Diffusivity-free turbulence in liquid metal rotating Rayleigh-Bénard convection experiments

Convection in planets and stars is predicted to occur in the “ultimate regime” of diffusivity-free, rapidly rotating turbulence in which flows are unaffected by viscous and thermal diffusion. Boundary layer diffusion, however, has historically hindered experimental access to this regime. Here, we utilize the boundary-independent oscillatory thermal-inertial mode of rotating convection to realize the diffusivity-free scaling in liquid metal laboratory experiments with additional validation from direct numerical simulations. This oscillatory style of convection arises in rotating liquid metals (low Prandtl number fluids) and is driven by the temperature gradient in the fluid bulk, thus remaining independent of diffusive boundary dynamics. We triply verify the existence of the diffusivity-free regime via measurements of heat transfer efficiency 𝑁⁢𝑢, dimensionless flow velocities 𝑅⁢𝑒, and internal temperature anomalies 𝜃, all of which are in quantitative agreement with planar asymptotically reduced models. Realizing theoretical diffusivity-free scalings in desktop-sized laboratory experiments provides the validation necessary to extrapolate and predict the convective flows in remote geophysical and astrophysical systems.