Oscillatory thermal-inertial flows in liquid metal rotating convection

TL;DR

This study characterizes oscillatory convection in a rotating liquid metal tank, revealing inertial, thermal wind flows, wall modes, and helicity patterns that inform planetary dynamo models.

Contribution

First detailed experimental and numerical analysis of oscillatory convection in liquid metal under rotation before steady modes develop.

Findings

Inertial, thermal wind flows reach velocities comparable to non-rotating cases.

Oscillatory bulk and wall modes coexist over a wide parameter range.

Flows exhibit significant time-mean helicity, supporting dynamo processes.

Abstract

We present the first detailed thermal and velocity field characterization of convection in a rotating cylindrical tank of liquid gallium, which has thermophysical properties similar to those of planetary core fluids. Our laboratory experiments, and a closely associated direct numerical simulation, are all carried out in the regime prior to the onset of steady convective modes. This allows us to study the oscillatory convective modes, sidewall modes and broadband turbulent flow that develop in liquid metals before the advent of steady columnar modes. Our thermo-velocimetric measurements show that strongly inertial, thermal wind flows develop, with velocities reaching those of comparable non-rotating cases. Oscillatory bulk convection and wall modes coexist across a wide range of our experiments, along with strong zonal flows that peak in the Stewartson layer, but that extend deep into the fluid bulk in the higher supercriticality cases. The flows contain significant time-mean helicity that is anti-symmetric across the midplane, demonstrating that oscillatory liquid metal convection contains the kinematic components to sustain system-scale dynamo generation.