Order Out of Chaos: Slowly Reversing Mean Flows Emerge from Turbulently Generated Internal Waves

TL;DR

This paper uses direct numerical simulations to show how internal waves generated by turbulence can spontaneously produce slowly reversing mean flows in a convective-stably stratified fluid system, revealing a generic three-scale dynamic relevant to astrophysical and geophysical fluids.

Contribution

It demonstrates for the first time that turbulence-driven internal waves can generate reversing mean flows through nonlinear interactions, highlighting the importance of wave intermittency in this process.

Findings

Mean flows spontaneously develop from turbulent internal waves.

Wave intermittency is crucial for mean-flow dynamics.

The three-scale convection-wave-mean flow process is generic.

Abstract

We demonstrate via direct numerical simulations that a periodic, oscillating mean flow spontaneously develops from turbulently generated internal waves. We consider a minimal physical model where the fluid self-organizes in a convective layer adjacent to a stably stratified one. Internal waves are excited by turbulent convective motions, then nonlinearly interact to produce a mean flow reversing on timescales much longer than the waves' period. Our results demonstrate for the first time that the three-scale dynamics due to convection, waves, and mean flow is generic and hence can occur in many astrophysical and geophysical fluids. We discuss efforts to reproduce the mean flow in reduced models, where the turbulence is bypassed. We demonstrate that wave intermittency, resulting from the chaotic nature of convection, plays a key role in the mean-flow dynamics, which thus cannot be captured using only second-order statistics of the turbulent motions.