Study on corner vortex enlarging process of 2D square Rayleigh-Bénard cells filled with air in transient states

In Rayleigh-Benard convection (RBC), it has been confirmed that the enlargement of the corner vortexes directly causes the reversals of mean wind in transient states. But when and how the size of corner vortexes start to increase are still unclear. In this paper, both experiments and simulations (Lattice Boltzmann Method) are applied to the research on corner-vortex enlargement in 2D square RBC cells filled with air (Pr = 0.7, Ra = 104–108). The results show that the dominant frequencies of RBC are divided into three levels with the increasing Ra. The wind reversals caused by corner-vortex enlargement are confirmed in transient states where the three frequency levels coexist. And obvious frequency-jump from the lower level to the higher level is detected before the corner vortexes grow up, which is absent in the cases without reversals. Further analysis based on the temperature contours ascribes these three frequency levels to different temperature-structure motions. And it is observed that the periodic change of corner-vortex volume dynamically keeps balance with the pulsating flow featuring these frequencies from the opposite conductive walls. In transient states, when the pulsating flow jumps into a new frequency, the previous heat balance inside the corner vortexes is broken, which immediately triggers the size enlargement of the corner vortexes.