Improved molecular collision models for nonequilibrium rarefied gases

The Direct Simulation Monte Carlo (DSMC) method typically used to model thermochemical nonequilibrium rarefied gases requires accurate total collision cross sections, reaction probabilities, and molecular internal energy exchange models. However, the baseline total cross sections are often determined from extrapolations of relatively low-temperature viscosity data, reaction probabilities are defined such that experimentally determined equilibrium reaction rates are replicated, and internal energy relaxation models are phenomenological in nature. Therefore, these models have questionable validity in modeling strongly nonequilibrium gases with temperatures greater than those possible in experimental test facilities. To rectify this deficiency, the Molecular Dynamics/Quasi-Classical Trajectories (MD/QCT) method can be used to accurately compute total collision cross sections, reaction probabilities, and internal energy exchange models based on first principles for hypervelocity collision conditions. In this thesis, MD/QCT-based models were used to improve simulations of two unique nonequilibrium rarefied gas systems: the Ionian atmosphere and hypersonic shocks in Earth's atmosphere.;The Jovian plasma torus flows over Io at ≈ 57 km/s, inducing high-speed collisions between atmospheric SO2 and the hypervelocity plasma's O atoms and ions. The DSMC method is well-suited to model the rarefied atmosphere, so MD/QCT studies are therefore conducted to improve DSMC collision models of the critical SO2-O collision pair. The MD/QCT trajectory simulations employed a new potential energy surface that was developed using a ReaxFF fit to a set of ab initio calculations. Compared to the MD/QCT results, the baseline DSMC models are found to significantly under-predict total cross sections, use reaction probabilities that are unrealistically high, and give unphysical internal energies above the dissociation energy for non-reacting inelastic collisions and under-predicts post-dissociation SO internal energy. Implemented into DSMC, the MD/QCT-based models had a significant effect on simulations of simple, thermal nonequilibrium heat bath and 2D counterflow cases approximating the upper atmospheric conditions of Io. In high-fidelity 1D simulations of the atmosphere of Io, the MD/QCT models predicted approximately half the SO2 atmospheric dissociation due to O and O+ bombardment and a temperature rise due to plasma heating further from the Ionian surface than the existing baseline methodologies.;Hypersonic spacecraftre-entering Earth's atmosphere experience significant heating from the post-shock gas. The DSMC method is used to model hypersonic shocks during the early stages of re-entry because of the rarefied nature of the atmosphere at high altitudes. Improved modeling of the N-N 2 and N2-N 2 collision pairs are thus generated with MD/QCT. For the N-N2 pair, a potential energy surface developed at NASA Ames is used and, for the N2-N2 pair, a new potential energy surface is developed using a ReaxFF fit to recent advanced ab initio computations. The MD/QCT-computed total cross sections agreed well with the baseline models, but the MD/QCT reaction probabilities exhibited better physical behavior, a stronger dependence on initial molecular internal energy, and were generally lower than the baseline DSMC chemistry models for strong nonequilibrium conditions, but higher for equilibrium conditions. Furthermore, the MD/QCT results predicted faster rotational-translational energy relaxation for the N-N2 pair and faster vibrational-translational energy relaxation for the N2-N2 pair. The MD/QCT models were tested in DSMC simulations of 2D axisymmetric hypersonic flow over a blunt body and thermal nonequilibrium heat bath cases. The MD/QCT models led to increased post-shock N2 dissociation and faster rates of internal energy relaxation, each of which led to corresponding decreases in translational temperature.