On the lack of universality in decaying magnetohydrodynamic turbulence

TL;DR

This study demonstrates that in decaying MHD turbulence, different initial magnetic fields lead to multiple inertial ranges in the energy spectrum, highlighting the non-universality of turbulence behavior depending on initial conditions.

Contribution

It reveals that the energy spectrum in decaying MHD turbulence varies with initial magnetic field conditions, emphasizing the role of intrinsic nonlinear dynamics rather than external forcing.

Findings

Multiple inertial ranges observed for different initial magnetic fields.

Results are consistent across various Reynolds numbers.

Energy spectrum variability depends on initial conditions, not just external parameters.

Abstract

Using computations of three-dimensional magnetohydrodynamic (MHD) turbulence with a Taylor-Green flow, whose inherent time-independent symmetries are implemented numerically, and in the absence of either a forcing function or an imposed uniform magnetic field, we show that three different inertial ranges for the energy spectrum may emerge for three different initial magnetic fields, the selecting parameter being the ratio of the Alfven to the eddy turnover time. Equivalent computational grids range from 128^3 to 2048^3 points with a unit magnetic Prandtl number and a Taylor Reynolds number of up to 1500 at the peak of dissipation. We also show convergence of our results with Reynolds number. Our study is consistent with previous findings of a variety of energy spectra in MHD turbulence by studies performed in the presence of both a forcing term with a given correlation time and a strong, uniform magnetic field. In contrast to the previous studies, however, the ratio of characteristic time scales here can only be ascribed to the intrinsic nonlinear dynamics of the flows under study.