Do magnetic fields enhance turbulence at low magnetic Reynolds number ?

TL;DR

This paper demonstrates that magnetic fields can enhance turbulence at low magnetic Reynolds numbers by promoting large, nearly 2D structures that offset Joule dissipation, challenging traditional views.

Contribution

It reveals that magnetic fields can increase turbulence intensity by fostering large-scale structures, when turbulence is directly forced in the magnetic field, not just generated by it.

Findings

Magnetic fields promote large, nearly 2D turbulent structures.

These structures can offset Joule dissipation and increase turbulence.

The effect has implications for planetary cores and engineering applications.

Abstract

Imposing a magnetic field on a turbulent flow of electrically conducting fluid incurs the Joule effect. A current paradigm is that the corresponding dissipation increases with the intensity of the magnetic field, and as a result turbulent fluctuations are all the more damped as the magnetic field is strong. While this idea finds apparent support in the phenomenology of decaying turbulence, measurements of turbulence in duct flows and other, more complex configurations have produced seemingly contradicting results. The root of the controversy is that magnetic fields promote sufficient scale-dependent anisotropy to profoundly reorganise the structure of turbulence, so their net effect cannot be understood in terms of the additional dissipation only. Here we show that when turbulence is forced in a magnetic field that acts on turbulence itself rather than on the mechanisms that generate it, the field promotes large, nearly 2D structures capturing sufficient energy to offset the loss due to Joule dissipation, with the net effect of increasing the intensity of turbulent fluctuations. This change of paradigm potentially carries important consequences for systems as diverse as the liquid cores of planets, accretion disks and a wide range of metallurgical and nuclear engineering applications.