Kinematic dynamo action of a precession driven flow based on the results of water experiments and hydrodynamic simulations

TL;DR

This study combines water experiments and hydrodynamic simulations to explore precession-driven flows in liquid sodium, identifying conditions conducive to magnetic field generation relevant for geophysical and astrophysical dynamos.

Contribution

It provides new insights into the flow dynamics and magnetic field onset in a precession-driven dynamo experiment, supported by both experimental and numerical analyses.

Findings

Flow is dominated by Kelvin and inertial modes in nonlinear regime.

Resonance-like axisymmetric mode enhances dynamo potential.

Critical magnetic Reynolds number around 430 for magnetic field excitation.

Abstract

The project DRESDYN (DREsden Sodium facility for DYNamo and thermohydraulic studies) conducted at Helmholtz-Zentrum Dresden-Rossendorf (HZDR) provides a new platform for a variety of liquid sodium experiments devoted to problems of geo- and astrophysical magnetohydrodynamics. The most ambitious experiment within this project is a precession driven dynamo experiment that currently is under construction. It consists of a cylinder filled with liquid sodium that simultaneously rotates around two axes. The experiment is motivated by the idea of a precession-driven flow as a complementary energy source for the geodynamo or the ancient lunar dynamo. In the present study we address numerical and experimental examinations in order to identify parameter regions where the onset of magnetic field excitation will be most probable. Both approaches show that in the strongly nonlinear regime the flow is essentially composed of the directly forced primary Kelvin mode and higher modes in terms of standing inertial waves that arise from nonlinear self-interactions. A peculiarity is the resonance-like emergence of an axisymmetric mode that represents a double roll structure in the meridional plane, which, however, only occurs in a very limited range of the precession ratio. This axisymmetric mode turns out to be beneficial for dynamo action, and kinematic simulations of the magnetic field evolution induced by the time-averaged flow exhibit magnetic field excitation at critical magnetic Reynolds numbers around ${\rm{Rm}}^{\rm{c}}\approx 430$, which is well within the range of the planned liquid sodium experiment.