Stability of the first-order unified gas-kinetic scheme based on a linear kinetic model

TL;DR

This paper analytically proves the weighted L2-stability of the first-order unified gas-kinetic scheme (UGKS) for a linear kinetic model, showing it remains stable under CFL conditions without being constrained by particle collision times.

Contribution

It provides the first rigorous stability analysis of UGKS applied to a linear kinetic model, revealing stability under broader conditions than previously known.

Findings

Proves weighted L2-stability of UGKS under CFL conditions.

Shows time step is not limited by particle collision time.

Demonstrates stability through convex combinations of physics-related sub-methods.

Abstract

The unified gas-kinetic scheme (UGKS) is becoming increasingly popular for multiscale simulations in all flow regimes. This paper provides the first analytical study on the stability of the UGKS applied to a linear kinetic model, which is able to reproduce the one-dimensional linear scalar advection-diffusion equation via the Chapman-Enskog expansion method. Adopting periodic boundary conditions and neglecting the error from numerical integration, this paper rigorously proves the weighted $L^2$-stability of the first-order UGKS under the Courant-Friedrichs-Lewy (CFL) conditions. It is shown that the time step of the method is not constrained by being less than the particle collision time, nor is it limited by parabolic type CFL conditions typically applied in solving diffusion equations. The novelty of the proof lies in that based on the ratio of the time step to the particle collision time, the update of distribution functions is viewed as a convex combinations of sub-methods related to various physics processes, such as the particle free transport and collisions. The weighted $L^2$-stability of the sub-methods is obtained by considering them as discretizations to corresponding linear hyperbolic systems and utilizing the associated Riemann invariants. Finally, the strong stability preserving property of the UGKS leads to the desired weighted $L^2$-stability.