Particle and continuum method comparison of a high-altitude, extreme-mach-number reentry flow

Stardust reentry flows have been simulated at an altitude of 80 km for a freestream velocity of 12:8 km=s using direct simulation Monte Carlo (DSMC) and computational fluid dynamics (CFD). Five ions and electrons 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 flow regime. Accurate electron-heavy particle collision cross sections and an electron-vibration 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. At 80 km, the degree of ionization predicted by DSMC was found to be approximately 5%, but in CFD, the degree of ionization is greater than 25% for the case of T_e = T_tr and 9% for the case of T_e = T_vib. In DSMC, the electron-vibration relaxation model was found to be important to predict electron and vibrational temperatures at this altitude, and the 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. In addition, using the Mott-Smith model, good agreement was obtained between the analytical bimodal distribution functions and DSMC velocity distributions. An effective temperature correction in the relaxation and chemical reaction models using the Mott-Smith model may reduce the continuum breakdown discrepancy between DSMC and CFD inside the shock in terms of degree of ionization and temperatures, but a general implementation is not clear.