Rotations and cessations of the large-scale circulation in turbulent   Rayleigh-Benard convection
  We present a broad range of measurements of the angular orientation theta_0(t) of the large-scale circulation (LSC) of turbulent Rayleigh-Benard convection as a function of time. We used two cylindrical samples of different overall sizes, but each with its diameter nearly equal to its height. The fluid was water with a Prandtl number of 4.38. The time series theta_0(t) consisted of meanderings similar to a diffusive process, but in addition contained large and irregular spontaneous reorientation events through angles Delta theta. We found that reorientations can occur by two distinct mechanisms. One consists of a rotation of the circulation plane without any major reduction of the circulation strength. The other involves a cessation of the circulation, followed by a restart in a randomly chosen new direction. Rotations occurred an order of magnitude more frequently than cessations. Rotations occurred with a monotonically decreasing probability distribution p(Delta theta), i.e. there was no dominant value of Delta theta and small Delta theta were more common than large ones. For cessations p(Delta theta) was uniform, suggesting that information of theta_0(t) before the cessation is lost. Both rotations and cessations have Poissonian statistics in time, and can occur at any theta_0. The average azimuthal rotation rate increased as the circulation strength of the LSC decreased. Tilting the sample relative to gravity significantly reduced the frequency of occurrence of both rotations and cessations. 
Eric Brown and Guenter Ahlers