Azimuthal motion, reorientation, cessation, and reversal of the large-scale circulation in turbulent thermal convection: a comparative study in aspect ratio one and one-half geometries.

We report a systematic experimental study of the orientation and the flow strength of the large-scale circulation (LSC) in water-filled cylindrical Rayleigh-Bénard convection cells with aspect ratios 2.3, 1, and 0.5 by both direct velocity measurement and the indirect multithermal-probe measurement. Unlike its weak effect in the system's global heat transport, the aspect ratio Gamma is found to play an important role in the dynamics of the azimuthal motion of the LSC. It is found that in larger Gamma geometries the azimuthal motion of the LSC's vertical plane is confined in smaller azimuthal region than that in smaller Gamma geometries. The twisting motion between top and bottom parts of the LSC observed in the Gamma=1 geometry is found to be absent in the Gamma=1/2 case. It is found that in the Gamma=1/2 geometry the orientational change mid R:Deltavarphimid R: through a reorientation has an exponential distribution, in contrast to the power-law distribution for the Gamma=1 case. Despite the difference in orientational change, the occurrence of the reorientations is a Poisson process in both geometries. Using the conditional average of the time interval between adjacent cessations or reversals on the rebound flow strength, we demonstrate the possibility to empirically predict when the next cessation or reversal will most likely occur if the rebound flow strength of the preceding cessation or reversal is given.