Reversals of the large-scale circulation in quasi-2D Rayleigh-Benard convection

We report an experimental study of the large-scale circulation (LSC) reversal in quasi-2D turbulent thermal convection, in which the aspect ratio Gamma (= height/length of a rectangular box) is used as a parameter to perturb the stability of the LSC. It is found that the mean time interval <tau > between two successive reversals increases strongly with increasing Gamma. A stochastic model is proposed to incorporate the effect of the corner rolls. In the model, the aspect ratio serves as a tuning parameter for the relative weight of the corner rolls that damp the LSC. The model predictions for the shape of the bistable states of the system and <tau > agree excellently with the experimental results, with <tau > having an unexpected stretched exponential Rayleigh number dependence, similar to exp(Ra-alpha). We further show quantitatively that the main damping force of the LSC in a quasi-2D system is from the corner rolls rather than the viscous drag from the sidewalls, which bridges the difference found in quasi-2D and 3D systems.