Large-scale circulation and Nusselt number in turbulent rotating Rayleigh-B\'enard convection.

We present measurements of the large-scale circulation (LSC) and the Nusselt number $Nu$ of turbulent Rayleigh-B\'enard convection in a cylindrical cell of aspect ratio 1 and rotated about a vertical axis at a rate $\Omega$. The side-wall temperatures at eight equally spaced azimuthal positions in the horizontal mid- plane were fit to a cosine function that gave the azimuthal LSC orientation $\theta(t)$ ($t$ is the time), the temperature amplitude $\delta(t)$, and the rms amplitude $\delta T(t)$ of the fluctuations about the fits. The LSC precessed in an azimuthal direction opposite to that of the imposed rotation. The precession rate $\omega= d\theta/dt$ showed a sharp transition at a Rossby number $Ro^* \simeq 2.5$. As $\Omega$ increased, $<\delta(t)>_t$ decreased and $<\delta T(t)>_t$ increased beginning at $Ro^*$. At $Ro^*$ $Nu$ began to increase with increasing $\Omega$. At high $Ro$ $| \omega |$ was proportional to but much smaller than $\Omega$.