The stability of stratified spatially periodic shear flows at low P\'eclet number

TL;DR

This paper investigates the stability of stratified periodic shear flows at low Péclet numbers, revealing conditions for linear and nonlinear stability, and demonstrating the existence of finite-amplitude instabilities through numerical simulations.

Contribution

It extends stability analysis to low-Péclet-number flows, including a formal extension of Squire's theorem and the identification of finite-amplitude instabilities.

Findings

Flow is linearly stable under certain stratification conditions.

Energy stability analysis shows flows can be stable to all perturbations.

Finite-amplitude instabilities can exist even in linearly stable regimes.

Abstract

This work addresses the question of the stability of stratified, spatially periodic shear flows at low P\'eclet number but high Reynolds number. This little-studied limit is motivated by astrophysical systems, where the Prandtl number is often very small. Furthermore, it can be studied using a reduced set of "low-P\'eclet-number equations" proposed by Lignieres [Astronomy & Astrophysics, 348, 933-939, 1999]. Through a linear stability analysis, we first determine the conditions for instability to infinitesimal perturbations. We formally extend Squire's theorem to the low-P\'eclet-number equations, which shows that the first unstable mode is always two-dimensional. We then perform an energy stability analysis of the low-P\'eclet-number equations and prove that for a given value of the Reynolds number, above a critical strength of the stratification, any smooth periodic shear flow is stable to perturbations of arbitrary amplitude. In that parameter regime, the flow can only be laminar and turbulent mixing does not take place. Finding that the conditions for linear and energy stability are different, we thus identify a region in parameter space where finite-amplitude instabilities could exist. Using direct numerical simulations, we indeed find that the system is subject to such finite-amplitude instabilities. We determine numerically how far into the linearly stable region of parameter space turbulence can be sustained.