High Rayleigh Number Cryogenic Facility
Italy
Publications

ArticleNature. 2000, Vol. 404, Issue 6780, p. 837840. DOI: 10.1038/35009036
 Abstract Turbulent convection occurs when the Rayleigh number (Ra)â€”which quantifies the relative magnitude of thermal driving to dissipative forces in the fluid motionâ€”becomes sufficiently high. Although many theoretical and experimental studies of turbulent convection exist, the basic properties of heat transport remain unclear. One important question concerns the existence of an asymptotic regime that is supposed to occur at very high Ra. Theory predicts that in such a state the Nusselt number (Nu), representing the global heat transport, should scale as \( \text{Nu} \propto \text{Ra}^{\beta }\) with \( \beta = 1/2 \). Here we investigate thermal transport over eleven orders of magnitude of the Rayleigh number (\( 10^{6} \leq \text{Ra} \leq 10^{17} \)), using cryogenic helium gas as the working fluid. Our data, over the entire range of Ra, can be described to the lowest order by a single powerlaw with scaling exponent \( \beta \) close to 0.31. In particular, we find no evidence for a transition to the \( \text{Ra}^{1/2} \) regime. We also study the variation of internal temperature fluctuations with Ra, and probe velocity statistics indirectly.