DSMC-CFD comparison of a high altitude, extreme-mach number reentry flow

Stardust reentry ows have been simulated at 80 km altitude, 12.8 km/s, using the direct simulation Monte Carlo (DSMC) and computational uid dynamics (CFD). Five ions and electron were considered in the flowfield, and ionization processes were modeled in DSMC. The ion-averaged velocity method in DSMC was validated to maintain charge-neutrality in the shock. Collision and energy exchange models for DSMC were reviewed to ensure adequacy for the high-energy ow regime. Accurate electron-heavy particle collision cross sections and an electron-vibration (e-V) relaxation model using Lee's relaxation time were implemented in DSMC. Although the DSMC results agreed well with CFD for the collision only case, discrepancies between DSMC and CFD were observed in the shock with the relaxation model activated. Furthermore, with full chemical reactions and ionization processes, DSMC results were compared with CFD. It was found that the assumption of electron temperature is crucial for the prediction of degree of ionization (DOI). At 80 km, the DOI predicted by DSMC was found to be approximately 3 %, but in CFD, the DOI is greater than 20 % for the case of T_e = T_tr and 9 % for the case of T_e = T _vib. In DSMC, the e-V relaxation model was found to be important to predict electron and vibrational temperatures at this altitude, and electron temperature is the same order as the vibrational temperature. Therefore, compared to the DSMC solution, the assumption of T_e = T_vib is preferable in CFD. Using the Mott-Smith (M-S) model, good agreement was obtained between the analytical bimodal distribution functions and DSMC velocity distributions. A effective temperature correction in the relaxation and chemical reaction models in CFD using the M-S model may reduce the breakdown discrepancy between DSMC and CFD in the high gradient region.