RAYLEIGH-BENARD CONVECTION IN LIQUID HELIUM-4 AND A HELIUM-3-HELIUM-4 MIXTURE

A study of Rayleigh-Benard convection has been conducted in liquid {dollar}sp{lcub}4{rcub}{dollar}He and a {dollar}sp{lcub}3{rcub}{dollar}He-{dollar}sp{lcub}4{rcub}{dollar}He mixture (c = 0.038) confined in a cylindrical container with variable height. In {dollar}sp{lcub}4{rcub}{dollar}He, measured critical Rayleigh numbers R{dollar}sb{lcub}rm c{rcub}{dollar} are in agreement with the predictions. The data for the initial slope of the Nusselt curve show that the convection pattern in the fluid layer can be described by predictions for parallel rolls. We have also studied the onset of time-dependence which is most likely caused by the skew varicose instability. Two types of time-dependence were found at onset. One with vanishing amplitude and the other with vanishing frequency of the oscillation. Time-dependent and turbulent convection were studied at the aspect ratio {dollar}lceil{dollar} = 15.06. A kind of intermittency dominates the route to turbulence. More than two stationary convection states were found for {dollar}lceil < 6.15,{dollar} whereas above 6.15 only one can be found at low Prandtl number P. Results for the Rayleigh number at the onset of time-dependence are obtained for 0.48 {dollar}<{dollar} P {dollar}<{dollar} 0.94 and they qualitatively agree with the prediction.; In the {dollar}sp{lcub}3{rcub}{dollar}He-{dollar}sp{lcub}4{rcub}{dollar}He mixture, when the separation ratio S is below the polycritical point, no time-dependent oscillations were found at the onset of convection as predicted by theory. But Nusselt curves show several discontinuous transitions at and above the onset of convection. Time-dependence does occur for R {dollar}>{dollar} R{dollar}sb{lcub}rm c{rcub}{dollar}. For S {dollar}>{dollar} O, the results for the Nusselt curves can be described reasonably well by the five-mode model. The convective wave number, probed by the onset Rayleigh number and the amplitude of oscillations for the secondary instability, does not decrease with increasing S as much as predicted.