Prandtl and Rayleigh number dependence of heat transport in high Rayleigh number thermal convection

Results from direct numerical simulation for three-dimensional Rayleigh-Benard convection in samples of aspect ratio $Gamma=0.23$ and $Gamma=0.5$ up to Rayleigh number $Ra=2times10^{12}$ are presented. The broad range of Prandtl numbers $0.5<Pr<10$ is considered. In contrast to some experiments, we do not see any increase in $Nu/Ra^{1/3}$, neither due to $Pr$ number effects, nor due to a constant heat flux boundary condition at the bottom plate instead of constant temperature boundary conditions. Even at these very high $Ra$, both the thermal and kinetic boundary layer thicknesses obey Prandtl-Blasius scaling.