Effect of weak rotation on the large-scale circulation in turbulent convection with a Prandtl number $Pr = 12.3$

We report measurements of large-scale circulation properties for high-Rayleigh-number convection in a rotating cylindrical sample with aspect ratio $\Gamma = D/L = 1.00$ (D is the diameter and L the height). The Prandtl number was $Pr = 12.3$. The measurements covered the Rayleigh-number range $2\times 10^{10} \leq Ra \leq 4\times10^{11}$ and the inverse Rossby-number range $0 \leq 1/Ro \leq 1/Ro_c = 0.28$ where the LSC was present. The azimuthal orientation $\theta_0$ of the LSC circulation plane remained fixed in the frame of the rotating sample for $Ra < Ra_{0} \simeq 5\times10^{10}$. The sloshing motion of the LSC showed oscillations with a short time period $\tau^{pl}$ of several tens of seconds. The temperature amplitude $<\delta>$ of the LSC increased as $1/Ro$ approached $1/Ro_c$, and decreased rapidly beyond it. For $Ra > Ra_{0}$, the circulation plane underwent retrograde rotation and hence caused time-periodic temperature oscillations near the side wall with a large period $\tau_{ac}$ of hundreds of seconds. Remarkably, $\tau_{ac}$ persisted without a discontinuity even for $1/Ro > 1/Ro_c$ where the LSC ceased to exist, indicating that vortex structures in that regime undergo the same retrograde rotation as the LSC.