding these mechanisms for effective TPS design. Hence, sensor calibration is a critical aspect of the project, ensuring accurate measurements under the challenging conditions of Titan's atmosphere.

The presentation will elaborate on the instrumentation design for each COSSTA location, detailing the integration of the radiometers, as well as the current status of the design, characterization and calibration processes. By providing high-quality measurements of radiative heating, COSSTA will play a pivotal role in enhancing our understanding of Titan atmospheric entry and contribute valuable data to the broader field of entry science.

References:
1. A. Gülhan et al. “Aerothermal Measurements from the ExoMars Schiaparelli Capsule Entry,” Journal of Spacecraft and Rockets, 2018.

Speaker: Mr Pascal zur Nieden (DLR)

Characterization and Calibration of the COSSTA Radiative Heating Measurements.pptx

Radiation modeling and simulation

33 Investigation of Shock Speed Variation Effects on Expanding Flow Experiments in the NASA Electric Arc Shock Tube

Introduction

The study of thermochemical non-equilibrium effects in hypersonic flows is valuable for the development of predictive tools for atmospheric entry vehicles and high-speed propulsion systems. Facilities like the Electric Arc Shock Tube (EAST) at NASA Ames Research Center play a vital role in investigating these phenomena by generating high-enthalpy flows. Recent experiments conducted by Tibère-Inglesse et al. at EAST with an expanding nozzle revealed notable discrepancies between computational fluid dynamics (CFD) predictions and experimental measurements of radiance. Underpredictions were observed in both the ultraviolet (UV) and visible spectra, attributed to inaccuracies in modelling N$_2$$^+$ densities and limitations in the kinetic models used for number density and temperature predictions.

Previous shock tube experiments in air at 10 km/s have demonstrated shock deceleration effects. This raises questions about the accuracy of current simulation methodologies and their ability to capture the complex flow physics present in shock tube experiments.
Recent studies by Collen et al. and Satchell et al. have emphasised the critical role of shock speed attenuation in determining test slug properties. Their research demonstrates that variations in shock speed can cause substantial changes in pressure, temperature, and species densities within the test gas. To address these complexities, the Lagrangian Shock Tube Analysis (LASTA) 2.0 code has been recently developed. This new code incorporates non-equilibrium thermochemistry effects, offering a more comprehensive tool for analysing shock tube flows.

This work aims to address whether shock speed variation significantly effects predictions of expanding flow experiments previously conducted in the EAST facility. The intended outcome of this work is to develop the understanding of high-enthalpy flow physics and improve the accuracy of computational models used in hypersonic research and design, ultimately contributing to the advancement of hypersonic technologies. To investigate shock speed variation effects, the Eilmer CFD code is used as the simulation platform, with additional input conditions derived from the Non-Equilibrium Shock Solver (NESS) and LASTA to account for non-equilibrium effects in the shock tube prior to the expanding nozzle section. Experimental data from EAST Test 63 will be presented for comparison, which used shock velocity targets of 10 km/s and 11 km/s at the nozzle entrance. Shock speeds within the expanding nozzle section will be compared, along with the use of NASA's NEQAIR radiation analysis code to compare simulation results with experimental measurements of radiance and electron number density from EAST.

Current work has been focused on refining the CFD setup with input conditions from NESS to ensure high-quality numerical results. Further simulations with LASTA conditions will commence shortly after, followed by analysis of these datasets with EAST data.

Experimental Overview

Facility Description

The EAST facility has been described by Cruden, with further details relevant for Test 63 from Tibère-Inglesse. Some details are repeated here for completeness.

The facility used a driver section with a trigger wire and pin assembly connected to a 1.3 mF capacitor bank storing 1.2 MJ of energy with a 40 kV electric potential.
An electric arc across the wire and pin initiates current flow as high as 1 MA to the ground electrode, heating the driver gas of choice to ultimately generate a shock wave downstream. The driver assembly was connected to an aluminium tube with a diameter of 101.6 mm. An expanding nozzle with a length of 1.9 m and a half-angle of 10 degrees was connected to the driven tube, with the nozzle entrance located 12.5 m downstream of the primary diaphragm. The nozzle diameter linearly expanded from 101.6 mm at the nozzle entrance to 762 mm at the nozzle exit. Rectangular window ports of length 148.6 mm are situated 292.3 mm downstream of the nozzle entrance, creating a parallel-wall section in the horizontal plane only for the length of the ports. These ports were coupled to four spectrometers, each capturing a different wavelength region, corresponding to Vacuum Ultraviolet ("VUV", 120-200 nm), UV/Visible ("Blue", 200-500 nm), Visible/Near-Infrared ("Red", 500-900 nm), and Mid-Wave-Infrared ("IR", 1600-5500 nm), respectively. Piezoelectric transducers (PCB 132A) were mounted along the length of the shock tube to measure the shock velocity with respect to time, enabling the calculation of the shock velocity at the nozzle entrance. A pitot rake was mounted 517 mm downstream of the nozzle entrance to measure the shock profile. The shock velocity at the window is calculated the shock positions with respect to time measured at 60 mm and 430 mm from the nozzle entrance, using a PCB and/or the pitot rake.

Test Conditions

EAST Test 63 focused on conditions relevant for lunar return missions. Shock velocities of 10 km/s and 11 km/s were targeted, with a free-stream pressure of 0.2 Torr. The test gas used was synthetic air. Test conditions varied from the target conditions due to variability associated with the electric arc system.

Numerical Overview

CFD simulations of the EAST facility with the expanding nozzle are currently in progress. The flow domain considered consists of the expanding nozzle, the test window section, and a section of the shock tube upstream of the expanding nozzle entrance to allow for appropriate flow development. The numerical setup also considers a 2D axisymmetric grid. Two grids will be considered; one with the test window section and one without, reproducing two extremes of the real geometry. Current simulations are being conducted using Eilmer 4 with input conditions from NESS to model a steady shock entering the shock tube with non-equilibrium thermochemistry and shock curvature. Beyond this, input conditions from LASTA will be used to model shock speed variation in the test slug. The completion date is expected to be in August 2024 for the simulations.

Speaker: Kanishk Ganga (University of Oxford)

2024-09-11 RHTG-10 Shock Speed Variation in EAST.pptx

12:40 Walk to Southwell and Lunch provided at Southwell Building 13:00 - 14:00

This is a 20-25min walk from the OeRC and we walk people over to Osney as a group. Maps will also be available at the Registration Desk. If you feel unable to walk this distance please let us know and we can organise a taxi for you.

https://www.google.com/maps/search/?api=1&query=51.746661447995926,-1.2711846828460693

Lab Tour: Overview of lab and experimental facilities

Coffee Break

High speed facilities, flight testing and propulsion

34 Laser-Based Characterization of Reflected Shock Tunnel Freestream Velocity and Multi-Species Thermal Nonequilibrium with Comparison to Modeling

Background of the study

Reflected shock tunnels are an excellent tool for studying high-temperature, hypersonic problems such as boundary layer transition, ablation, and shock-induced flow separation. These phenomena are known to be sensitive to the thermochemical state of the tunnel freestream. Although shock tunnels are a mature technology, much uncertainty in the tunnel freestream conditions remains. The temperature behind the reflected shock wave in the plenum results in dissociation of diatomic species, such as O2, and the production of additional species, such as nitric oxide (NO). The subsequent rapid expansion through the nozzle results in pronounced thermodynamic nonequilibrium where the vibrational temperature, Tv, of diatomic molecules can be much higher than the translational, Tt, and rotational temperature, Tr, at the nozzle exit. Moreover, the chemistry during the expansion can also remain ‘frozen’ where the freestream maintains a chemical composition having NO and high concentrations of atomic species akin to that in the nozzle throat. Thus, detailed multi-species, multi-temperature measurements are required to inform and validate predictive models for the nozzle expansion process and characterize the test section freestream.
The millisecond test times, high background emission, and low freestream densities in shock tunnels make measurements challenging. With recent advances in frequency-tunable, burst-mode laser diagnostics, time-resolved CARS measurements of homonuclear molecular temperatures are now possible. This presentation will present measurements of N2 and O2 Tr and Tv in the air freestream of Sandia’s Hypersonic Shock Tunnel (HST), while comparing to complimentary measurements of NO Tr and Tv temperatures acquired using laser absorption spectroscopy (LAS). Additionally, simultaneous freestream velocity measurements are performed using NO MTV along with concurrent measurement of stagnation and pitot pressures.

Methodology

The HST at Sandia National Laboratories is a research-scale, free-piston facility capable of producing high-enthalpy hypersonic flow with freestream velocities ranging from 3 to 5 km/s. A free-piston shock tube generated the high temperature and pressure stagnation reservoir gas for the shock tunnel. The shock tunnel nozzle has a throat diameter of 12.7 mm, with a circular throat cross-section blended to a conical expansion of 7.9 degrees. The area ratio was 784 corresponding to a 35.6-cm exit diameter. The core-flow diameter, as determined by a pitot rake, was approximately 25 cm. Pitot pressures are measured using a rake containing 17 PCB sensors in the test section. HST experiments were performed at four conditions to produce freestream velocities of nominally 3 km/s and 4 km/s with a test gas of either synthetic air (80% N¬2, 20% O2) or pure N2.
The CARS technique was used to measure rotational and vibrational temperatures of N2 and O2 in the freestream of the shock tunnel. The setup used a Spectral Energies “Quasimodo” burst-mode laser, which provided both the 532- and 355-nm, 10-ns pulses at 100 kHz for a burst duration of ~1.2 milliseconds (ms). To generate the broadband Stokes beam for the CARS process, a noncolinear optical parametric oscillator (NOPO) was pumped by the 355-nm output from the burst-mode laser. The output wavelength was centered at 580 nm for O2 CARS and 607 nm for N2 CARS. The CARS setup utilized the 532-nm from the burst-mode to form the CARS pump beams, and the broadband output from the NOPO was used for the Stokes beam to measure single-shot measurements of N2 and O2 temperature measurements.
The NO MTV setup was operated simultaneously with the CARS by splitting the main 355-nm output and pumping a secondary OPO to generate 622-nm output. The 622-nm output and the residual 355-nm pump beam are used for sum-frequency generation (SFG) to generate a laser at 226 nm with a bandwidth of ≈ 15 cm-1 to excite multiple rotational levels near the (0,0) bandhead of the NO A2Σ - X2Π system. The NO MTV beam is overlapped with the CARS set-up to generate NO LIF signal colinear with the CARS measurement and captured using a UV-sensitive image intensifier (LaVision HS-IRO S20) coupled to a high-speed Phantom TMX 7510 mono-chrome camera.
The LAS diagnostic used two quantum cascade lasers to measure rotational and vibrational temperatures and the partial pressure of NO at 25 or 100 kHz. The beams were both fiber coupled into a single-mode fiber. The collimated, collinear beams were pitched through the test section where 3D printed flow cutters were used to isolate the quasi-uniform core flow. The beams were positioned 2 cm from the nozzle exit and the absorbing path length was 23.2 cm.

Results

Ensemble-averaged temporal profiles of stagnation pressure, pitot pressure, freestream temperatures, and freestream velocity were taken for all four cases. Fig. 1 shows the temporal profiles for the 3 km/s – air test condition. SPARC CFD predictions initialized with NASA Chemical Equilibrium with Applications (CEA) of the stagnation reservoir are also shown on Fig. 1. For each case, there is pronounced thermal nonequilibrium observed between the rotational and vibrational temperature of all three species. The SPARC model accurately predicts the rotational temperature and the freestream velocity for each case. SPARC CFD predictions show that vibrational relaxation is fastest for NO and slowest for N2, an observation consistent with the experimental measurements. The current multi-vibrational temperature model captures this trend, but underpredicts NO Tv¬ while overpredicting N2 and O2 Tv¬.
To isolate oxygen chemistry effects on the N2 thermal relaxation during the flow expansion, pure nitrogen was also used as a test gas. With the lack of other collisional partners, the pure N2 condition showed elevated vibrational temperatures for both the 3 and 4 km/s conditions. Comparison of the experimental data with the CFD predictions showed good agreement with the rotational temperatures. The measured N2 vibrational temperatures came in lower than the CFD predictions at both conditions, consistent with the air cases.
An additional case was run using humid air to test the effect water has on N2 nonequilibrium at the 3 km/s condition. With 3600 ppm of water present, both the rotational and vibrational temperatures of N2 were nearly identical to the previous 3 km/s synthetic air cases showing little dependence on water at these levels.

Conclusion

A novel combination of high-speed laser spectroscopy measurements for freestream velocimetry and multi-species internal temperatures has been performed over an extensive number of repeat experiments in Sandia’s reflected shock tunnel. The resulting dataset allowed for meaningful trends between experimental conditions and molecular temperatures to be discovered. Additionally, the combined dataset served as a benchmark for comparison to nonequilibrium freestream modeling using the SPARC CFD code.
Sandia National Laboratories is a multimission laboratory managed and operated by National Technology & Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA0003525.

Fig. 1. Time-synchronized plot of pressures, velocity, and temperatures for the 3 km/s condition. First row, stagnation pressure. Second row, pitot pressure measured on centerline probe. Third row, velocity measured from MTV. Fourth row, rotational temperature of diatomic nitrogen (from CARS) and nitric oxide (from LAS). Fifth row, vibrational temperature of diatomic nitrogen and oxygen (both from CARS) and nitric oxide (from LAS). Shading indicates variability between runs, plotted as +/- 1 standard uncertainty of the ensemble. Dashed horizontal lines indicate SPARC predictions.

Speaker: Elijah Jans (Sandia National Labs)

RTHG_CARS.pptx

35 Radiation of contaminant species in the High Enthalpy Shock Tunnel Göttingen (HEG)

Background
The development of re-entry aerodynamic configurations requires examination of surface heat flux. Surface heating due to radiation at high-enthalpy stagnation conditions has been shown to comprise a significant portion of the total measured surface heat flux on re-entry configurations [1] [2]. Earlier studies in high-enthalpy flows on blunt sphere-cones [3] and capsules [4] indicated substantially increased surface heat flux levels at or near the stagnation region than predicted numerically. This presents difficulties with numerical reconstruction of such test cases, and also regarding formulation of experiments serving as validation cases. It has been hypothesised that the extra heating component measured was due to radiation of contaminant species, in addition to flow species, within the shock layer [1]. This was termed radiation augmentation, the source of which is currently unclear. Systematic investigation of this would benefit from further tests under conditions of the High Enthalpy Shock Tunnel (HEG) [5], which can reproduce a variety of re-entry-relevant freestream conditions.
Methodology
A flat-faced cylindrical probe to generate a bow shock layer upstream of the model face has been designed and implemented in the HEG. The probe houses an array of surface-mounted sensors including a radiative heat flux sensor, pressure sensors, temperature sensors, and fibre optic components for in-situ optical emission spectroscopic (OES) measurements within the shock layer. This work will focus on the in-situ OES measurements, which were made at up to 4 kHz and with different line-of-sight orientations. Furthermore, the use of a mass spectrometer and a scanning electron microscope enabled investigation of contaminants obtained post-test from the HEG tunnel walls and assisted in understanding possible sources of contaminant species. The HEG was used at various reservoir conditions (low- and high-enthalpies) up to a reservoir pressure of 44 MPa and a specific reservoir enthalpy of 12 MJ/kg.
Results
Results will be presented for surface sensors on the probe, indicating the establishment of the shock layer and the detection of total and radiative heat flux components. The latter was recorded for different wavelength bands and comparisons will be drawn from measurements made in the UV-VIS to the NIR regions. Results from the in-situ OES will be used to assess the presence of contaminant species and mass spectrometry result will assist in ascertaining the possible source of these contaminants.
Conclusion
A flat-faced cylindrical probe has been successfully implemented in the HEG and first results of its usage will be presented. A workflow combining in-situ OES and mass spectrometry has resulted in increased certainty in identifying sources of contaminant species in high-enthalpy hypersonic radiative flows.

References

[1] H. Tanno, T. Komuro, R.P.Lillard und J. Olejniczak, „Experimental study of high-enthalpy heat flux augmentation in shock tunnels,“ Journal of Thermophysics and Heat Transfer, pp. 858-862, Oct - Dec 2015.
[2] B. Cruden, C. Tang, J. Olejniczak, A. Amar und H. Tanno, „Characterization of radiative heating anomaly in high enthalpy shock tunnels,“ Experiments in Fluids, 2021.
[3] B. Hollis und D. Prabhu, „Assessment if laminar, convective aeroheating uncertainties for Mars-entry vehicles,“ Journal of Spacecraft and Rockets, pp. 56 - 68, Jan - Feb 2013.
[4] E. Marineau, D. Lewis, M. Smith, J. Lafferty, M. White und A. Amar, „Investigation of hypersonic laminar heating augmentation in the stagnation region,“ presented at the 51st AIAA Aerospace Sciences Meeting including the New Horizons Forum and Aerospace Exposition, Jan 2013.
[5] J. M. Schramm, A. Wagner, D. Surujhlal, G. Camillo und T. Ecker, „The High Enthalpy Shock Tunnel Göttingen of the German Aerospace Center (DLR),“ Journal of Large-Scale Research Facilities, in print, 2024.

Speaker: Dr Divek Surujhlal (German Aerospace Center (DLR))

Presentation_RHTG_2024.pptx

36 Numerical Study on Hypersonic Boundary Layer Behind a Propagating Shock Wave in an Expansion Tube

Background of the study

An expansion tube is one of the ground-based experimental facilities that can generate a high-enthalpy hypersonic flow. A standard expansion tube consists of a compression tube, a shock tube, an acceleration tube, and a test section. In the expansion tube, the acceleration tube at extremely low pressure is connected to the shock tube with a thin diaphragm. The shock wave driven by the shock tube is further accelerated by the acceleration tube, resulting in an extremely high-speed flow. However, measurements in the expansion tube have difficulties since a test time of the generated flow is quite short. Therefore, a numerical simulation of the expansion tube is important to support the measurement [D. E. Gildfind et al. 2018].

The shock wave traveling in a tube is gradually weakened by the interaction with the boundary layer. The shock speed attenuation observed in the expansion tube is quite large [H. Tanno et al. 2016], and it was not reproduced by the numerical simulation [K. Kitazono et al. 2019]. This discrepancy in the shock speed attenuation between the experiment and the numerical simulation has been a problem in recent years since the large shock speed attenuation causes a decrease in the velocity of the test flow, and also undermines the reliability of the numerical simulation used to identify the test time. Recently, a numerical simulation using a turbulence model such as Reynolds-averaged Navier-Stokes model has successfully reproduced the experimentally observed shock speed attenuation [H. Sakamoto et al. 2021]. As a result, it was suggested that the large attenuation of shock speed observed in the expansion tube is caused by the turbulent transition of the boundary layer behind the shock wave. However, the turbulent transition in the actual expansion tube facility is still controversial.

Methodology

In this study, we use an analysis of the linear stability theory to investigate the stability of the boundary layer that develops behind a shock wave propagating inside an expansion tube. Specifically, we consider two base flows for the linear stability analysis: an adiabatic wall condition and an isothermal wall condition. The Chebyshev spectral collocation discretization method was used to discretize the governing equations used in the linear stability analysis. The collocation point number is $N=121$. The solver for the linear stability analysis is an in-house code.

For the base flow used in the linear stability analysis, a self-similar solution of the boundary layer behind the shock wave propagating in the acceleration tube of the HEK-X expansion tube was used. In this study, the analysis was performed in a shock stationary frame (SSF), an inertial system in which the shock wave is almost stationary. This condition simulates the experiment[H. Tanno et al. 2016] conducted in JAXA's free piston type expansion tube HEK-X. Note that in this study, only the airflow behind the shock wave is considered, and the presence of the contact discontinuity is ignored. Under the isothermal wall condition, the heat capacity of the wall is sufficiently large and the wall temperature is assumed to be about room temperature, 300 K.

Results

The stability of the boundary layer flow behind a shock wave propagating inside the acceleration tube of an expansion tube was investigated using linear stability theory. A self-similar solution was used for the base flow in the linear stability analysis, and two conditions, an adiabatic wall condition and an isothermal wall condition, were prepared. The wave number $\alpha$ is normalized by the 99\% velocity boundary layer thickness $y_{99}$, i.e., $\alpha^*y_{99}$. We also used the 99% velocity boundary layer thickness as the characteristic length for the Reynolds number $\operatorname{Re}$.

Under the adiabatic wall condition, an unstable mode was found in which the time growth rate of the disturbance was positive. However, when checking the eigenfunction of this unstable mode, it was found to oscillate violently outside the boundary layer at the grid point interval. This behavior is unlikely to be a physically unstable mode, and it is suggested that the unstable mode found under the adiabatic wall condition is a numerical mode, not a physically unstable mode.

Under the isothermal wall condition, an unstable mode with wave number $\alpha$=5 was found when $x=0.3$ m behind the shock wave. When checking the eigenfunction of this unstable mode, it was found that the amplitude oscillated violently within the boundary layer and gradually decayed outside the boundary layer. The characteristics of this eigenfunction are similar to the supersonic mode, an unstable mode that appears when the wall cooling effect is large[C. P. Knisely & X. Zhong 2019]. The unstable mode was also found to be a high wave number unstable mode, with a frequency equivalent to 3.1 MHz.

The sensitivity of grid resolution on the unstable modes found under the isothermal wall condition was also investigated. The collocation point numbers are $N=241, \,361$. The unstable modes found under the isothermal wall condition shifted to the higher wavenumber side as the grid resolution was increased. In other words, it was found that the unstable modes found under the isothermal wall condition are greatly affected by the grid resolution. It is possible that if the grid resolution is increased sufficiently, the unstable mode will converge at a certain high wavenumber, but such an unstable mode would be on the order of megahertz and is unlikely to physically exist. Therefore, the unstable mode found is considered to be numerical mode under the isothermal wall condition.

In this study, we investigated the stability of the boundary layer behind a shock wave propagating in an expansion tube using the analysis of the linear stability theory. We assumed an ideal gas and investigated the time instability of the wave number in the $x$ direction under the parallel flow approximation. However, it is not appropriate to apply these assumptions to actual flow fields. In particular, the influence of real gas effects is thought to be large, and it is desirable to take these effects into account. Therefore, more advanced analysis is required, as it is possible that the flow field may become unstable due to the influence of the wave number in the spanwise direction, spatial instability, and axisymmetric effects, as well as analyses that take into account real gas effects and stability analyses that do not use parallel flow approximations.

Conclusion

In this study, the analysis of the linear stability theory was used to investigate the stability of the boundary layer behind a shock wave propagating in the expansion tube. Two base flows were prepared for an adiabatic wall and an isothermal wall condition. The analysis was based on the assumption of an ideal gas, and the temporal instability of the wave number in the streamwise direction was investigated.

The unstable modes found under adiabatic wall conditions are likely to be numerical modes since the eigenfunctions had odd shapes from a physical point of view. Under isothermal wall conditions, the unstable mode at high wavenumber was found. The eigenfunction shape of the high-wavenumber unstable mode is similar to the supersonic mode. However, as the grid resolution was increased, the time growth rate became smaller and the unstable mode shifted to higher wavenumber. If the grid resolution was increased sufficiently, the unstable mode may converge at a certain high wavenumber. However, the frequency of the unstable mode would be on the order of megahertz, making it unlikely to be physical.
Therefore, the high-wavenumber unstable mode found under the isothermal wall condition is likely to be a numerical mode, not physical unstable mode.

Reference

D. E. Gildfind et al.: Scramjet Test Flow Reconstruction for a Large-Scale Expansion Tube, Part 2: Axisymmetric CFD Analysis. Shock Waves 28, 899 (2018).
H. Tanno et al.: Basic Characteristics of the Free-Piston Driven Expansion Tube JAXA HEK-X. AIAA Paper 2016-3817 (2016).
K. Kitazono et al.: Numerical Study of Unsteady High Enthalpy Flow in an Expansion Tube. AIAA Paper 2019-1392 (2019).
H. Sakamoto et al.: Numerical Analysis of Shock Speed Attenuation in Expansion Tube. AIAA Paper 2021-0058 (2021).
C. P. Knisely and X. Zhong: Sound Radiation by Supersonic Unstable Modes in Hypersonic Blunt Cone Boundary Layers. I. Linear Stability Theory. Physics of Fluids, 31, 024103, (2019).

Speaker: Hiroki Sakamoto (Tohoku University)

20230908_hsakamoto_WRHTG_slide_v.3.0.share.key

20230908_hsakamoto_WRHTG_slide_v.3.0.share.pdf

37 Uncertainty analysis of optical emission spectroscopy measurements in the Electric Arc Shock Tube

Background
The Electric Arc Shock Tube (EAST) at NASA Ames Research Center produces hypersonic shock waves for validation and improvement of non-equilibrium chemical kinetics and radiation models relevant to atmospheric entry flows. The main experimental technique employs four spectrometers, capable of achieving simultaneous spatially resolved emission spectra of the radiating gas from the vacuum ultraviolet to the mid-infrared wavelength range [1, 2]. Calibrated frames map the radiance emitted along the probing volume across the tube diameter, resolved in wavelength and spatial dimensions along the view window. The similarity to the stagnation line flow over a blunt body at equivalent free-stream conditions is exploited to numerically simulate the hypersonic shock wave and evaluate the agreement with radiative simulations using the Nonequilibrium Radiative Transport and Spectra Program (NEQAIR v15.3) [3]. Experimental uncertainties on the measured radiance are important to quantify the discrepancy with numerical models, and directly propagate to inferred quantities, e.g., when performing calibration of rate parameters.

Methodology
Leveraging recent experimental data collected in Test Series 66 [3], which included incident air shocks at nominal velocities of 6 km/s and 7 km/s, and fill pressures of 2 Torr and 1.4 Torr, respectively, this work analyzes the uncertainty on the experimental radiance, and its impact on the measured temperature and number densities. Comparison to concurrent laser absorption measurements, as well as to data collected at similar test conditions in the Oxford T6 facility, will be also presented.
A previous work from Brandis et al. [4] characterized the uncertainty on emission spectroscopy measurement in EAST based on the scatter among data points with respect to a global fit of different conditions. This work focuses on the individual contributions to the OES calibration uncertainty and propagates their impact to the measured spectral radiance for each single shot. These include the calibration source intensity, its spatial non-uniformity over the sensor field, as well as the camera non-reciprocity and intensity non-linearity. Measurement and background noise are characterized by means of statistical sampling. Uncertainties are propagated with a random sampling of the distributions through the calibration steps.

Additionally, the effect of different analytical representations of the Instrument Line Shape (ILS) and Spatial Resolution Function (SRF) is studied when comparing with simulated post-shock equilibrium spectra and spatial radiance profiles. We consider the effect of reducing the slit width on the background continuum level, as well as hardware binning the CCD sensors to reduce the gate time and improve the spatial resolution.
Finally, the work investigates the impact of the measurement uncertainties on temperature and species number density obtained through fitting of the measured spectra, and evaluates their agreement with concurrent laser absorption measurements, in view of providing a consistent description of the post-shock conditions.

Preliminary results
Preliminary analysis provides standard deviations close to 10% of the measured local radiance below 900 nm. The relative contribution of the different terms ultimately depends on the measured signal level, with calibration and measurement noise being the largest terms for intensified CCD cameras at the observed conditions. In this regard, the noise equivalent radiance is a useful quantity to define the lower detectability threshold, and relative uncertainties increase significantly when the measured signal reaches this level.

Broadening of traditional analytical shapes, such as the square root of the Voigt profile, or the mixed Gaussian-Lorentzian profiles, with an additional square function improves the fitting of the measured ILS in case of wide slit widths. Optical raytracing across the tube diameter defines the optical resolution function, providing consistent results with previous analytical derivations and showing negligible spherical aberration effects. The camera gating function dominates the SRF overall.

Spectral fitting of $\textrm{NO}$, $\textrm{N}_2$ and $\textrm{N}_2^+$ excited states is in progress. Due to the fact the measurements are affected by the spatial resolution of the optical system, a deconvolution procedure is attempted to retrieve local values.

Conclusions
This work quantifies the experimental uncertainty on spatially resolved radiance maps obtained by optical emission spectroscopy measurements of incident shock radiation in the EAST facility. The aim is to determine bounds to the local spectral radiance, which can better inform comparison to model predictions and calibration of rate parameters. The sources of uncertainty at each calibration step are identified and propagated to the measured radiance. Analysis of fitted quantities is in progress.

References
[1] Cruden, Brett, Ramon Martinez, Jay Grinstead, and Joeseph Olejniczak. “Simultaneous Vacuum-Ultraviolet Through Near-IR Absolute Radiation Measurement with Spatiotemporal Resolution in An Electric Arc Shock Tube.” In 41st AIAA Thermophysics Conference. San Antonio, TX, USA, 2009.
[2] Cruden, Brett A. “Absolute Radiation Measurements in Earth and Mars Entry Conditions.” Radiation and Gas-Surface Interaction Phenomena in High-Speed Reentry, VKI Lecture Series, 2014.
[3] Cruden, Brett A, and Aaron M Brandis. “Updates to the NEQAIR Radiation Solver.” In 6th International Workshop on Radiation of High Temperature Gas, St. Andrews, UK, 2014.
[4] Cruden, Brett A, and Augustin Tibere-Inglesse. “Radiative Emission in Incident Air Shocks from 3-7 Km/s.” to be presented at AIAA Aviation Forum, Las Vegas, NV, USA, 2024.
[5] Brandis, A. M., C. O. Johnston, B. A. Cruden, D. Prabhu, and D. Bose. “Uncertainty Analysis and Validation of Radiation Measurements for Earth Reentry.” Journal of Thermophysics and Heat Transfer 29, no. 2 (April 2015): 209–21.

Speaker: Andrea Fagnani (Oak Ridge Associates Universities at NASA Ames Research Center)

Fagnani_RHTG24_v4.pdf

Thursday 12 September

Coffee Break

State to state and Collisional Radiative Modelling

38 Multi-temperature model for giant planet atmospheres

\section{Introduction}
%\label{sec:intro}

In the last years, state-to-state (StS) chemical kinetics have been used to model high-enthalpy flows in 2D configurations in dissociating air~\cite{bonelli2024finite,guo2024investigation,wang2023high}. The recent interest in the exploration of ice giant planets~\cite{blanc2021science} needs the construction of kinetic schemes for Hydrogen/Helium mixture plus some impurities (CH$_{4}$), which give a relevant contribution to the radiation emission in the shock layer.

A pure H$_{2}$/He StS kinetic scheme, originally developed to describe volume sources of negative ion production~\cite{celiberto2023advanced,colonna2017vibrational}, has been used to model EAST shock tube experiment in conditions of Saturn entry~\cite{cruden2017shock}, reproducing the ionisation profile~\cite{colonna2020ionization}. It has been shown that ionisation is more efficiently initiated by the reaction $2\text{H}_{2}\rightleftharpoons \text{H}_{3}^{+}+\text{H}^{-}$ than by ionising recombination ($2\text{H}\rightleftharpoons \text{H}_{2}^{+}+e^{-}$), the latter limited by the dissociation kinetics.

A StS model accounting for ionisation should also describe the free electron kinetics, through the solution of the Boltzmann equation~\cite{colonna2022two}, and, together with the vibrational levels, the evolution of electronically excited states of atoms and molecules~\cite{colonna2001influence}.

A multi-temperature (mT) model rigorously derived from the StS is here presented. The state-specific cross section database has been updated so as to include the most accurate data available for the electron-impact induced inelastic and dissociative processes of H$_2$~\cite{scarlett2021complete} and the mT model still follows the kinetics of selected electronically excited states of molecular hydrogen. The equation for vibrational energy relaxation includes also the contribution of chemical processes.

\section{From State-to-State to multi-Temperature}
\label{sec:StS-to-mT}
A mT model assumes that the internal distributions follow the Boltzmann function with independent internal temperatures $T_{v}$. Given a process like $\text{H}_{2}(v)+X \rightleftharpoons \cdots$
with $k_{v}^{d}$ and $k_{v}^{r}$ direct and reverse rate respectively, the kinetic equation contains the term proportional to $k_{v}^{d}(T)f_{v}(T_{v})$ and to $k_{v}^{r}(T)$, where $f_{v}$ is the vibrational distribution. The global rates are given by
\begin{equation}
\begin{array}{ll}
K^{d}(T,T_{v})&=\sum_{v}k_{v}^{d}(T)f_{v}(T_{v})\
K^{r}(T)&=\sum_{v}k_{v}^{r}(T)
\end{array}
\label{e:colonna:globalRateGen}
\end{equation}
and the contribution to the internal energy by
\begin{equation}
\begin{array}{ll}
\mathcal{L}(T,T_{v})&=\sum_{v}\varepsilon_{v}k_{v}^{d}(T)f_{v}(T_{v})\
\mathcal{G}(T)&=\sum_{v}\varepsilon_{v}k_{v}^{r}(T)
\end{array}
\label{e:colonna:globalGainLoss}
\end{equation}
where the symbols $\mathcal{L}$ and $\mathcal{G}$ indicate the rate of energy loss and gain respectively.
It should be noted that only $K^{d}$ and $\mathcal{L}$ depend on $T_{v}$, while $K^{r}$ and $\mathcal{G}$ are only a function of $T$. The reverse rates are not independent and can be calculated from the direct ones using the detailed balance principle
\begin{equation}
K^{r}(T)=K^{d}(T,T)/K_{eq}(T).
\label{e:colonna:detBal}
\end{equation}

A similar procedure is followed to calculate global rates for processes induced by electron collision,
$e^{-}+\text{H}_{2}(v)$.
The first step consists in calculating the StS rates as a function of the electron temperature $T_{e}$
\begin{equation}
k^{d}{v}(T{e})=\int_{\varepsilon^{\star}{v}}^{\infty}f^{M}{e}(\varepsilon,T_{e})u(\varepsilon)\sigma_{v}(\varepsilon)\varepsilon
\label{e:colonna:electronRateTe}
\end{equation}
where $f^{M}_{e}$ is a Maxwell electron energy distribution function, $\varepsilon$ the electron energy, $u$ the electron velocity and $\sigma_{v}$ the cross section of the process. The quantity $\varepsilon^{\star}_{v}$ is the threshold corresponding to the amount of energy lost by electrons in the process. Then the global rates as a function of $T_{e}$ and $T_{v}$ can be calculated using Eqs.~\ref{e:colonna:globalRateGen},\ref{e:colonna:globalGainLoss}. In the absence of an applied electromagnetic field, it is reasonable to assume that free electrons are in equilibrium with the gas temperature~\cite{colonna2020ionization}. For the sake of completeness, the contribution of these processes to the electron energy equation are given by
\begin{equation}
\begin{array}{ll}
\mathcal{L}(T,T_{v})&=\sum_{v}\varepsilon_{v}^{\star}k_{v}^{d}(T)f_{v}(T_{v})\
\mathcal{G}(T)&=\sum_{v}\varepsilon_{v}^{\star}k_{v}^{r}(T)
\end{array}
\label{e:colonna:elEnGainLoss}
\end{equation}
that should be added to the Landau-Teller terms due to elastic collisions with heavy particles.

The numerical values of the $K^{d}$ have been fitted as a function of $T$ and $T_{v}$ with a 2D best fitting procedure. Quantum effects and anharmonicity of the vibrational levels make the Arrhenius expression not accurate enough to fit the data in the long range. In order to preserve continuity full-range expressions have been used, accurate in the temperature interval $100\div 10^{5}$. The fitting expression is generally given as a function $T$
\begin{equation}
K^{d}(T,T_{v})=f(T;{c_{i}(T_{v})})
\label{e:colonna:FittingFunc}
\end{equation}
whose coefficients $c_{i}$ are functions of $T_{v}$. The functions used are generally the sum of Arrhenius or of sigmoids
\begin{equation}
\sigma_{f}(x)=a\frac{e^{(x-c)/\ell}}{e^{(x-c)/\ell}+e^{-(x-c)/\ell}}.
\label{e:colonna:FittingFuncBis}
\end{equation}

\section{Conclusions}
\label{sec:conclusion}
A novel mT model has been derived from accurate StS dynamical data. This is the first step toward a mT model of the entry in Ice Giants, including impurities already present in the atmosphere or evaporated from the vehicle surface. As in the traditional mT approaches, this model cannot take into account the departure from the Boltzmann distributions. However, corrections are possible, by adding an equation for the tail of the distributions~\cite{colonna2008recombination} to correct the rates with the contribution of highly-excited vibrational levels.

\subsection*{\sc acknowledgements}
This research has been funded under ESA Contract No. 4000139351/22/NL/MG "Development of a state-to-state CFD code for the characterization of the aerothermal environment of Ice Giants planets entry capsules".

%\bibliographystyle{IEEEbib}
%\bibliography{refs}

\begin{thebibliography}{10}

\bibitem{bonelli2024finite}
Francesco Bonelli, Davide Ninni, Gianpiero Colonna, and Giuseppe Pascazio,
\newblock ``A finite-volume hybrid weno/central-difference shock capturing
approach with detailed state-to-state kinetics for high-enthalpy flows,''
\newblock {\em Aeronautics and Astronautics}, p. 170, 2024.

\bibitem{guo2024investigation}
Jinghui Guo, Xiaoyong Wang, and Sijia Li,
\newblock ``Investigation of high enthalpy thermochemical nonequilibrium flow
over spheres,''
\newblock {\em Physics of Fluids}, vol. 36, no. 1, 2024.

\bibitem{wang2023high}
Xiaoyong Wang, Jinghui Guo, Qizhen Hong, and Sijia Li,
\newblock ``High-fidelity state-to-state modeling of hypersonic flow over a
double cone,''
\newblock {\em Physics of Fluids}, vol. 35, no. 11, 2023.

\bibitem{blanc2021science}
Michel Blanc, Kathleen Mandt, Olivier Mousis, Nicolas Andr{\'e}, Alexis
Bouquet, S{\'e}bastien Charnoz, Kathleen~L Craft, Magali Deleuil, L{\'e}a
Griton, Ravit Helled, et~al.,
\newblock ``Science goals and mission objectives for the future exploration of
ice giants systems: a horizon 2061 perspective,''
\newblock {\em Space Science Reviews}, vol. 217, pp. 1--59, 2021.

\bibitem{celiberto2023advanced}
Roberto Celiberto, Mario Capitelli, Annarita Laricchiuta, Lucia~Daniela
Pietanza, and Gianpiero Colonna,
\newblock ``Advanced models for negative ion production in hydrogen ion
sources,''
\newblock in {\em Physics and Applications of Hydrogen Negative Ion Sources},
pp. 167--188. Springer International Publishing Cham, 2023.

\bibitem{colonna2017vibrational}
Gianpiero Colonna, Lucia~D Pietanza, Giuliano D'Ammando, Roberto Celiberto,
Mario Capitelli, and Annarita Laricchiuta,
\newblock ``Vibrational kinetics of electronically excited states in {H$_2$}
discharges,''
\newblock {\em The European Physical Journal D}, vol. 71, pp. 1--8, 2017.

\bibitem{cruden2017shock}
Brett~A Cruden and David~W Bogdanoff,
\newblock ``Shock radiation tests for saturn and uranus entry probes,''
\newblock {\em Journal of Spacecraft and Rockets}, vol. 54, no. 6, pp.
1246--1257, 2017.

\bibitem{colonna2020ionization}
Gianpiero Colonna, Lucia~Daniela Pietanza, and Annarita Laricchiuta,
\newblock ``Ionization kinetic model for hydrogen-helium atmospheres in
hypersonic shock tubes,''
\newblock {\em International Journal of Heat and Mass Transfer}, vol. 156, pp.
119916, 2020.

\bibitem{colonna2022two}
Gianpiero Colonna and Antonio D'Angola,
\newblock ``Two-term boltzmann equation,''
\newblock in {\em Plasma Modeling (Second Edition) Methods and applications},
pp. 2--1. IOP Publishing Bristol, UK, 2022.

\bibitem{colonna2001influence}
G~Colonna and M~Capitelli,
\newblock ``The influence of atomic and molecular metastable states in
high-enthalpy nozzle expansion nitrogen flows,''
\newblock {\em Journal of Physics D: Applied Physics}, vol. 34, no. 12, pp.
1812, 2001.

\bibitem{scarlett2021complete}
Liam~H Scarlett, Dmitry~V Fursa, Mark~C Zammit, Igor Bray, Yuri Ralchenko, and
Kayla~D Davie,
\newblock ``Complete collision data set for electrons scattering on molecular
hydrogen and its isotopologues: I. fully vibrationally-resolved electronic
excitation of {H$_2$ ($X^1\Sigma_g^+$)},''
\newblock {\em Atomic Data and Nuclear Data Tables}, vol. 137, pp. 101361,
2021.

\bibitem{colonna2008recombination}
Gianpiero Colonna, Lucia~Daniela Pietanza, and Mario Capitelli,
\newblock ``Recombination-assisted nitrogen dissociation rates under
nonequilibrium conditions,''
\newblock {\em Journal of Thermophysics and Heat Transfer}, vol. 22, no. 3, pp.
399--406, 2008.

\end{thebibliography}

% References should be produced using the bibtex program from suitable
% BiBTeX files (here: refs). The IEEEbib.bst bibliography
% style file from IEEE produces unsorted bibliography list.
% -------------------------------------------------------------------------

Speaker: Gianpiero Colonna (PLASMI Lab at CNR-NANOTEC)

ColonnaRHTG10.pptx

39 Thermodynamics and transport properties for the characterization of ice giant entry conditions

\section{Background of the study}
\label{sec:intro}
The planned NASA missions to the ice-giants, i.e. Uranus and Neptune, push the space exploration to the edge of the solar system and commit to new challenges the research activities meant to address all the related engineering issues. Fluid-dynamic models of hypersonic entry conditions allows the estimation of heat load at the surface of the vehicle and represent the most valuable tool to perform virtual experiments and assist the design of thermal protection systems.
The reliable modelling of hypersonic entry of bodies in the atmosphere of the ice giants needs information relevant to the thermodynamics and transport properties of the plasma formed in the shock layer. The chemistry of the major atmosphere-components, i.e. H$_2$ and He, is affected by the presence of CH$_4$ as minority though not-negligible chemical species, furthermore the region close to the vehicle surface is complicated by the contamination of carbon-oxygen-compounds from the surface, thus requiring their characterisation in terms of single-species thermodynamic quantities and collision integrals for binary interactions.
This contribution focusses on the derivation of these properties within an hybrid theoretical framework and on the assessment of their accuracy for the construction of a database to be included in CFD codes.

\section{Methodology}
\label{sec:method}
The calculation of chemical equilibrium, thermodynamics and transport is performed with the web-access tool EquilTheTA (EQUILibrium for plasma THErmodynamics and Transport Applications)~\cite{d2018equiltheta} designed and implemented in the framework of the cooperation between the CNR ISTP Bari and University of Basilicata. The tool, accurate, stable and reliable in wide temperature and pressure ranges, derives the quantities from core databases of atomic and molecular energy levels and collision integrals, in the frame of the classical theory of statistical thermodynamics and the Chapman-Enskog theory, respectively.

The creation of a complete database of collision integrals for binary heavy-particle interactions in complex mixtures including large number of species has been successfully tackled adopting a hybrid procedure that combines the traditional {\sl multi-potential} with the {\sl phenomenological} approaches~\cite{capitelli2007possibility,laricchiuta2007classical,pirani2023intermolecular}. In the multi-potential method the effective collision integrals for a given interaction results from the averaging procedure of terms corresponding to each allowed interaction between the two colliding partners. The phenomenological approach, allowing the derivation of complete and consistent datasets of collision integrals for possibly any interaction, is very attractive, estimating the interaction potential on a physically sound basis. In fact, the average interaction is modeled by an Improved Lennard Jones (ILJ) potential whose features (depth and position of the well) are derived by correlation formulas given in terms of fundamental physical properties of interacting partners (dipole polarizability, charge, number of electrons effective in polarization). These approaches combined with the asymptotic approach for the estimation of the resonant charge-exchange contribution to odd-order collision integrals, represent a powerful strategy to extend the collision integral database. The electron-neutral interactions require the integration of the quantum differential elastic cross section for electron scattering.

\section{Results}
\label{sec:results}
The single-species thermodynamic properties are based on partition functions in a direct-level-summation scheme for diatomic molecules and within a RRHO (rigid rotator-harmonic oscillator) approximation for polyatomic molecules. The upgraded database of EquilTheTA, including the newly-calculated partition functions, has been compared with the most recent references in the literature~\cite{barklem2016partition,gamache2021total}, this last relying on experimental (HITRAN database) compilation of levels, finding a good agreement.
The transport cross sections for binary interactions, involving neutral-neutral and neutral-ion collision systems, have been derived within the hybrid approach, while for electron-neutral interactions the knowledge about the energy dependence of the differential elastic cross section has been constructed combining experimental and theoretical information in the literature. For interactions relevant to a dissociation regime, the comparison with the NASA results~\cite{bellas2022transport} shows a substantial agreement of the empirical formula for the estimation of phenomenological parameters, namely the effective number of electrons~\cite{cambi91}, with respect to the ab-initio approach adopted in the NASA study~\cite{bellas2022transport}.
The extended core databases for the H$_2$/He/C/O system allowed to perform a systematic comparison of the EquilTheTA results, in terms of equilibrium composition, thermodynamics and transport coefficients, with a number of papers in the literature discussing H/O and H/C/O subsystems~\cite{aubreton2009transport,kvrenek2008thermophysical,wang2012thermophysical,wang2012thermophysical_2}.

\section{Conclusions}
\label{sec:Conclusions}
The databases for H$_2$/He system developed for the atmosphere of the gas giants (Jupiter and Saturn) of the solar system have been extended to allow the thermodynamic and transport characterisation of the ice giants in presence of carbon-oxygen contaminants.

\subsection*{\sc acknowledgements}
This research has been funded under ESA Contract No. 4000139351/22/NL/MG "Development of a state-to-state CFD code for the characterization of the aerothermal environment of Ice Giants planets entry capsules".

%\bibliographystyle{IEEEbib}
%\bibliography{refs}

\begin{thebibliography}{10}

\bibitem{d2018equiltheta}
A~D’Angola, A~Laricchiuta, and G~Colonna,
\newblock ``Equiltheta: a web-access tool for lte plasma thermodynamics and
transport properties,''
\newblock {\em 45th EPS Conference on Plasma Physics}, 2018.

\bibitem{capitelli2007possibility}
M~Capitelli, D~Cappelletti, G~Colonna, C~Gorse, A~Laricchiuta, G~Liuti,
S~Longo, and F~Pirani,
\newblock ``On the possibility of using model potentials for collision integral
calculations of interest for planetary atmospheres,''
\newblock {\em Chemical Physics}, vol. 338, no. 1, pp. 62--68, 2007.

\bibitem{laricchiuta2007classical}
A~Laricchiuta, G~Colonna, D~Bruno, R~Celiberto, C~Gorse, F~Pirani, and
M~Capitelli,
\newblock ``Classical transport collision integrals for a {Lennard-Jones} like
phenomenological model potential,''
\newblock {\em Chemical Physics Letters}, vol. 445, no. 4, pp. 133--139, 2007.

\bibitem{pirani2023intermolecular}
Fernando Pirani, Stefano Falcinelli, Franco Vecchiocattivi, Vincenzo Aquilanti,
Annarita Laricchiuta, Gianpiero Colonna, and Mario Capitelli,
\newblock ``Intermolecular interactions and the weakly bound precursor states
of elementary physicochemical processes,''
\newblock {\em Rendiconti Lincei. Scienze Fisiche e Naturali}, vol. 34, no. 4,
pp. 983--995, 2023.

\bibitem{barklem2016partition}
Paul~S Barklem and Remo Collet,
\newblock ``Partition functions and equilibrium constants for diatomic
molecules and atoms of astrophysical interest,''
\newblock {\em Astronomy \& Astrophysics}, vol. 588, pp. A96, 2016.

\bibitem{gamache2021total}
Robert~R Gamache, Bastien Vispoel, Micha{\"e}l Rey, Andrei Nikitin, Vladimir
Tyuterev, Oleg Egorov, Iouli~E Gordon, and Vincent Boudon,
\newblock ``Total internal partition sums for the hitran2020 database,''
\newblock {\em Journal of Quantitative Spectroscopy and Radiative Transfer},
vol. 271, pp. 107713, 2021.

\bibitem{bellas2022transport}
Georgios Bellas~Chatzigeorgis, Justin~B Haskins, and James~B Scoggins,
\newblock ``Transport properties for neutral {C, H, N, O, and Si}-containing
species and mixtures from the gordon and mcbride thermodynamic database,''
\newblock {\em Physics of Fluids}, vol. 34, no. 8, pp. 087106, 2022.

\bibitem{cambi91}
R.~Cambi, D.~Cappelletti, G.~Liuti, and F.~Pirani,
\newblock ``Generalized correlations in terms of polarizability for van der
waals interaction potential parameter calculations,''
\newblock {\em Journal Chemical Physics}, vol. 95, no. 3, pp. 1852--1861, Aug.
1991.

\bibitem{aubreton2009transport}
Jacques Aubreton, Marie-Fran{\c{c}}oise Elchinger, and J~M Vinson,
\newblock ``Transport coefficients in water plasma: part i: equilibrium
plasma,''
\newblock {\em Plasma Chemistry and Plasma Processing}, vol. 29, pp. 149--171,
2009.

\bibitem{kvrenek2008thermophysical}
Petr K{\v{r}}enek,
\newblock ``Thermophysical properties of {H$_2$O-Ar} plasmas at temperatures
400--50,000 k and pressure 0.1 mpa,''
\newblock {\em Plasma Chemistry and Plasma Processing}, vol. 28, pp. 107--122,
2008.

\bibitem{wang2012thermophysical}
Wei~Zong Wang, AB~Murphy, JD~Yan, Ming~Zhe Rong, JW~Spencer, and MTC Fang,
\newblock ``Thermophysical properties of high-temperature reacting mixtures of
carbon and water in the range 400--30,000 {K} and 0.1--10 atm. part 1:
equilibrium composition and thermodynamic properties,''
\newblock {\em Plasma Chemistry and Plasma Processing}, vol. 32, pp. 75--96,
2012.

\bibitem{wang2012thermophysical_2}
WeiZong Wang, Joseph~D Yan, MingZhe Rong, AB~Murphy, and Joseph~W Spencer,
\newblock ``Thermophysical properties of high temperature reacting mixtures of
carbon and water in the range 400--30,000 {K} and 0.1--10 atm. part 2:
Transport coefficients,''
\newblock {\em Plasma Chemistry and Plasma Processing}, vol. 32, pp. 495--518,
2012.

\end{thebibliography}

Speaker: Annarita Laricchiuta (CNR ISTP Bari)

Laricchiuta_Thursday12Sept2024.pdf

40 Comparative assessment of state-to-state and macroscopic multi-temperature modeling for atmospheric re-entry conditions on ice giants

Background of the study

The continuous study of ice giants, Uranus and Neptune is critical to advance our understanding of the solar system's origin and evolution. Despite being subject of numerous previous studies, fundamental questions still remain about the composition and thermal behaviour of their atmosphere and about the planets’ bulk composition. It is therefore paramount to develop new missions and dedicate additional research efforts to further our knowledge of these planets.

Key to the exploration of the ice giants is the ability to ensure the survivability of probes through the challenging aerothermodynamic environment experienced during atmospheric re-entry. The scope of the present work is to present a comparison of the results of numerical codes employing different physical models and numerical methods for the prediction of the aerodynamic and thermal loads acting on re-entering objects.

Methodology

Non-equilibrium hypersonic flows typical of atmospheric entry on the ice giants will be modelled using two different approaches under the assumption of continuum regime. The first approach will adopt a detailed state-to-state modeling for the thermochemical processes and will take advantage of GPU to address the large computational burden of explicitly modeling non-equilibrium aerothermodynamics. In order to reduce the computational cost, a new reduced electronically-specific state-to-state model was derived at CNR-ISTP for the H2/He mixture in the two-temperature approach.

The second approach, making use of the open-source code SU2-NEMO, will follow a more standard scheme where a so-called macroscopic approach will be used to deal with the thermochemical mechanisms. The latter will consider explicitly the pseudo-species in terms of their mass balance but will use a macroscopic two-temperature model for the energy exchange as implemented in the VKI Mutation++ library. By taking advantage of the existing integration between the open-source CFD software SU2-NEMO and the thermo-chemical library Mutation++, this model enables simulating the atmospheric entry into Ice Giants.

Test case: Galileo probe atmospheric entry on Ice Giants

Numerical simulations of hypersonic flows past Galileo probe will be presented in the final manuscript for entry on the atmospheres of Uranus and Neptune. In order to simplify the geometrical configuration, a null angle of attack will be considered, so that axis-symmetric simulations can be executed.

Relevant atmospheric entry conditions have been identified by using a low fidelity approach based on local panel inclination methods and simplified correlations such as the Van Driest and Fay-Riddel model for heat fluxes. The results obtained with the different approaches to model the non-equilibrium processes will be compared in terms of the evaluation of stagnation line quantities such as temperatures and species mass fractions and in terms of surface aerothermal loads.

Speaker: Dr Marco Fossati (University of Strathclyde)

2024-09-Oxford-Stat2State-NEMO.pptx

41 Simplifying Chemical Kinetics in Hypersonics Non-Equilibrium Flows for Earth Re-entry

Background to the study

Thermal-chemical kinetics for non-equilibrium flows can be accurately represented through state-to-state (StS) models, where each molecular energy state is treated as a distinct pseudo-species. However, StS modeling requires a significantly larger number of variables than multi-temperature models, as it involves hundreds of internal energy states and thousands of kinetic processes, resulting in high computational demands.

This study introduces a new computational approach to simplify chemical kinetics in nonequilibrium hypersonic flows by identifying and reducing the number of dominant reaction mechanisms for a given flow condition. By recognizing that chemical reactions and molecular transport operate at vastly different time scales, it is possible to separate fast reactions that reach local equilibrium from those that impact the local flow dynamics.

Maas and Pope proposed the Intrinsic Lower Dimensional Manifold (ILDM) as a reduction technique to simplify the chemical kinetics of combustion problems in subsonic conditions. This method is based on the principle of timescale separation, assuming that fast time scales have equilibrated, resulting in the creation of a lower dimensional space governed by slow reactions. The ILDM method simplifies the conservation equations for a spatially homogeneous, adiabatic, and isobaric system. By linearizing the solution around an initial state with small perturbations, the problem can be formulated as an ordinary differential equation. The solution is then expressed as a linear combination of exponential terms, with the eigenvalues of the Jacobian matrix determining the time scale of the mechanism and the eigenvectors indicating the associated response direction.
Nevertheless, the assumptions of adiabatic and isobaric conditions are not representative of the conditions experienced in a reactive hypersonic nonequilibrium flow, such as those occurring in the shock layer or along a stagnation line.

In this paper, the constancy of pressure and energy is not assumed; instead, they are bounded by the conservation equations along an isentropic path. A generalized eigenvalue problem is proposed based on the linearized reactive Euler equations with Park's two-temperature model. The specific case of the eigenvalue problem corresponding to a stationary disturbance is analysed, unveiling the relationship between lower-order dimensional manifolds and the reaction mechanisms investigated. The methodology discussed can be applied to identify the relevant reaction mechanisms and reduce the computational cost of nonequilibrium hypersonic simulations for spacecraft re-entry applications.

Methodology

The conservation equations for a chemically reacting, non-conducting, inviscid one-dimensional flow incorporating vibrational relaxation are expressed below in vector conservative form, where $U$ is the vector of conserved variables, $F$ is the inviscid flux vector and the source term $W$ is the vector of production rates.

$\frac{\partial U}{\partial t}+\frac{\partial F}{\partial x}=W$

The instantaneous flow is defined as the sum of the mean state variables $\hat{Q}$ and their fluctuating disturbance $\hat{Q}$, where the latter is expressed as a harmonic wave of angular frequency $\omega$ and wave number $\nu$.

$ Q=\bar{Q}+\hat{Q} e^{j(\omega t+ \nu x)}$

The linearization of the governing equations around the equilibrium state is justifiable for small perturbations, yielding the formulation given as follows, which is of analogous type as a generalised eigenvalue problem $(B\xi=\lambda A\xi)$:

$\left(j \omega \frac{\partial U}{\partial Q} - \frac{\partial W}{\partial Q} \right) \hat{Q} = -j \nu \frac{\partial F}{\partial Q} \hat{Q}$

In the case of $\omega=0$, a stationary disturbance is obtained. The eigenvalue problem thus reduces to \autoref{eq:firstorder}, yielding eigenvalues of zeros for the conservation laws of mass, momentum and energy. The remaining non-zero eigenvalues $s$ are due to the presence of chemical source terms and describe the relaxation process towards equilibrium of the perturbed flow.

$ \frac{\partial W}{\partial Q} \hat{Q} = j \nu \frac{\partial F}{\partial Q} \hat{Q}$

The first-order linear differential equation has the straightforward exponential solution given below, where the initial conditions are given by the jump in properties across the shock $\Delta Q$. The physical meaning of the eigenvalues $s$ clearly appears to define the length-scale over which the relaxation process occurs for a flow that is travelling at a finite speed, whereas the right-eigenvectors $V_R$ define the characteristic direction along which the relaxation occurs. The components with large, negative, $s$ are in equilibrium, while those with small $s$ evolve slowly and govern the flow.

$ q(x)= \sum V_{R} e^{-s x} V_{L} \Delta Q $

Results and Conclusions

The paper will present a manifold analysis based on rate Jacobian for mechanism reduction at hypersonic conditions subject to dissociation, ionization and relaxation through a shock. The methodology is applied to 5-species and 11-species Park 2-Temperature model to demonstrate that a reduced number of independent mechanisms is sufficient to simplify and accurately describe the flow relaxation. Calculations of such reduced mechanisms for Earth re-entry scenarios are presented at different altitudes and speeds. The extracted reduced mechanisms are detailed and their validity is investigated. Comparisons are drawn with commonly used mechanisms for air using reduced species sets.

Speaker: Claudio Rapisarda (University of Oxford)

RHTG.v4.pptx

10:40 Coffee Break

High speed facilities, flight testing and propulsion

42 A Semi-Universal Spectroscopy System for Hypersonic Impulse Facilities

Background of the study:

This work demonstrates a versatile optical system capable of providing simultaneous radiation measurements from the vacuum ultraviolet (VUV) to the mid-wave infrared (MWIR). The system consists of several easily swapable configurations that aim to not only minimise experimental setup and alignment time but also increase the scientific output of the X2 expansion tube facility.

In experimental facilities studying shock layer radiation, measurements are commonly obtained using emission spectroscopy. The simplest optical configuration required for these measurements is based on the thin lens equation, in which the system properties, such as magnification and total system length, are fixed by the selection of just three variables: the lens focal length (f), distance from the object to the lens (u), and the distance from the lens to the image (v). Despite the formulation using lenses, facilities typically implement focusing mirrors instead to limit optical aberrations. In practice several additional optical components such as flat mirrors and periscopes are also required to overcome spatial restraints and to provide the desired image orientation. For a facility that specialises in radiation measurements over models, such as the X2 expansion tube, a given experiment’s field of view requirements can range anywhere from 1 mm to upwards of 50 mm. This often necessitates custom optical setups for each campaign, which in turn results in further facility downtime as new systems are installed, aligned, and calibrated. Not only does this downtime reduce the scientific output of the facility but, for the study of shock layer radiation, the lengthy setup time and associated complexity often limits the implementation of additional optical systems that could be used to obtain measurements over a larger range of wavelengths.

In choosing to instead base an optical system on a two-lens combination system, the relative simplicity of a one lens system is sacrificed in favour of the increased flexibility that the additional variables offer to the design space. This flexibility enables the design of an optical system in which the relative positions of all optical components are fixed, and instead system magnification can be altered by simply swapping between different combinations of the two focal mirrors.

To extend the versatility of this optical system dichroic mirrors have also been implemented. These operate by splitting the incoming light by wavelength region with minimal loss in signal intensity. If placed near the focal plane of the two-lens optical system, these mirrors subsequently allow for light from a single optical path to be split and sent into a series of spectrographs. This has been implemented on the X2 facility to provide simultaneous radiation measurements from the VUV to MWIR along the same line-of-sight for up to four spectral regions at a time.

Methodology:

In contrast to a traditional one lens optical setup, when approaching the additional design variables required for a two-lens system the larger parameter space can be more difficult to optimise. To alleviate this, several constraints were introduced to the system design process in the hope of an optical system that offers a range of magnifications predominantly controlled by modifying the focal lengths of the two main mirrors in lieu of large changes to mirror positions. However, whilst custom focal length mirrors allow for a fully fixed optical configuration, in practice this would require many custom mirrors which could be cost prohibitive. As such, the first constraint implemented for the primary optical system was that the focal mirrors had to be standard focal lengths that were commercially available. Subsequent constraints involved limiting total system length based on the available space and ensuring all angles between mirrors were kept as small as possible to limit optical aberrations such as astigmatism. The problem was approached iteratively, with an algorithm designed to sort through the given constraints and output viable systems that covered a wide range of magnifications whilst sharing similar optical layouts. For each system that satisfied the constraints a series of radiation measurements were undertaken using the X2 facility to test and showcase each optical configuration.

Results:

An optical layout was chosen to enable four easily interchangeable configurations, allowing for the study of radiation in X2 along 5, 8, 13, and 50 mm long fields of view with minimal changes to optical layout. Tests are currently underway using the X2 facility to test each configuration, providing radiation data from the VUV up to MWIR for a range of planetary entries. Analysis of the system and the resulting radiation data for the case of a high-speed Earth re-entry will be presented during the conference.

Conclusion:

A robust optical system was developed that enabled the study of radiation over flow scales ranging from 5 to 50 mm by utilising a two-focal mirror system to minimise large changes in mirror and spectrograph placement. This was paired with a set of dichroic mirrors to provide radiation measurements at these length scales for the case of a high-speed Earth re-entry simultaneously from the VUV to the MWIR spectral regions.

Speaker: Samuel Lock (The University of Queensland)

RHTG10_Samuel_Lock_Semi_Universal_Spectroscopy_System_for_Hypersonic_Impulse_Facilities.pptx

43 Validation and Characterisation of CO 2 Recombination in Expanding Flows for Low Velocity, High Density Mars Entry Conditions

1 Background of the study
Mars has been the major focus of space exploration and interplanetary travel next only to
Earth. The current design process of thermal protection systems for Mars atmospheric
entry has large tolerances and uncertainities, which result an the excessive mass of heat
shield. One major contributing factor to the uncertainties is the lack of understanding of
backshell radiative heating. Mars’ atmosphere is 95.7% CO2, 2.7% N2 and 1.6% Ar by
volume. In the later parts of the trajectory, closer to the surface of the planet, infrared
radiative heating due to CO2 is a major contributor towards total radiative heating. The
conditions here have a relatively low speed (3-5 km/s) of the spacecraft and a higher
atmospheric density. These conditions induce recombination of the dissociated CO2 in the
bow shock, resulting in its increased number density along the shoulder and the backshell
of the spacecraft. Hence, the total radiative heating here is dominated by radiation due to
vibrationally excited recombined CO2. With future missions to Mars most likely being
low speed, human crewed spacecraft, it is crucial to design a safe and efficient spacecraft
through better understanding the phenomena taking place in the expanding region.
Due to the limited available literature on the experimental validation of CO2 in the
expanding recombining flows under a non-equilibrium regime, the contribution of CO2
towards total radiative heating is not fully understood. Expansion tubes offer a powerful
tool for generating the real flight aerothermodynamics experienced during planetary entry.
The experimental data generated pertaining to such conditions can contribute towards the
comprehension of the phenomena taking place in the shock layer around the spacecraft
including the radiative heating. This can lead to a reduction in the uncertainties in the
modeling.
The goal of this study is to simulate and characterise CO2 recombination in the
conditions representative of Mars low velocity and high density atmospheric entry. The
experimental data is taken in UQ’s X2 expansion tube. Midwave infrared emission
spectroscopy was used to capture and quantify the 4.3 μm band of CO2 on a scaled wedge
model. To characterise the CO2 recombination in expanding flows, a 15 species model is
developed and added to the Eilmer4 database. The species included are CO2, CO, O2,
O, N2, NO, N, C, O, C2, CN, C+, O+, C+, e– . Cruden (2018) proposed the most up
to date reaction scheme relevant to Mars atmospheric entry. In this work, Park’s and
Cruden’s chemical kinetics models are used to predict the non-equilibrium shock layer in
the experiments. The numerically predicted radiation will be compared with the measured
data for validation purposes. NASA’s radiation modeling code NEQAIR is used to predict
and validate the radiation for the proposed conditions.
2 Methodology
2.1 Experimental Setup
To simulate the non-equilibrium expanding flows, a 54◦ two-dimensional wedge was used
to generate a strong shock, post shock compression region and expanding region. The
simulated phenomena are representative of the strong bow shock and the corresponding
expansion along the shoulder of a spacecraft making an atmospheric entry. The X2
expansion tube is used to create super-orbital flows representative of a planetary entry.
The conditions were designed to generate a free stream velocity of 4000 m/s, pressure of
400 Pa and density of 2 × 10−3 kg/m3, which is representative of BET (Best Estimated
Trajectory) of the MSL (Mars Science Laboratory) between 80 and 85 seconds.
To capture the 4.3 μm band of CO2, midwave infrared emission spectroscopy was used.
The flow was imaged along a field of view of about 35 mm, which captured the freestream,
post shock compression region, the expansion fan and the post-expansion region. The
imaging system as designed were set to capture the region 4 mm above the shoulder of
the wedge model.
2.2 Flowfield and Radiation Modeling
UQ’s in-house numerical simulation code, Eilmer4 is used to model the flow using the
most updated transport property model and the reaction schemes. The collision integrals’
fit coefficients were generated by curve-fitting the expression proposed by Gupta and Yos
using the data provided by Wright et al. NASA’s radiation modeling code, NEQAIR is
used for radiation modeling.
3 Results
Experimental spectra have been generated and calibrated. The calibrated spectra of both
the conditions showed an increase in the 4.3 μm band radiance in the shoulder region as
compared to the radiance in post shock region, that could be due to recombining CO2.
The authors are working on the validation of the most up-to-date reaction scheme and
transport property model using the data obtained through X2 experimentation using
Eilmer4 and NEQAIR. This will result in the characterisation of CO2 in the expanding
recombining region. The results will be included in the final presentation.
4 Conclusion
The conditions representative of low velocity, high density Mars atmospheric entry were
designed and generated in the X2 expansion tube in a cold driver configuration. Midwave
spectroscopy was used to capture the 4.3 μm band of CO2. The spectra showed an
increased band radiance in the expanding region as compared to post shock region. The
results from the flowfield and radiation modeling will help validate and characterise the
CO2 recombination for our conditions. The results will allow validation of the proposed
numerical scheme

Speaker: Mragank Singh (University of Queensland)

RHTG10_Mragank.pptx

44 Test trials of the ESTHER Shock Tube

ESTHER is a two stage shock-tube. It comprises a \SI{1.6}{\metre} length and \SI{200}{\milli\metre} diameter combustion driver where He/\ce{H2}/\ce{O2} and \ce{N2}/\ce{H2}/\ce{O2} mixtures are injected by an automated gas filling system at initial pressures up to \SI{100}{\bar}. These mixtures are ignited through a Nd:Yag laser shooting on the back plate through a thick sapphire window, reaching final pressures up to \SI{600}{\bar} for typical deflagrations (subsonic combustion). Occasionally, detonations (supersonic combustion) may occur, leading to higher transient pressures (up to \SI{2.4}{\kilo\bar} reflected pressures). The combustion chamber is designed accounting for such maximum pressure requirements. It is manufactured from low carbon super-duplex steel which has high mechanical strength and is tolerant for \ce{H2} presence because of minimized adsorption.

An intermediary compression tube is connected to the combustion chamber through a diaphragm designed to burst at a predetermined pressure. The compression tube is filled with He gas at pressures of about 0.01-\SI{1}{\bar}. The shock-wave propagates in this section leading to transient reflected pressures of \SI{70}{\bar}. The tube end sections are made in super-duplex stainless steel, while the middle sections are made in duplex stainless steel, which also has a low rate of Carbon, limiting adsorption. The compression tube section has an internal diameter of \SI{130}{\milli\metre}, and a length of about \SI{6.5}{\metre}.

The compression tube is connected to the shock-tube test section through a second diaphragm designed to burst at a predetermined pressure. The shock-tube is filled with a test gas at pressures of about 0.1 mbar. The shock-wave propagates in this section at velocities that can exceed \SI{10}{\kilo\metre\per\second}, leading to transient reflected pressures of no more than \SI{20}{\bar}. The tube is manufactured in duplex stainless steel. The shock-tube section has an internal diameter of \SI{80}{\milli\metre}, and a length of about \SI{5.9}{\metre}. Pressure sensor stations are located at different stages of the shock-tube, detecting the rise of pressure in the wake of the shock-wave. This allows for developing a triggering system initiating high-speed (10--100MHz rated), time-dependent spectroscopic measurements at the test-section windows (\SI{25}{\milli\metre} diameter) of the radiation emitted and absorbed in the wake of the shockwave.

A dump tank recovers all the gases flowing in the wake of the shock-wave. The \ce{H2O} liquid phase is drained off, while the remaining contaminated He mixture is evacuated by the pumping system, after which the shock-tube can be opened for cleaning operations and the replacement of the diaphragms.\

This work presents the final test-trials of the facility.

Speaker: Mario Lino da Silva (Instituto de Plasmas e Fusão Nuclear - Instituto Superior Tecnico)

12:25 Lunch

Radiation modeling and simulation: Missions

45 Electron Number Density Measurement in the T6 Stalker Tunnel utilising Langmuir Probes

Measuring electron number densities in hypersonic plasma flows is crucial for understanding aerothermodynamic phenomena, where traditional fluid dynamics principles no longer apply due to high speeds. The gas around the vehicle becomes hot enough to cause dissociation and ionisation of molecules, creating a chemically reacting flow [1]. The aerothermodynamic phenomena include chemical reactions, thermodynamic non-equilibrium, plasma formation, strong shock waves, and viscous interactions, which influence the communication blackout during re-entry along with aerodynamic heating, and material degradation [2], [3]. Ground testing facilities, such as the T6 Stalker Tunnel at the University of Oxford, provide a means to investigate ionisation rates, equilibrium ionisation percentages, and electron number densities under controlled conditions [4]. Both intrusive and non-intrusive diagnostic techniques can be employed to measure these parameters in hypersonic flows. Non-intrusive methods, such as microwave interferometry and spectroscopic techniques, offer the advantage of not disturbing the flow field [3]. However, intrusive measurements, particularly Langmuir probes, remain widely used in plasma facilities due to their simplicity, reliability, and ability to provide localised measurements [2], [5]. Their use in impulse facilities has been limited due to implementation and analysis restrictions. This work seeks to demonstrate the functionality of the probes in hypersonic facilities.

Langmuir probes, particularly triple probes, first demonstrated by Chen and Sekiguchi in 1964 [6], use three electrodes, with two forming a constant voltage circuit and the other floating at the plasma potential. These probes are employed in plasma facilities to measure electron number densities in highly ionised plasmas [7]. Goekce [5] previously attempted to use Langmuir probes in hypersonic plasma flows, but the results were challenging and the conclusions were limited. This highlights the complexity of adapting this diagnostic technique to the transient and high-enthalpy conditions characteristic of impulse facilities, further underscoring the innovative nature of the present research.

For the first time, electron number densities are measured in the University of Oxford's T6 Stalker tunnel. Intrusive measurements using Langmuir probes have been performed, and non-intrusive measurements using a Michelson interferometer, are planned. This work presents the Langmuir probe measurement results obtained so far. The objectives of the experiments are twofold: (i) to assess the operational efficacy of the Langmuir probes and validate their performance, and (ii) to quantify electron number densities behind shock waves across a spectrum of hypersonic flow conditions, specifically for shock speeds ranging from 4 to 7 km/s and freestream pressures between 13 and 133 Pa within the T6 Stalker Tunnel.

Benchtop tests, replicating the cabling and instrumentation layout within the tunnel, revealed a battery response time of 1.5 μs, with the overall system response time measured at 3 μs. These response times are deemed sufficiently rapid, given the described test times of 20 μs in [5]. The results of all benchtop tests demonstrated the Langmuir probes' proper functionality, confirming their readiness for experiments in the T6 Stalker Tunnel. The experiments in T6 acquired data in synthetic air, using the flush-mounted Langmuir probes, and operated in the aluminium shock tube mode (AST). A pair of rake-mounted Langmuir probes were positioned below the shock tube centreline at the dump tank exit, with concurrent Pitot pressure measurements for freestream pressure and test time characterisation. The probes were powered by a battery system operating at 9V.

Preliminary results from both Langmuir probes suggest repeatable current measurements and current-voltage (I-V) characterisation. Further testing and analysis are required to establish the consistency and reliability of the measurements. The calculated electron number density in the equilibrium region on the probe surface lies in the range of 1014/cm3. The expected electron number density in the given regime however is expected to be in the range between 1017/cm3 and 1018/cm3 [8], which lies in the post-shock equilibrium. The obtained electron number density measurements correspond to the secondary shocked gas near the probe surface, rather than the primary post-shock flow. This measured density differs from the predicted values by two orders of magnitude. This discrepancy likely arises from the presence of a non-equilibrium region between the primary post-shock flow and the probe surface. In this region, the gas undergoes additional processing due to the probe's presence, altering its properties. Further investigation into this phenomenon and its effects on electron number density measurements will be focused on in subsequent stages of this research.

To date, a solid foundation to work towards fulfilling the aims of this study could be established. The Langmuir probes are operational and generating data in the T6 Stalker Tunnel. Further experiments will provide more data with which the accuracy and functionality of the Langmuir probes can be evaluated further. These investigations will also investigate the reasoning why the observed order of magnitude is smaller than the expected order of magnitude in electron number density.

References
[1] Anderson, J. D. (2006). Hypersonic and High-Temperature Gas Dynamics, Second Edition. AIAA Education Series.
[2] Dunn, M. G., and Lordi, J. A., “Measurement of Electron Temperature and Number Density in Shock-Tunnel Flows. Part I: Development of Free-Molecular Langmuir Probes,” AIAA Journal, Vol. 7, No. 8, 1969, pp. 1458–1465. https:doi.org/10.2514/3.5415
[3] Yamada, G., Kawazoe, H., and Obayashi, S., “Electron density measurements behind a hypersonic shock wave in argon,” Journal of Fluid Science and Technology, Vol. 11, No. 1, 2016, p. JFST0005. https://doi.org/10.1299/jfst.2016jfst0005
[4] Collen, P., Doherty, L. J., Subiah, S. D., and McGilvray, M., “Development and commissioning of the T6 Stalker Tunnel,” Experiments in Fluids, Vol. 62, 2021, p. 225. https://doi.org/10.1007/s00348-021-03298-1
[5] Goekce, S., “Development of Langmuir Probes for Hypersonic Plasma Flow Diagnostics,” Master’s thesis, University of Queensland, 2009
[6] Chen, S. L., and Sekiguchi, T., “Instantaneous Direct-Display System of Plasma Parameters by Means of Triple Probe,” Journal of Applied Physics, Vol. 36, No. 8, 1965, pp. 2363–2375. https://doi.org/10.1063/1.1714492.
[7] Lindner, J. et al, “Langmuir analysis of electron beam induced plasma in environmental TEM”, Ultramicroscopy, Vol. 243, 2023, 113629.
[8] Iain D. Boyd; Modelling of associative ionization reactions in hypersonic rarefied flows. Physics of Fluids, 1 September 2007; 19 (9): 096102. https://doi.org/10.1063/1.2771662

Speaker: Nathalie Nick (University of Oxford)

2024_RHTG_Oxford_Electron_Number_Density_NickN.pptx

46 MITIGATING DRAGONFLY TPS DESIGN RISKS THROUGH IMPROVED AEROTHERMAL ENVIRONMENTS

The critical design review status of the Dragonfly aerothermal environments and simulations will be presented. Titan’s atmosphere predominantly consists of nitrogen (~98% by mole) with small amounts of methane (~2% by mole) and other trace gases. CN is a strong radiator and is found in nonequilibrium concentrations for Titan entry, and is of particular importance on the backshell, where radiation dominates the heat flux.

The presentation will discuss the simulation methodology and assumptions, as well as the margin process in determining the aerothermal environments for Dragonfly’s entry at Titan. In particular, the presentation will focus on refining the aerothermal environments to mitigate thermostructural risks for the PICA-D heatshield. The PICA-D risks are related to possible in-plane stress/strain failure modes and has been the focus of several experimental campaigns. Typically, the initial approach for margining aerothermal environments for heatshield design is conservative. As the mission design matures, refinements to this process are investigated, when and if, the aerothermal loads preclude the design from closing. This submission will detail the various physics and parameters investigated to lower the total margined heat flux. These physics include surface catalysis, summation of aerothermal uncertainties, flowfield/radiation coupling, non-equilibrium radiative heat flux model improvements, choice of design trajectory and radiative heating margin reduction utilizing shock tube informed bias. Shock tube informed bias being a relatively new and innovative methodology based on comparison to shock tube experiments performed in EAST at NASA Ames. These EAST experiments were also critical for reductions in the nominal radiative heat flux predictions.

Speaker: Dr Aaron Brandis (NASA Ames Research Center)

RHTG-Dragonfly-AerothermalUpdates.pptx

RHTG-Dragonfly-AerothermalUpdates-v2.pptx

47 Ultra-Highspeed Optical Emission Spectroscopy: Benefits and Difficulties

Shock and expansion tubes are important facilities used to investigate hypersonic conditions experienced at high Mach numbers and during planetary entry. However, they are limited by short test times depending on the facility design and test conditions used. The NASA Ames EAST shock tube, for example, has test times between 3 and 20 µs whereas the Chinese JF-12 facility has test times between 100 to 130 ms.
Optical emission spectroscopy (OES) is the primary tool used to capture spectral data on hypersonic impulse testing facilities. Short test times restrict the amount and quality of spectral data collected from the emitting test flow due to limitations in spectrometers.
The CCD and ICCD cameras used in spectrometers to capture spectral data on shock and expansion tubes only capture one image due to the low frame rate and sensitivity. Meaning multiple tests are often required to properly understand time history during experiments.
There are three important axes to consider when collecting spectral data: spectral, spatial and temporal. Spectrally resolved data helps identify the chemical species radiating in the flow, with an analysis of intensity providing temperature and number densities. Spatially resolved data allows for investigation of the post-shock relaxation region and other spatially distributed quantities. Temporally resolved data provides insight into the properties of the test gas and how it is changing over the test time; this would be particularly useful with quasi-steady, non-equilibrium, recirculating, turbulent and reacting flows.
An ideal spectrometer would be able to capture spectral data that is temporally, spatially and temporally resolved, without sacrificing its sensor’s sensitivity to low intensity emissions. Current setups with CCD and ICCD’s are only capable of spectrally and spatially resolved spectral data. Other sensor types have been used to collect temporally resolved spectral data with varying success.
Photomultiplier tubes (PMT) have been used as sensors in OES systems to collect temporally resolved data since becoming widely available in the 1930s. While being extremely sensitive and capable of high temporal resolution, the limitation comes from lack of a spatial and spectral dimension that modern ICCD based systems provide, rather providing intensity as a function of time at one location and wavelength.
Just like PMT's, streak cameras are also able to collect temporally resolved data. They can have both temporal and spectral dimensions. However, they do not simultaneously have a spatial dimension, meaning the streak camera can only spectrally resolve radiation from single point.
CCD and ICCD sensors spatial dimension makes them the best candidate for an ideal spectrometer, as seen in imaging spectrometers around the world. Now that ultra-highspeed cameras and image intensifiers have improved, and become more available, an optical emission spectrometer can now be designed allowing spectral data that is spectrally, spatially and temporally resolved to be captured on hypersonic impulse test facilities.
An ultra-highspeed optical emission spectrometer utilising a Phantom v2012 ultra-highspeed camera and a HiCATT 25 image intensifier has been used to successfully capture data that is spectrally, spatially and temporally resolved on the X2 expansion tube at the University of Queensland. This facility’s longest test time has been recorded at 100 µs. The spectrometer successfully captured spatially, spectrally and temporally resolved data at a frame rate of 100 kHz (100,000 FPS) on X2, and it is expected that a frame rate of 300 kHz could achieved.

The spectrometer achieved this speed when investigating contamination on X2, determining contaminant species. Due to the temporal resolved nature of the spectrometer, when sodium was deliberately added to certain locations in X2’s shock tube, it could be seen when it arrived at the test section, and how bright it was, at different times during the test flow.
After the experimental campaign, rigorous bench testing was conducted to fully understand the capabilities and how the spectral system worked. Upon bench testing of the system, discrepancies were noticed when the spectrometer was exposed to constant bright light source. The system would capture 500 frames of data, depending on the exposure time and gain setting of the intensifier, the number of counts detected by the sensor would decrease, despite a constant light source.
It was determined that the microchannel plate (MCP) in the intensifier, used to multiply the number of electrons after converting photons into electrons, was responsible for the drop in counts. The gain setting on the intensifier sets the level of electron multiplication. If the intensifier is unable to recharge the electrons in the MCP after a multiplication event, the gain will start to ‘drop’ reducing the multiplication effect. Over extended periods of time the gain drop can affect the intensity of light outputted by the intensifier. If this is not accounted for in intensity calibration of spectral data, it will increase uncertainties.
The investigation into contamination on X2 is just one example of the usefulness of having a spectrometer that can be spectrally, spatially and temporally resolved on hypersonic impulse test facilities. These types of spectrometers will help reduce the need for repeated shots, while increasing understanding of test flows in such facilities.

Speaker: Nathan Lu (The University of Queensland Centre for Hypersonics)

RHTG_talk_2024_NL.pptx

Coffee Break

High speed facilities, flight testing and propulsion

48 QUALIFICATION OF THE NASA AMES EAST LOW DENSITY SHOCK TUBE

QUALIFICATION OF THE NASA AMES EAST LOW DENSITY SHOCK TUBE

Brett A. Cruden(1)

(1)AMA Inc, NASA Ames Research Center, Moffett Field, CA 94035, USA, E-mail:Brett.A.Cruden@nasa.gov

The Electric Arc Shock Tube Facility (EAST) at NASA Ames Research Center is a well established shock tube facility for quantifying shock layer radiation phenomena. First operated in 1965, EAST consisted of an arc driver into a 10 cm (4”) diameter stainless steel driven tube. A second stainless steel 60 cm (24”) diameter tube, utilizing the same driver, was installed in 1971. The 24” tube saw limited use over the next 20 years, but some results were reported by Sharma and Park in 1989.[1] As a result of this study, the 4” diameter stainless tube was replaced with an aluminum tube and the 24” diameter tube was mothballed for the foreseeable future.

In 2012, the 24” diameter tube was recommissioned and dubbed the Low Density Shock Tube (LDST). Results from these tests in LDST were reported in [2, 3]. Findings during these tests were that the tube, due to sub-standard machining and acceptance criteria, generated a number of shock patterns which disrupted useful radiation measurements. Some re-machining and re-arrangement of the tube was conducted, which enabled a reasonable throughput of test data to be obtained [4]. At the completion of these tests in 2015, the LDST was decommissioned, and work begun on a replacement facility for LDST.

The new LDST is an 18.6 m long, 54.1 cm (21.3”) diameter aluminum tube that couples to the existing EAST driver. The test section is an expanded version of the HVST test section, with 32 round diagnostic ports for pressure and line of sight optical measurements (e.g. laser absorption) and two oblong ports for imaging spectroscopy of the shock structure along the tube centerline. The EAST driver discharges into a 3.7m long, 10 cm diameter tube before expanding through a 15º cone into the larger diameter. The observation window is approximately 16m from the beginning of the larger diameter and 21m downstream from the cone. The initial tests in LDST are scheduled to begin in the summer of 2024 and will focus on conditions relevant to the earlier part of the Dragonfly entry to Titan.

This paper will report on status and development of the LDST, including previous studies that lead to its replacement, specification of the new tube, and initial testing in LDST. Overlap conditions of LDST to HVST will be presented.

1. Sharma, S. P., and Park, C. "Operating Characteristics of a 60-and 10-cm Electric Arc-Driven Shock Tube, Part II: The Driven Section," Journal of thermophysics and heat transfer Vol. 4, No. 3, 1990, pp. 266-272
2. Bogdanoff, D. W., and Cruden , B. A. "Optimizing Facility Configurations and Operating Conditions for Improved Performance in the NASA Ames EAST 24 Inch Shock Tube," Vol. NASA/TM-2016-219164, NASA/TM-2016-219164, No. NASA/TM-2016-219164, 2016,
3. Cruden, B. A. "Radiance Measurement for Low Density Mars Entry," 43rd AIAA Thermophysics Conference 2012-2742, 2012,
4. Cruden, B. A., and Brandis, A. M. "Measurement of Radiative Non-equilibrium for Air Shocks Between 7-9 km/s," 47th AIAA Thermophysics Conference 2017-4535, 2017. doi: 10.2514/6.2017-4535

Speaker: Brett Cruden (AMA Inc/NASA Ames)

RHTG 2024 LDST.pptx

49 ULTRAVIOLET LASER ABSORPTION SPECTROSCOPY IN SHOCK TUBE FLOWS

1. INTRODUCTION
   The complexity of the flowfield encountered around reentering
   vehicles poses significant problems to the design
   of spacecraft thermal protection systems. One
   large source of uncertainty is linked to thermochemical
   non-equilibrium. Ground testing is conducted to generate
   flows similar to those encountered in flight, replicating
   the essential features of non-equilibrium flows.
   One such canonical experimental setup is the shock
   tube, where a normal shock transiently passes through a
   straight tube [CB17]. Although subject to several facilityrelated
   artefacts [CMM23, CSMM22], this setup represents
   one of the most fundamental fluid mechanical
   processes, which allows the isolation of thermochemical
   non-equilibrium from other aspects of complex flowfields.
   As such, shock tubes provide the opportunity of
   studying fundamentals of thermochemical reactions for
   flight-relevant enthalpies.
   Optical diagnostics provide the ideal vehicle to interrogate
   these flows with respect to their thermochemical
   state. As one such technique, laser absorption spectroscopy
   (LAS) is seeing a rise in use due to the recent
   improvement in high-speed tunable diode lasers and
   quantum cascade lasers [SKW+19, GGD+24]. LAS targets
   the lower state of radiative transitions and can therefore
   provide absolute number density measurements of
   low-energy quantum states. If ground states are measured,
   absolute particle densities of the probed species
   can be inferred with high accuracy. Scanning absorption
   spectroscopy techniques utilise a spectrally narrow light
   source whose central wavelength is changed very rapidly.
   By scanning over an absorption line in this fashion and
   recording the transmitted light with a detector, the line
   profile and absolute absorbance are measured and can be
   utilised to infer the translational temperature and lower
   state density. Broad band light sources can be utilised as
   well, however, the absorption features need to be spectrally
   resolved by a spectrometer. In the current work, a
   broad band light source is used, as it provides an instantaneous
   snapshot of the passing shock wave and does not
   require tuning of the wavelength during the experiment.

2. METHODOLOGY
   Shock tube flows are generated in the Oxford T6 Stalker
   Tunnel in aluminium shock tube mode [GCM22]. Optical
   measurements are taken through windows set in the
   shock tube wall, and acquisition is triggered to record
   as it passes this location. Flow conditions investigated
   in this work will consist of a set of velocities between
   5.5 km.s􀀀1 and 6.5 km.s􀀀1 [GCM22]. The laser absorption
   spectroscopy system utilises a bespoke modeless
   laser based on the work by Ewart [Ewa85]. A pulsed
   laser source provides a beam with a high degree of collimation
   that is advantageous for propagation over a long
   path, accurate steerability through the region of interest
   and efficient illumination of the spectrometer for recording
   the absorption spectrum. Laser sources, however,
   are usually characterized by a spectrum of longitudinal
   modes with frequency separation related to the laser cavity
   length. The spectral gaps between modes and their
   fluctuation in amplitude and frequency leads to difficulties
   in recording absorption spectra consisting of narrow
   spectral features. Some molecular absorption lines that
   may fall between the modes will not be recorded or distorted
   as a result of the amplitude and frequency fluctuations.
   The modeless laser, since it operates without a resonant
   cavity, is free from longitudinal mode-structure and
   provides an essentially continuous spectrum with spectral
   noise determined basically by quantum fluctuations in
   the amplified spontaneous emission from the amplifying
   medium. The unique advantage of this system is that it
   provides a tunable centre wavelength with variable bandwidth
   and a continuous spectrum that eliminates mode
   noise [SSE91, EAB+05, KE97].
   A flashlamp-pumped nanosecond Surelite I-10 Nd:YAG
   laser is used in its third harmonic mode producing radiation
   at 355 nm which is passed through a set of lenses
   to control the beam size. The Nd:YAG fundamental and
   second harmonic (1064 nm and 532 nm) are dumped in
   an enclosure outside of the Nd:YAG laser head, with
   only the third harmonic propagating into the modeless
   laser system. The third harmonic beam is separated into
   four beams by a four-faceted prism which are each absorbed
   by a dye cell at different heights [Ewa85]. The
   dye cell features a continuous flow of ethanol containing
   0.28 g.L􀀀1 of Coumarin dye. The dye produces a spectrally
   broad output at each of the four pumped locations.
   The spontaneous emission from the four pumped strips
   in the dye cell are amplified as a travelling wave by refection
   at two totally internally reflecting (TIR) prisms
   with apexes slightly displaced relative to each other. The
   dispersing prism selects a band of wavelengths from the
   fluorescence spectrum of the dye. The orientation of the
   right-hand TIR prism is used to select a band centred on
   452 nm. The output beam is subsequently frequency doubled
   in a critically phase-matched crystal of BBO (Beta
   Barium Borate) to 226 nm with a bandwidth (FWHM)
   of approximately 2 nm. This ultra-violet beam is separated
   from the fundamental at 452 nm using a Pellin-
   Broca prims and directed to the shock tube.
   Once the beam is produced with the aforementioned
   spectral properties, it is passed through a system of turning
   mirrors and relay lenses to the side of the T6 tunnel.
   At this location it is expanded and collimated by a twolenses
   system containing a cylindrical and a spherical
   lens, resulting in a laser sheet. This sheet is aligned with
   the field of view of a telecentric imaging system. As the
   shock wave travel through this field of view, the laser is
   activated and partially absorbed by the nitric oxide in the
   flow. The laser sheet will be arranged in such a way that it
   covers the freestream, non-equilibrium region and equilibrium
   region behind the shock front. The transmitted
   radiation of the sheet is imaged onto the entrance slit of
   the spectrometer. Hence, the resulting spectral image corresponds
   to a spatially resolved image along one dimension.
   This way, the absorbance can be simultaneously
   measured at different locations across the shock wave, resulting
   in a resolution of the non-equilibrium layer. The
   data acquired in this way will be used to infer NO number
   densities and excitation temperatures through spectral
   fitting methods.

3. RESULTS
   The full paper will present the data of the currently ongoing
   test campaign and will contain setup, calibration and
   post-processing. Raw absorbance data will be shown, as
   well as the post-processed properties of nitric oxide in the
   shock layer. The post-processing will be undertaken by
   comparing the measured absorbance to a computational
   model which simulates the absorption through a hightemperature
   gas. This will allow the determination of
   nitric oxide ground-state densities, as well as vibrational
   and rotational temperature.

Speaker: Maïlys Buquet

12_09_MB_RHTG_v2.pptx

50 Emission spectroscopy of ablative heat shield shock layers for earth entry in the T6 free piston driven shock tube

See attached

Speaker: Mr Aaron Kennedy (University of Oxford)

Oxford CO2 RHTG 2024-No notes-AK.pptx

Wrap-up Closure, RHTG-11

Dinner at St Hilda's College

On arrival, please report to the College Lodge on Cowley Place where you will be directed to the dinner.