Experimental study of bubble-drag interaction in a Taylor-Couette flow

This study is an experimental investigation of the interactions between the bubbles, the coherent motion and the viscous drag in a Taylor Couette flow, for the outer cylinder at rest. The cylinder radius ratio is 0.9. Bubbles are injected through a needle at the bottom of the apparatus inside the gap. Different bubble sizes are investigated (ratio between the bubble size and the gap width 0.05 and 0.12) for very small void fraction (≤0.012). Different flow regimes are studied corresponding to Reynolds number Re based on the gap width and the velocity of the inner cylinder ranging from 400 to 20000. For these Re values, Taylor vortices are persistent leading to an axial periodicity of the flow. PIV measurements of the liquid flow features, bubble tracking in a meridian plane and viscous torque of the inner cylinder measurements are performed. This study provides a first evidence of the link between the bubble localisation, the Taylor vortices and viscous torque modifications. Bubbles are attracted towards the inner cylinder, due to the rotation of the cylinder. For small buoyancy effect, bubbles are trapped and induce a decrease in the outflow intensity, thus leading to an increase of the viscous torque. When buoyancy induced bubble motion, by comparison to the coherent motion of the liquid is increased, a decrease in the viscous torque is suspected

Experimental study of bubble-drag interaction in a Taylor-Couette flow

This study is an experimental investigation of the interactions between the bubbles, the coherent motion and the viscous drag in a Taylor Couette flow, for the outer cylinder at rest. The cylinder radius ratio η is 0.9. Bubbles are injected through a needle at the bottom of the apparatus inside the gap. Different bubble sizes are investigated (ratio between the bubble size and the gap width 0.05 and 0.12) for very small void fraction(α≤0.012). Different flow regimes are studied corresponding to Reynolds number Re based on the gap width and the velocity of the inner cylinder ranging from 400 to 20000. For these Re values, Taylor vortices are persistent leading to an axial periodicity of the flow. PIV measurements of the liquid flow features, bubble tracking in a meridian plane and viscous torque of the inner cylinder measurements are performed. This study provides a first evidence of the link between the bubble localisation, the Taylor vortices and viscous torque modifications. Bubbles are attracted towards the inner cylinder, due to the rotation of the cylinder. For small buoyancy effect, bubbles are trapped and induce a decrease in the outflow intensity, thus leading to an increase of the viscous torque. When buoyancy induced bubble motion, by comparison to the coherent motion of the liquid is increased, a decrease in the viscous torque is suspected

Larger, diversified and courted : the new triad of local firms of infrastructure in transition ?

International audienceFollowing Jaglin and Verdeil’s argument (2013), this article considers that a certain obsession for energy transition may overlook important transformations of energy systems. A focus on largely understudies local utilities in three very different contexts (Grenoble, Magdeburg, Medellin) reveals important though silent changes that indicate new emerging infrastructure regimes and complete the literature on infrastructure firms

Numerical and experimental studies of bubble dispersion and bubble induced drag modulation in a Turbulent Taylor Couette flow

This study deals with numerical and experimental investigations of the bubble dispersion and the viscous torque induced modifications in a Taylor-Couette flow for the outer cylinder at rest

From passive tracer to bubbles dispersion in Taylor-Couette flows

We investigate dispersion of passive tracer and bubble preferential accumulation in the flow between two concentric cylinders. The dispersive characteristics are analysed for Taylor Vortex Flow, Wavy Vortex Flows and fully Turbulent Taylor-Couette flows. Experiments based on flow visualization, PIV and PLIF measurements are compared to direct numerical simulations of Navier-Stokes equations coupled to Lagrangian tracking of fluid elements and bubbles. In vortical flows, bubble accumulation is driven by a competition between added-mass effect, lift and buoyancy forces. At low to moderate Reynolds numbers, the flow is strongly coherent and bubble accumulation patterns can be predicted theoretically (stability analysis of fixed points). When turbulence sets in, small scale structures enhance dispersion. This complex situation where large-scale coherent structures interact with fine scale turbulence leads to bubble mixing which have been analyzed by numerical simulations. Several distributions of bubbles are observed depending on the respective magnitude of turbulence and buoyancy force

Magnetic field effect on the dielectric constant of glasses: Evidence of disorder within tunneling barriers

The magnetic field dependence of the low frequency dielectric constant $e_r$(H) of a structural glass a - SiO2 + xCyHz was studied from 400 mK to 50 mK and for H up to 3T. Measurement of both the real and the imaginary parts of $e_r$ is used to eliminate the difficult question of keeping constant the temperature of the sample while increasing H: a non-zero $e_r$(H) dependence is reported in the same range as that one very recently reported on multicomponent glasses. In addition to the recently proposed explanation based on interactions, the reported $e_r$(H) is interpreted quantitatively as a consequence of the disorder lying within the nanometric barriers of the elementary tunneling systems of the glass.Comment: latex Bcorrige1.tex, 5 files, 4 figures, 7 pages [SPEC-S02/009

Contribution to the MHD modeling in low speed radial flux AC machines with air-gaps filled with conductive fluids

This work deals with the modeling of the magnetohydrodynamic (MHD) phenomena in the air-gaps of low speed radial flux AC electrical machines filled with incompressible and electrically conductive fluids. The proposed model concerns laminar flows and it is based on a weak MHD coupling at the steady state regimes. The MHD power losses are evaluated and discussed. The model is easy to implement and could be a useful tool for the design and the optimization. An application to marine current turbine is considered

L14. Immunomodulatory properties of apoptotic cells.

International audienc

Caractérisation expérimentale de l’effet de l’injection de bulles dans une couche limite turbulente

To investigate the effect of intermediate sized bubble injection into a turbulent boundary layer under a strong gravity effect, we measure the velocity field of two-phase flow in a horizontal cavitation tunnel with 2D Particle Tracking Velocimetry in vertical plane perpendicular to the upper wall and 1D Laser Doppler Velocimetry technique for longitudinal velocity component. In this study, we focus on an horizontal boundary layer where the Reynolds number, based on the thickness of momentum and external velocity is Re 3976. The air injection panel is located at the upper wall of the tunnel’s test section and it allows to cover the injection of a large range of bubble sizes (from 40 to 300 viscous lengths) as well as varying the void fraction between 0.01% and 0.11%. The results reveal that two flow regimes exist: with the increase of the global air injection, a longitudinal speed deficit is observed in the log zone altogether with a longitudinal speed increase in the sub-viscous layer and a Reynolds stress drop. This is in agreement with a phenomena of aspiration caused by the bubble buoyancy. Beyond a critical bubble size, wake bubbles are found in the flow and contribute to a drop in viscous friction and in turbulent shear stress in the near wall region, this phenomenon could be associated to a blowing effect.Cette étude a pour objectif de caractériser expérimentalement les effets de l’injection de bulles de tailles intermédiaire (millimétriques), soumises à un fort effet de gravité, sur un écoulement de couche limite turbulente. La configuration expérimentale est celle d’une couche limite horizontale en développement pour un nombre de Reynolds caractéristique, basé sur l’épaisseur de quantité de mouvement et la vitesse externe, Re= 3976. Le système d’injection d’air localisé sur la paroi supérieure du tunnel a permis de faire varier la taille des bulles dans une large gamme (de 40 à 300 longueurs visqueuses), et de faire varier le taux de vide moyen entre 0.01% et 0.11%. Nous avons mesuré la vitesse du liquide dans la zone interne de la couche par Vélocimétrie par Images de Particules en 2D, pour les composantes de vitesse dans le plan vertical et Vélocimétrie par effet Doppler en 1D pour la composante longitudinale. Les résultats montrent l’existence de deux régimes d’écoulement. Avec l’augmentation du débit global d’air injecté, un déficit de vitesse longitudinale se manifeste dans la zone logarithmique, associé à un excès de vitesse longitudinale dans la sous couche visqueuse et une diminution des tensions de Reynolds. Ceci est en accord avec un phénomène d’aspiration dû à la flottabilité induite par les bulles. Au-delà d’une certaine taille de bulles, des bulles de sillage sont présentes dans l’écoulement, contribuant ainsi à une diminution tant du frottement visqueux que du frottement turbulent en très proche paroi, associé à un effet de soufflage.ANR-ASTRID F-DRAIH

Caractérisation expérimentale de l’effet de l’injection de bulles dans une couche limite turbulente

To investigate the effect of intermediate sized bubble injection into a turbulent boundary layer under a strong gravity effect, we measure the velocity field of two-phase flow in a horizontal cavitation tunnel with 2D Particle Tracking Velocimetry in vertical plane perpendicular to the upper wall and 1D Laser Doppler Velocimetry technique for longitudinal velocity component. In this study, we focus on an horizontal boundary layer where the Reynolds number, based on the thickness of momentum and external velocity is Re 3976. The air injection panel is located at the upper wall of the tunnel’s test section and it allows to cover the injection of a large range of bubble sizes (from 40 to 300 viscous lengths) as well as varying the void fraction between 0.01% and 0.11%. The results reveal that two flow regimes exist: with the increase of the global air injection, a longitudinal speed deficit is observed in the log zone altogether with a longitudinal speed increase in the sub-viscous layer and a Reynolds stress drop. This is in agreement with a phenomena of aspiration caused by the bubble buoyancy. Beyond a critical bubble size, wake bubbles are found in the flow and contribute to a drop in viscous friction and in turbulent shear stress in the near wall region, this phenomenon could be associated to a blowing effect.Cette étude a pour objectif de caractériser expérimentalement les effets de l’injection de bulles de tailles intermédiaire (millimétriques), soumises à un fort effet de gravité, sur un écoulement de couche limite turbulente. La configuration expérimentale est celle d’une couche limite horizontale en développement pour un nombre de Reynolds caractéristique, basé sur l’épaisseur de quantité de mouvement et la vitesse externe, Re= 3976. Le système d’injection d’air localisé sur la paroi supérieure du tunnel a permis de faire varier la taille des bulles dans une large gamme (de 40 à 300 longueurs visqueuses), et de faire varier le taux de vide moyen entre 0.01% et 0.11%. Nous avons mesuré la vitesse du liquide dans la zone interne de la couche par Vélocimétrie par Images de Particules en 2D, pour les composantes de vitesse dans le plan vertical et Vélocimétrie par effet Doppler en 1D pour la composante longitudinale. Les résultats montrent l’existence de deux régimes d’écoulement. Avec l’augmentation du débit global d’air injecté, un déficit de vitesse longitudinale se manifeste dans la zone logarithmique, associé à un excès de vitesse longitudinale dans la sous couche visqueuse et une diminution des tensions de Reynolds. Ceci est en accord avec un phénomène d’aspiration dû à la flottabilité induite par les bulles. Au-delà d’une certaine taille de bulles, des bulles de sillage sont présentes dans l’écoulement, contribuant ainsi à une diminution tant du frottement visqueux que du frottement turbulent en très proche paroi, associé à un effet de soufflage.ANR-ASTRID F-DRAIH