Identification of vortexes obstructing the dynamo mechanism in laboratory experiments

TL;DR

This paper uses numerical simulations and advanced analysis to identify vortexes that hinder magnetic field generation in laboratory dynamo experiments, explaining previous experimental challenges.

Contribution

It introduces a novel application of Singular Value Decomposition to identify vortexes affecting the dynamo mechanism in spherical geometry.

Findings

Identified an axisymmetric vortex that disrupts the dynamo effect.

Demonstrated similar vortex dynamics in cylindrical experiments.

Provided insights into experimental difficulties in achieving dynamo action.

Abstract

The magnetohydrodynamic dynamo effect explains the generation of self-sustained magnetic fields in electrically conducting flows, especially in geo- and astrophysical environments. Yet the details of this mechanism are still unknown, e.g., how and to which extent the geometry, the fluid topology, the forcing mechanism and the turbulence can have a negative effect on this process. We report on numerical simulations carried out in spherical geometry, analyzing the predicted velocity flow with the so-called Singular Value Decomposition, a powerful technique that allows us to precisely identify vortexes in the flow which would be difficult to characterize with conventional spectral methods. We then quantify the contribution of these vortexes to the growth rate of the magnetic energy in the system. We identify an axisymmetric vortex, whose rotational direction changes periodically in time, and whose dynamics are decoupled from those of the large scale background flow, is detrimental for the dynamo effect. A comparison with experiments is carried out, showing that similar dynamics were observed in cylindrical geometry. These previously unexpected eddies, which impede the dynamo effect, offer an explanation for the experimental difficulties in attaining a dynamo in spherical geometry.