This thesis describes research in modeling rarefied, nonequilibrium hypersonic flows using the direct simulation Monte Carlo (DSMC) method. Modeling of chemical reactions and ionization processes in highly nonequilibrium flows is an important aspect in the simulation of flow fields and radiation of high-speed reentry vehicles. To develop a physically accurate chemical reaction model for use in DSMC, the molecular dynamics/quasi-classical trajectory (MD/QCT) method was utilized. For modeling rarefied, ionized hypersonic flows, a charge neutrality approach was followed, and an improved chemistry model involving electron collision and energy exchange mechanisms was developed.;The MD/QCT chemical reaction model was applied for the O+HCl → OH+Cl reaction, which is an important exchange reaction in atmospheric-side jet interaction flows. It was found that the MD/QCT model using a recent state-of-the-art potential energy surface predicted good agreement with the total collision energy model because this reaction is a low enthalpy reaction and does not show strong favoring of internal modes. For strong favoring reactions, one should verify both reaction and collision cross sections using the MD/QCT method if an accurate potential energy surface is available.;Ionized hypersonic flows for the Stardust blunt body were simulated in DSMC between 68.9 and 100 km altitudes for a free stream velocity higher than 10 km/s. The flow modeling included ionization processes and energy exchange assuming that charge neutrality exists in the bow-shock region. Accurate modeling of electron scattering collision processes and electron-vibration energy exchange using Lee's relaxation time for the first time in DSMC is presented and was found to significantly influence the vibrational and electron temperatures. For further analysis, the DSMC Stardust simulations were compared with computational fluid dynamics (CFD) results at 68.9 and 80 km altitudes. Breakdown effects were investigated, and influences of shock stand-off distance and temperature profiles were observed. Radiation was calculated using the Nonequilibrium Air Radiation code, and it was found that the new DSMC chemistry and excitation models significantly affected the N and O radiation in the ultraviolet range. Preliminary results show that CFD predicts one order of magnitude higher radiation than DSMC overall.
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