The Transition to the Ultimate State in Turbulent Thermal Convection

Abstract eng:
We report measurements on turbulent Rayleigh-B´enard convection using three cylindrical samples with aspect ratios (diameter/height) Γ = 1.00, 0.50 and 0.33. All samples had the same diameter D = 1.12 m, but different heights L. Compressed sulfur hexafluoride gas (SF6 ) at pressures up to 19 bar was used as the fluid at the G¨ottingen Turbulence Facility (see www.EuHIT.org). The 15 measurements were conducted over the Rayleigh-number range 1012 < ∼ 4 × 10 and for Prandtl numbers P r near 0.8. In three ∼ Ra < independent measurements, namely global heat transport, local turbulent Reynolds numbers, and large-scale-circulation dynamics, we observed a transition over a range of Ra from the classical regime to a new regime which has been referred to as “ultimate”. We discuss the properties of the classical and the ultimate state, as well as the influence of rotation on this system.

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