Combining confinement and rotation to improve heat transport in Rayleigh-Bénard convection*

It has been shown by Chong et al., (2017) that different stabilizing mechanisms similary increase heat transport in Rayleigh-Bénard convection (RBC) compared to the standard non-stabilized system. Here, we study the combination of two stabilizing mechanisms, namely rotation, and confinement, by performing direct numerical simulations of confined rotating RBC in a cylindrical cell for Prandtl number $Pr=4.38$ and various Rayleigh numbers $Ra\in[2\cdot10^8,7\cdot10^9]$. In the absence of rotation, flow confinement can result in a higher heat transport as it makes the large scale circulation more effective in transporting heat, while system rotation can increase heat transport due to a process known as Ekman pumping. We find that Ekman pumping becomes more effective at a certain aspect ratio, i.e. diameter over the height of the sample, when the flow forms a stable structure of two vortices. In addition, we find that for high Ra and very low aspect ratio cells, a third distinct maximum evolves, caused by the formation of a singular strong vortex. We show that for relatively low $Ra<2\cdot10^9$ the combination of confinement and rotation results in the highest heat transport. For $Ra>2\cdot10^9$ the use of confinement is the most effective way to increase heat transport.