The Effect of Bubbles on Vortical Flows
Grétar Tryggvason2002, Volume 2: Symposia and General Papers, Parts A and B
Sign up for access to the world's latest research
checkGet notified about relevant papers
checkSave papers to use in your research
checkJoin the discussion with peers
checkTrack your impact
Abstract
The effect of bubbles on the evolution of vortical flows near a wall are studied by direct numerical simulations, using a finite volume/front tracking technique that accounts fully for the effect of fluid inertia, viscosity, bubble deformability, and surface tension. Two problems have been studied. In one, the interaction of bubbles with a welldefined vortical flow, consisting of a parabolic velocity profile and a pair of counter-rotating straight vortex filaments near a wall, parallel to the flow direction, is followed. For a wide range of injection sites and bubble sizes, as well as for different number of bubbles, the motion of the bubbles into the vortex core leads to a cancellation of the original vorticity with secondary wall vorticity, resulting in a small transient reduction of the wall shear. In the other study, bubbles are injected near the wall in a turbulent channel flow. The evolution of the bubbles and the modification of the flow is followed as the bubbles migrate away from the wall.
Related papers
Streamwise Vorticity in the Wake of a Sliding Bubble
World Academy of Science, Engineering and Technology, International Journal of Mechanical, Aerospace, Industrial, Mechatronic and Manufacturing Engineering, 2016
The dynamics of bubbly flows are rich and varied, with the understanding of these flows a major issue for many engineering applications. Although a large body of work exists for bubbles rising in an unbounded medium, that of bubbles rising in constricted geometries has received less attention. The particular case of a bubble sliding underneath an inclined surface is common to two-phase flow systems such as shell and tube heat exchangers, mineral flotation and oxidation in water treatment. Prior work by the authors has quantified the surface heat transfer of a single air bubble sliding under a heated surface, showing that enhanced heat transfer is achievable for gas bubbles, even at low wall superheats. The current study intends to expand this knowledge by performing experiments to quantify the streamwise flow structures associated with a single sliding air bubble under an inclined surface in quiescent water and adiabatic conditions. This is achieved by means of two-dimensional, two-component particle image velocimetry (PIV), performed with a continuous wave laser and high-speed camera. PIV velocity and vorticity fields obtained in a plane perpendicular to the sliding surface show that there is significant bulk fluid motion away from the surface, likely contributing to the aforementioned heat transfer enhancement. Additionally, the associated momentum of the bubble means that this wake motion persists for a significant time before viscous dissipation. Bubbles in the shape regime defined by the Reynolds, Morton and Weber numbers are ellipsoidal and experience path oscillations. The magnitude and direction of the flow structures in the streamwise measurement plane are found to depend on the point on its path through which the bubble enters the plane. This entry point, represented by a phase angle, affects the nature and strength of the vortical structures. Previous work has shown that streamwise vorticity downstream of the bubble is key to understanding these flow structures and their effects on fluid mixing. The current study reconstructs the vorticity field in the wake of the bubble, converting the field at different instances in time to slices of a large-scale wake structure. This is, in essence, Taylor's "frozen turbulence" hypothesis. Applying this to the obtained vorticity fields provides a pseudo three-dimensional representation from 2-D data, allowing for a more intuitive understanding of the bubble wake. This study provides insights into bubble behaviour for a configuration that is ubiquitous to engineering applications, and will aid in the interpretation of previously obtained surface heat transfer measurements.
The role of coherent structures in bubble transport by turbulent shear flows
Journal of Fluid Mechanics, 1994
Using Auton's force law for the unsteady motion of a spherical bubble in inhomogeneous unsteady flow, two key dimensionless groups are deduced which determine whether isolated vortices or shear-layer vortices can trap bubbles. These groups represent the ratio of inertial to buoyancy forces as a relaxation parameter 17 = AU2/2gx and a trapping parameter r = AUI VT where AUis the velocity difference across the vortex or the shear layer, x is streamwise distance measured from the effective origin of the mixing layer and VT is the terminal slip speed of the bubble or particle. It is shown here that whilst buoyancy and drag forces can lead to bubbles moving in closed orbits in the vortex flows (either free or forced), only inertial forces result in convergent trajectories. Bubbles converge on the downflow side of the vortex at a location that depends on the inertial and lift forces. It is important to note that the latter have been omitted from many earlier studies.
The effect of bubbles on the wall drag in a turbulent channel flow
Physics of Fluids, 2005
The effect of a few relatively large bubbles injected near the walls on the wall drag in the "minimum turbulent channel" is examined by direct numerical simulations. A front-tracking/finite-volume method is used to fully resolve all flow scales including the bubbles and the flow around them. The Reynolds number, using the friction velocity and the channel half-height, is 135 and the bubbles are 54 wall units in diameter. The results show that deformable bubbles can lead to significant reduction of the wall drag by suppression of streamwise vorticity. Less deformable bubbles, on the other hand, are slowed down by the viscous sublayer and lead to a large increase in drag.
Non-spherical bubble behavior in vortex flow fields
Computational Mechanics, 2003
The boundary element method (BEM) is applied to solve the unsteady behavior of a bubble placed in a vortex flow field. The steady vortex field is given in terms of the viscous core radius and the circulation, both of which may vary along the vortex axis. For this study, 2DYNA YNAFS Ó , an axisymmetric potential flow code which has been verified successfully for diverse type of fluid dynamic problems, is extended. The modifications to accommodate the ambient vortex flow field and to model the extreme deformations of the bubble are presented. Through the numerical simulations, the time history of the bubble geometry and the corresponding pressure signal at a fixed field point are obtained. A special effort is made to continue the numerical simulation after the bubble splits into two or more sub-bubbles. Indeed, it is found that an elongated bubble sometimes splits into smaller bubbles, which then collapse with the emission of strong pressure signals. The behavior of the axial jets after the split is also studied in more detail.
Flow structures and dynamics in the wakes of sliding bubbles
International Journal of Multiphase Flow, 2016
An experimental investigation is reported for the flow structures in the wake of an air bubble sliding under an inclined surface in quiescent water. Time-resolved particle image velocimetry (PIV) is used to study the wakes of sliding bubbles for a range of measurement planes, bubble diameters and surface inclination angles. Additionally, key aspects of the bubble's motion are measured simultaneously using a novel method that accounts for the motion of the bubble's interface. Thus, vortex shedding may be linked to changes in the bubble shape and path. Analysis of the measured velocity and vorticity fields reveals a wake structure consisting of a near wake that moves in close proximity to the bubble, shedding vorticity at the inversion points of the bubble path. Downstream of the bubble in the far wake, these structures evolve into asymmetrical, oppositelyoriented hairpin vortices that are generated in the near wake. These hairpin vortices bear similarities to those observed behind freely rising bubbles and near-wall bluff bodies and are found to cause significant motion of the bulk fluid. This bulk fluid motion has the potential to offer significant convective cooling of adjacent heated surfaces, such as submerged electronics components.
Measurements of the streamwise vorticity in the wake of an oscillating bubble
International Journal of Multiphase Flow, 2009
Interaction and ordering of bubbles levitated in vortical flow
Microgravity Science and Technology, 2007
Hydrodynamics features of dispersed bubbles in the ventilated wake flow of a cylinder
Chinese Journal of Chemical Engineering, 2018
An experimental study was conducted to investigate the 2D bubbly flow downstream of a cylinder. Sparsely distributed bubbles were produced using the ventilation method. The carrier flow was measured using the particle image velocimetry (PIV) technique. The shadow imaging technique was used to capture instantaneous bubbly flow images. An image-processing code was compiled to identify bubbles in acquired image, calculate the bubble equivalent diameter and the bubble velocity. The effects of Reynolds number and the flow rate of the injected air were considered. The result indicates that the carrier flow is featured by distinct flow structures and the wake region is suppressed as the upstream velocity increases. Regarding the bubbles trapped in the wake flow, the number of small bubbles increases with the upstream velocity. On the whole, the bubble velocity is slightly lower than that of the carrier flow. The consistency between small bubbles and the carrier flow is high in terms of velocity magnitude, which is justified near the wake edge. The difference between the bubble velocity and the carrier flow velocity is remarkable near the wake centerline. For certain Reynolds number, with the increase in the air flow rate, the bubble equivalent diameter increases and the bubble void fraction is elevated.
Modification of flow transition for a vortex ring by a bubble released at the axis
The interactions between a vortex ring and a gas bubble released at the axis are studied numerically, which shed light on understanding more complicated bubble-turbulence coupling. We fix the Reynolds number at Re τ = 7500 and consider various Weber numbers in the range of We = 130-870. We find that the translating speed of the vortex ring is substantially lower than the case of the vortex ring without a bubble. It is explained with two different mechanisms, depending on the Weber number. In the low-We range, the reduction of translating speed of the ring is due to the capture of bubbles into the ring core, leading to significant changes in the vorticity distribution within the core. In the high-We range, the repeatedly generated secondary vortex rings perturb the primary one, which bring about an earlier flow transition, thereby reducing the translating speed of the vortex ring. On the other hand, the evolution of a gas bubble is also affected by the presence of the vortex ring. In the low-We range, we observe binary breakup of the bubble after it is captured by the primary vortex ring. In contrast, in the high-We range, it is interesting to find that the bubble experiences sequentially stretching, spreading and breakup stages. In the high-We range, the numbers of smaller bubbles predicted by the classical Rayleigh-Plateau instability of a stretched cylindrical bubble agree well with our numerical simulations. Consistent with the previous experiments, this number keeps unchanged at 16 as We further increases. An additional comparison is made between two higher Reynolds numbers, indicating that the finer eddies in a vortical field with a higher Reynolds number tend to tear the bubble into more fragments.
Preferential concentration and rise velocity reduction of bubbles immersed in a homogeneous and isotropic turbulent flow
Physics of Fluids, 2011
The behavior of small spherical bubbles immersed in a homogeneous isotropic turbulent carrier flow of a heavier fluid has been experimentally studied. Air bubbles with diameters between 10 and 900 lm were injected in the test section of a horizontal water channel and allowed to interact with the turbulence induced by a grid located at the entrance to the test section. Point measurements of the bubble diameter and convective and rise velocities were taken from light interferometry data, together with flow visualizations that showed the instantaneous concentration field of bubbles in the carrier flow. The effect of the turbulence on the bubbles was found to alter the concentration field of bubbles leading to preferential accumulation at small scales, a phenomenon referred to as clustering, and to a decrease in the rise velocity of bubbles in the flow below the value measured and predicted for bubbles in a stationary fluid. These results are interpreted in terms of the different forces acting on the bubble in an inhomogeneous flow and in particular as the effect of pressure fluctuations that drive the bubbles preferentially to the core of vortices.
Discrete-vortex simulation of a turbulent separation bubble
Journal of Fluid Mechanics, 1982
The discrete-vortex model is applied to simulate the separation bubble over a two- dimensional blunt flat plate with finite thickness and right-angled corners, which is aligned parallel to a uniform approaching stream. This flow situation is chosen because, unlike most previous applications of the model, the separation bubble is supposed to be strongly affected by a nearby solid surface. The major objective of this paper is to examine to what extent the discrete-vortex model is effective for such a flow. A simple procedure is employed to represent the effect of viscosity near the solid surface; in particular, the no-slip condition on the solid surface. A reduction in the circulation of elemental vortices is introduced as a function of their ages in order to represent the three-dimensional deformation of vortex filaments, An experiment was also performed for comparison purposes.The calculation yielded reasonable predictions of the time-mean and r.m.s. values of the velocity and the s...
Flow Modulation of a Planar Free Shear-Layer With Large Bubbles - Direct Numerical Simulations
1994
Al~ract--The flow of a planar free shear layer with cylindrical bubbles is simulated using a finite difference/front tracking scheme. This approach allows direct numerical simulation of the multiphase flow by wholly incorporating the local bubble flow field in conjunction with the large scale vortical structures of the liquid. The role of large bubbles in modifying low Reynolds number (~ 250) shear flow structures is investigated, specifically for bubbles whose diameter approaches the scale of the largest liquid eddies.
Capture and inception of bubbles near line vortices
Physics of Fluids, 2005
Motivated by the need to predict vortex cavitation inception, a study has been conducted to investigate bubble capture by a concentrated line vortex of core size r c and circulation ⌫ 0 under noncavitating and cavitating conditions. Direct numerical simulations that solve simultaneously for the two phase flow field, as well as a simpler one-way coupled point-particle-tracking model ͑PTM͒ were used to investigate the capture process. The capture times were compared to experimental observations. It was found that the point-particle-tracking model can successfully predict the capture of noncavitating small nuclei by a line vortex released far from the vortex axis. The nucleus grows very slowly during capture until the late stages of the process, where bubble/vortex interaction and bubble deformation become important. Consequently, PTM can be used to study the capture of cavitating nuclei by dividing the process into the noncavitating capture of the nucleus, and then the growth of the nucleus in the low-pressure core region. Bubble growth and deformation act to speed up the capture process.
Direct Numerical Simulation of Deformable Bubbles in Wall-Bounded Shear Flows
We present a method for fully-resolved simulations of bubbly flows using a front-tracking/front-capturing technique. The method is a modification of a marker-andcell method developed previously for free-surface flows. The basic approach is somehow similar to the front-tracking method of Tryggvason: the continuity and Navier-Stokes equations are solved in a single Eulerian grid, and the interface is represented by an unstructured Lagrangian grid moving through the Eulerian grid, with the velocities at the Lagrangian grid obtained by interpolation from the Eulerian grid. However, contrary to Tryggvason's method, our method uses a sharp interface between the gas and the liquid, since it "captures" the interface within one cell of the Eulerian grid. The surface tension is expressed as a body-force in the Eulerian grid, and is computed using a least-squares fit, together with a mass-conserving filter to remove "sub-grid oscillations". The Navier-Stokes equations are solved using a finite difference scheme on the Eulerian grid and the continuity is enforced using a standard projection technique, with the resulting Poisson equation being solved by a conjugate gradient method. We present some results of the simulations of single bubbles of different sizes in laminar wall-bounded uniform-shear flows at moderate Reynolds numbers (bubble-Reynolds-number in the range of 20-50). In wallbounded shear flows, it has been observed experimentally that depending on the bubble size the lift force can push the bubble either towards or away from the wall. Our simulations show a good agreement with the experiments, both qualitatively and quantitatively, and an explanation for the lift inversion mechanism is provided based on the analysis of the forces acting on the bubble.
Experimental study of bubble injection in a turbulent boundary layer
International Journal of Multiphase Flow, 2002
A bubbly flow experiment has been performed in a horizontal channel in order to simulate the dynamical effects of the nucleation of bubbles and their departure from the wall in boiling flows. Bubbles were injected through a porous plate located on the lower wall. The void fraction, bubble velocity and diameter were measured with a fibre-optic probe and the liquid flow in the bubble layer was studied with a hot film anemometer. The void fraction profiles are nearly self-similar. The expansion of the bubble layer is quasilinear with the distance downstream, with a rate of expansion depending on the bubble diameter. Comparison with a simple model of bubble trajectories highlights the role of the lift force in the development of the bubble layer. The mean velocity in the bubble layer does not differ greatly from that measured in singlephase flow, except near the wall. The velocity profiles follow a logarithmic law similar to that for turbulent flow over a rough surface suggesting that bubbles attached to the wall act as roughness elements on the liquid flow. The turbulent kinetic energy is greater than in single-phase flow. The additional turbulence is analysed and attributed partly to the relative motion of the bubbles and partly to the augmentation of the turbulent shear stress in the bubble layer.
Hydrodynamic forces on a clean spherical bubble translating in a wall-bounded linear shear flow
Physical Review Fluids
The three-dimensional flow around a spherical clean bubble translating steadily in a wall-bounded linear shear flow is studied numerically. The present work is concerned with the drag and lift forces experienced by the bubble over a wide range of Reynolds number (0.1 Re 10 3 , Re being based on the bubble diameter and relative velocity with respect to the ambient fluid), wall distance (1.5 L R 8, L R being the distance from the bubble center to the wall normalized by the bubble radius), and relative shear rate (−0.5 Sr 0.5, Sr being the ratio between the velocity difference across the bubble and the relative velocity). Based on the above range of parameters, situations where the bubble is repelled from or attracted to the wall are both covered. The flow structure and vorticity field are analyzed to obtain qualitative insight into the interaction mechanisms at work. The drag and lift forces are computed as well. Their variations agree well with theoretical predictions available in the limit of low-but-finite Reynolds number and, when the fluid is at rest, in the potential flow limit. Numerical results and analytical expressions are combined to provide accurate semiempirical expressions for the drag and lift forces at arbitrary Reynolds number and separation distance.
LES of bubble dynamics in wake flows
Computers & Fluids, 2005
The results of large eddy simulations (LES) of turbulent bubbly wake flows are presented. The LES technique was applied together with the Lagrangian particle dynamics method and a random flow generation (RFG) technique to the cases of a two-phase bubbly mixing layer and the high-Reynolds number bubbly ship-wake flows. The validation was performed on the experimental data for the bubbly mixing layer. Instantaneous distributions and probability density functions of bubbles in the wake were obtained using a joint LES/RFG approach. Separate estimates of bubble decay due to dissolution and buoyancy effects were obtained. The analysis of bubble agglomeration effects was done on the basis of experimental data for a turbulent vortex to satisfy one-way coupling that is used in this study.
Numerical simulation of bubble dynamics in a vortex flow using Navier-Stokes computations and moving Chimera grid scheme
2001
A numerical scheme using Navier-Stokes computations was applied to simulate bubble dynamics in a vortex flow. A moving grid generation scheme and a Chimera grid scheme were integrated with the Navier-Stokes solver to automatically generate the appropriate grids based on the moving bubble surface. A general free surface boundary condition was implemented to describe the bubble surface motion. The numerical scheme was validated by comparing results with those obtained from the Rayleigh-Plesset equation and from the potential flow solver, 3DynaFS, for bubble dynamics in an infinite volume of quiescent water with/without the gravity effect. Imp ortant numerical factors that influenced the accuracy of solution are reported. Finally, the computations were conducted for bubbles released within a Rankine vortex. Different bubble behaviors were observed for different cavitation numbers.
Velocity fluctuations resulting from the interaction of a bubble with a vertical wall
Physics of Fluids, 2005
The interaction and motion of a spherical bubble with a vertical wall moving in an ideal fluid are analyzed. This case is representative of a bubble that moves at a high Reynolds number and has a small Weber number. Using potential flow theory, the velocity potential is obtained using a double expansion in spherical harmonics. The presence of the wall is approximated by considering the motion of a bubble and its virtual image. The motion equations, obtained considering an energy conservation argument, are solved numerically for a range of initial conditions. The mean and variance of the bubble velocity are calculated. In particular, the bubble velocity variance is obtained as a measure of the bubble velocity fluctuations resulting from the interaction with the wall. The predictions of this model are in good qualitative agreement with some recent experimental results.
Loading Preview
Sorry, preview is currently unavailable. You can download the paper by clicking the button above.