Speaker
Description
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.
Summary
The Direct Simulation Monte Carlo (DSMC) method is used to simulate flow conditions in Sandia's Hypersonic Shock Tunnel (HST) facility and generate numerical estimates of flow macroparameters and NO emission in the shock layer. The modeled setup consists of a cylinder geometry placed in a hypersonic flow in a state of thermal non-equilibrium with freestream velocities of 4km/s and 4.5km/s. Flowfield distributions of specie-specific temperatures highlights regions of thermal non-equilibrium and different vibrational relaxation time scales associated with each specie. The DSMC solutions also predict significant quantities of NO in the expansion and wake regions with the spatial variation in distributions governed solely by flow transport properties. The final presentation will show comparisons of fitted temperatures from experiments and DSMC results as well as comparisons of experimental emission data with numerical estimates of emission spectra.
