Quasi-static magnetohydrodynamic turbulence at high Reynolds number

TL;DR

This paper investigates the anisotropic behavior of high Reynolds number quasi-static magnetohydrodynamic turbulence, revealing how linear and nonlinear effects combine to shape turbulence structure and energy distribution.

Contribution

It introduces a combined linear and nonlinear analysis using DNS and EDQNM models to explain the transition to two-and-a-half-dimensional turbulence in quasi-static MHD.

Findings

Transition to two-and-a-half-dimensional flow due to combined effects

Strong anisotropy in correlation spectra aligned with magnetic field

Spectral scalings consistent with high Reynolds number turbulence

Abstract

We analyse the anisotropy of homogeneous turbulence in an electrically conducting fluid submitted to a uniform magnetic field, for low magnetic Reynolds number, in the quasi- static approximation. We interpret disagreeing previous predictions between linearized theory and simulations: in the linear limit, the kinetic energy of transverse velocity components, normal to the magnetic field, decays faster than the kinetic energy of the axial component, along the magnetic field (Moffatt (1967)); whereas many numerical studies predict a final state characterised by dominant energy of transverse velocity components. We investigate the corresponding nonlinear phenomenon using Direct Numerical Simulations of freely-decaying turbulence, and a two-point statistical spectral closure based on the Eddy Damped Quasi-Normal Markovian model. The transition from the three-dimensional turbulent flow to a "two-and-a-half-dimensional" flow (Montgomery & Turner (1982)) is a result of the combined effects of short-time linear Joule dissipation and longer time nonlinear creation of polarisation anisotropy. It is this combination of linear and nonlinear effects which explains the disagreement between predictions from linearized theory and results from numerical simulations. The transition is characterized by the elongation of turbulent structures along the applied magnetic field, and by the strong anisotropy of directional two-point correlation spectra, in agreement with experimental evidence. Inertial equatorial transfers in both DNS and the model are presented to describe in detail the most important equilibrium dynamics. Spectral scalings are maintained in high Reynolds number turbulence attainable only with the EDQNM model, which also provides simplified modelling of the asymptotic state of quasi-static MHD turbulence.