Linear Rayleigh-Taylor instability for viscous, compressible fluids

TL;DR

This paper analyzes the linearized compressible Navier-Stokes equations for Rayleigh-Taylor instability, developing a new variational approach to identify maximal growth modes and establishing conditions for stability or instability.

Contribution

It introduces a novel method of studying modified variational problems to construct maximal growing modes in viscous, compressible fluids with or without surface tension.

Findings

Constructed smooth solutions exhibiting exponential growth in Sobolev spaces.

Established an estimate relating solution growth to the maximal growth rate of modes.

Showed that small periodicity can prevent instability when surface tension is present.

Abstract

We study the equations obtained from linearizing the compressible Navier-Stokes equations around a steady-state profile with a heavier fluid lying above a lighter fluid along a planar interface, i.e. a Rayleigh-Taylor instability. We consider the equations with or without surface tension, with the viscosity allowed to depend on the density, and in both periodic and non-periodic settings. In the presence of viscosity there is no natural variational framework for constructing growing mode solutions to the linearized problem. We develop a general method of studying a family of modified variational problems in order to produce maximal growing modes. Using these growing modes, we construct smooth (when restricted to each fluid domain) solutions to the linear equations that grow exponentially in time in Sobolev spaces. We then prove an estimate for arbitrary solutions to the linearized equations in terms of the fastest possible growth rate for the growing modes. In the periodic setting, we show that sufficiently small periodicity avoids instability in the presence of surface tension.