Flow dynamics and heat transport in turbulent thermal convection

An experimental study of flow structure and dynamics and their relationship with the heat transport in turbulent thermal convection is reported. It is shown that the instantaneous large-scale flow structure in a cylindrical convection cell consists of an elliptical circulatory roll and two smaller counter-rotating rolls. As the quasi-2D instantaneous flow structure rotates azimuthally, it generates a time-averaged 3D flow pattern featuring two toroidal rings near the top and bottom plates respectively. It is found that the apparently stochastic azimuthal motion of the flow structure, which generates a net rotation on average, has the characters of a Brownian ratchet. We also provide convincing evidence that a particular value of the heat transport efficiency Nu can be associated with a specific internal flow mode of the system.