Particle image velocimetry measurement of the velocity field in turbulent thermal convection

The spatial structure of the velocity field in turbulent Rayleigh-Benard convection in water has been measured using the particle image velocimetry technique, with the Rayleigh number Ra varying from 9x10(8) to 9x10(11) and the Prandtl number remaining approximately constant (Prsimilar or equal to4). The study provides a direct confirmation that a rotatory mean wind indeed persists for the highest value of Ra reached in the experiment. The measurement reveals that the mean flow in the central region of the convection cell is of the shape of a coherent elliptical rotating core for Ra below 1x10(10). Above this Ra, the orientation of the elliptical core changes by a 90degrees angle and an inner core rotating at a lower rate inside the original bulk core emerges. It is further found that the rotation frequencies of the inner core and the outer shell have distinct scalings with Ra; the scaling exponent for the outer-shell is 0.5 and it is 0.4 for the inner core. From the measured rms and skewness distributions of the velocity field, we find that velocity fluctuations at the cell center are neither homogenous nor isotropic. The turbulent energy production fields further reveal that the mean wind is not driven by turbulent fluctuations associated with Reynolds stress.