Free-fall velocities and heat transport enhancement in liquid metal magneto-convection

TL;DR

This study reveals that in liquid metal magneto-convection, applying an optimal static magnetic field enhances heat transport and convective velocities to their theoretical maximum, despite magnetic stabilization effects.

Contribution

It demonstrates the existence of an optimal magnetic field strength that maximizes heat transport and flow velocities in low Prandtl number magneto-convective flows.

Findings

Convective velocities reach the free-fall limit at optimal magnetic field strength.

Magnetic fields induce anisotropic, ordered flow structures with reduced turbulence.

Beyond the optimal magnetic field, Hartmann braking reduces heat and momentum transport.

Abstract

In geo- and astrophysics, low Prandtl number convective flows often interact with magnetic fields. Although a static magnetic field acts as a stabilizing force on such flow fields, we find that self-organized convective flow structures reach an optimal state where the heat transport significantly increases and convective velocities reach the theoretical free-fall limit, i.e. the maximum possible velocity a fluid parcel can achieve when its potential buoyant energy is fully converted into kinetic energy. Our measurements show that the application of a static magnetic field leads to an anisotropic, highly ordered flow structure and a decrease of the turbulent fluctuations. When the magnetic field strength is increased beyond the optimum, Hartmann braking becomes dominant and leads to a reduction of the heat and momentum transport. The results are relevant for the understanding of magneto-hydrodynamic convective flows in planetary cores and stellar interiors in regions with strong toroidal magnetic fields oriented perpendicular to temperature gradients.