Local fluxes in MHD Turbulence

TL;DR

This study uses high-resolution simulations to analyze local energy fluxes in MHD turbulence, revealing extreme event statistics, flux contributions, and correlations with magnetic field gradients, with implications for subgrid modeling.

Contribution

It provides a detailed statistical analysis of local energy fluxes in MHD turbulence, highlighting differences from hydrodynamics and implications for modeling.

Findings

Local energy flux distributions have long tails indicating extreme events.

Different flux components show flow-dependent properties.

Flux correlates with magnetic field gradients and local magnetic field amplitude.

Abstract

Using highly resolved direct numerical simulations we examine the statistical properties of the local energy flux rate $\Pi_\ell(x)$ towards small scales for three isotropic turbulent magnetohydrodynamic flows, which differ in strength and structure of the magnetic field. We analyse the cascade process both in the kinetic and magnetic energy, disentangling the different flux contributions to the overall energy dynamics. The results show that the probability distribution of the local energy flux develops long tails related to extreme events, similar to the hydrodynamic case. The different terms of the energy flux display different properties and show sensitivity on the type of the flow examined. We further examine the joint pdf between the local energy flux and the gradients of the involved fields. The results point out a correlation with the magnetic field gradients, showing however a dispersion much stronger than what is observed in hydrodynamic flows. Finally, it is also shown that the local energy flux shows some dependence on the local amplitude of the magnetic field. The present results have implications for subgrid scale models that we discuss.