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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; 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.

This paper presents an original panel method to treat partially cavitating two-and three-dimensional hydrofoil problems. The method uses a velocity-based boundary element method (VBEM) but its originality lies in the deformation process. Indeed, the cavitation number is imposed, and the cavity shape is calculated in the course of iterations (free cavity length method). In fact, considering the boundary layers, a normal velocity is observed on the cavity surface which supplies the new stream-surface shape. For any imposed closure condition, the cavity shape (thickness and length) is adapted in such a way that it closes on the hydrofoil. The use of parabolic dipoles, constant source (panel method minimization process) allows a good description of the pressure field. The two-and three-dimensional results obtained are compared with experimental measurements and also with the results of earlier former methods.

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.

19 pages; International audience; The numerical modeling of partially cavitating foils under a confined flow configuration is described. A complete study of previous numerical models highlights that the presence of a turbulent and two-phase wake, at the rear of the cavity, has a nonnegligible effect on the local pressure coefficient, the cavitation number, the cavity length and the lift coefficient; hence viscous effects must be included. Two potential methods are used, each being coupled with a calculation of the boundary layer developed downstream of the cavity. So, an open cavity numerical model, as it is called, was developed and tested with two types of foil: a NACA classic foil and a foil of which the profile is obtained performing an inverse calculation on a propeller blade test section. On the other hand, under noncavitating conditions, for each method, the results are compared with the results obtained by the Navier-Stokes solver FLUENT. The cavitating flow configurations presented herein were carried out using the small hydrodynamic tunnel at Bassin d'Essais des Carènes [Val de Reuil, France]. The results obtained by the two methods are compared with experimental measurements.

Partially cavitating flow around a hydrofoil in a confined two-dimensional flow is presented. The calculation method, based on the singularities technique combined with a minimisation method, is adapted to open configurations. With this extension, cavity wakes not necessarily merging with the upper-side of the foil can be treated. In the case of subcavitating flow, a boundary layer calculated is made, indicating a separation point downstream of which the flow becomes separated. In this area, an imaginary streamline (wake) is introduced to simulate the effect of separation. The choice of different forms of wake clearly shows the influence of wake form on the value of results. The process is extended to the case of cavitating flow for wakes developing behind the cavity. The method is applied to a test cavitating hydrofoil placed in a tunnel. Several cavity wakes progressively diverging from the foil were tested. The results obtained, compared with experimental results, show the great importance of achieving more accurate modelling of flow conditions behind cavities.

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.