From gradual to sharp transitions, heat transport in rapidly rotating Rayleigh-Bénard Convection

We present experimental measurements of the heat transport in rapidly rotating Rayleigh-Bénard convection for cylindrical samples of water (Pr=4.38) with diameter D=24.0cm and various aspect ratios of 0.4≤Γ≤4.0. Results for the Nu as a function of Ra reveal a gradual transition from weakly rotating convection to rotation-dominated geostrophic convection in a low-Ra regime Ra≤5.0*10⁷. However, in a high-Ra regime with Ra≥1.0*10⁹, a sequence of sharp transitions appears in Nu(Ra) when Ra is increased. With Γ=0.4 data of Nu(Ra) in the geostrophic turbulence regime (Ek≤10^-6 and Ra≥3Ra_c with Ra_c=8.7*Ek^-4/3) suggest a universal scaling relationship Nu=(Ra/Rac)^1.4 independent on the Ek number.