Thermovelocimetric Characterization of Liquid Metal Convection in a Rotating Slender Cylinder

TL;DR

This study introduces a novel thermovelocimetric method to analyze liquid metal convection in a rotating slender cylinder, revealing large-scale vortical structures and unique scaling behaviors in low-Prandtl-number fluids.

Contribution

It presents the first experimental thermovelocimetric characterization of liquid metal rotating convection, highlighting the formation of azimuthal vortices and new scaling laws in low-Prandtl-number regimes.

Findings

Identification of a stable azimuthal wavenumber 2 vortical structure

Evidence of different wall mode precession frequency scaling in liquid metals

Extension of previous high-Pr studies to low-Pr rotating convection

Abstract

Rotating turbulent convection occurs ubiquitously in natural convective systems encompassing planetary cores, oceans, and atmospheres, as well as in many industrial applications. While the global heat and mass transfer of water-like rotating Rayleigh-B\'enard convection is well-documented, the characteristics of rotating convection in liquid metals remain less well understood. In this study, we characterize rotating Rayleigh-B\'enard convection in liquid gallium (Prandtl number $Pr \approx 0.027$) within a slender cylinder (diameter-to-height aspect ratio $\Gamma = D/H = 1/2$) using novel thermovelocimetric diagnostic techniques that integrate simultaneous multi-point thermometry and ultrasonic Doppler velocity measurements. This approach experimentally reveals the formation of a stable azimuthal wavenumber $m = 2$ global-scale vortical structure at low supercriticality. We propose that enhanced wall modes facilitated by the slender cylinder geometry interact with the bulk flow to create these large-scale axialized vortices. Our findings extend results from the previous $Pr \sim 1$ studies across various cylindrical aspect ratios. In particular, we find evidence of a different scaling for wall mode precession frequency that possibly exists in liquid metal, offering new insights into the coupling effects in low-$Pr$ rotating convective turbulence.