What is turbulence, what is fossil turbulence, and which ways do they cascade?

TL;DR

This paper defines turbulence and fossil turbulence, explains their cascade process from small to large scales, and discusses their detection in cosmic and fluid flows, highlighting their persistent and information-preserving nature.

Contribution

It introduces a clear definition of fossil turbulence, details the cascade process, and presents evidence of fossil turbulence in cosmic and fluid flow phenomena.

Findings

Turbulence cascades from small to large scales.

Fossil turbulence persists after turbulence ceases.

Cosmic microwave background shows fossil turbulence patterns.

Abstract

Turbulence is defined as an eddy-like state of fluid motion where the inertial-vortex forces of the eddies are larger than any other forces that tend to damp the eddies out. By this definition, turbulence always cascades from small scales (where the vorticity is created) to larger scales (where other forces dominate and the turbulence fossilizes). Fossil turbulence is any perturbation in a hydrophysical field produced by turbulence that persists after the fluid is no longer turbulent at the scale of the perturbation. Fossil turbulence patterns and fossil turbulence waves preserve and propagate information about previous turbulence to larger and smaller length scales. Big bang fossil turbulence patterns are identified in anisotropies of temperature detected by space telescopes in the cosmic microwave background. Direct numerical simulations of stratified shear flows and wakes show that turbulence and fossil turbulence interactions are recognizable and persistent.