Monte Carlo sensitivity analyses of DSMC parameters for ionizing hypersonic flows

Higdon, Kyle J.
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This work focuses on the development and sensitivity analyses of a direct simulation Monte Carlo (DSMC) code to understand the complex physical processes that occur during hypersonic entry into a rarefied atmosphere. Simulations are performed on 1-dimensional hypersonic shock scenarios that mimic the conditions of high altitude atmospheric entry to Earth and Saturn with the Computation of Hypersonic Ionizing Particles in Shocks (CHIPS) code. To model hypersonic entry problems accurately, the CHIPS code must resolve nonequilibrium flows and account for a number of complex gas dynamics processes at the molecular level. In this thesis, several high temperature models are added to the CHIPS code including charged particle models and electronic excitation. These models are refined using preliminary sensitivity analyses resulting in improved electronic excitation models and a new backward chemical reaction model. The CHIPS simulations completed in this work reproduce rarefied hypersonic shock tube experiments performed in the Electric Arc Shock Tube (EAST) at NASA Ames Research Center. The CHIPS results are post-processed by the NEQAIR line-by-line radiative solver to compare directly to spectra measured experimentally in EAST. The DSMC techniques used to model hypersonic phenomena require numerous experimentally calibrated parameters. Many of these parameters are inferred from lower temperature experiments, resulting in an unknown amount of uncertainty in the simulated results at the extreme conditions of hypersonic flow. A global Monte Carlo sensitivity analysis is performed by simultaneously varying the CHIPS input parameter values to understand the sensitivity of experimentally measured quantities simulated by the CHIPS and NEQAIR codes. The sensitivity of several of these output quantities is used to rank the input parameters, identifying the most important parameters for the simulation of the hypersonic scenario. It was concluded that experimentally measured radiation intensity is most sensitive to the following key processes: N+e⁻⇌N⁺+e⁻+e⁻, NO+N⁺⇌N+NO⁺, N₂+N⇌N+N+N, N+O⇌NO⁺+e⁻, N+N⇌N₂⁺+e⁻, and Z [subscript elec] for N, O, and N₂⁺. In the future, this ranking can be used to identify which input parameters should be experimentally investigated, where model improvements could be beneficial, and aid in reducing the parameter space for DSMC calibrations to experimental data.