Turbulent Rayleigh–Bénard convection for a Prandtl number of 0.67

For the Rayleigh-number range 10 Ra 10 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 Γ ≡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 δ of the azimuthal temperature variation induced by the LSC at the sidewall, and the LSC circulation-plane orientation θ0, 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 δ the probability distribution p(δ) increases linearly from zero; for Γ =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 Γ =0.5 and Pr=0.7, where a one-roll LSC was found to exist only for Ra 10 to 10, and from measurements for Γ =0.5 and Pr 5, where oneand two-roll structures were observed with transitions between them at random time intervals.