Linear stability analysis of the homogeneous Couette flow in a 2D isentropic compressible fluid

TL;DR

This paper analyzes the linear stability of 2D isentropic compressible Couette flow, revealing instabilities in density and irrotational velocity, while solenoidal velocity experiences damping, and identifies enhanced dissipation phenomena in viscous cases.

Contribution

It provides the first detection of enhanced dissipation mechanisms in compressible fluids and characterizes the different stability behaviors of flow components.

Findings

Inviscid density and irrotational velocity grow as t^{1/2}.

Solenoidal velocity decays due to inviscid damping.

Viscous perturbations exhibit transient growth followed by exponential decay.

Abstract

In this paper, we study the linear stability properties of perturbations around the homogeneous Couette flow for a 2D isentropic compressible fluid in the domain $\mathbb{T}\times \mathbb{R}$. In the inviscid case there is a generic Lyapunov type instability for the density and the irrotational component of the velocity field. More precisely, we prove that their $L^2$ norm grows as $t^{1/2}$ and this confirms previous observations in the physics literature. Instead, the solenoidal component of the velocity field experience inviscid damping, meaning that it decays to zero even in the absence of viscosity. For a viscous compressible fluid, we show that the perturbations may have a transient growth of order $\nu^{-1/6}$ (with $\nu^{-1}$ being proportional to the Reynolds number) on a time-scale $\nu^{-1/3}$, after which it decays exponentially fast. This phenomenon is also called enhanced dissipation and our result appears to be the first to detect this mechanism for a compressible fluid, where an exponential decay for the density is not a priori trivial given the absence of dissipation in the continuity equation.