Heat transfer in rotating Rayleigh-Benard convection with rough plates

This experimental study focuses on the effect of horizontal boundaries with pyramid-shaped roughness elements on the heat transfer in rotating Rayleigh-Benard convection. It is shown that the Ekman pumping mechanism, which is responsible for the heat transfer enhancement under rotation in the case of smooth top and bottom surfaces, is unaffected by the roughness as long as the Ekman layer thickness delta(E) is significantly larger than the roughness height k. As the rotation rate increases, and thus delta(E) decreases, the roughness elements penetrate the radially inward flow in the interior of the Ekman boundary layer that feeds the columnar Ekman vortices. This perturbation generates additional thermal disturbances which are found to increase the heat transfer efficiency even further. However, when delta(E) approximate to k, the Ekman boundary layer is strongly perturbed by the roughness elements and the Ekman pumping mechanism is suppressed. The results suggest that the Ekman pumping is re-established for delta(E) << k as the faces of the pyramidal roughness elements then act locally as a sloping boundary on which an Ekman layer can be formed.