Azimuthal Motion of the Mean Wind in Turbulent Thermal Convection

We report an experimental study of the azimuthal motion of the circulation plane of the mean wind in turbulent Rayleigh-B{\'e} nard convection in water. Measurements were made in both aspect ratio $\Gamma = 1$ and $0.5$ cylindrical cells. The results show that for $\Gamma = 1$ the orientation of the wind fluctuates over an azimuthal angular range of $\sim \pm 100$ degrees about a preferred direction for over 90$\%$ of the time. In contrast, for $\Gamma =0.5$ the orientation of the wind shows no preferred direction. For $\Gamma = 1$ the observed azimuthal motion of the wind is a superposition of a periodic oscillation in short timescale and chaotic fluctuation in longtime scale. For both $\Gamma =$ 1 and 0.5 the apparently stochastic azimuthal motion of the wind generates a net-rotation on average, with the $\Gamma =$ 0.5 cell having a much larger net-rotation rate. Measurements with varying values of the Rayleigh number Ra is made for the $\Gamma =0.5$ case, and it is found that the net rotation rate diminishes with increasing Ra, reaching a vanishing value around $Ra = 1\times 10^{11}$.