The dynamics of neutrally buoyant particles in isotropic turbulence : an experimental study

Le but de cette étude expérimentale est de caractériser la dynamique de particules solides, à flottabilité nulle, incluse dans un écoulement turbulent isotrope en décroissance libre. Les particules utilisées sont de forme sphérique et ont un diamètre de 4 à 5 fois plus grand que l'échelle spatial de Kolmogorov de l'écoulement. De part leur taille, les particules ont également un nombre de Stokes proche de l'unité. On s'attend alors à ce que ces particules aient une dynamique différente de celle du fluide environnant. Dans cette étude, ont se propose de quantifier les différences de vitesses entre les deux phases à l'aide d'une technique de vélocimétrie simultanée. ABSTRACT : In this experimental study, the focus is made on the characterization of the dynamics of solid neutrally buoyant particles embedded in a freely decaying, nearly isotropic turbulence, with a weak mean flow. The particles are spherical with diameters several times larger than the Kolmogorov scale. The study of this flow configuration is still challenging both theoretically and numerically. Due to large particle sizes, the local flow around particles can not be considered as uniform and due to fluid-particle density ratio of around unity, the history and Basset forces cannot be neglected in comparison with the viscous drag force. Particle equation of motion is then fully non-linear, in contrast to the equation for heavy particles with diameters smaller then the Kolmogorov scale, for which only the Stokes drag is considered. In several experimental and numerical studies, the effect of particle size on velocity and acceleration statistics has been investigated (Homann and Bec 2010 ; Qureshi et al. 2008 ; Ouellette et al. 2008 ; Xu and Bodenschatz 2008). In the case of isotropic turbulence, Homann and Bec (2010) show that while the PDF of the particle velocity normalized by the square root of its variance does not vary with particle size, the variance itself is size dependent. A scaling relation for particle velocity variance has been proposed by using the Faxen correction (Gatignol 1983) which takes into account the non uniformity of the fluid flow at the scale of the particle. The aim of our research is to further study the dependence of particle dynamics on particle size. To that purpose, a turbulence generator has been set-up and the resulting turbulence is characterized. Then the flow was seeded with millimeter sized, neutrally-buoyant particles and the velocity of the two phases have been measured simultaneously. Simultaneous measurements of particle and surrounding fluid velocities show that although the global velocity statistics of the two phases have comparable values, the particles may have different local velocity from the velocity of the neighboring fluid

Coherent and turbulent processes in the bistable regime around a tandem of cylinders including reattached flow dynamics by means of high-speed PIV

The turbulent flow around two cylinders in tandem at the sub-critical Reynolds number range of order of 105 and pitch to diameter ratio of 3.7 is investigated by using time-resolved Particle Image Velocimetry (TRPIV) of 1 kHz and 8 kHz. The bi-stable flow regimes including a flow pattern I with a strong vortex shedding past the upstream and the downstream cylinder, as well as a flow pattern II corresponding to a weak alternating vortex shedding with reattachment past the upstream cylinder are investigated. The structure of this “reattachment regime” has been analyzed in association with the vortex dynamics past the downstream cylinder, by means of POD and phase-average decomposition. These elements allowed interconnection among all the measured PIV planes and hence analysis of the reattachment structure and the flow dynamics past both cylinders. The results highlight fundamental differences of the flow structure and dynamics around each cylinder and provide the ‘gap’ flow nature between the cylinders. Thanks to a high-speed camera of 8 kHz, the shear-layer vortices tracking has been possible downstream of the separation point and the quantification of their shedding frequency at the present high Reynolds number range has been achieved. This issue is important regarding fluid instabilities involved in the fluid–structure interaction of cylinder arrays in nuclear reactor systems, as well as acoustic noise generated from the tandem cylinders of a landing gear in aeronautics

Simultaneous PIV/PTV velocimetry technique in a turbulent particle-laden flow

In a three-dimensional two-phase flow, accessing the velocity fields of the two phases simultaneously is challenging. Nevertheless, information about the local relative motion between the two phases is particularly valuable to quantify the inter-phase interactions. In this article, the dynamics of neutrally buoyant finite-sized particles embedded in a three-dimensional turbulence is investigated using a simultaneous particle image velocimetry (PIV) and particle tracking velocimetry (PTV) measurement technique based on optical discrimination of the two phases prior to image acquisition. The implementation of this dual whole-field velocimetry technique is presented and detailed. With this technique, we were able to measure the instantaneous and local velocity differences between the particles and the underlying fluid. Our results show that, whereas the single-point velocity statistics of the two phases are very similar, the particles often have different local velocity than the velocity of the neighboring fluid. The relative velocity increases with the relative size of the particle to the Kolmogorov scale. In addition, the relative velocity exhibits an intermittent distribution

Etude numérique et expérimentale d’impacts hydrodynamiques sur eau aérée.

National audienc

Effet de l’aération lors d’impacts hydrodynamiques : essais et simulations

National audienc

On the influence of aeration on water entry problems: experiments and simulations

Dans cette étude, nous présentons les résultats d'une campagne d'essais d'impacts hydrodynamiques réalisés grâce à la machine de choc de l'ENSTA Bretagne, en intégrant un dispositif permettant de générer des bulles de manière contrôlée dans le bassin. Préalablement à ces essais, les taux de vide générés par l'aérateur ont été caractérisés en fonction du débit d'air injecté à l'aide d'une sonde optique. Au cours de ces essais, le débit d'air injecté a été varié pour obtenir des taux de vide moyens α0 compris entre 0,35 et 1,1 %. Les résultats mettent en évidence une réduction sensible de la force d'impact avec l'augmentation du taux de vide.In this study, we presents the results of hydrodynamic impact tests performed with the shock machine at ENSTA Brittany, by integrating a device allowing to generate bubbles in the tank. During these tests, the average void fractions considered are between 0.35 and 1.1%. Our results highlight a significant reduction of the impact force with the increase of the void fraction

Effet de l'aération lors d'impacts hydrodynamiques : essais et simulations

International audienceIn this study, we presents the results of hydrodynamic impact tests performed with the shock machine at ENSTA Brittany, by integrating a device allowing to generate bubbles in the tank. During these tests, the average void fractions considered are between 0.35 and 1.1%. Our results highlight a significant reduction of the impact force with the increase of the void fraction.Dans cette étude, nous présentons les résultats d'une campagne d'essais d'impacts hydrodynamiques réalisés grâce à la machine de choc de l'ENSTA Bretagne, en intégrant un dispositif permettant de générer des bulles de manière contrôlée dans le bassin. Préalablement à ces essais, les taux de vide générés par l'aérateur ont été caractérisés en fonction du débit d'air injecté à l'aide d'une sonde optique. Au cours de ces essais, le débit d'air injecté a été varié pour obtenir des taux de vide moyens α0 compris entre 0,35 et 1,1 %. Les résultats mettent en évidence une réduction sensible de la force d'impact avec l'augmentation du taux de vide

The influence of aeration and compressibility on slamming loads during cone water entry

The problem of the impact between a rigid body and a gas-liquid mixture is relevant to various engineering applications, including the design of breakwaters and LNG containers. In the present study, the particular problem of the impact of a rigid cone upon the surface of an aerated liquid is investigated. Numerical simulations of water entry of cones with different deadrise angles (7° and 15°) were performed using an explicit finite element method. The air-water mixture is modelled as a homogeneous fluid with a specific equation of state. In addition, experimental tests of the impact of a cone with a deadrise angle of 7° on the surface of bubbly water were performed. The air volume fraction was measured prior to the impact tests using optical probes, and the instantaneous impact force on the cone was measured using strain gauges. The results highlight a significant reduction of the impact load with the increase of the air volume fraction. Moreover, the numerical results show that this reduction is also dependent on the impact velocity. This phenomenon is found to be related to the nonlinearity of the equation of state of the air-water mixture

Measurements of pressure during the forced water entry of a cone into pure and aerated water

International audienc

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