Thermal fluctuations and their ordering in turbulent convection

In highly nonlinear thermal convection, random fluctuations within the flow organize themselves into a self-sustaining large-scale circulation. We experimentally study some properties of this mean “wind”, including the irregular reversal of its direction. We show that the energy in the organized motion is a small fraction of the thermal energy input, and becomes increasingly smaller as the Rayleigh number increases. A rough correspondence can be made to self-organized criticality from the analysis of various probability density functions associated with the fluctuating wind.