BOUNDARY-LAYER LENGTH SCALES IN THERMAL TURBULENCE

We present measurements of local temperature fluctuations from a conve ction cell of aspect ratio 1, using gas at room temperature, for Rayle igh numbers (Ra) ranging from 5 x 10(5) to 1 x 10(11). The thermal bou ndary layer thickness is measured; its scaling is close to Ra-2/7 for Ra > 2 x 10(7). A second length scale is measured through the cutoff f requency of the power spectrum, which we relate to the velocity of the large scale circulation. The Rayleigh number dependence of this lengt h changes at Ra=2 x 10(9).