Temperature power spectra and the viscous boundary layer in thermal turbulence: the role of Prandtl number

We report an experimental investigation on the relationship between the thickness of the viscous boundary layer and the highest excitation (or cutoff) frequency of the temperature power spectrum in turbulent convection, using water as working fluid. For the range of the Rayleigh number spanned in the experiment (108–1010), we confirm that the peak position in the cutoff frequency profile can be used as an alternative measure of the viscous boundary layer thickness [A. Belmonte et al. Phys. Rev. Lett. 70 (1993) 4067]. But a previous justification given for this method is not borne out in our system. The implications of the current work will be discussed with respect to results from other experiments using fluids with Prandtl numbers different than that of water.