Turbulent Rayleigh-Bénard convection in water has been studied using a newly developed technique of incoherent cross-correlation spectroscopy. At high Rayleigh number Ra (hard turbulence), local velocity fluctuations are isotropic, and their probability density functions in the center region have an invariant Gaussian form, with the rms velocity ${\upsilon }_0\sim {\mathrm{Ra}}^{0.44}$. At low Ra in the soft turbulence regime, the probability density functions for the vertical velocity fluctuations do not have a universal form and appear to depend on the coherence of thermal plumes emitted from the boundary layers.

• Received 7 March 1995

DOI:https://doi.org/10.1103/PhysRevLett.75.437