Reversals of the Large-scale Circulation in Turbulent Thermal Convection in Small Aspect-ratio Geometry

We present experimental studies of flow reversals and cessations in turbulent thermal convection in cells of aspect ratio ($\Gamma )$ less than one. It is found that cessations of the large-scale circulation (LSC) occur not only in $\Gamma $ =1 cells but also in $\Gamma $ = 1/2 and 1/3 cells, and they occur in these small aspect ratio cells an order of magnitude more frequently than in $\Gamma $ =1 cells. The experiment reveals that in these small aspect ratio cells 180$^{o}$ is the most probable orientational angular change of the LSC's circulation plane after a cessation. The time interval of successive cessations is found to distribute exponentially. It is further found that a cessation correspond to decoherence of the LSC.