Speakers
Description
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.
Summary
A new spectral fitting routine is validated and applied to real shock tube data
