Three-dimensional flow structures and dynamics of turbulent thermal convection in a cylindrical cell.

The technique of particle image velocimetry is used to study the velocity field of turbulent Rayleigh-Bénard convection in an aspect-ratio-1 cylindrical cell filled with water. By measuring the two-dimensional (2D) velocity vector map in different vertical cross sections of the cell, we investigate the 3D structures and dynamics of turbulent thermal convection. The experiment reveals how thermal plumes synchronize their emissions and organize their motions spatially between the top and bottom plates, leading to an oscillatory motion in the bulk region of the fluid with a period equal to twice the plume's cell-crossing time. From the measured instantaneous velocity vector map, we find the phase relationship between the velocity components along different directions and at different positions in a 2D plane. These phase relations illustrate how the convecting fluid in different regions of the cell interact with each other and generate a synchronized and coherent motion in a closed system.