Model for the dynamics of the large-scale circulations in two-layer turbulent convection

We present a physically motivated low-dimensional model for the dynamics of two interacting large-scale circulations (LSC) in two-layer turbulent convection. Inspired by our experimental results of the flow dynamics and coupling in two-layer turbulent convection [J. Fluid Mech. 728, R1 (2013)], the model extends previous studies of single-LSC dynamics to incorporate four stochastic ordinary differential equations describing the strength delta and azimuthal orientations 0 of two vertically aligned LSCs. The interaction terms of the two LSCs, i.e., thermal and viscous coupling terms, are predicted based on the influence of the fluid temperature by the other LSC through heat advection and thermal diffusion, and the enhanced (reduced) viscous damping across the interface between the two LSCs. Our model produces two stable LSC rolls and predicts their preferred flow states for the thermal and viscous couplings. The model describes properly the diffusive motion of both delta and 0 of the two LSCs, and the Poissonian distribution of time interval between LSC cessations. More importantly, our study reveals that flow reversals and cessations in two-layer convection can be achieved when turbulent fluctuations drive the azimuthal diffusion of the two LSCs into a flow state that the two LSC planes are orthogonal to each other, the strength of the LSC in the fluid layer with a relatively larger Rayleigh number reduces to zero deterministically, owing to the unbalanced buoyancy forcing. Our model provides accurate predictions for the enhanced occurrence frequency of flow reversals observed in the experiment, and it suggests a new dynamical process of flow reversals in multilayer turbulent convection.