(2020-12-04) Poondla, Yasvanth Kumar; Varghese, Philip L.; Goldstein, David Benjamin, doctor of aeronautics; Raja, Laxminarayan; Liechty, Derek; Moore, Christopher

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The goal of this work is to build up the capability of Quasi-Particle Simulation (QuiPS), a novel flow solver, such that it can adequately model the rarefied portion of an atmospheric reentry trajectory. Direct Simulation Monte Carlo (DSMC) is the conventional solver for such conditions, but struggles to resolve transient
flows, trace species, and high level internal energy states due to stochastic noise. Quasi-Particle Simulation (QuiPS) is a novel Boltzmann solver that
describes a system with a discretized, truncated velocity distribution
function. The resulting fixed-velocity, variable weight quasi-particles enable
smooth variation of macroscopic properties. The distribution function
description enables use of a variance reduced collision model, greatly
minimizing expense near equilibrium.
Improvements made to the method in this work include parallelization of the collision integral routine, modification of the velocity space definition to improve performance and resolution of the distribution function, and the addition of a neutral chemistry model. Chemistry's dependence on the tail of a distribution function necessitates accurate resolution of said tail, a computationally challenging proposition. The effects of these additions are verified and studied
through a number of 0D calculations, including simulations for which analytic
solutions exist and model simulations intended to capture relevant physics
present in more complicated problems. The explicit representation of internal
distributions in QuiPS reveals some of the flaws in existing physics models.
Variance reduction, a key feature of QuiPS can greatly reduce expense of
multi-dimensional calculations, but is only cheaper when the gas composition is near chemical equilibrium.