Velocity and Heat Transfer Measurements in Turbulent Liquid Metal Rotating Convection Experiments

Earth’s magnetic field is generated by turbulent, convective motions in its liquid iron outer core. This rotating, liquid metal system is characterized by a low Prandtl number, Pr = O(10^-2). Global heat transfer in low Pr fluids has been well-characterized across numerous studies. However, interior bulk thermal and velocity scaling behavior has not been as thoroughly explored. Here we present results from laboratory experiments of rotating Rayleigh-Bénard convection in liquid gallium in an aspect ratio Γ = D/H = 1/2 cylinder. The experiments cover a range of Rayleigh number (non-dimensional buoyancy forcing) 8 × 10⁶ < Ra < 1 × 10⁹, Ekman number (non-dimensional rotation rate) 3 × 10^-7 < E < 1 × 10^-5, and convective Rossby number (buoyancy/rotation) 2 × 10^-2 < Ro_c < 2. We measure global heat transfer efficiency, Nu, non-dimensional vertical flow velocities, Re, and the internal local temperature fluctuation, θ, to characterize the flow. We find that our measurements are well-described by the Coriolis-Inertia-Archimedean (CIA) triple torque balance, similar to previous studies in water (Pr = 7). We additionally compare these results to those of prior rotating RBC liquid gallium studies in wider aspect ratio cylinders (Γ = 1 and Γ = 2), from which we find that interior bulk quantities (Re, θ) are largely unaffected by the enhanced wall mode effects that otherwise contaminate global heat transfer (Nu) in low aspect ratio, slender cells.