The effect of Prandtl number on turbulent sheared thermal convection

In turbulent wall sheared thermal convection, there are three different flow regimes, depending on the relative relevance of thermal forcing and wall shear. In this paper, we report the results of direct numerical simulations of such sheared Rayleigh-Benard convection, at fixed Rayleigh number Ra = 10(6), varying the wall Reynolds number in the range 0 <= Re-w <= 4000 and Prandtl number 0.22 <= Pr <= 4.6, extending our prior work by Blass et al. (J. Fluid Mech., vol. 897, 2020, A22), where Pr was kept constant at unity and the thermal forcing (Ra) varied. We cover a wide span of bulk Richardson numbers 0.014 <= Ri <= 100 and show that the Prandtl number strongly influences the morphology and dynamics of the flow structures. In particular, at fixed Ra and Rew, a high Prandtl number causes stronger momentum transport from the walls and therefore yields a greater impact of the wall shear on the flow structures, resulting in an increased effect of Rew on the Nusselt number. Furthermore, we analyse the thermal and kinetic boundary layer thicknesses and relate their behaviour to the resulting flow regimes. For the largest shear rates and Pr numbers, we observe the emergence of a Prandtl-von Karman log layer, signalling the onset of turbulent dynamics in the boundary layer.