Turbulent thermal convection in a closed domain: viscous boundary layer and mean flow effects

In this paper the effects of viscous boundary layers and mean flow structures on the heat transfer of a flow in a slender cylindrical cell are analysed using the direct numerical simulation of the Navier-Stokes equations with the Boussinesq approximation. Ideal flows are produced by suppressing the viscous boundary layers and by artificially enforcing the flow axisymmetry with the aim of checking some proposed explanations for the Nusselt number dependence on the Rayleigh number. The emerging picture suggests that, in this slender geometry, the presence of the viscous boundary layers does not have appreciable impact on the slope of the Nu. vs. Ra relation while a transition of the mean flow is most likely the reason for the slope increase observed around Ra = 2 x 10(9).