Helicity effect on turbulent passive and active scalar diffusivities

TL;DR

This paper investigates how helicity influences turbulent scalar diffusivities, revealing that helicity reduces magnetic diffusivity but enhances passive scalar diffusivity, with simulations showing increased velocity correlation time and decreased scalar diffusivity with rotation.

Contribution

It demonstrates the opposite effects of helicity on magnetic and passive scalar diffusivities and highlights the role of velocity correlation time in these phenomena.

Findings

Helicity reduces magnetic diffusivity.

Helicity enhances passive scalar diffusivity.

Velocity correlation time increases with helicity.

Abstract

Turbulent flows are known to produce enhanced effective magnetic and passive scalar diffusivities, which can fairly accurately be determined with numerical methods. It is now known that, if the flow is also helical, the effective magnetic diffusivity is reduced relative to the nonhelical value. Neither the usual second-order correlation approximation nor the various $\tau$ approaches have been able to capture this. Here we show that the helicity effect on the turbulent passive scalar diffusivity works in the opposite sense and leads to an enhancement. We have also demonstrated that the correlation time of the turbulent velocity field increases with the kinetic helicity. This is a key point in the theoretical interpretation of the obtained numerical results. Simulations in which helicity is being produced self-consistently by stratified rotating turbulence resulted in a turbulent passive scalar diffusivity that was found to be decreasing with increasing rotation rate.