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

We report an experimental study of the large-scale circulation (LSC) reversal in quasi-2D turbulent thermal convection, in which the aspect ratio Γ (= 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 (τ) between two successive reversals increases strongly with increasing Γ. 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 (τ) agree excellently with the experimental results, with (τ) having an unexpected stretched exponential Rayleigh number dependence, ∼exp(Raα). 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.