L'étude du comportement d'un fluide proche de son point critique et soumis à des vibrations ou une variation rapide de gravité/acceleration est un sujet extrêmement intéressant. Les phénomènes physiques impliqués sont d'un grand intérêt non seulement pour la physique fondamentale mais aussi pour l'industrie spatiale. Dans cette thèse, trois problèmes sont principalement trait&s: (i) Etude de l'interaction de vibrations harmoniques avec une couche limite thermique dans un fluide supercritique en absence de gravité, (ii) Etude de l'interaction de vibrations avec une interface liquide/vapeur d'un fluide sous−critique sous plusieurs niveaux de gravité (les instabilités de Faraday et d'onde gelée, l'équilibre dynamique d'une interface) et (iii) Etude du phénomène de geyser à l'intérieur d'un réservoir partiellement rempli d'oxygène lorsqu'il est soumis à une variation rapide de la gravité (ou accélération). La thèse comporte une partie expérimentale et une partie numérique. Des expériences ont été réalisées sur les installations HYLDE et OLGA du CEA Grenoble utilisant respectivement les fluides H2 et O2 dans la zone sous−critique. Des simulations numériques sont réalisées pour étudier la stabilité d'une couche limite thermique et la dynamique d'une interface fluide soumise à une variation rapide de la gravité en utilisant des codes numériques basées sur le méthode volumes finis utilisant les algorithmes SIMPLER et VOF−PLIC respectivement. Plusieurs résultats intéressants ont été obtenus. Différents phénomènes ont été étudiés et quantifiés, comme l'instabilité de Faraday et l'instabilité d'onde gelée dans le domaine sous−critique et l'instabilité parametrique et l'instabilité Rayleigh−vibrationnelle dans le domaine supercritique. Les expériences ont permis de bien comprendre les raisons de la transition de l'instabilité de Faraday vers une structuration en bandes verticales très près du point critique. Les expériences et les simulations numériques sur le phénomène de geyser ont aidé à développer des corrélations empiriques pour les vitesses de la bulle et du geyser en prenant en compte les effets des parois. / The behavior of a near-critical fluid subjected to vibration or a rapid variation of acceleration is an extremely interesting topic of research. The resulting physical phenomena are of great interest in view of the fundamental physics involved and have great relevance to the space industry. The thesis addresses mainly three problems: (i) study of the interaction of harmonic vibration with a thermal boundary layer of a supercritical fluid under the absence of gravity, (ii) study of the inter- action of vibration with the liquid−vapor interface of a near−critical fluid under various gravity levels (Faraday and frozen wave instabili- ties, dynamic equilibrium of the interface) (iii) study of the geysering phenomenon inside a reservoir partially filled with a liquid when it is subjected to a rapid variation of gravity. Experiments are conducted onboard the zero−g installations HYLDE and OLGA developed by CEA Grenoble using H2 and O2 as the work- ing fluids. Numerical simulations are carried out using finite volume codes based on SIMPLER (for the problem involving the supercriti- cal fluid) and VOF−PLIC (for the interface dynamics problem under rapid variation of gravity). New and interesting results have been obtained. Various phenom- ena like the Faraday instability and the frozen wave instability in the sub−critical region and the parametric instability and the Rayleigh−vibrational instability in the supercritical region have been quantified. The experiments have successfully explained the reason behind the transition of the Faraday instability into vertical band pattern very close to the critical point. Experiments and numerical simulation of the geysering phenomenon have helped to evolve empir- ical correlations for the bubble rise and geyser edge velocities taking into account the effect of walls on these velocities.

The accuracy of the numerical simulation in the prediction of cavitation in cryogenic fluids is of critical importance for the efficient design and performance of turbopumps in rocket propulsion systems. One of the main challenges remains the efficiency in modeling the physics, handling the multiscale properties and developing robust numerical methodologies. Such flows involve thermodynamic phase transition and cavitation bubbles smaller than the global flow structure. Cryogenic fluids are thermo-sensible, then thermal effects and strong variations in fluid properties can alter the cavity properties. The aim of this work is to address the challenge of efficiently modeling cavitating flows when using water and cryogenic fluids. Because of the complexity of the phenomenon, we focus on improving accuracy of the numerical simulation and on proposing some approaches for a strong coupling between numerics and experiments. We first discuss how to simulate cavitation by means of a mixture model. We specifically address two challenges. The first one is associated with the prediction of thermal effect during the phase transition, requiring the solution of the energy conservation equation. The second challenge is associated to the prediction of the number of bubbles, by considering a transport equations for the bubble density. This study is applied to the numerical simulation of a cavitating flow in a Venturi configuration. We observe an improved estimation of temperature and pressure profiles by using the energy equation and the nucleation model. Secondly, we focus on bubble dynamics. Several forms of Rayleigh-Plesset (RP) equations are solved in order to estimate the temperature and pressure during the collapse of the bubble. We observe that, for high Mach number flows, RP modified with a compressible term can predict the bubble behavior more accurately than the classical form of RP. It is necessary to use a complex equation of state for non-condensable gas (van der Waals) in order to have an accurate estimation of the bubble temperature during the collapse phase. We first apply this approach to the water treatment with cavitation, by proposing a model for the estimation of radicals developed during the collapse of the bubble. Secondly, this equation is modified by adding a term of convective heat transfer at the interface between liquid and bubble and it is coupled with a bubbly flow model in order to assess the prediction of thermal effect. We perform a parametric study by considering several values and models for the convective heat transfer coefficient, hb, and we compare temperature and pressure profiles with respect to the experimental data. We observe the importance of the choice of hb for correctly predicting the temperature drop in the cavitating region and we assess the most efficient models. In addition, we perform an experimental study on nitrogen cavitating flows in order to validate numerical prediction of thermal effect, and in order to assess the fundamental characteristics of the nucleation and the transient growth process of the bubble.