High-order lattice Boltzmann models for wall-bounded flows at finite Knudsen numbers

TL;DR

This paper investigates high-order lattice Boltzmann models for finite Knudsen number flows, emphasizing the importance of boundary condition accuracy and quadrature order to reliably simulate slip and rarefied gas flows.

Contribution

It introduces a systematic method to generate high-order discrete velocity models that accurately implement Maxwell boundary conditions for rarefied flows.

Findings

High-order models can recover flow regimes beyond Navier-Stokes.

Accurate boundary conditions are essential for reliable results.

The models reproduce mass flow and slip velocity up to Kn=1.

Abstract

We analyze a large number of high-order discrete velocity models for solving the Boltzmann-BGK equation for finite Knudsen number flows. Using the Chapman-Enskog formalism, we prove for isothermal flows a relation identifying the resolved flow regimes for low Mach numbers. Although high-order lattice Boltzmann models recover flow regimes beyond the Navier-Stokes level we observe for several models significant deviations from reference results. We found this to be caused by their inability to recover the Maxwell boundary condition exactly. By using supplementary conditions for the gas-surface interaction it is shown how to systematically generate discrete velocity models of any order with the inherent ability to fulfill the diffuse Maxwell boundary condition accurately. Both high-order quadratures and an exact representation of the boundary condition turn out to be crucial for achieving reliable results. For Poiseuille flow, we can reproduce the mass flow and slip velocity up to the Knudsen number of 1. Moreover, for small Knudsen numbers, the Knudsen layer behavior is recovered.