Cross-helicity effects and turbulent transport in magnetohydrodynamic flow

TL;DR

This paper explores how turbulent cross helicity influences magnetic and momentum transport in magnetohydrodynamic flows, potentially suppressing turbulence effects and impacting dynamo processes, validated through direct numerical simulations.

Contribution

It introduces the role of cross-helicity effects in turbulent transport and distinguishes them from traditional helicity effects in dynamo theory.

Findings

Cross-helicity effects can suppress turbulent transport.

DNS validates the significance of cross-helicity in dynamo action.

Mechanisms generating turbulence with cross helicity are discussed.

Abstract

In the presence of large-scale vortical motions and/or magnetic-field strains, the turbulent cross helicity (velocity--magnetic-field correlation in fluctuations) may contribute to the turbulent electromotive force and the Reynolds stress. These effects of cross helicity are considered to balance the primary effects of turbulence such as the turbulent magnetic diffusivity in magnetic-field evolution and the eddy viscosity in the momentum transport. The cross-helicity effects may suppress the enhanced transports due to turbulence. Physical interpretation of the effects is presented with special emphasis on the difference between the cross-helicity effect and the usual $\alpha$ or helicity effect in the dynamo action. The relative importance of the cross-helicity effect in dynamo action is validated with the aid of a direct numerical simulation (DNS) of the Kolmogorov flow with an imposed magnetic field. Several mechanisms that provide turbulence with the cross helicity are also discussed.