# Permanence of large eddies in variable-density homogeneous turbulence

## TL;DR

This paper investigates how variable-density affects the permanence of large eddies in turbulence, showing that density variations alter energy transfer and restrict the conditions under which large eddies persist.

## Contribution

It demonstrates that density non-uniformities modify large eddy permanence criteria, extending understanding from constant to variable-density turbulence.

## Key findings

- Large eddies in variable-density flows are less persistent.
- Permanence applies to initial spectra with infrared exponent less than 2.
- Simulations confirm theoretical predictions.

## Abstract

The principle of permanence of large eddies is one of the central pillars onto which our understanding of decaying homogeneous turbulence is built. The validity conditions of this principle have been thoroughly discussed for constant density flows, but not for variable-density ones. In this work, we show that density non-uniformities modify the remote action of the pressure field. It results into distant velocity correlations being submitted to a stronger non-linear transfer of energy. A simple example is proposed to illustrate this property and a spectral analysis of non-linear transfer terms is undertook to further characterize it. From there, we derive that large eddies in variable density flows remain permanent for a smaller set of initial conditions than when density is constant. Permanence strictly applies to initial spectra having an infrared exponent smaller than 2 instead of 4. Implicit large-eddy simulations are performed to verify the main predictions of this work.