Transition from random emission of plumes to coherent circulation promoted by a free rotor driven by Rayleigh–Bénard convection

We investigate the flow states in Rayleigh–Bénard convection influenced by a free body (termed “rotor”), which is immersed in a horizontal cylindrical cell. In the absence of rotor, the randomly emitted thermal plumes would self-organize and make a transition to form an ordered large-scale circulation when the Rayleigh number (Ra) exceeds a threshold Ra_t. When this happens, an order parameter Φ, defined based on the difference in the numbers of plumes with positive and negative angular velocities, exhibits a sharp increase from 0 to 0.8. When the rotor is present and is driven to spin by the flow, it regulates the emission of the thermal plumes by invading the thermal boundary layers. Over the entire parameter range of 0.2Ra_t≤Ra≤5Ra_t, we find Φ∼1.0, indicating the prevalence of the ordered flow state. We further demonstrate the effectiveness of flow rectification, by tracking the motion of the thermal plumes, thanks to the fluid-structure interaction between the rotor and the convection flows.