Mechanism of large-scale flow reversals in turbulent thermal convection

We report a new kind of convective instability for turbulent thermal convection in a closed thin disk cell. It is found that the convective flow stays over a long steady “quiet period” having a minute amount of heat accumulation, followed by a short and intermittent “active period” with a massive eruption of thermal plumes to release the accumulated heat. The rare massive eruption of thermal plumes disrupts the existing large-scale circulation across the cell and resets its rotational direction. The distribution function of the plume eruption amplitude is found to follow the generalized extreme value statistics with an upper bound, which changes with the physical properties of the convecting fluid. The experimental findings have important implications to many closed convection systems of geophysical scale, in which massive eruptions and sudden changes in large-scale flow pattern are often observed.

This research was supported by the Research Grants Council of Hong Kong SAR.