Stability of the first-order unified gas-kinetic scheme based on a linear kinetic model
Tuowei Chen, Kun Xu
TL;DR
This paper analyzes the stability of a first-order unified gas-kinetic scheme (UGKS) built on a linear BGK kinetic model that reproduces the 1-D linear advection-diffusion equation $\partial_t u + a\,\partial_x u = \nu\,\partial_{xx} u$ via Chapman–Enskog expansion. It develops a convex-combination decomposition of the scheme into sub-methods corresponding to free transport and collisions, and proves weighted $L^2$-stability under a CFL-type condition using Riemann invariants and SSP arguments. The key finding is that the time step is governed by a hyperbolic CFL bound, not by collision time or diffusion parabolic CFL conditions. The results provide a rigorous analytical framework for the stability of multiscale kinetic methods and lay groundwork toward stability analysis of the full UGKS for compressible flows.
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.