High-Reynolds-number thermal turbulence in mercury

We have studied the scaling properties of turbulence in a Rayleigh-Benard convection cell. using a low-Prandtl-number (Pr) fluid. Hg (Pr=0.024). The length scales of thermal and viscous layers, analyzed from time series of movable thermistors near the cold upper plate, revealed that the viscous boundary layer is thinner than the thermal one over the observed range of Rayleigh numbers (10(6) < Ra < 10(8)). This feature, peculiar to low Pr fluid convection. differed from what is commonly observed in He. water, or other fluids. The scaling of Reynolds number Re and Nusselt number Nu has been investigated using different aspect ratio cells (Gamma = 0.5, 1, and 2) for a wider range of Ra. The hypothetical ultimate regime (Nu similar to Ra-1/2) was not observed in the range 10(5) < Ra < 2 x 10(9) even though the two boundary layers are inverted.