Direct Simulation Monte Carlo Shock Simulation of Saturn Entry Probe Conditions

Abstract

This work describes the initial efforts of a direct simulation Monte Carlo investigation of bow shock physics for Saturn entry probe scenarios and the influence of nonequilibrium phenomena on Saturn entry conditions. Therefore, the purpose of this paper is to employ models that are sufficient for a typical hypersonic entry direct simulation Monte Carlo simulation and identify the areas where improvement is required. The Computation of Hypersonic Ionizing Particles in Shocks (CHIPS) direct simulation Monte Carlo code is used to simulate two rarefied hypersonic shock tube experiments on a hydrogen–helium mixture performed in the Electric Arc Shock Tube (EAST) at NASA Ames Research Center. The CHIPS simulations are postprocessed through the NEQAIR line-by-line radiation code to compare directly to the experimental results. Improved collision cross-sections, inelastic collision parameters, and reaction rates are determined for a high-temperature direct simulation Monte Carlo simulation of a seven-species hydrogen–helium mixture, and an electronic excitation model is implemented in the CHIPS code. Simulation results for 27.8 and shock waves are obtained at 0.2 and 0.1 Torr, respectively, and compared to measured spectra in the vacuum ultraviolet, ultraviolet, visible, and infrared ranges. These results confirm the persistence of nonequilibrium for several centimeters behind the shock and the diffusion of atomic hydrogen upstream of the shock wave. Although the magnitude of the radiance does not match experiments and an ionization inductance period is not observed in the simulations, the discrepancies indicate where improvements are needed in the CHIPS and NEQAIR models.