Nusselt number enhancement and flow structures in Rotating Rayleigh-Benard convection

It is well known that external rotation may enhance the heat transfer in Rayleigh-Benard (RB) convection through Ekman pumping. In this talk we focus on the Nusselt numbers enhancement regime of the rotating RB flow. We show that for low Rayleigh number, the optimal rotating rate which generates the highest heat transfer can be well explained by a cross-over of the boundary layer thicknesses of the momentum and thermal field. However, when Rayleigh number is larger than a critical value, such explanation breaks down. The optimal rotating rate is different for periodic and cylindrical domains, as well as cylindrical domains with different aspect ratios. By looking into the autocorrelation of the vertical velocity we found that characteristic height of the coherent structures in the bulk is strongly affected by the horizontal boundary conditions. Therefore different domains reach optimal heat transfer at different rotating rates.