Roberto Zenit - Academia.edu (original) (raw)

Papers by Roberto Zenit

Aps Division of Fluid Dynamics Meeting Abstracts, Nov 1, 2002

Experiments on a vertical channel were performed to to study the behavior of a monodispersed bubb... more Experiments on a vertical channel were performed to to study the behavior of a monodispersed bubble suspension. Using water and water-glycerin mixtures, we were able to obtain measurements for a range of Reynolds and Weber numbers. To generate a uniform stream of bubbles an array of identical capillaries was used. To avoid the coalescence effects, a small amount of salt was added to the interstitial fluid, which did not affect the fluid properties significantly. Measurements of the bubble phase velocity were obtained using a dual impedance probe and through high speed digital video processing. We also obtained measurements of the bubble size and shape as a function of the gas volume fraction for the different flow regimes. We found that, for all cases, the bubble velocity decreases as mean gas volume fraction increases. The flow agitation, characterized with the bubble velocity variance, increases with bubble concentration. The flow becomes unstable for lower gas concentrations as the viscosity of the interstitial fluid increases.

The flow around gastropod shells used by hermit crabs (Calcinus californiensis) was visualized ex... more The flow around gastropod shells used by hermit crabs (Calcinus californiensis) was visualized experimentally. These crabs choose their shells according to many factors; we found that the choice of shell (shape and weight) is directly related to the drag caused over them by the exposure to wave action. Tests were conducted in a wind tunnel to investigate flow differences for shells of various shapes. A particle image velocimetry (PIV) system was used to visualize the flow field. The images above show the flow field around two types of shells (Thais speciosa and Nerita scabircosta) for Reynolds numbers of O(10^5). Using a control volume analysis, the drag coefficient was inferred. Several shell geometries, orientations and mean flow velocities were tested. In this talk, the flow and drag force will be shown for the different arrangements. A discussion of the relation between drag and shape will be presented.

An experimental study was conducted to analyze the stability and breakup of a viscous thread in a... more An experimental study was conducted to analyze the stability and breakup of a viscous thread in an isotropic turbulent flow. The motivation for this study arises from the need to understand the mechanisms that control the formation of emulsions of very viscous liquids. Experiments were performed in an isotropic turbulence chamber, in which a single thread was injected. The fluid disturbances on the thread's surface were studied for filaments of different diameters and lengths. The turbulence intensity was varied for each case. The fluid velocity was characterized using a Particle Image Velocimetry (PIV) system. The threads and their temporal evolution were visualized with a high speed camera. We observed that for most conditions the filaments are surprisingly stable; they are largely elongated until their diameter is very small. A dimensionless analysis indicated that at large diameters the filaments respond to turbulent fluctuation, while at small diameters capillary forces dominate.

Physics of Fluids, 2011

We report on the results of a combined experimental and numerical study on the fluid motion gener... more We report on the results of a combined experimental and numerical study on the fluid motion generated by the controlled approach and arrest of a solid sphere moving towards a solid wall at moderate Reynolds number. The experiments are performed in a small tank filled with water for a range of Reynolds numbers for which the flow remains axisymmetric. The fluid agitation of the fluid related to the kinetic energy is obtained as function of time in the experiment in a volume located around the impact point. The same quantities are obtained from the numerical simulations for the same volume of integration as in the experiments and also for the entire volume of the container. As shown in previous studies, this flow is characterized by a vortex ring, initially in the wake of the sphere, that spreads radially along the wall, generating secondary vorticity of opposite sign at the sphere surface and wall. It is also observed that before the impact, the kinetic energy increases sharply for a small period of time and then decreases gradually as the fluid motion dies out. The measure of the relative agitation of the collision is found to increase weakly with the Reynolds number Re. The close agreement between the numerics and experiments is indicative of the robustness of the results. These results may be useful in light of a potential modelling of particle-laden flows. Movies illustrating the spatio-temporal dynamics are provided with the online version of this paper.

Oil dispersions in aqueous media produced in stirred tanks are part of many industrial processes.... more Oil dispersions in aqueous media produced in stirred tanks are part of many industrial processes. The oil drops size and dispersion stability are determined by the impeller geometry, stirring velocity and the physicochemical properties of the mixture. A critical parameter is the total interfacial area which is increased as the drop size is decreased. The mechanism that disperses the oil and generates the drops has not been completely explained. In the present work, castor oil (1% v/v, viscosity 500mPa) and water are stirred with a Scaba impeller in a flat bottom cylindrical tank. The process was recorded with high-speed video and the Reynolds number was fixed to 24,000. Before the stirring, the oil is added at the air water interface. At the beginning of the stirring, the oil is suctioned at the impeller shaft and incorporated into the flow ejected by the impeller. In this region, the flow is turbulent and exhibits velocity gradients that elongate the oil phase. Viscous thin filaments are generated and expelled from the impeller. Thereafter, the filaments are elongated and break to form drops. This process is repeated in all the oil phase and drops are incorporated into the dispersion. Two main zones can be identified in the tank: the impeller discharge characterized by high turbulence and the rest of the flow where low velocity gradients appear. In this region surface forces dominate the inertial ones, and drops became spheroidal.

Physics of Fluids, Apr 1, 2006

This paper presents results from hard-particle discrete element simulations of a two-dimensional ... more This paper presents results from hard-particle discrete element simulations of a two-dimensional dilute stream of particles accelerating past an immersed fixed cylinder. Simulation measurements of the drag force Fd are expressed in terms of a dimensionless drag coefficient, Cd=Fd/[1/2 ρνU2(D+d)], where ρ is the particle density, ν is the upstream solid fraction, U is the upstream instantaneous velocity, and D and d are the cylinder and particle diameters, respectively. Measurements indicate that the cylinder's unsteady drag coefficient does not vary significantly from its steady (nonaccelerating) drag coefficient for both frictionless and frictional particles implying that the added mass for the flow is negligible. However, the drag coefficient is larger than its nominal value during an initial transient stage, during which a shock wave develops in front of the cylinder. Once the shock has developed, the drag coefficient remains constant despite the stream's acceleration. The duration of the shock development transient stage is a function of the number of particle/cylinder collisions.

The motion of air bubbles undergoing collisions with solid walls was studied experimentally. Usin... more The motion of air bubbles undergoing collisions with solid walls was studied experimentally. Using a high speed camera, the processes of approach, contact and rebound was recorded for a wide range of fluid properties. The process is characterized considering a modified Stokes number, St^*=(CAMρdeqUo)/(9μ), which compares the inertia associated with the bubble (added mass) and viscous dissipation. We found that the dependence of the coefficient of restitution, ɛ=-Ureb/Uo, with the impact Stokes number can be approximated by ɛ˜(St^*)-1/2, which is different from that found for the case of solid spheres are fluid drops. A discussion of the nature of this dependence is presented.

ABSTRACT Vertical polydisperse liquid-gas flow was experimentally studied to analyze the regime t... more ABSTRACT Vertical polydisperse liquid-gas flow was experimentally studied to analyze the regime transition from bubbly to slug flows. Laser Doppler Anemometry (LDA) and multi-tip conductivity probes were used to measure local flow parameters as liquid velocity, void fraction and superficial gas velocity in a cylindrical pipe. Particle Reynolds number was ranged from 100 to 10000 and Weber number from near to 0 to 100. The void fraction was progressively increased to obtain bubbly, transition and slug flow regimes. The power spectral density obtained from liquid velocity shows a nearly constant decay energy exponent when radial void fraction distribution shows a wall-peak in bubbly regime, whereas in transition and slug regimes the exponent decay shows a non-constant value when void fraction increases. Critical flow conditions defined by Reynolds and Weber numbers are suggested to identify regime transition from bubbly to slug. Turbulence intensity and other local flow parameters are analyzed to validate the proposed criteria.

ABSTRACT Particle aggregation is a common phenomenon observed in viscoelastic multiphase flows. I... more ABSTRACT Particle aggregation is a common phenomenon observed in viscoelastic multiphase flows. In this work a new effect has been observed to occur in monodispersed bubbly flows in a Boger-type fluid. It was found that the dispersion of bubble changes dramatically depending on the bubble size: if the diameter of the bubbles is small, large vertical clusters are formed; on the other hand, the bubble assembly rises in a dispersed manner if the bubble size is increased. To understand the condition for which agglomeration occurs two additional experiments were conducted: the interaction of two side-by-side bubble chains was analyzed; and, the unsteady behavior of the first normal stress difference was studied in a rheometric flow. These analyses suggest that there is a process of accumulation of elastic stress; when the accumulated elastic stress surpasses the viscous repulsive stress, aggregation can occur even at supercritical speeds. Interestingly, the two bubble diameters tested in the bubbly flow experiments are above and below the critical diameter for which the velocity of an isolated bubble becomes discontinuous, the so-called bubble velocity discontinuity. This suggests that the bubble dispersion improvement could result from the modification of the gas-liquid interface.

A modified hot-film anemometry technique was used to measure liquid velocity fluctuations resulti... more A modified hot-film anemometry technique was used to measure liquid velocity fluctuations resulting from bubble agitation in a liquid-gas flow. The first modification aims to remedy the main drawback in hot-film anemometry measurements in liquid-gas flow: bubble-probe interaction. To improve bubble detection, optical fibers were installed in close proximity to the anemometer sensing element; in this way, the collisions of bubbles with the probe can be detected and removed from the signal. The second modification resolves the poor performance of the probe at small mean liquid velocity. The sensing element is moved at a known rate; subsequently, this translation velocity is removed from the signal leaving only the fluctuating velocity of the liquid. Furthermore, an analysis of the effect of the signal processing parameters, such as detection and signal length threshold, is conducted. The flow conditions at which this technique was tested covered void fractions up to 6% in nearly monodispersed bubbly flows. The results obtained show good agreement with reported data by other authors in both, variance and spectral density of the liquid velocity. This technique can be used to measure psuedoturbulence in on bubbly flows.

The flow around a fixed cylinder immersed in a uniform granular flow is studied experimentally. E... more The flow around a fixed cylinder immersed in a uniform granular flow is studied experimentally. Experiments are performed in a tall vertical chute that produces a quasi two-dimensional granular flow. A storage bin at the top of the chute feeds glass particles into the channel while the mean velocity of the flow is controlled by varying the width of a hopper located at the channel exit. Measurements of the drag force acting on a fixed cylinder are made using a strain gauge force measurement system. The flow velocity field is measured through a transparent wall using particle image velocimetry analyses of high speed video recordings of the flow. Experiments are performed for a range of upstream particle velocities, cylinder diameters, and two diameters of glass particles. For the range of velocities studied, the drag force acting on the cylinder is independent of the mean flow velocity, contrary to what is expected from any ordinary fluid. The drag force scales with the asymptotic static stress state in a tall granular bed. The drag coefficient, defined in terms of a dynamic pressure, scales with the flow Froude number and a length scale parameter that accounts for the effective cylinder size. Although the drag force on the cylinder does not change with the upstream flow velocity, the flow streamlines do, in fact, change with velocity. A large stagnation zone forms at the leading edge of the cylinder while at the trailing edge an empty wake is observed. The wake size increases with flow velocity.

Metallurgical and Materials Transactions B, 2013

ABSTRACT In the current study a transparent water physical model was developed to study fluid flo... more ABSTRACT In the current study a transparent water physical model was developed to study fluid flow and turbulent structure of aluminum ladles for degassing treatment with a rotating impeller and gas injection. Flow patterns and turbulent structure in the ladle were measured with the particle image velocimetry technique. The effects of process parameters such as rotor speed, gas flow rate, and type of rotor on the flow patterns and on the vortex formation were analyzed using this model, which control degassing kinetics. In addition, a comparison between two points of gas injection was performed: (a) conventional gas injection through the shaft and (b) a “novel” gas injection technique through the bottom of the ladle. Results show that the most significant process variable on the stirring degree of the bath was the angular speed of the impeller, which promotes better stirred baths with smaller and better distributed bubbles. A gas flow rate increment is detrimental to stirring. Finally, although the injection point was the less-significant variable, it was found that the “novel” injection from the bottom of the ladle improves the stirring in the ladle, promotes a better distribution of bubbles, and shows to be a promising alternative for gas injection.

Physics of Fluids, Jun 1, 2003

The flow around a fixed cylinder immersed in a uniform granular flow is studied experimentally. E... more The flow around a fixed cylinder immersed in a uniform granular flow is studied experimentally. Experiments are performed in a tall vertical chute producing a quasi two-dimensional granular flow. A storage bin at the top of the chute feeds glass particles into the channel while the mean velocity of the flow is controlled by varying the exit width of a hopper located at the channel bottom. Measurements of the drag force acting on a fixed cylinder are made using a strain gauge force measurement system. The flow velocity field is measured through a transparent wall using a particle image velocimetry analysis of high speed video recordings of the flow. Experiments are performed for a range of upstream particle velocities, cylinder diameters, and two sizes of glass particles. For the range of velocities studied, the mean drag force acting on the cylinder is independent of the mean flow velocity, contrary to what is expected from any ordinary fluid. The drag force increases with cylinder diameter and decreases with particle diameter. The drag force scales with the asymptotic static stress state in a tall granular bed. The drag coefficient, defined in terms of a dynamic pressure and an effective cylinder diameter, scales with the flow Froude number based on the hydraulic diameter of the channel. This analysis indicates that the drag acting on the cylinder is strongly affected by the surrounding channel geometry. Although the drag force on the cylinder does not change with the upstream flow velocity, the flow streamlines do change with velocity. A large stagnation zone forms at the leading edge of the cylinder while at the trailing edge an empty wake is observed. The wake size increases with flow velocity. Measurements of the flow vorticity and granular temperature are also presented and discussed.

68th Annual Meeting of the APS Division of Fluid Dynamics - Gallery of Fluid Motion, 2015

We have studied the motion of bubbles colliding with solid walls both numerically and experimenta... more We have studied the motion of bubbles colliding with solid walls both numerically and experimentally. The simulations were performed considering a VOF method of the JADIM code (Bonometti & Mahnaudet IJMF 2007) that permits to reproduce accurately motion bubbles for a large range of Eotvos and Morton numbers. To be able to compare with the numerical results, the experiments were carried out using silicon oils, for which the interface remains clean under ordinary laboratory conditions. By measuring the approach and rebound velocities, we calculated the coefficient of restitution of the collision, ɛ, which was found to scale as -ɛ˜(Ca/St^*)^1/2, as suggested by Zenit and Legendre (PoF, 2009) (where Ca is the capillary number and St^* is a modified Stokes number). Since the numerical results were validated (through direct comparisons with experiments), we conducted a vast parametric study of the coefficient of restitution, varying all the fluid properties in an independent manner. We will discuss these results and their implications in the study of solid/fluid particle collisions in general.

Physics of Fluids, Mar 1, 2009

A study on falling spheres descending in associative polymers with sphere-container ratios of 0.0... more A study on falling spheres descending in associative polymers with sphere-container ratios of 0.05-0.15 for various polymer concentrations and Weissenberg numbers is presented. The fluid exhibits constant viscosity over a wide range of small to moderate shear rates, and shear thinning for large shear rates. The simple shear rheology and linear viscoelasticity of these polymers are modeled with the BMP equation of state [F. Bautista, J. M. de Santos, J. E. Puig, and O. Manero, J. Non-Newtonian Fluid Mech. 80, 93 (1999); O. Manero, F. Bautista, J. F. A. Soltero, and J. E. Puig, J. Non-Newtonian Fluid Mech. 106, 1 (2002)], which enables the prediction of the extensional viscosity as a function of the strain rate. The particle image velocimetry technique allows the measurement of the velocity field in the rear of the sphere. The container wall affects the formation of the negative wake at a critical Weissenberg number, which closely corresponds to the region around the peak of extension thickening of the Trouton ratio in the solution. A characteristic strain rate is estimated from the distance of the sphere surface to the stagnant point where the velocity changes direction. Using these data, various criteria for the appearance of the negative wake are discussed. Conclusions reached by Dou and Phan-Thien [Rheol. Acta 43, 203 (2004)] on the physical mechanisms for negative wake generation, are in agreement with the results exposed in this work.

International Journal of Multiphase Flow, 2009

Review of Scientific Instruments, Apr 30, 2003

We have developed a dual impedance-based probe that can simultaneously measure the bubble velocit... more We have developed a dual impedance-based probe that can simultaneously measure the bubble velocity and the gas volume fraction in length scales comparable to the bubble diameter. The accurate determination of the profiles is very important for comparisons with existing theories that describe the rheological behavior of bubbly liquids. The gas volume fraction is determined by the residence time of bubble within the measuring volume of the probe. We have found that the details of the bubble-probe interactions must be taken into account to obtain an accurate measure of the gas volume fraction at a point. We are able to predict the apparent nonlinear behavior of the gas volume fraction measurement at large concentrations. The bubble velocity is obtained from the cross correlation of the signals of two closely spaced identical probes. Performance tests and results are shown for bubble velocity and bubble concentration profiles in a gravity driven shear flow of a bubbly liquid.

International Journal of Multiphase Flow, Sep 1, 2011

Collisional phenomena in a solid-liquid flow were studied in terms of two parameters: the collisi... more Collisional phenomena in a solid-liquid flow were studied in terms of two parameters: the collision frequency and the coefficient of restitution. Experimental measurements of these parameters were conducted inside a liquid fluidized bed by particle tracking in an index-matched array. Collision detection was based on the use of a peak acceleration threshold of the instantaneous speed of colored tracers. The measurements of collision frequency were compared with the theoretical expression derived from the kinetic theory for granular flow (KTGF). The normal and tangential restitution coefficients were measured from the trajectories before and after contact for both particle-particle and particle-wall collisions. A comparison with previous theoretical and experimental works is presented and discussed.

Aps Division of Fluid Dynamics Meeting Abstracts, Nov 1, 2010

In many instances of biological relevance, self-propelled cells have to swim through non-Newtonia... more In many instances of biological relevance, self-propelled cells have to swim through non-Newtonian fluids. In order to provide fundamental understanding on the effect of such non-Newtonian stresses on locomotion, we have studied the motion an oscillating magnetic swimmer immersed in both Newtonian and non-Newtonian liquids at small Reynolds numbers. The swimmer is made with a small rare earth (Neodymium-Iron-Boron) magnetic rod (3 mm) to which a flexible tail was glued. This array was immersed in cylindrical container (50 mm diameter) in which the test fluid was contained. A nearly uniform oscillating magnetic field was created with a Helmholtz coil (R=200mm) and a AC power supply. For the Newtonian case, a 30,000 cSt silicon oil was used. In the non-Newtonian case, a fluid with nearly constant viscosity and large first normal stress difference (highly elastic) was used; this fluid was made with Corn syrup with a small amount of polyacrylamide. The swimming speed was measured, for different amplitudes and frequencies, using a digital image analysis. The objective of the present investigation is to determine whether the elastic effects of the fluid improve or not the swimming performance. Some preliminary results will be presented and discussed.

Aps Division of Fluid Dynamics Meeting Abstracts, Nov 1, 2002

Experiments on a vertical channel were performed to to study the behavior of a monodispersed bubb... more Experiments on a vertical channel were performed to to study the behavior of a monodispersed bubble suspension. Using water and water-glycerin mixtures, we were able to obtain measurements for a range of Reynolds and Weber numbers. To generate a uniform stream of bubbles an array of identical capillaries was used. To avoid the coalescence effects, a small amount of salt was added to the interstitial fluid, which did not affect the fluid properties significantly. Measurements of the bubble phase velocity were obtained using a dual impedance probe and through high speed digital video processing. We also obtained measurements of the bubble size and shape as a function of the gas volume fraction for the different flow regimes. We found that, for all cases, the bubble velocity decreases as mean gas volume fraction increases. The flow agitation, characterized with the bubble velocity variance, increases with bubble concentration. The flow becomes unstable for lower gas concentrations as the viscosity of the interstitial fluid increases.

The flow around gastropod shells used by hermit crabs (Calcinus californiensis) was visualized ex... more The flow around gastropod shells used by hermit crabs (Calcinus californiensis) was visualized experimentally. These crabs choose their shells according to many factors; we found that the choice of shell (shape and weight) is directly related to the drag caused over them by the exposure to wave action. Tests were conducted in a wind tunnel to investigate flow differences for shells of various shapes. A particle image velocimetry (PIV) system was used to visualize the flow field. The images above show the flow field around two types of shells (Thais speciosa and Nerita scabircosta) for Reynolds numbers of O(10^5). Using a control volume analysis, the drag coefficient was inferred. Several shell geometries, orientations and mean flow velocities were tested. In this talk, the flow and drag force will be shown for the different arrangements. A discussion of the relation between drag and shape will be presented.

An experimental study was conducted to analyze the stability and breakup of a viscous thread in a... more An experimental study was conducted to analyze the stability and breakup of a viscous thread in an isotropic turbulent flow. The motivation for this study arises from the need to understand the mechanisms that control the formation of emulsions of very viscous liquids. Experiments were performed in an isotropic turbulence chamber, in which a single thread was injected. The fluid disturbances on the thread's surface were studied for filaments of different diameters and lengths. The turbulence intensity was varied for each case. The fluid velocity was characterized using a Particle Image Velocimetry (PIV) system. The threads and their temporal evolution were visualized with a high speed camera. We observed that for most conditions the filaments are surprisingly stable; they are largely elongated until their diameter is very small. A dimensionless analysis indicated that at large diameters the filaments respond to turbulent fluctuation, while at small diameters capillary forces dominate.

Physics of Fluids, 2011

We report on the results of a combined experimental and numerical study on the fluid motion gener... more We report on the results of a combined experimental and numerical study on the fluid motion generated by the controlled approach and arrest of a solid sphere moving towards a solid wall at moderate Reynolds number. The experiments are performed in a small tank filled with water for a range of Reynolds numbers for which the flow remains axisymmetric. The fluid agitation of the fluid related to the kinetic energy is obtained as function of time in the experiment in a volume located around the impact point. The same quantities are obtained from the numerical simulations for the same volume of integration as in the experiments and also for the entire volume of the container. As shown in previous studies, this flow is characterized by a vortex ring, initially in the wake of the sphere, that spreads radially along the wall, generating secondary vorticity of opposite sign at the sphere surface and wall. It is also observed that before the impact, the kinetic energy increases sharply for a small period of time and then decreases gradually as the fluid motion dies out. The measure of the relative agitation of the collision is found to increase weakly with the Reynolds number Re. The close agreement between the numerics and experiments is indicative of the robustness of the results. These results may be useful in light of a potential modelling of particle-laden flows. Movies illustrating the spatio-temporal dynamics are provided with the online version of this paper.

Oil dispersions in aqueous media produced in stirred tanks are part of many industrial processes.... more Oil dispersions in aqueous media produced in stirred tanks are part of many industrial processes. The oil drops size and dispersion stability are determined by the impeller geometry, stirring velocity and the physicochemical properties of the mixture. A critical parameter is the total interfacial area which is increased as the drop size is decreased. The mechanism that disperses the oil and generates the drops has not been completely explained. In the present work, castor oil (1% v/v, viscosity 500mPa) and water are stirred with a Scaba impeller in a flat bottom cylindrical tank. The process was recorded with high-speed video and the Reynolds number was fixed to 24,000. Before the stirring, the oil is added at the air water interface. At the beginning of the stirring, the oil is suctioned at the impeller shaft and incorporated into the flow ejected by the impeller. In this region, the flow is turbulent and exhibits velocity gradients that elongate the oil phase. Viscous thin filaments are generated and expelled from the impeller. Thereafter, the filaments are elongated and break to form drops. This process is repeated in all the oil phase and drops are incorporated into the dispersion. Two main zones can be identified in the tank: the impeller discharge characterized by high turbulence and the rest of the flow where low velocity gradients appear. In this region surface forces dominate the inertial ones, and drops became spheroidal.

Physics of Fluids, Apr 1, 2006

This paper presents results from hard-particle discrete element simulations of a two-dimensional ... more This paper presents results from hard-particle discrete element simulations of a two-dimensional dilute stream of particles accelerating past an immersed fixed cylinder. Simulation measurements of the drag force Fd are expressed in terms of a dimensionless drag coefficient, Cd=Fd/[1/2 ρνU2(D+d)], where ρ is the particle density, ν is the upstream solid fraction, U is the upstream instantaneous velocity, and D and d are the cylinder and particle diameters, respectively. Measurements indicate that the cylinder's unsteady drag coefficient does not vary significantly from its steady (nonaccelerating) drag coefficient for both frictionless and frictional particles implying that the added mass for the flow is negligible. However, the drag coefficient is larger than its nominal value during an initial transient stage, during which a shock wave develops in front of the cylinder. Once the shock has developed, the drag coefficient remains constant despite the stream's acceleration. The duration of the shock development transient stage is a function of the number of particle/cylinder collisions.

The motion of air bubbles undergoing collisions with solid walls was studied experimentally. Usin... more The motion of air bubbles undergoing collisions with solid walls was studied experimentally. Using a high speed camera, the processes of approach, contact and rebound was recorded for a wide range of fluid properties. The process is characterized considering a modified Stokes number, St^*=(CAMρdeqUo)/(9μ), which compares the inertia associated with the bubble (added mass) and viscous dissipation. We found that the dependence of the coefficient of restitution, ɛ=-Ureb/Uo, with the impact Stokes number can be approximated by ɛ˜(St^*)-1/2, which is different from that found for the case of solid spheres are fluid drops. A discussion of the nature of this dependence is presented.

ABSTRACT Vertical polydisperse liquid-gas flow was experimentally studied to analyze the regime t... more ABSTRACT Vertical polydisperse liquid-gas flow was experimentally studied to analyze the regime transition from bubbly to slug flows. Laser Doppler Anemometry (LDA) and multi-tip conductivity probes were used to measure local flow parameters as liquid velocity, void fraction and superficial gas velocity in a cylindrical pipe. Particle Reynolds number was ranged from 100 to 10000 and Weber number from near to 0 to 100. The void fraction was progressively increased to obtain bubbly, transition and slug flow regimes. The power spectral density obtained from liquid velocity shows a nearly constant decay energy exponent when radial void fraction distribution shows a wall-peak in bubbly regime, whereas in transition and slug regimes the exponent decay shows a non-constant value when void fraction increases. Critical flow conditions defined by Reynolds and Weber numbers are suggested to identify regime transition from bubbly to slug. Turbulence intensity and other local flow parameters are analyzed to validate the proposed criteria.

ABSTRACT Particle aggregation is a common phenomenon observed in viscoelastic multiphase flows. I... more ABSTRACT Particle aggregation is a common phenomenon observed in viscoelastic multiphase flows. In this work a new effect has been observed to occur in monodispersed bubbly flows in a Boger-type fluid. It was found that the dispersion of bubble changes dramatically depending on the bubble size: if the diameter of the bubbles is small, large vertical clusters are formed; on the other hand, the bubble assembly rises in a dispersed manner if the bubble size is increased. To understand the condition for which agglomeration occurs two additional experiments were conducted: the interaction of two side-by-side bubble chains was analyzed; and, the unsteady behavior of the first normal stress difference was studied in a rheometric flow. These analyses suggest that there is a process of accumulation of elastic stress; when the accumulated elastic stress surpasses the viscous repulsive stress, aggregation can occur even at supercritical speeds. Interestingly, the two bubble diameters tested in the bubbly flow experiments are above and below the critical diameter for which the velocity of an isolated bubble becomes discontinuous, the so-called bubble velocity discontinuity. This suggests that the bubble dispersion improvement could result from the modification of the gas-liquid interface.

A modified hot-film anemometry technique was used to measure liquid velocity fluctuations resulti... more A modified hot-film anemometry technique was used to measure liquid velocity fluctuations resulting from bubble agitation in a liquid-gas flow. The first modification aims to remedy the main drawback in hot-film anemometry measurements in liquid-gas flow: bubble-probe interaction. To improve bubble detection, optical fibers were installed in close proximity to the anemometer sensing element; in this way, the collisions of bubbles with the probe can be detected and removed from the signal. The second modification resolves the poor performance of the probe at small mean liquid velocity. The sensing element is moved at a known rate; subsequently, this translation velocity is removed from the signal leaving only the fluctuating velocity of the liquid. Furthermore, an analysis of the effect of the signal processing parameters, such as detection and signal length threshold, is conducted. The flow conditions at which this technique was tested covered void fractions up to 6% in nearly monodispersed bubbly flows. The results obtained show good agreement with reported data by other authors in both, variance and spectral density of the liquid velocity. This technique can be used to measure psuedoturbulence in on bubbly flows.

The flow around a fixed cylinder immersed in a uniform granular flow is studied experimentally. E... more The flow around a fixed cylinder immersed in a uniform granular flow is studied experimentally. Experiments are performed in a tall vertical chute that produces a quasi two-dimensional granular flow. A storage bin at the top of the chute feeds glass particles into the channel while the mean velocity of the flow is controlled by varying the width of a hopper located at the channel exit. Measurements of the drag force acting on a fixed cylinder are made using a strain gauge force measurement system. The flow velocity field is measured through a transparent wall using particle image velocimetry analyses of high speed video recordings of the flow. Experiments are performed for a range of upstream particle velocities, cylinder diameters, and two diameters of glass particles. For the range of velocities studied, the drag force acting on the cylinder is independent of the mean flow velocity, contrary to what is expected from any ordinary fluid. The drag force scales with the asymptotic static stress state in a tall granular bed. The drag coefficient, defined in terms of a dynamic pressure, scales with the flow Froude number and a length scale parameter that accounts for the effective cylinder size. Although the drag force on the cylinder does not change with the upstream flow velocity, the flow streamlines do, in fact, change with velocity. A large stagnation zone forms at the leading edge of the cylinder while at the trailing edge an empty wake is observed. The wake size increases with flow velocity.

Metallurgical and Materials Transactions B, 2013

ABSTRACT In the current study a transparent water physical model was developed to study fluid flo... more ABSTRACT In the current study a transparent water physical model was developed to study fluid flow and turbulent structure of aluminum ladles for degassing treatment with a rotating impeller and gas injection. Flow patterns and turbulent structure in the ladle were measured with the particle image velocimetry technique. The effects of process parameters such as rotor speed, gas flow rate, and type of rotor on the flow patterns and on the vortex formation were analyzed using this model, which control degassing kinetics. In addition, a comparison between two points of gas injection was performed: (a) conventional gas injection through the shaft and (b) a “novel” gas injection technique through the bottom of the ladle. Results show that the most significant process variable on the stirring degree of the bath was the angular speed of the impeller, which promotes better stirred baths with smaller and better distributed bubbles. A gas flow rate increment is detrimental to stirring. Finally, although the injection point was the less-significant variable, it was found that the “novel” injection from the bottom of the ladle improves the stirring in the ladle, promotes a better distribution of bubbles, and shows to be a promising alternative for gas injection.

Physics of Fluids, Jun 1, 2003

The flow around a fixed cylinder immersed in a uniform granular flow is studied experimentally. E... more The flow around a fixed cylinder immersed in a uniform granular flow is studied experimentally. Experiments are performed in a tall vertical chute producing a quasi two-dimensional granular flow. A storage bin at the top of the chute feeds glass particles into the channel while the mean velocity of the flow is controlled by varying the exit width of a hopper located at the channel bottom. Measurements of the drag force acting on a fixed cylinder are made using a strain gauge force measurement system. The flow velocity field is measured through a transparent wall using a particle image velocimetry analysis of high speed video recordings of the flow. Experiments are performed for a range of upstream particle velocities, cylinder diameters, and two sizes of glass particles. For the range of velocities studied, the mean drag force acting on the cylinder is independent of the mean flow velocity, contrary to what is expected from any ordinary fluid. The drag force increases with cylinder diameter and decreases with particle diameter. The drag force scales with the asymptotic static stress state in a tall granular bed. The drag coefficient, defined in terms of a dynamic pressure and an effective cylinder diameter, scales with the flow Froude number based on the hydraulic diameter of the channel. This analysis indicates that the drag acting on the cylinder is strongly affected by the surrounding channel geometry. Although the drag force on the cylinder does not change with the upstream flow velocity, the flow streamlines do change with velocity. A large stagnation zone forms at the leading edge of the cylinder while at the trailing edge an empty wake is observed. The wake size increases with flow velocity. Measurements of the flow vorticity and granular temperature are also presented and discussed.

68th Annual Meeting of the APS Division of Fluid Dynamics - Gallery of Fluid Motion, 2015

We have studied the motion of bubbles colliding with solid walls both numerically and experimenta... more We have studied the motion of bubbles colliding with solid walls both numerically and experimentally. The simulations were performed considering a VOF method of the JADIM code (Bonometti & Mahnaudet IJMF 2007) that permits to reproduce accurately motion bubbles for a large range of Eotvos and Morton numbers. To be able to compare with the numerical results, the experiments were carried out using silicon oils, for which the interface remains clean under ordinary laboratory conditions. By measuring the approach and rebound velocities, we calculated the coefficient of restitution of the collision, ɛ, which was found to scale as -ɛ˜(Ca/St^*)^1/2, as suggested by Zenit and Legendre (PoF, 2009) (where Ca is the capillary number and St^* is a modified Stokes number). Since the numerical results were validated (through direct comparisons with experiments), we conducted a vast parametric study of the coefficient of restitution, varying all the fluid properties in an independent manner. We will discuss these results and their implications in the study of solid/fluid particle collisions in general.

Physics of Fluids, Mar 1, 2009

A study on falling spheres descending in associative polymers with sphere-container ratios of 0.0... more A study on falling spheres descending in associative polymers with sphere-container ratios of 0.05-0.15 for various polymer concentrations and Weissenberg numbers is presented. The fluid exhibits constant viscosity over a wide range of small to moderate shear rates, and shear thinning for large shear rates. The simple shear rheology and linear viscoelasticity of these polymers are modeled with the BMP equation of state [F. Bautista, J. M. de Santos, J. E. Puig, and O. Manero, J. Non-Newtonian Fluid Mech. 80, 93 (1999); O. Manero, F. Bautista, J. F. A. Soltero, and J. E. Puig, J. Non-Newtonian Fluid Mech. 106, 1 (2002)], which enables the prediction of the extensional viscosity as a function of the strain rate. The particle image velocimetry technique allows the measurement of the velocity field in the rear of the sphere. The container wall affects the formation of the negative wake at a critical Weissenberg number, which closely corresponds to the region around the peak of extension thickening of the Trouton ratio in the solution. A characteristic strain rate is estimated from the distance of the sphere surface to the stagnant point where the velocity changes direction. Using these data, various criteria for the appearance of the negative wake are discussed. Conclusions reached by Dou and Phan-Thien [Rheol. Acta 43, 203 (2004)] on the physical mechanisms for negative wake generation, are in agreement with the results exposed in this work.

International Journal of Multiphase Flow, 2009

Review of Scientific Instruments, Apr 30, 2003

We have developed a dual impedance-based probe that can simultaneously measure the bubble velocit... more We have developed a dual impedance-based probe that can simultaneously measure the bubble velocity and the gas volume fraction in length scales comparable to the bubble diameter. The accurate determination of the profiles is very important for comparisons with existing theories that describe the rheological behavior of bubbly liquids. The gas volume fraction is determined by the residence time of bubble within the measuring volume of the probe. We have found that the details of the bubble-probe interactions must be taken into account to obtain an accurate measure of the gas volume fraction at a point. We are able to predict the apparent nonlinear behavior of the gas volume fraction measurement at large concentrations. The bubble velocity is obtained from the cross correlation of the signals of two closely spaced identical probes. Performance tests and results are shown for bubble velocity and bubble concentration profiles in a gravity driven shear flow of a bubbly liquid.

International Journal of Multiphase Flow, Sep 1, 2011

Collisional phenomena in a solid-liquid flow were studied in terms of two parameters: the collisi... more Collisional phenomena in a solid-liquid flow were studied in terms of two parameters: the collision frequency and the coefficient of restitution. Experimental measurements of these parameters were conducted inside a liquid fluidized bed by particle tracking in an index-matched array. Collision detection was based on the use of a peak acceleration threshold of the instantaneous speed of colored tracers. The measurements of collision frequency were compared with the theoretical expression derived from the kinetic theory for granular flow (KTGF). The normal and tangential restitution coefficients were measured from the trajectories before and after contact for both particle-particle and particle-wall collisions. A comparison with previous theoretical and experimental works is presented and discussed.

Aps Division of Fluid Dynamics Meeting Abstracts, Nov 1, 2010

In many instances of biological relevance, self-propelled cells have to swim through non-Newtonia... more In many instances of biological relevance, self-propelled cells have to swim through non-Newtonian fluids. In order to provide fundamental understanding on the effect of such non-Newtonian stresses on locomotion, we have studied the motion an oscillating magnetic swimmer immersed in both Newtonian and non-Newtonian liquids at small Reynolds numbers. The swimmer is made with a small rare earth (Neodymium-Iron-Boron) magnetic rod (3 mm) to which a flexible tail was glued. This array was immersed in cylindrical container (50 mm diameter) in which the test fluid was contained. A nearly uniform oscillating magnetic field was created with a Helmholtz coil (R=200mm) and a AC power supply. For the Newtonian case, a 30,000 cSt silicon oil was used. In the non-Newtonian case, a fluid with nearly constant viscosity and large first normal stress difference (highly elastic) was used; this fluid was made with Corn syrup with a small amount of polyacrylamide. The swimming speed was measured, for different amplitudes and frequencies, using a digital image analysis. The objective of the present investigation is to determine whether the elastic effects of the fluid improve or not the swimming performance. Some preliminary results will be presented and discussed.