Influence of an external magnetic field on forced turbulence in a swirling flow of liquid metal

TL;DR

This experimental study examines how an external magnetic field affects forced turbulence in liquid gallium, revealing exponential damping of velocity fluctuations and flow anisotropy without becoming bidimensional.

Contribution

It provides new insights into the damping mechanisms of turbulence under magnetic influence and the weak correlation between magnetic fields and velocity fluctuations in liquid metal flows.

Findings

Exponential damping of turbulence with increasing magnetic field

Flow develops anisotropy but remains three-dimensional

Weak correlation between magnetic field and velocity fluctuations

Abstract

We report an experimental investigation on the influence of an external magnetic field on forced 3D turbulence of liquid gallium in a closed vessel. We observe an exponential damping of the turbulent velocity fluctuations as a function of the interaction parameter N (ratio of Lorentz force over inertial terms of the Navier-Stokes equation). The flow structures develop some anisotropy but do not become bidimensional. From a dynamical viewpoint, the damping first occurs homogeneously over the whole spectrum of frequencies. For larger values of N, a very strong additional damping occurs at the highest frequencies. However, the injected mechanical power remains independent of the applied magnetic field. The simultaneous measurement of induced magnetic field and electrical potential differences shows a very weak correlation between magnetic field and velocity fluctuations. The observed reduction of the fluctuations is in agreement with a previously proposed mechanism for the saturation of turbulent dynamos and with the order of magnitude of the Von Karman Sodium dynamo magnetic field.