Plume motion and large-scale circulation in a cylindrical Rayleigh-Benard cell

We used the time correlation of shadowgraph images to determine the angle Theta of the horizontal component of the plume velocity above ( below) the center of the bottom ( top) plate of a cylindrical Rayleigh-Benard cell of aspect ratio Gamma equivalent to D/L = 1 (D is the diameter and L similar or equal to 87 mm is the height) in the Rayleigh-number range 7 x 10(7) less than or equal to R less than or equal to 3 x 10(9) for a Prandtl number sigma = 6. We expect that Theta gives the direction of the large-scale circulation. It oscillates time periodically. Near the top and bottom plates Theta(t) has the same frequency but is anticorrelated.