From laminar plumes to organized flows: the onset of large-scale circulation in turbulent thermal convection

We report an experimental study on the onset of the large-scale coherent mean flow in Rayleigh-Benard turbulent convection. Shadowgraph and particle image velocimetry techniques are used to visualize the motion of thermal plumes and measure the velocity of the plumes and of the 'background' flow field, as the fluid motion evolves from quiescent to steady state. The experiment reveals the dynamical origin of the initial horizontal motion required by the large-scale flow: the fluid entrainment caused by the plume's vertical motion generates vortices surrounding the plume itself. These vortices in turn generate the initial horizontal motion of the flow field. Two types of interactions have been identified: (i) direct plume-vortex interaction; and (ii) plume-plume interaction via vortices. These interactions and the interaction and merging of the vortices from neighbouring plumes lead to groupings and/or merging of plumes, which in turn generate vortices of even larger scale. As a result of these interactions, the convective flow evolves into a coherent rotatory motion consisting of mainly the plumes themselves and spanning the whole convection box. This study clearly demonstrates that it is the thermal plumes that initiate the horizontal large-scale flow across the top and bottom conducting plates.