Modélisation et simulation des écoulements de contre-courant de l'hélium superfluide par la méthode Boltzmann sur réseau

Abstract : The exceptional thermal properties of superfluid helium, or He-II, are exploited to the cryogenic refrigeration of high power installations, although the underlying physical mechanisms remain poorly understood. The He-II can be described macroscopically as the superposition of two fluids in interaction: a normal fluid behaves as an ordinary liquid, and a superfluid without viscosity. In the presence of a heat source, a counterflow established between these two components. The heat dissipation by this counterflow is limited by the occurrence of instabilities in misunderstood condition; the wide dispersion of experimental data does not allow to discriminate between the different theoretical models. This thesis examines using numerical simulations the role of boundary conditions and the mutual coupling between the two components of the He-II in triggering instabilities in counterflow.An innovative lattice Boltzmann type approach was developed to model the mesoscopic scale interaction between the two components of the He-II. A code reproducing counterflow in 2D and 3D conducts has been developed and validated. The results obtained indicate amplified entrance effects for superfluid component, which generate abnormally high pressure drops. The head of these entrance effects mechanism has been studied and it is shown that it can distort the detection of the transition threshold in too short pipes; This may partly explain the dispersion of experimental data.To illustrate the power of the approach in a complex geometry, the wake of an obstacle in a counterflow was simulated. The presence of recirculation areas on both sides of the obstacle, already observed experimentally, is found and explained by a new mechanism using "virtual walls".
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Jonathan Bertolaccini. Modélisation et simulation des écoulements de contre-courant de l'hélium superfluide par la méthode Boltzmann sur réseau. Physique Numérique [physics.comp-ph]. Ecole normale supérieure de lyon - ENS LYON, 2015. Français. ⟨NNT : 2015ENSL1063⟩. ⟨tel-01267233⟩

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