Turbulent Rayleigh-Benard convection in gaseous and liquid He

In this article we deal with the turbulent regimes of Rayleigh-Benard conve ction, namely the 2/7 regime and beyond. An experiment with He at low tempe rature allows us to explore a large Rayleigh number (Ra) range up to 2x10(1 4), under Boussinesq conditions, while the Prandtl number (Pr) is equal to and larger than 0.7. Calorimetric measurements evidence a departure from th e 2/7 regime above Ra=10(11) toward a new regime where the heat transfer is enhanced. Local measurements with two nearby thermometers allows us to rel ate this change to a laminar-turbulent transition of the velocity boundary layer induced by the large-scale flow near the walls of the cell. The featu res of the observed new regime match those of the ultimate regime predicted by R. Kraichnan [Phys. Fluids 5, 1374 (1962)] at moderate Pr; in particula r, our experimental data show that the thermal boundary layer lies inside t he viscous sublayer of the turbulent boundary layer. (C) 2001 American Inst itute of Physics.