Contribution à l'étude de la propagation des ultrasons en milieu cavitant

Abstract : The main limitation of ultrasonic processes is the lack of knowledge spatial distribution of the acoustic energy in the insonified volume, because the presence of cavitation bubbles puts the problem out of the validity range of linear acoustics. The nonlinear behaviour of bubbles radial oscillations is the first difficulty in modelling the propagation of power ultrasounds. First, by using a bubbly liquid model, the nonlinearity of the acoustic wave is studied numerically in a monodimensional geometry, with a finite-elements code developed for this purpose. The bubble population is assumed to be monodisperse and spatially uniform, and the code if used for different ratios between the acoustic frequency and the resonance frequency of the bubbles. The results show different couplings between the harmonic content of the various quantities and the length of the cavity, but most of all, that the spatial damping of the wave increases appreciably with the magnitude of the acoustic source. However, the bubble population is an unknown of the problem, and the assumption of monodispersity and spatial uniformity is relaxed, setting aside in other respects the nonlinear aspects. The growth, migration, fragmentation and coalescence phenomena are quantified and combined in a conservation equation of the bubble size distribution function, which describes the self-organisation of the bubble population under the influence of the acoustic wave. The system is closed by the linearised propagation equation in a bubbly liquid. The set of equations obtained constitutes a difficult problem, but its resolution may be conceivable for a steady state. For lack of a solution, a qualitative comparison of the model with the screen effect is proposed, and shows that a dissolved gas transport equation should be added to the model,and that the quantification of the various phenomena should be reconsidered within a nonlinear framework.
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Olivier Louisnard. Contribution à l'étude de la propagation des ultrasons en milieu cavitant. Génie des procédés. Ecole des mines de Paris, 1998. Français. ⟨NNT : 1998ENMP0817⟩. ⟨tel-01857809⟩

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