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International audience; Un modèle simple basé sur la résolution de l'équation de Rayleigh décrivant l'évolution d'une bulle sphérique isolée est développé pour analyser les effets thermiques en cavitation. Deux approches sont utilisées pour modéliser le transfert de chaleur à travers l'interface liquide-vapeur. La première est basée sur un modèle de type convectif, la seconde sur la résolution de l'équation de diffusion de la chaleur dans le liquide entourant la bulle. Les deux modèles sont appliqués à un inducteur cavitant; les formes de cavités ainsi que les distributions de température sont comparées. Les évolutions de la longueur de cavité en fonction du nombre de cavitation pour l'eau froide (sans effet thermique) et pour le fluide réfrigérant R114 à deux températures différentes sont confrontées aux résultats expérimentaux.

International audience; This paper is devoted to cavitation erosion modeling. It presents some recent numerical developments made in the code initially developed in collaboration with E.Johnsen and collaborators at University of Michigan [1] in order to account for fluid-structure interaction. The considered test case is that of a single air bubble collapsing near a wall due to an incident shock wave in the surrounding water. In our investigation, we focus on the code implementation and optimization and the bubble implosion mechanism. The paper is focused on the various events occurring during bubble collapse and the computation of the time evolution of the pressure distribution. The influence of the amplitude of the incident wave and the distance of the bubble to the wall are investigated.

International audience; Numerical prediction of mass loss due to cavitation erosion requires the knowledge of the hydrodynamic impact loads generated by cavitation bubble collapses. Experimental measurements of such impact loads using conventional pressure sensors are not reliable (if not impossible) due to the micron size and the very small duration of the loading. In this paper, a new method to estimate these loading conditions is proposed based on cavitation pitting tests and an iterative inverse finite element modeling. The principle of the method is as follows. First, numerous pits corresponding to localized plastically deformed regions are identified from a cavitation test performed in a dedicated tunnel. Then each pit is numerically reproduced by finite element simulations of the material response to a representative Gaussian pressure field supposed to mimic a single bubble collapse. This gives the size and pressure distribution of the bubble impacts. The prime objective of this study is to find out if the target material itself could be used as a pressure sensor or not, i.e., if the cavitation pits left on the surface of the tested specimen could provide the characteristics of the cavitating flow in terms of pressure fields independently of the target material. Pitting tests were done on three materials, namely, 7075 Aluminum alloy (Al-7075), 2205 duplex stainless steel (A-2205), and Nickel-Aluminum Bronze (NAB) at three different flow conditions and the impact loads have been estimated for each identified pit. Very interestingly, a statistical analysis shows that the estimated impact loads are material independent at all flow conditions, provided the material properties are characterized properly. It is also shown that for some materials, the constitutive parameters obtained from compression tests are not satisfactory.

International audience; A simple model for two-dimensional cavity flows is presented. It is based upon the Logvinovich independence principle. Each section of the cavity is assumed to behave independently of the neighbouring ones. The equation of evolution of the cavity interface is derived. It mainly takes into account an added mass effect and is similar to the well-known Rayleigh–Plesset equation relative to spherical bubbles. The dynamics of the 2D cavity is controlled by the pressure difference between infinity and the cavity. The model proves to be in good agreement with Tulin's solution for a steady cavity flow and easily applicable to unsteady cavity flows.; Une modélisation simple des écoulements cavitants bidimensionnels est proposée. Elle est basée sur le principe d'indépendance de Logvinovich qui suppose que chaque section de cavité se comporte indépendamment des voisines. L'équation d'évolution de l'interface est présentée dans cette Note. Elle prend essentiellement en compte un effet de masse ajoutée et est comparable à l'équation de Rayleigh-Plesset qui régit l'évolution d'une bulle sphérique. La dynamique d'une cavité bidimensionnelle est contrôlée par la différence entre la pression de cavité et la pression à l'infini. Le modèle est en bon accord avec la solution de Tulin pour un écoulement supercavitant stationnaire et est facilement applicable à une configuration instationnaire.

La presente etude concerne une approche de modelisation numerique des ecoulements partiellement cavitants bidimensionnels en milieu confine. Une etude synthetique des modeles deja existants montre la necessite de faire intervenir, de facon aussi deterministe que possible, les effets visqueux dus a la presence d'un sillage diphasique et turbulent developpe a l'aval des poches. Ces effets sont introduits dans deux modeles de calcul potentiel (methode des elements finis et methode integrale) par la prise en compte de la couche limite generee a l'arriere des poches. Un modele numerique dit a cavite ouverte a ete ainsi elabore et teste pour les deux methodes sur deux types de profil. Le choix s'est porte sur un profil classique NACA et sur un profil obtenu par une methode inverse a partir d'une section de pale d'helice. Les cas de figures presentes correspondent a des cas experimentaux etudies dans le tunnel hydrodynamique du Bassin d'Essais des Carenes du Val de Reuil. Les methodes sont comparees entre elles et quelques resultats numeriques sont confrontes aux resultats experimentaux.

International audience; Numerical calculations in the single liquid phase and optical observations in the two-phase cavitating flow regime have been performed on microdiaphragms and microventuris fed with deionized water. Simulations have confirmed the influence of the shape of the shrinkage upon the contraction of the jet, and so on the localisation of possible cavitating area downstream. Observations of cavitating flow patterns through hybrid silicon-pyrex microdevices have been performed either via a laser excitation with a pulse duration of 6 ns, or with the help of a high-speed camera. Recorded snapshots and movies are presented. Concerning microdiaphragms, it is confirmed that very high shear rates downstream the diaphragms are the cause of bubbly flows. Concerning microventuris, a gaseous cavity forms on a boundary downstream the throat. As a consequence of a microsystem instability, the cavity displays a high frequency pulsation. Low values Strouhal numbers are associated to such a sheet cavitation. Moreover, when the intensity of the cavitating flow is reduced, there is a mismatch between the frequency of the pulsation of the cavity and the frequency of shedded clouds downstream the channel. That may be the consequence of viscous effects limiting the impingement of a re-entrant liquid jet on the attached cavity.

International audience; An experimental study of hydrodynamic cavitation downstream microdiaphragms and microventuris is presented. Deionized water and nanofluids have been characterized within silicon-Pyrex micromachined devices with hydraulic diameters ranging from 51 μm to 104 μm. The input pressure could reach up to 10 bars, and the flow rate was below 1 liter per hour. The output pressure of the devices was fixed at values ranging from 0.3 bar to 2 bars, so that it was possible to study the evolution of the cavitation number as a function of the Reynolds number in the orifice of the diaphragms or in the throat of the venturis. A delay on the onset of cavitation has been recorded for all the devices when they are fed with deionized water, because of the metastability of the liquid and because of the lack of roughness of the walls. For the first time, hydrodynamic cavitation of nanofluids (nanoparticles dispersed into the liquid) has been considered. The presence of nano-aggregates in the liquid does not exhibit any noticeable effect on the cavitation threshold through the venturis. However, such a presence has a strong influence on the cavitation onset in microdiaphragms: above a critical volume solid concentration of ≈10−5, the metastability is broken and the nanofluids behave as tap water filled up with large nuclei. These microdevices, where a low amount of fluid is required to reach cavitating flows, appear to be useful tools in order to study cavitating phenomena in localized area with specific fluids.

International audience; This work aims at evaluating the capacity and limitations of conventional Reynolds-averaged Navier-Stokes (RANS) techniques to numerically simulate supercavity flows. The configuration is that of a two-dimensional (2D) symmetrical supercavitating wedge investigated experimentally by Michel (1974). Mesh effect is studied in detail under noncavitating conditions. The computational grid is refined in the region where cavitation develops in order to accurately track the supercavity. The effect of tunnel height is also analyzed, and the height finally chosen was large enough to simulate an infinite flow-field. The cavitating flow is treated as a homogeneous mixture of variable density. To account for vaporization and condensation, an additional continuity equation for the vapor (or the liquid) is solved with an appropriate source term expressing mass transfer between the two phases. The effect of nuclei concentration on the vaporization rate and then on the development of the supercavity is investigated. Results obtained using two different RANS codes are compared. They are also compared with experimental data and with inviscid solutions including a nonlinear boundary element method. They concern in particular cavity length and shape, pressure distribution, and drag coefficient. Under unsteady conditions, a special attention is paid to the characteristic growth time of the supercavity following a sudden pressure drop, from noncavitating conditions.

A simple model based on the resolution of Rayleigh equation is used to analyze thermal effects in cavitation. Two different assumptions are considered for the modeling of heat transfer toward the liquid∕vapor interface. One is based upon a convective type approach using a convection heat transfer coefficient or the equivalent Nusselt number. The other one is based upon the resolution of the heat diffusion equation in the liquid surrounding the bubble. This conductive-type approach requires one to specify the eddy thermal diffusivity or the equivalent Peclet number. Both models are applied to a cavitating inducer. The basic pressure distribution on the blades is determined from a potential flow computation in a two-dimensional cascade of flat plates. The sheet cavity, which develops from the leading edge, is approximated by the envelope of a hemispherical bubble traveling on the suction side of the blade. Cavity shape and temperature distribution predicted by both models are compared. The evolutions of cavity length with the cavitation number for cold water (without thermal effects) and for Refrigerant 114 at two different temperatures is compared to experimental data. Such a simple model is easy to apply and appears to be quite pertinent for the analysis of thermal effects in a cavitating inducer.

International audience; The aim of this work is to provide a detailed experimental and numerical analysis of the flow in a fixed bucket of a Pelton turbine. The head, jet incidence, and flow rate have been varied to cover a wide range of the turbine functioning points. The experimental analysis provides measurements of pressure and torque as well as flow visualization. The numerical analysis is performed with the FLUENT code using the two-phase flow volume of fluid method. The results present a good consistency with experimental data. In particular, the pressure distribution is very well predicted for the whole range of the studied parameters. A detailed analysis of torque and thrust allows evaluating the losses due to the edge and the cutout of the bucket. These results give insight into the benefit we can expect of steady flow calculations through the optimization process of the design of Pelton turbines.

L'ecoulement stationaire tridimensionnel autour d'un hydrofoil de forme plan elliptique est modelise numeriquement a l'aide d'une methode integrale aux frontieres. L'effet d'une paroi de confinement sur la portance et la distribution de circulation en envergure est plus particulierement etudie. Le but de cette etude est in fine de caracteriser le fonctionnement des helices carenees, et notamment les conditions d'apparition de la cavitation. L'etude experimentale du probleme a ete menee en parallele dans le tunnel hydrodynamique du LEGI, ce qui a permis d'effectuer de solides comparaisons entre resultats numeriques et mesures experimentales.

Pb, Zn, Cd and Cu have been measured using ultraclean procedures in various sections of a 70.3-m snow/ice core covering the past 220 years (including the Industrial Revolution) drilled at Summit, central Greenland. These time series are the first reliable ones ever published for Zn, Cd, and Cu; for Pb they are the first verification of the pioneering data published more than two decades ago by C. Patterson and his coworkers [Murozumi et al., 1969]. For all four heavy metals, concentrations are found to have markedly increased up until the 1960s and 1970s before decreasing significantly during the following few decades. The timing and the amplitude of the observed changes differ significantly however from one metal to another. Comparison with concentration values obtained by analyzing ancient Holocene ice dated 7760 years B.P., that is, before humans started to impact on the atmosphere, show that no detectable increase occurred for Zn, Cd, and Cu before the Industrial Revolution. On the other hand, Pb concentrations were already one order of magnitude above natural values in late 18th century ice. Cumulative deposition of heavy metals to the whole Greenland ice cap since the Industrial Revolution ranges from 3200 t for Pb to 60 t for Cd.

Technical progress based on the use of supercavitating wings with wetted upper surface is expected. An essential difficulty for the correct use of this type of wing is the determination of the free field of carvitation. This field requires knowledge of the pressure coefficients in the region of the leading edge of the wing ; this can be obtained from experiments or calculation. With regard to calculation, the fact that the cavity is of unknown shape, while determination of the pressure coefficient at the leading edge requires a non-linear procedure, creates a considerable problem. The calculation method used is that of discretized particularities. If we assume initially that the geometry of the cavity is known, we can solve the problem by distributing a single layer on the wing, a double layer on the wing, the cavity and the wake, and distributing linear pits behind the cavity. Solution of the problem leads to non-uniform pressure in the cavity because of the arbitrary nature of the geometry chosen to start off the calculation. When the cavity pressure is made uniform by an iterative procedure, the length of the cavity can be adjusted in terms of the span of the wing. We then start again with this shape of cavity, and the iterations are continued to detail the shape in the three space dimensions, in order that the slip condition may be satisfied everywhere on it. Experimentally, investigations on wings of limited span require a large installation in order to minimize the influence of the side walls, the effects of the boundary layer, and the effects of the housing, as well as the errors arising from the geometrical definition of the wing itself. The series of tests reported on here were carried out on a wing of fixed profile whose sweep-back was varied by rotation around an axis at the root, and whose aspect ratio was varied by successive cutting back. The tests were carried out in the TH2-TH8 hydrodynamic tunnel of the NEYRTEC Company, whose smallest water-vein permits studies on three-dimensional flows under acceptable conditions. The theoretical and experimental results were subjected to critical analysis of the influence of the principal geometrical parameters and the area of non-cavitation. In particular, when then aspect ratio of a ventilated-base wing is reduced, a study of the slope of the curve, the coefficient of lift in terms of the angle of attack, and of the spread of non-cavitation reveals a significant increase in the spread of non-cavitation. This points to the advantage of using wings of very low aspect ratio in order to increase the possible incidence deflections of the wing. Tith regard to the localization of the points where cavitation appears, for swept-back wings cavitation on the upper surface appears at about two thirds of the span ; on the under surface, cavitation appears first at the wing root. This result, confirmed by experience, shows that there may be an advantage in using a twisted wing, with the a smaller angle for the end sections of the wing than for those at the root.

The general context of the present thesis is renewable energies within the HARVEST program initialized at LEGI laboratory, which consists in developing a Darrieus-type water current turbine (WCT). The WTC can be equipped with a channelling device which allows transforming a bigger amount of the kinetic energy contained in the flowstream into electricity. The present work is focused on the channelling devices. Studies concern three main topics: the explanation of the channelling device hydrodynamic functioning, the evaluation of the performance of the shrouded WCT and the revealing of the system geometrical parameters which allow its improvement or optimisation. All studies have been carried out by 2D RANS calculations and available experimental data, and have been compared to bare WTC results., Cette thèse s'inscrit dans le cadre des énergies renouvelables au sein du programme HARVEST du laboratoire LEGI, qui consiste à développer un concept d'hydrolienne de type Darrieus. L'hydrolienne peut être équipée d'un dispositif appelé carénage afin de transformer une portion plus grande de l'énergie cinétique contenue dans le courant d'eau en électricité. Les travaux présentés se sont focalisés sur ces systèmes de carénage, autour de trois axes : l'explication du principe de fonctionnement hydrodynamique du carénage, la quantification des performances de l'hydrolienne carénée et la mise en évidence des grandeurs géométriques clés du carénage permettant d'améliorer ou d'optimiser la performance du système. Toutes les études ont été réalisées à l'aide des calculs RANS 2D et des données expérimentales mises à disposition et, comparées aux résultats obtenus pour une hydrolienne non-carénée.

The general context of the present thesis is renewable energies within the HARVEST project, which consists in a water current turbine (WCT) development, inspired from the Darrieus and Gorlov geometries. The main advantage of the HARVEST WCT is the introduction of a channelling device, which allows extracting a bigger amount of the kinetic energy contained in the flowstream. However, the shrouding device can eventually increase cavitating risks, which generally damage the WCT itself and its performance. The main topic of this work is cavitation. The hydrodynamic behavior of the WCT is analyzed both numerically and experimentally, in non cavitating and cavitating conditions. For this analysis, two devices have been developed. On the one hand, the LEGI hydrodynamic channel is equipped with a measurement platform which provides the instantaneous and average measurements of two dimensional thrusts as well as the hydrodynamic torque. On the other hand, in the numerical domain, the work has been oriented to the improvement and the development of a CFD code, named Cavka. The intensive utilisation of these two devices, coupled to theoretical models, allow highlighting the functioning of the bare and shrouded WCTs and their limits in cavitating conditions., Cette thèse s'inscrit dans le cadre des énergies renouvelables au sein du programme HARVEST centré sur le développement d'un concept d'hydrolienne dérivé des turbines Darrieus et Gorlov. L'ajout d'un dispositif appelé carénage à la turbine permet à celle-ci d'extraire une portion de l'énergie cinétique du courant plus grande. Toutefois ce dernier peut favoriser la cavitation qui nuit à la turbine. Parmi les différents axes du programme, les travaux de thèse se situent dans cette problématique. En régime subcavitant et cavitant, l'analyse de l'hydrolienne a été menée suivant une approche numérique et expérimentale. Pour ce faire deux outils ont été mis en place. Du coté expérimental, le tunnel hydrodynamique du LEGI a été équipé d'une balance qui donne la mesure instantanée des forces et du couple qui s'exercent sur la turbine. Du coté numérique, les efforts ont été orientés sur l'amélioration et le développement du code de calcul universitaire, CAVKA. L'utilisation intensive de ces deux moyens, couplée à des modèles théoriques, a permis de mettre en évidence d'une part le fonctionnement de la turbine libre ou carénée et, d'autre part, les limites de fonctionnement vis-à-vis de la cavitation.