Convection turbulente dans une cellule de Rayleigh-Bénard cryogénique : de nouveaux éléments en faveur du Régime Ultime de Kraichnan

Abstract : This thesis deals with turbulent Rayleigh-Bénard convection for Rayleigh numbers (Ra) up to 10^14, thanks to the use of cryogenic helium. We focused on the convection regime appearing at Ra≈10^12. This regime has been observed and interpreted 10 years ago in Grenoble, as the transition to the R. Kraichnan Ultimate Regime of convection (1962). Two signatures of this regime had been reported : an increase of the Nusselt number (Nu) and a qualitative change of the temperature fluctuations in the bulk of the cell. This last signature has been questionned by invoking a size effect of the sensors. First, we confirm the transition previously observed, with local temperature measurements, using a thermometer ten times smaller. This work is complemented with a study of the probe size effect on the temperature measurement. Thermometers, whose size ranges from 17 microns to 2 mm, were designed for this purpose. Then we indirectly observe an instability of the thermal boundary layer, which is in agreement with predictions on the Ultimate Regime. This observation is based on temperature fluctuations measurements of the bottom plate of the cell. This transition and the one observed on the heat transfer occur at the same Ra. Finally, we exclude the hypothesis that the transition observed on the heat transfer is due to a transition of the large scale circulation in the cell, thanks to the characterization of this circulation before and after the transition.
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Frédéric Gauthier. Convection turbulente dans une cellule de Rayleigh-Bénard cryogénique : de nouveaux éléments en faveur du Régime Ultime de Kraichnan. Dynamique des Fluides [physics.flu-dyn]. Université Joseph-Fourier - Grenoble I, 2008. Français. ⟨tel-00366089⟩

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