An experimental study of Rayleigh-Be'nard convection in helium gas at roughly 5 K is performed in a cell with aspect ratio 1. Data are analysed in a ' hard turbulence ' region (4 x 10' < Ra < 6 x 10l2) in which the F'randtl number remains between 0.65 and 1.5, The main observation is a simple scaling behaviour over this entire range of Ra. However the results are not the same as in previous theories. For example, a classical result gives the dimensionless heat flux, Nu, proportional to R d while experiment gives an index much closer to 5. A new scaling theory is described. This new approach suggests scaling indices very close to the observed ones. The new approach is based upon the assumption that the boundary layer remains in existence even though its Rayleigh number is considerably greater than unity and is, in fact, diverging. A stability analysis of the boundary layer is performed which indicates that the boundary layer may be stabilized by the interaction of buoyancy driven effects and a fluctuating wind.

No abstract

No abstract

In this article we deal with the turbulent regimes of Rayleigh–Bénard convection, namely the 2/7 regime and beyond. An experiment with He at low temperature allows us to explore a large Rayleigh number (Ra) range up to 2×1014, under Boussinesq conditions, while the Prandtl number (Pr) is equal to and larger than 0.7. Calorimetric measurements evidence a departure from the 2/7 regime above Ra=1011 toward a new regime where the heat transfer is enhanced. Local measurements with two nearby thermometers allows us to relate 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 features of the observed new regime match those of the ultimate regime predicted by R. Kraichnan [Phys. Fluids 5, 1374 (1962)] at moderate Pr; in particular, our experimental data show that the thermal boundary layer lies inside the viscous sublayer of the turbulent boundary layer.