Reynolds number dependence of Lagrangian dispersion in direct numerical simulations of anisotropic magnetohydrodynamic turbulence

TL;DR

This study uses high-resolution direct numerical simulations to analyze how the Reynolds number influences Lagrangian dispersion in anisotropic magnetohydrodynamic turbulence, revealing anisotropic diffusion behaviors and steeper-than-Richardson scalings.

Contribution

It provides new insights into Reynolds number effects on particle dispersion in anisotropic MHD turbulence through extensive simulations up to Re=21000.

Findings

Single-particle diffusion shows mildly superdiffusive behavior.

Particle pair dispersion is affected by magnetic field alignment.

Dispersion scalings are steeper than Richardson prediction at high Reynolds numbers.

Abstract

Large-scale magnetic fields thread through the electrically conducting matter of the interplanetary and interstellar medium, stellar interiors, and other astrophysical plasmas, producing anisotropic flows with regions of high-Reynolds-number turbulence. It is common to encounter turbulent flows structured by a magnetic field with a strength approximately equal to the root-mean-square magnetic fluctuations. In this work, direct numerical simulations of anisotropic magnetohydrodynamic (MHD) turbulence influenced by such a magnetic field are conducted for a series of cases that have identical resolution, and increasing grid sizes up to $2048^3$. The result is a series of closely comparable simulations at Reynolds numbers ranging from 1,400 up to 21,000. We investigate the influence of the Reynolds number from the Lagrangian viewpoint by tracking fluid particles and calculating single-particle and two-particle statistics. The influence of Alfv\'enic fluctuations and the fundamental anisotropy on the MHD turbulence in these statistics is discussed. Single-particle diffusion curves exhibit mildly superdiffusive behaviors that differ in the direction aligned with the magnetic field and the direction perpendicular to it. Competing alignment processes affect the dispersion of particle pairs, in particular at the beginning of the inertial subrange of time scales. Scalings for relative dispersion, which become clearer in the inertial subrange for larger Reynolds number, can be observed that are steeper than indicated by the Richardson prediction.