Multiple heat transport maxima in confined-rotating Rayleigh-Benard convection

Moderate rotation and moderate horizontal confinement similarly enhance the heat transport in Rayleigh-Benard convection (RBC). Here, we systematically investigate how these two types of flow stabilization together affect the heat transport. We conduct direct numerical simulations of confined-rotating RBC in a cylindrical set-up at Prandtl number Pr = 4.38, and various Rayleigh numbers 2 x 10(8) <= Ra <= 7 x 10(9). Within the parameter space of rotation (given as inverse Rossby number 0 <= Ro(-1) <= 40) and confinement (given as height-to-diameter aspect ratio 2 <= Gamma(-1) <= 32), we observe three heat transport maxima. At lower Ra, the combination of rotation and confinement can achieve larger heat transport than either rotation or confinement individually, whereas at higher Ra, confinement alone is most effective in enhancing the heat transport. Further, we identify two effects enhancing the heat transport: (i) the ratio of kinetic and thermal boundary layer thicknesses controlling the efficiency of Ekman pumping, and (ii) the formation of a stable domain-spanning flow for an efficient vertical transport of the heat through the bulk. Their interfering efficiencies generate the multiple heat transport maxima.