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

VMS- and OES-based hybrid simulations of bluff body flows

International audienceFlows past bluff bodies show turbulent near wall behavior in various conditions. For example, for a high Reynolds incident flow, the boundary layer at front side of a circular cylinder may show a transition to a turbulent boundary layer. After separation, the back of the cylinder is in contact with the turbulent wake. Analogously, the turbulent wake of a first obstacle can hit the front of a second one

Dynamique de particules à flottabilité nulle suspendues dans une turbulence isotrope : une étude expérimentale

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 simultanéeIn 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 flui

Dynamique de particules à flottabilité nulle suspendues dans une turbulence isotrope (une étude expérimentale)

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 simultanéeIn 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 fluidTOULOUSE-INP (315552154) / SudocSudocFranceF

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