A discrete velocity method for the Boltzmann equation with internal energy and stochastic variance reduction



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The goal of this work is to develop an accurate and efficient flow solver based upon a discrete velocity description of the Boltzmann equation. Standard particle based methods such as Direct Simulation Monte Carlo (DSMC) have a number of difficulties with complex and transient flows, stochastic noise, trace species, and high level internal energy states. To address these issues, a discrete velocity method (DVM) was developed which models the evolution of a flow through the collisions and motion of variable mass quasi-particles defined as delta functions on a truncated, discrete velocity domain.

The work is an extension of a previous method developed for a single, monatomic species solved on a uniformly spaced velocity grid. The collision integral was computed using a variance reduced stochastic model where the deviation from equilibrium was calculated and operated upon. This method produces fast, smooth solutions of near-equilibrium flows.

Improvements to the method include additional cross-section models, diffuse boundary conditions, simple realignment of velocity grid lines into non-uniform grids, the capability to handle multiple species (specifically trace species or species with large molecular mass ratios), and both a single valued rotational energy model and a quantized rotational and vibrational model. A variance reduced form is presented for multi-species gases and gases with internal energy in order to maintain the computational benefits of the method. Every advance in the method allows for more complex flow simulations either by extending the available physics or by increasing computational efficiency. Each addition is tested and verified for an accurate implementation through homogeneous simulations where analytic solutions exist, and the efficiency and stochastic noise are inspected for many of the cases. Further simulations are run using a variety of classical one-dimensional flow problems such as normal shock waves and channel flows.