Numerical study of large-scale vorticity generation in shear-flow turbulence

TL;DR

This study uses simulations to explore how large-scale vorticity patterns spontaneously form in shear-flow turbulence, suggesting a mean-vorticity dynamo mechanism driven by anisotropic eddy viscosity.

Contribution

It demonstrates the potential role of anisotropic eddy viscosity in generating large-scale flows in shear turbulence, based on numerical simulations and analysis.

Findings

Large-scale flow patterns can spontaneously emerge in shear turbulence.

The eddy viscosity tensor's anisotropy is crucial for vorticity dynamo action.

Diagonal eddy viscosity component is proportional to turbulent velocity and eddy scale.

Abstract

Simulations of stochastically forced shear-flow turbulence in a shearing-periodic domain are used to study the spontaneous generation of large-scale flow patterns in the direction perpendicular to the plane of the shear. Based on an analysis of the resulting large-scale velocity correlations it is argued that the mechanism behind this phenomenon could be the mean-vorticity dynamo effect pioneered by Elperin, Kleeorin, and Rogachevskii in 2003 (Phys. Rev. E 68, 016311). This effect is based on the anisotropy of the eddy viscosity tensor. One of its components may be able to replenish cross-stream mean flows by acting upon the streamwise component of the mean flow. Shear, in turn, closes the loop by acting upon the cross-stream mean flow to produce stronger streamwise mean flows. The diagonal component of the eddy viscosity is found to be of the order of the rms turbulent velocity divided by the wavenumber of the energy-carrying eddies.