Thermal boundary layers and heat flux in turbulent convection: The role of recirculating flows

We experimentally examine the hypothesis that the scaling of the Nusselt number Nu (nondimensional heat flux) with Rayleigh number Ra in turbulent thermal convection is affected by large-scale flows near and within the boundary layers. The natural flows are enhanced artificially by a moving layer of mercury beneath the convecting fluid. Recirculating large-scale flows are thereby induced near the lower thermal boundary layer, and Nu is enhanced by as much as 70%. The response to forcing allows a determination of the large-scale flows needed to account for the observed scaling of Nu with Ra in the unperturbed situation. The required flows are found to be comparable to those present naturally, about 1.5 mm/s at Ra=${10}^8$ and a Prandtl number near 5. The experiments thus indicate that Nu is at least largely determined by a balance between diffusion of heat into the boundary layers and advection from them by large-scale flows. In an appendix, the form of the temperature probability distribution in the interior is shown to be related to the coherence of the thermal plumes.