Oscillating convection and reversal flow in connected cavities

Abstract

The two-dimensional natural convection of air in upper and a lower square cavities connected with a vertical vent is studied numerically and theoretically. At high Rayleigh numbers (Ra), it is revealed by the dynamic-mode-decomposition method that the plumes yield to a new sway mode, where the plumes oscillate in the azimuthal direction around the vent ends. In particular, when Ra is high enough, a reversal convection occurs, i.e., the dominating large-scale circulation changes its flow direction quasiperiodically. A van der Pol--type model is developed based on the momentum and the energy equations, and the reversal phenomenon is explained in terms of the relaxation oscillation caused by the competition between the nonlinear advection and the thermal diffusion.