Large amplitude problem of BGK model: Relaxation to quadratic nonlinearity

TL;DR

This paper investigates the asymptotic stability of the BGK model for kinetic flows, showing that despite initial nonlinearity, the system eventually transitions to quadratic nonlinearity after a finite time, ensuring stability.

Contribution

It introduces a refined analysis demonstrating the transition from highly nonlinear to quadratic nonlinear regimes in the BGK model, addressing stability without initial proximity to equilibrium.

Findings

System transitions from nonlinear to quadratic regime after finite time

Refined macroscopic analysis guarantees stability in highly nonlinear setting

Perturbations relax to quadratic nonlinearity, preventing blow-up

Abstract

Bhatnagar-Gross-Krook (BGK) equation is a relaxation model of the Boltzmann equation which is widely used in place of the Boltzmann equation for the simulation of various kinetic flow problems. In this work, we study the asymptotic stability of the BGK model when the initial data is not necessarily close to the global equilibrium pointwisely. Due to the highly nonlinear structure of the relaxation operator, the argument developed to derive the bootstrap estimate for the Boltzmann equation leads to a weaker estimate in the case of the BGK model, which does not exclude the possible blow-up of the perturbation. To overcome this issue, we carry out a refined analysis of the macroscopic fields to guarantee that the system transits from a highly nonlinear regime into a quadratic nonlinear regime after a long but finite time, in which the highly nonlinear perturbative term relaxes to essentially quadratic nonlinearity.