Turbulent Rayleigh-Benard convection for a Prandtl number of 0.67

For the Rayleigh-number range 10(7) less than or similar to Ra less than or similar to 10(11) we report measurements of the Nusselt number Nu and of properties of the large-scale circulation (LSC) for cylindrical samples of helium gas (Prandtl number Pr = 0.674) that have aspect ratio Gamma equivalent to D/L = 0.50 (D and L are the diameter and the height respectively) and are heated from below. The results for Nu are consistent with recent direct numerical Simulations. We measured the amplitude delta of the azimuthal temperature variation induced by the LSC at the sidewall, and the LSC circulation-plane orientation theta(0), at three vertical positions. For the entire Ra range the LSC involves a convection roll that is coherent over the height of the system. However, this structure frequently collapses completely at irregular time intervals and then reorganizes from the incoherent flow. At small delta the probability distribution p(delta) increases linearly from zero; for Gamma = 1 and Pr = 4.38 this increase is exponential. No evidence of a two-roll structure, with one above the other, was observed. This differs from recent direct numerical simulations for Gamma = 0.5 and Pr = 0.7, where a one-roll LSC was Found to exist only for Ra less than or similar to 10(9) to 10(10), and from measurements for Gamma = 0.5 and Pr similar or equal to 5, where one- and two-roll structures were observed with transitions between them at random time intervals.