Large-scale flow generation by inhomogeneous helicity

TL;DR

This paper demonstrates through direct numerical simulations that inhomogeneous kinetic helicity in rotating turbulence can generate large-scale flows, confirming the inhomogeneous helicity effect's role in flow generation.

Contribution

It provides the first direct numerical evidence that inhomogeneous helicity induces large-scale flow in rotating turbulence, expanding understanding of vortex dynamo mechanisms.

Findings

Large-scale flow aligns with angular velocity in inhomogeneous helicity turbulence.

Homogeneous helicity does not produce large-scale flow.

Supports the concept of a vortex dynamo in incompressible turbulence.

Abstract

The effect of kinetic helicity (velocity--vorticity correlation) on turbulent momentum transport is investigated. The turbulent kinetic helicity (pseudoscalar) enters the Reynolds stress (mirrorsymmetric tensor) expression in the form of a helicity gradient as the coupling coefficient for the mean vorticity and/or the angular velocity (axial vector), which suggests the possibility of mean-flow generation in the presence of inhomogeneous helicity. This inhomogeneous helicity effect, which was previously confirmed at the level of a turbulence- or closure-model simulation, is examined with the aid of direct numerical simulations of rotating turbulence with non-uniform helicity sustained by an external forcing. The numerical simulations show that the spatial distribution of the Reynolds stress is in agreement with the helicity-related term coupled with the angular velocity, and that a large-scale flow is generated in the direction of angular velocity. Such a large-scale flow is not induced in the case of homogeneous turbulent helicity. This result confirms the validity of the inhomogeneous helicity effect in large-scale flow generation and suggests that a vortex dynamo is possible even in incompressible turbulence where there is no baroclinicity effect.