Experimental study of multiphase flows within a separated laminar boundary layer.

La cavitation hydrodynamique, et plus particulièrement la cavitation à poche attachée, peut apparaitre et se développer dans des écoulements turbulents complexes à l’intérieur de décollements de la couche limite laminaire. Ce phénomène s’avère être également sensible aux autres gaz présents dans l’écoulement comme l’air. Pour mieux comprendre l’attachement de poches de cavitation dans des décollements laminaires et l’influence de l’air sur celles-ci, nous proposons d’étudier des écoulements laminaires décollés d’huiles silicones visqueuses, contenant une grande quantité d’air, autour d’une géométrie Venturi lisse. Dans notre étude nous observons l’apparition de plusieurs types de poches, d’air ou de vapeur, qui peuvent s’attacher dans différents décollements de l’écoulement laminaire. Le dégazage joue alors un rôle important à hautes pressions, générant des poches d’air attachées présentant des dynamiques particulièrement intéressantes.À très basses pressions, des poches de cavitations peuvent s’attacher provoquantselon la stabilité de l’écoulement une transition à un régime transitionnel laminaire/turbulent dans leurs sillage. Cette même transition peut également apparaitre de façon intermittente à plus hautes pressions dans le sillage d’une bulle d’air recirculante, caractéristique du dégazage dans les écoulement laminaires décollés. Le régime transitionnel laminaire/turbulent, beaucoup moins sensible au dégazage, est caractérisé par de la cavitation de tourbillons, générés à hautes fréquences, dans le sillage d’un bulbe de décollement laminaire “court” le long de la pente du Venturi. Le bulbe se développe jusqu’à transitionner brutalement en bulbe “long” pour une taille de poche assez élevée, on peut associer ce phénomène à la supercavitation.Hydrodynamic cavitation, more specifically attached cavitation, can emerge et develop in complex turbulent flows within laminar boundary layer separations. This phenomenon might be extremely sensitive to the gaz content in the flow. For an easier understanding of the attachment of cavities into laminar separated flows within the influence of air content, we propose to focus our study on viscous silicon oil laminar separated flows, presenting high gas content, within a smooth Venturi geometry. In this study, the inception of several types of attached cavities, filled with air or oil vapor, can be observed into different laminar flow separations. For high pressures, the degassing phenomenon is dominant in the flow, generating attached cavities filled with air presenting interesting dynamics. For low pressures, attach vapor cavities can emerge inducing, if the flow is unstable, the transition to laminar/turbulent transitioning regime in their wake. This transition can also occurs intermittently at higher pressures in the wake of a recirculating air bubble, characteristic to degassing into laminar separated flows. The laminar/turbulent transitioning regime, less sensitive to degassing, is characterized by vortex cavitation, occurring at high frequencies, at the rear of a “short” laminar separation bubble along the divergent Venturi slope. The “short” laminar separation bubble grows until transitioning to a “long” bubble within an large attached cavity. This transition can be associate to thesupercavitation phenomenon

Étude expérimentale des écoulements multiphasiques dans une couche limite laminaire décollée.

Hydrodynamic cavitation, more specifically attached cavitation, can emerge et develop in complex turbulent flows within laminar boundary layer separations. This phenomenon might be extremely sensitive to the gaz content in the flow. For an easier understanding of the attachment of cavities into laminar separated flows within the influence of air content, we propose to focus our study on viscous silicon oil laminar separated flows, presenting high gas content, within a smooth Venturi geometry. In this study, the inception of several types of attached cavities, filled with air or oil vapor, can be observed into different laminar flow separations. For high pressures, the degassing phenomenon is dominant in the flow, generating attached cavities filled with air presenting interesting dynamics. For low pressures, attach vapor cavities can emerge inducing, if the flow is unstable, the transition to laminar/turbulent transitioning regime in their wake. This transition can also occurs intermittently at higher pressures in the wake of a recirculating air bubble, characteristic to degassing into laminar separated flows. The laminar/turbulent transitioning regime, less sensitive to degassing, is characterized by vortex cavitation, occurring at high frequencies, at the rear of a “short” laminar separation bubble along the divergent Venturi slope. The “short” laminar separation bubble grows until transitioning to a “long” bubble within an large attached cavity. This transition can be associate to thesupercavitation phenomenon.La cavitation hydrodynamique, et plus particulièrement la cavitation à poche attachée, peut apparaitre et se développer dans des écoulements turbulents complexes à l’intérieur de décollements de la couche limite laminaire. Ce phénomène s’avère être également sensible aux autres gaz présents dans l’écoulement comme l’air. Pour mieux comprendre l’attachement de poches de cavitation dans des décollements laminaires et l’influence de l’air sur celles-ci, nous proposons d’étudier des écoulements laminaires décollés d’huiles silicones visqueuses, contenant une grande quantité d’air, autour d’une géométrie Venturi lisse. Dans notre étude nous observons l’apparition de plusieurs types de poches, d’air ou de vapeur, qui peuvent s’attacher dans différents décollements de l’écoulement laminaire. Le dégazage joue alors un rôle important à hautes pressions, générant des poches d’air attachées présentant des dynamiques particulièrement intéressantes.À très basses pressions, des poches de cavitations peuvent s’attacher provoquantselon la stabilité de l’écoulement une transition à un régime transitionnel laminaire/turbulent dans leurs sillage. Cette même transition peut également apparaitre de façon intermittente à plus hautes pressions dans le sillage d’une bulle d’air recirculante, caractéristique du dégazage dans les écoulement laminaires décollés. Le régime transitionnel laminaire/turbulent, beaucoup moins sensible au dégazage, est caractérisé par de la cavitation de tourbillons, générés à hautes fréquences, dans le sillage d’un bulbe de décollement laminaire “court” le long de la pente du Venturi. Le bulbe se développe jusqu’à transitionner brutalement en bulbe “long” pour une taille de poche assez élevée, on peut associer ce phénomène à la supercavitation

Experimental Study of Cavitation in Laminar Flow

International audienceAn experimental setup has been especially developed in order to observe cavitation in laminar flows. Experiments have been carried out with a silicon oil of viscosity υ = 100cSt passing through a Venturi-type geometry with 18°/8° convergent/divergent angles respectively. The range of Reynolds numbers at the inlet section is between 350 and 1000. Two dynamic regimes are identified. They are characterized by two critical Reynolds numbers, induced by major hydrodynamic changes in the flow, in addition to a hysteresis effect between the inception and the disappearance of cavitation

Attached cavitation in laminar separations within a transition to unsteadiness

Attached sheet cavitation is usually observed in turbulent water flows within small laminar separation bubbles which can provide favorable conditions for inception and attachment of cavities. In the present study, viscous silicone oils are used within a small scale Venturi geometry to investigate attached cavitation into laminar separated flows for Reynolds numbers from 346 to 2188. Numerical simulations about single phase flows are performed with steady simulations for a Reynolds number range Re > [50; 1400] and with unsteady simulations for Re > [1000; 2000]. They reveal the emergence of two large laminar boundary layer separations downstream of the Venturi throat in addition to low pressure zones which can possibly induce both degassing or cavitation features. Experiments are performed with high-speed photography, and several multiphase dynamics are observed in these viscous flows, which are considered as quasisteady flows at low Reynolds numbers Re B 1400. Degassing phenomenon with air bubble recirculation has been first observed at pressures far above liquid vapor pressure whereas typical attached cavities have been identified for low pressure conditions as “band” and “tadpole” cavities into the different separations of the laminar flows. For higher Reynolds numbers, a flow regime transition can be noticed in the wake of well-developed gas structures, characterized by wake instabilities, causing vortex cavitation above a critical Reynolds number associated with the bubble width Rebc 616. This regime transition can possibly occur either quasicontinuously in the wake of an attached “band” vapor cavity or intermittently behind a recirculating air bubble generated with degassing. This last phenomenon is associated in our study to classical “patch” cavitation

Numerical study of hydrodynamic impact on bubbly water

The phenomenon of slamming on a bubbly liquid has many occurrences in marine and costal engineering. However, experimental or numerical data on the effect of the presence of gas bubbles within the liquid on the impact loads are scarce and the related physical mechanisms are poorly understood. The aim of the present paper is to study numerically the relationship between the void volume fraction and the impact loads. For that purpose, numerical simulations of the impact of a cone on bubbly water have been performed using the finite element code ABAQUS/Explicit. The present results show the diminution ofthe impact loads with the increase of the void fraction. This effect appears to be related to the high compressibility of the liquid-gas mixture.The phenomenon of slamming on a bubbly liquid has many occurrences in marine and costal engineering. However, experimental or numerical data on the effect of the presence of gas bubbles within the liquid on the impact loads are scarce and the related physical mechanisms are poorly understood. The aim of the present paper is to study numerically the relationship between the void volume fraction and the impact loads. For that purpose, numerical simulations of the impact of a cone on bubbly water have been performed using the finite element code ABAQUS/Explicit. The present results show the diminution ofthe impact loads with the increase of the void fraction. This effect appears to be related to the high compressibility of the liquid-gas mixture

Development of Attached Cavitation at Very Low Reynolds Numbers from Partial to Super-Cavitation

The present study focuses on the inception, the growth, and the potential unsteady dynamics of attached vapor cavities into laminar separation bubbles. A viscous silicon oil has been used in a Venturi geometry to explore the flow for Reynolds numbers ranging from Re=800 to Re=2000. Special care has been taken to extract the maximum amount of dissolved air. At the lowest Reynolds numbers the cavities are steady and grow regularly with decreasing ambient pressure. A transition takes place between Re=1200 and Re=1400 for which different dynamical regimes are identified: a steady regime for tiny cavities, a periodical regime of attached cavity shrinking characterized by a very small Strouhal number for cavities of intermediate sizes, the bursting of aperiodical cavitational vortices which further lower the pressure, and finally steady super-cavitating sheets observed at the lowest of pressures. The growth of the cavity with the decrease of the cavitation number also becomes steeper. This scenario is then well established and similar for Reynolds numbers between Re=1400 and Re=2000

Étude de l'impact entre un noyau cylindrique fixe et une surface d'eau montante en milieu confiné avec des mesures PIV et de jauge PVDF

International audienc

Mitigation of underwater explosion effects by bubble curtains : experiments and modelling

Mine fields and UneXploded Ordnances (UXO) become a danger regarding maritime activities. Since UXOs are strongly affected by marine corrosion after decades, they cannot be handled safely. A safe solution to get rid of them would be to explode them in their locations. However, this method generates noise pollution and damaging shock waves. Mitigation of shocks and noises is made possible by the use of a bubble curtain set around the explosive charge. Physical aspects of shock propagation in bubbly flows have been the subject of numerous investigations in the past decades and theoretical models of aerated liquids now reproduce main shock features with acceptable accuracy in the case of a uniform distribution of bubbles of the same size. However, the bubble distribution obtained by air blown in a porous pipe is far to be monodisperse. So the modeling of the interaction of a shock wave with a polydisperse medium still remains a challenge. In the present study, the transmission of a shock wave propagating through a bubble curtain is investigated experimentally on a water filled tank. A microporous pipe, connected to a compressed air supply system and a flowmeter, is placed on the floor in the tank. A dual-tip fiber optical probe is used to measure the gas fraction distribution, bubble rising velocity and bubble size distribution in the curtain. A calibrated shock wave is generated by plate impact, upstream of the bubble curtain, and recorded downstream with a hydrophone. The mitigation of the pressure peak by the bubbly medium is evidenced by recorded pressure signals with and without bubble curtain. Experimental gas fraction profiles and bubble size distributions, measured in the bubble curtains, are finally used as input parameters in the numerical model developed by Grandjean et al. (2011). This numerical model enables prediction of shock wave mitigation and allows calibrating a suitable bubble curtain

Experimental and numerical investigation of two physical mechanisms influencing the cloud cavitation shedding dynamics

The paper presents numerical and experimental investigations of the existence of two different physical mechanisms as principal origin of cloud cavitation shedding. The two mechanisms are the re-entrant jet formed at the cavity closure region and the shock wave propagation due to the condensation of vapor structures. The experimental observations of these phenomena are done at a fixed Reynolds number of about 1.2 × 10 5 by means of a high-speed camera on a transparent horizontal Venturi nozzle with 18° /8° convergent/divergent angles, respectively. A wavelet analysis is applied with several cavitation numbers in order to associate some image series to the occurrence frequencies of the two shedding mechanisms. In complement, a numerical model is performed in order to access to a 3D representation of the different phenomena. The compressible Navier-Stokes equations coupled with the Homogeneous Equilibrium Mixture Model are solved with a Finite Volume solver based on Moving Least Squares approximations. A snapshot Proper Orthogonal Decomposition technique is applied on both numerical and experimental results. The energy levels of different modes from numerical and experimental data are found to be in a good agreement. Instantaneous pressure peaks of the order of 10 bar, associated with erosive condensation shock wave, are numerically identified. The 3D numerical simulations reveal also that side-entrant jet flow is partially responsible for the re-entrant jet influence on the cloud cavitation shedding