Impact of fluid flow on the thermal boundary layer dynamics in turbulent liquid metal Rayleigh–Bénard convection

Abstract

The dynamics of the thermal boundary layer are studied experimentally in turbulent Rayleigh–Bénard convection at $3\times 10^7\le Ra\le 3\times 10^9$. The local boundary layer (BL) height and the flow structures are measured in the ternary alloy GaInSn (Prandtl number $Pr=0.03$) contained in a cylindrical sample with an aspect ratio of $\Gamma =0.5$. The data, which are recorded at two measurement positions in the center of the plate and near the side wall, reveal disruptive disturbances of the thermal BL at the top plate, the intensity and frequency of which vary with the Rayleigh number $Ra$. We observe the occurrence of pronounced peaks in the BL thickness, whereby the number of these events relative to the free-fall time $t_{ff}$ decreases with increasing $Ra$ while their mean duration increases. The events of this intermittent significant broadening of the thermal boundary layer are detectable in the temporal behavior of the Nusselt number and correlate strongly with the occurrence of a vertical flow moving away from the plate surface. This suggests that the peaks in the thermal BL height coincide with the formation and detachment of thermal plumes on the copper plate. An interesting aspect of our measurements is the observation of BL disturbances at the measurement position in the center of the plate, which is dominated by a wall-parallel shear flow of large-scale circulation (LSC), just as frequently as in the region near the side wall, where impingement or detachment of thermal plumes should occur.