Item

Abstract

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–Bénard
convection, at fixed Rayleigh number Ra = 10^6, varying the wall Reynolds number in the
range 0 <= Rew <= 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 Kármán log layer,
signalling the onset of turbulent dynamics in the boundary layer.