Measured instantaneous viscous boundary layer in turbulent Rayleigh-Bénard convection.

We report measurements of the instantaneous viscous boundary layer (BL) thickness delta(v)(t) in turbulent Rayleigh-Bénard convection. It is found that delta(v)(t) obtained from the measured instantaneous two-dimensional velocity field exhibits intermittent fluctuations. For small values, delta(v)(t) obeys a lognormal distribution, whereas for large values, the distribution of delta(v)(t) exhibits an exponential tail. The variation of delta(v)(t) with time is found to be driven by the fluctuations of the large-scale mean-flow velocity, and the local horizontal velocities close to the plate can be used as an instant measure of this variation. It is further found that in the present parameter range of the experiment, the mean velocity profile measured in the laboratory frame can be brought into coincidence with the theoretical Prandtl-Blasius laminar BL profile, if it is resampled relative to the time-dependent frame of delta(v)(t).