The Stardust Sample Return Capsule (SRC) entered the Earth's atmosphere at a velocity of 12.6 km/s. At high altitude, the flow field is expected to be in a strong state of thermochemical nonequilibrium. In the present study, both continuum (CFD) and particle (DSMC) methods are used to analyze the forebody flow of the SRC at an altitude of 81 km where the freestream Knudsen number is about 0.005. The very large entry velocity represents a highly energetic condition for which the thermochemistry models are not well calibrated. Direct comparisons between baseline CFD and DSMC models give enormous differences in basic flow field properties. To study the discrepancy between the solutions, different methods for determining the temperature used by CFD to control the dissociation and ionization reactions are investigated. Also, a new model is introduced for the DSMC technique that makes it possible to simulate reverse direction chemical reactions in a manner more consistent with that used in CFD. While the revised CFD and DSMC results are in better agreement with each other, under these highly-energetic, near-continuum flow conditions, significant differences remain between continuum and particle solutions. Additional CFD computations performed at lower altitude indicate, as expected, that flow field results become less sensitive to details of the chemistry modeling further into the continuum regime.
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