Prandtl number dependence of the transition to the geostrophic regime of rotating Rayleigh-Bénard convection

In the rotating Rayleigh-Bénard (RRB) configuration different regimes of turbulent rotating convection can be reached depending on the strength of the buoyant forcing (Rayleigh number, Ra), the strength of rotation (Ekman number, Ek) and the fluid properties (Prandtl number, Pr). The transition between regimes has been widely studied for moderate values of the Prandtl number (Pr≥1). We directly simulate the RRB flow to specifically investigate the transition to the geostrophic turbulent regime for different Prandtl numbers. We include cases at Pr=0.1, relevant to liquid metals and to numerous geophysical and astrophysical scenarios, and Pr=1 and ∼5 that corresponds to water as used by many laboratory experiments and relevant to oceanic processes. For the different Prandtl numbers we find a transition between geostrophic turbulence and the rotation-affected regime. We highlight the presence of a large-scale vortical structure in the geostrophic turbulent regime for our lowest Pr case with no-slip top/bottom boundaries in the RRB setup. Similar large-scale coherent structures have been observed for stress-free boundaries, but not yet confirmed for the no-slip case until now. Our results expose the active role of the Prandtl number in the dynamics of the flow.