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http://hdl.handle.net/10985/11325
Experimental investigation of bubble induced modifications of a turbulent boundary layer; Caractérisation expérimentale de l’effet de l’injection de bulles dans une couche limite turbulente
ZHANG, Jishen; CLEMENT, Adrien; BILLARD, Jean-Yves; GABILLET, Céline
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.
Fri, 01 Jan 2016 00:00:00 GMThttp://hdl.handle.net/10985/113252016-01-01T00:00:00ZZHANG, JishenCLEMENT, AdrienBILLARD, Jean-YvesGABILLET, CélineTo 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.Quantitative visualization of swirl and cloud bubbles in Taylor–Couette flow
http://hdl.handle.net/10985/11324
Quantitative visualization of swirl and cloud bubbles in Taylor–Couette flow
VAN RUYMBEKE, Bruno; MURAI, Yuichi; TASAKA, Yuji; OISHI, Yoshihiko; COLIN, Catherine; LATRACHE, Noureddine; GABILLET, Céline
We develop a novel method to study the gas phase features in a bubbly Taylor–Couette flow when bubbles are arranged as elevated toroidal strings. The flow is recorded in the front view plane with a highspeed camera for a Reynolds number of 1500 and a global void fraction of 0.14 %. An image processing algorithm is developed to discriminate bubbles accumulated in clouds near the inner cylinder (cloud bubbles) from bubbles trapped in the bulk flow by vortices (swirl bubbles). The analysis of the preferential positions, azimuthal velocities, and equivalent void fraction of clouds and swirl bubbles separately provides a new insight into the dynamics of the bubble’s entrapment.
Fri, 01 Jan 2016 00:00:00 GMThttp://hdl.handle.net/10985/113242016-01-01T00:00:00ZVAN RUYMBEKE, BrunoMURAI, YuichiTASAKA, YujiOISHI, YoshihikoCOLIN, CatherineLATRACHE, NoureddineGABILLET, CélineWe develop a novel method to study the gas phase features in a bubbly Taylor–Couette flow when bubbles are arranged as elevated toroidal strings. The flow is recorded in the front view plane with a highspeed camera for a Reynolds number of 1500 and a global void fraction of 0.14 %. An image processing algorithm is developed to discriminate bubbles accumulated in clouds near the inner cylinder (cloud bubbles) from bubbles trapped in the bulk flow by vortices (swirl bubbles). The analysis of the preferential positions, azimuthal velocities, and equivalent void fraction of clouds and swirl bubbles separately provides a new insight into the dynamics of the bubble’s entrapment.The influence of aeration and compressibility on slamming loads during cone water entry
http://hdl.handle.net/10985/11777
The influence of aeration and compressibility on slamming loads during cone water entry
ELHIMER, Mehdi; JACQUES, Nicolas; EL MALKI ALAOUI, Aboulghit; GABILLET, Céline
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 specific 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 dead-rise 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 dead-rise 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 technique, 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.
Sun, 01 Jan 2017 00:00:00 GMThttp://hdl.handle.net/10985/117772017-01-01T00:00:00ZELHIMER, MehdiJACQUES, NicolasEL MALKI ALAOUI, AboulghitGABILLET, CélineThe 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 specific 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 dead-rise 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 dead-rise 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 technique, 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.Numerical simulations of drag modulation by microbubbles in a turbulent Taylor-Couette flow
http://hdl.handle.net/10985/8695
Numerical simulations of drag modulation by microbubbles in a turbulent Taylor-Couette flow
CHOUIPPE, Agathe; CLIMENT, Eric; LEGENDRE, Dominique; GABILLET, Céline
The aim of our study is to investigate numerically the interaction between a dispersed phase composed of microbubbles and a turbulent Taylor-Couette flow (flow within the gap between two cylinders). We use the Euler-Lagrange approach based on Direct Numerical Simulation of the continuous phase flow equations and a Lagrangian tracking for the dispersed phase. Each bubble trajectory is calculated by integrating the force balance equation accounting for buoyancy, drag, added-mass, pressure gradient, and the lift forces. The numerical method has been adapted in order to take into account the feed-back effect of the dispersed bubbles on the carrying flow. Our approach is based on local volume average of the two-phase Navier-Stokes equations. Local and temporal variations of the bubble concentration and momentum source terms are accounted for in mass and momentum balance equations. A number of reference cases have been tested to validate the modelling approach and its numerical implementation. Then, our previous study of bubble dispersion has been extended to two-way coupling simulations of turbulent Taylor-Couette flows (only inner cylinder is rotating). Modulation of the drag will be discussed for different geometries, Reynolds numbers and bubble sizes. The results show that near-wall turbulent structures are modified by the presence of bubbles.
Tue, 01 Jan 2013 00:00:00 GMThttp://hdl.handle.net/10985/86952013-01-01T00:00:00ZCHOUIPPE, AgatheCLIMENT, EricLEGENDRE, DominiqueGABILLET, CélineThe aim of our study is to investigate numerically the interaction between a dispersed phase composed of microbubbles and a turbulent Taylor-Couette flow (flow within the gap between two cylinders). We use the Euler-Lagrange approach based on Direct Numerical Simulation of the continuous phase flow equations and a Lagrangian tracking for the dispersed phase. Each bubble trajectory is calculated by integrating the force balance equation accounting for buoyancy, drag, added-mass, pressure gradient, and the lift forces. The numerical method has been adapted in order to take into account the feed-back effect of the dispersed bubbles on the carrying flow. Our approach is based on local volume average of the two-phase Navier-Stokes equations. Local and temporal variations of the bubble concentration and momentum source terms are accounted for in mass and momentum balance equations. A number of reference cases have been tested to validate the modelling approach and its numerical implementation. Then, our previous study of bubble dispersion has been extended to two-way coupling simulations of turbulent Taylor-Couette flows (only inner cylinder is rotating). Modulation of the drag will be discussed for different geometries, Reynolds numbers and bubble sizes. The results show that near-wall turbulent structures are modified by the presence of bubbles.Open ocean regimes of relative dispersion
http://hdl.handle.net/10985/10165
Open ocean regimes of relative dispersion
OLLITRAULT, Michel; COLIN DE VERDIERE, Alain; GABILLET, Céline
As two fluid particles separate in time, the entire spectrum of eddy motions is being sampled from the smallest to the largest scales. In large-scale geophysical systems for which the Earth rotation is important, it has been conjectured that the relative diffusivity should vary respectively as D2 and D4/3 for distances respectively smaller and larger than a well-defined forcing scale of the order of the internal Rossby radius (with D the r.m.s. separation distance). Particle paths data from a mid-latitude float experiment in the central part of the North Atlantic appear to support these statements partly: two particles initially separated by a few km within two distinct clusters west and east of the mid-Atlantic ridge, statistically dispersed following a Richardson regime (D2∼t3 asymptotically) for r.m.s. separation distances between 40 and 300 km, in agreement with a D4/3 law. At early times, and for smaller separation distances, an exponential growth, in agreement with a D2 law, was briefly observed but only for the eastern cluster (with an e-folding time around 6 days). After a few months or separation distances greater than 300 km, the relative dispersion slowed down naturally to the Taylor absolute dispersion regime.
Sat, 01 Jan 2005 00:00:00 GMThttp://hdl.handle.net/10985/101652005-01-01T00:00:00ZOLLITRAULT, MichelCOLIN DE VERDIERE, AlainGABILLET, CélineAs two fluid particles separate in time, the entire spectrum of eddy motions is being sampled from the smallest to the largest scales. In large-scale geophysical systems for which the Earth rotation is important, it has been conjectured that the relative diffusivity should vary respectively as D2 and D4/3 for distances respectively smaller and larger than a well-defined forcing scale of the order of the internal Rossby radius (with D the r.m.s. separation distance). Particle paths data from a mid-latitude float experiment in the central part of the North Atlantic appear to support these statements partly: two particles initially separated by a few km within two distinct clusters west and east of the mid-Atlantic ridge, statistically dispersed following a Richardson regime (D2∼t3 asymptotically) for r.m.s. separation distances between 40 and 300 km, in agreement with a D4/3 law. At early times, and for smaller separation distances, an exponential growth, in agreement with a D2 law, was briefly observed but only for the eastern cluster (with an e-folding time around 6 days). After a few months or separation distances greater than 300 km, the relative dispersion slowed down naturally to the Taylor absolute dispersion regime.Analysis of the flow pattern modifications in a bubbly Couette-Taylor flow
http://hdl.handle.net/10985/10301
Analysis of the flow pattern modifications in a bubbly Couette-Taylor flow
MEHEL, Amine; DJERIDI, Henda; GABILLET, Céline
The aim of this Brief Communication is to discuss the bubble effect on the Couette-Taylor flow patterns in the transition from laminar to turbulent flow, especially in the weakly turbulent regime. It is shown that bubble location and local void fractions both in the vortices cores and in the near wall regions directly influence the axial wavelength. Bubbles trapped in the vortices tend to increase the vorticity and reduce the axial diffusivity. Bubbles near the wall contribute to “shear induced” turbulence depending on the void fraction gradient near the wall and the bubble size.
Mon, 01 Jan 2007 00:00:00 GMThttp://hdl.handle.net/10985/103012007-01-01T00:00:00ZMEHEL, AmineDJERIDI, HendaGABILLET, CélineThe aim of this Brief Communication is to discuss the bubble effect on the Couette-Taylor flow patterns in the transition from laminar to turbulent flow, especially in the weakly turbulent regime. It is shown that bubble location and local void fractions both in the vortices cores and in the near wall regions directly influence the axial wavelength. Bubbles trapped in the vortices tend to increase the vorticity and reduce the axial diffusivity. Bubbles near the wall contribute to “shear induced” turbulence depending on the void fraction gradient near the wall and the bubble size.Time Resolved Two Dimensional X-Ray Densitometry of a Two Phase Flow Downstream of a Ventilated Cavity
http://hdl.handle.net/10985/8674
Time Resolved Two Dimensional X-Ray Densitometry of a Two Phase Flow Downstream of a Ventilated Cavity
MAKIHARJU, Simo A; PAIK, Bu-Geun; CHANG, Natasha A; PERLIN, Marc; CECCIO, Steven L; GABILLET, Céline
To measure the void fraction distribution in gas-liquid flows, a two-dimensional x-ray densitometry system was developed. This system is capable of acquiring a two-dimensional projection with a 225 cm2 area of measurement through 21 cm of water. The images can be acquired at rates on the order of 1 kHz. Common sources of error in x-ray imaging, such as x-ray scatter, image distortion, veiling glare, and beam hardening were considered, and mitigated. The measured average void fraction was compared success fully to that of a phantom target and found to be within 1%. To evaluate the performance of the new system, the flow in and downstream of a ventilated nominally two-dimensional partial cavity was investigated and compared to measurements from dual tip fiber optical probes and high speed video. The measurements were found to have satisfactory agreement for void fractions above 5% of the selected void fraction measurement range.
Tue, 01 Jan 2013 00:00:00 GMThttp://hdl.handle.net/10985/86742013-01-01T00:00:00ZMAKIHARJU, Simo APAIK, Bu-GeunCHANG, Natasha APERLIN, MarcCECCIO, Steven LGABILLET, CélineTo measure the void fraction distribution in gas-liquid flows, a two-dimensional x-ray densitometry system was developed. This system is capable of acquiring a two-dimensional projection with a 225 cm2 area of measurement through 21 cm of water. The images can be acquired at rates on the order of 1 kHz. Common sources of error in x-ray imaging, such as x-ray scatter, image distortion, veiling glare, and beam hardening were considered, and mitigated. The measured average void fraction was compared success fully to that of a phantom target and found to be within 1%. To evaluate the performance of the new system, the flow in and downstream of a ventilated nominally two-dimensional partial cavity was investigated and compared to measurements from dual tip fiber optical probes and high speed video. The measurements were found to have satisfactory agreement for void fractions above 5% of the selected void fraction measurement range.Numerical simulation of bubble dispersion in turbulent Taylor-Couette flow
http://hdl.handle.net/10985/8678
Numerical simulation of bubble dispersion in turbulent Taylor-Couette flow
CHOUIPPE, Agathe; CLIMENT, Eric; LEGENDRE, Dominique; GABILLET, Céline
We investigate bubble dispersion in turbulent Taylor-Couette flow. The aim of this study is to describe the main mechanisms yielding preferential bubble accumulation in near-wall structures of the flow. We first proceed to direct numerical simulation of Taylor-Couette flows for three different geometrical configurations (three radius ratios η = R 1/R 2: η = 0.5, η = 0.72, and η = 0.91 with the outer cylinder at rest) and Reynolds numbers corresponding to turbulent regime ranging from 3000 to 8000. The statistics of the flow are discussed using two different averaging procedures that permit to characterize the mean azimuthal velocity, the Taylor vortices contribution and the small-scale turbulent fluctuations. The simulations are compared and validated with experimental and numerical data from literature. The second part of this study is devoted to bubble dispersion. Bubble accumulation is analyzed by comparing the dispersion obtained with the full turbulent flow field to bubble dispersion occurring at lower Reynolds numbers in previous works. Several patterns of preferential accumulation of bubbles have been observed depending on bubble size and the effect of gravity. For the smaller size considered, bubbles disperse homogeneously throughout the gap, while for the larger size they accumulate along the inner wall for the large gap width (η = 0.5). Varying the intensity of buoyancy yields complex evolution of the bubble spatial distribution. For low gravity effect, bubble entrapment is strong leading to accumulation along the inner wall in outflow regions (streaks of low wall shear stress). When buoyancy effect dominates on vortex trapping, bubbles rise through the vortices, while spiral patterns stretched along the inner cylinder are clearly identified. Force balance is analyzed to identify dominating forces leading to this accumulation and accumulation patterns are compared with previous experiments.
Wed, 01 Jan 2014 00:00:00 GMThttp://hdl.handle.net/10985/86782014-01-01T00:00:00ZCHOUIPPE, AgatheCLIMENT, EricLEGENDRE, DominiqueGABILLET, CélineWe investigate bubble dispersion in turbulent Taylor-Couette flow. The aim of this study is to describe the main mechanisms yielding preferential bubble accumulation in near-wall structures of the flow. We first proceed to direct numerical simulation of Taylor-Couette flows for three different geometrical configurations (three radius ratios η = R 1/R 2: η = 0.5, η = 0.72, and η = 0.91 with the outer cylinder at rest) and Reynolds numbers corresponding to turbulent regime ranging from 3000 to 8000. The statistics of the flow are discussed using two different averaging procedures that permit to characterize the mean azimuthal velocity, the Taylor vortices contribution and the small-scale turbulent fluctuations. The simulations are compared and validated with experimental and numerical data from literature. The second part of this study is devoted to bubble dispersion. Bubble accumulation is analyzed by comparing the dispersion obtained with the full turbulent flow field to bubble dispersion occurring at lower Reynolds numbers in previous works. Several patterns of preferential accumulation of bubbles have been observed depending on bubble size and the effect of gravity. For the smaller size considered, bubbles disperse homogeneously throughout the gap, while for the larger size they accumulate along the inner wall for the large gap width (η = 0.5). Varying the intensity of buoyancy yields complex evolution of the bubble spatial distribution. For low gravity effect, bubble entrapment is strong leading to accumulation along the inner wall in outflow regions (streaks of low wall shear stress). When buoyancy effect dominates on vortex trapping, bubbles rise through the vortices, while spiral patterns stretched along the inner cylinder are clearly identified. Force balance is analyzed to identify dominating forces leading to this accumulation and accumulation patterns are compared with previous experiments.Experimental Study of the Flow in a Compact Heat Exchanger Channel with Embossed-type Vortex Generators
http://hdl.handle.net/10985/9495
Experimental Study of the Flow in a Compact Heat Exchanger Channel with Embossed-type Vortex Generators
DUPONT, Frédéric; BOT, Patrick; GABILLET, Céline
The isothermal flow in a model channel of plate-fin heat exchanger with periodically arranged embossed-like vortex generators is investigated. Velocity measurements are performed by LDA in the transitional regime (Reynolds number from 1000 up to 5000). Strong longitudinal vortices are observed downstream of each vortex generator. The vortex roll-up process is highlighted by the evolution of the velocity vector field in the cross section of the flow. The modifications of the vortex characteristics after successively encountered generators are investigated. This work shows most of the flow features which are known to produce heat transfer enhancement, and shows that these smooth shaped vortex generators are very promising for enhanced heat exchangers.
Wed, 01 Jan 2003 00:00:00 GMThttp://hdl.handle.net/10985/94952003-01-01T00:00:00ZDUPONT, FrédéricBOT, PatrickGABILLET, CélineThe isothermal flow in a model channel of plate-fin heat exchanger with periodically arranged embossed-like vortex generators is investigated. Velocity measurements are performed by LDA in the transitional regime (Reynolds number from 1000 up to 5000). Strong longitudinal vortices are observed downstream of each vortex generator. The vortex roll-up process is highlighted by the evolution of the velocity vector field in the cross section of the flow. The modifications of the vortex characteristics after successively encountered generators are investigated. This work shows most of the flow features which are known to produce heat transfer enhancement, and shows that these smooth shaped vortex generators are very promising for enhanced heat exchangers.Effect of bubble’s arrangement on the viscous torque in bubbly Taylor- Couette flow
http://hdl.handle.net/10985/9419
Effect of bubble’s arrangement on the viscous torque in bubbly Taylor- Couette flow
NDONGO FOKOUA, Georges; AUBERT, Adrien; COLIN, Catherine; GABILLET, Céline
An experimental investigation of the interactions between bubbles, coherent motion and viscous drag in a Taylor-Couette flow with the outer cylinder at rest is presented. The cylinder radii ratio η is 0.91. Bubbles are injected inside the gap through a needle at the bottom of the apparatus. Different bubbles sizes are investigated (ratio between the bubble diameter and the gap width ranges from 0.05 to 0.125) for very small void fraction (α<=0.23%). Different flow regimes are studied corresponding to Reynolds number Re based on the gap width and velocity of the inner cylinder, ranging from 600 to 20000. Regarding these Re values, Taylor vortices are persistent leading to an axial periodicity of the flow. A detailed characterization of the vortices is performed for the single-phase flow. The experiment also develops bubbles tracking in a meridian plane and viscous torque of the inner cylinder measurements. The findings of this study show evidence of the link between bubbles localisation, Taylor vortices and viscous torque modifications. We also highlight two regimes of viscous torque modification and various types of bubbles arrangements, depending on their size and on the Reynolds number. Bubbles can have a sliding and wavering motion near the inner cylinder and be either captured by the Taylor vortices or by the outflow areas near the inner cylinder. For small buoyancy effect, bubbles are trapped, leading to an increase of the viscous torque. When buoyancy induced bubbles motion is increased by comparison to the coherent motion of the liquid, a decrease in the viscous torque is rather observed. The type of bubble arrangement is parameterized by the two dimensionless parameters C and H introduced by Climent et al. [E. Climent, M. Simonnet and J. Magnaudet, Phys. Fluids 19, 083301(2007)]. Phase diagrams summarizing the various types of bubbles arrangements, viscous torque modifications and axial wavelength evolution are built.
Thu, 01 Jan 2015 00:00:00 GMThttp://hdl.handle.net/10985/94192015-01-01T00:00:00ZNDONGO FOKOUA, GeorgesAUBERT, AdrienCOLIN, CatherineGABILLET, CélineAn experimental investigation of the interactions between bubbles, coherent motion and viscous drag in a Taylor-Couette flow with the outer cylinder at rest is presented. The cylinder radii ratio η is 0.91. Bubbles are injected inside the gap through a needle at the bottom of the apparatus. Different bubbles sizes are investigated (ratio between the bubble diameter and the gap width ranges from 0.05 to 0.125) for very small void fraction (α<=0.23%). Different flow regimes are studied corresponding to Reynolds number Re based on the gap width and velocity of the inner cylinder, ranging from 600 to 20000. Regarding these Re values, Taylor vortices are persistent leading to an axial periodicity of the flow. A detailed characterization of the vortices is performed for the single-phase flow. The experiment also develops bubbles tracking in a meridian plane and viscous torque of the inner cylinder measurements. The findings of this study show evidence of the link between bubbles localisation, Taylor vortices and viscous torque modifications. We also highlight two regimes of viscous torque modification and various types of bubbles arrangements, depending on their size and on the Reynolds number. Bubbles can have a sliding and wavering motion near the inner cylinder and be either captured by the Taylor vortices or by the outflow areas near the inner cylinder. For small buoyancy effect, bubbles are trapped, leading to an increase of the viscous torque. When buoyancy induced bubbles motion is increased by comparison to the coherent motion of the liquid, a decrease in the viscous torque is rather observed. The type of bubble arrangement is parameterized by the two dimensionless parameters C and H introduced by Climent et al. [E. Climent, M. Simonnet and J. Magnaudet, Phys. Fluids 19, 083301(2007)]. Phase diagrams summarizing the various types of bubbles arrangements, viscous torque modifications and axial wavelength evolution are built.