Radial boundary layer structure and Nusselt number in Rayleigh-Benard convection

Results from direct numerical simulation (DNS) for three-dimensional Rayleigh-Benard convection in a cylindrical cell of aspect ratio 1/2 and Prandtl number Pr = 0.7 are presented. They span five decades of Rayleigh number Ra from 2 x 10(6) to 2 x 10(11). The results are in good agreement with the experimental data of Niemela et al. (Nature, vol. 404, 2000, p. 837). Previous DNS results from Amati et al. (Phys. Fluids, vol. 17, 2005, paper no. 121701) showed a heat transfer that was up to 30% higher than the experimental values. The Simulations presented in this paper are performed with a much higher resolution to properly resolve the plume dynamics. We find that in under-resolved simulations the hot (cold) plumes travel further from the bottom (top) plate than in the better-resolved ones, because of insufficient thermal dissipation mainly close to the sidewall (where the grid cells are largest), and therefore the Nusselt number in under-resolved simulations is overestimated. Furthermore, we compare the best resolved thermal boundary layer profile with the Prandtl-Blasius profile. We find that the boundary layer profile is closer to the Prandtl-Blasius profile at the cylinder axis than close to the sidewall, because of rising plumes close to the sidewall.