Bubble In Shear Flow Research Articles

Coalescence-induced jumping of bubbles in shear flow in microgravity

Bubble removal from a solid surface is of significant importance to many technical processes and applications. In addition to the conventional buoyancy-aided bubble removal, there is also a passive strategy to remove bubbles from a solid surface via coalescence. However, likewise several processes, the coalescence-induced removal of bubbles from the solid surface is masked by the dominant buoyancy, hence, difficult to observe in terrestrial conditions. Microgravity condition offers a unique opportunity to investigate such phenomenon in great detail that can significantly improve our fundamental understanding. In this work, we report coalescence-induced jumping of isolated vapor bubbles from the heated substrate during shear flow in microgravity condition. We show that, similar to the coalescence-induced jumping droplets, when two bubbles coalesce, the resulting big coalesced bubble jumps from the substrate due to the conversion of excess surface energy into the translational kinetic energy, which provides the requisite initial velocity for jumping. Jumping of bubbles over a wide range of bubble size (post-coalescence radius ≈0.9–3.4 mm) is observed. Bubbles oscillate continuously while rising through certain height post-coalescence. We perform force balance and scaling analysis to develop a model to predict the maximum jumping height of bubbles. We show that the jumping height is strongly related to the bubble size and the non-dimensional Ohnesorge number, which captures the role of fluid properties governing the coalescence. The physical insight presented in this work has implication for the design of energy systems and microfluidic devices for the earth and space-based applications.

Euler-Euler numerical simulations of upward turbulent bubbly flows in vertical pipes with low-Reynolds-number model

In this work, numerical simulations of upward turbulent bubbly flows in vertical pipes are conducted in the Euler-Euler framework with a low-Reynolds-number (LRN) model for liquid. It is found that the existing correlation for the drag coefficient of a single bubble in a shear flow, which has been successfully used along with the high-Reynolds-number (HRN) models, cannot be used with the LRN model. The reason is that drag forces of bubbles calculated using such correlation are enormous in near-wall cells (order of [Formula: see text]), which causes a divergence of numerical simulations with LRN. Therefore, a modified correlation for the drag coefficient of a single bubble in shear flow, that can be used successfully with the LRN model, has been proposed. The results of numerical simulations, performed with a new correlation for the drag coefficient, are analysed and compared to selected experimental measurements for different pipe diameters and different flow conditions of gas and liquid. It is shown that the largest effect of the application of the new correlation for the drag coefficient of a single bubble in shear flow in numerical simulations can be achieved on the reduction of the relative velocity between gas and liquid. The degree of this reduction depends on the pipe diameter and liquid volumetric flux.

Численно-экспериментальное исследование деформации пузырька на поверхности в сдвиговом потоке вязкой жидкости

The aim of the work is to develop an experimental and numerical approach to study the deformation of a surface attached bubble in shear flow of a viscous liquid. The numerical method is based on the boundary element method for Stokes flow. For the experimental study of bubble dynamics, the optical microscopy and high-speed recording are used. We studied the dynamics of the receding and advancing contact angles depending on the intensity of the shear flow of a viscous liquid. We obtained a qualitative and quantitative agreement between the results of numerical simulation and experiment for various capillary numbers.

The lateral migration of relative large bubble in simple shear flow in water

The lateral migration of relative large bubble in shear flow was essential for two phase flow modeling when relative large bubble was presented. To better understand the lift force for relative large bubble, the single air bubble motions with diameter ranged from 10mm to 20mm in shear flow in water were captured by high speed camera. The linear shear flow with low turbulence intensity was achieved by curved screen. The bubbles were collected in an inverted semi-spherical cup and released by rotating the cup. The lateral migration velocity and quasi-steady lift force were obtained by linear fitting of bubble trajectories. Reproducible results were obtained by measurements of many bubbles. The results showed that bubble shape parameters, rising velocity and drag force were stayed at same level in shear flow as in still water. The relative large bubble migrated to left side and the lift force was negative. A critical bubble diameter of 15mm was observed which was near the boundary between oblate bubble and spherical cap bubble. When the bubble equivalent diameter was lower than the critical value, the lift coefficient was decreasing with the increasing of shear stress magnitude. When the bubble diameter was larger than the critical diameter, the lift coefficient was monotonically decreased with the bubble equivalent diameter and the shear stress magnitude had no effect in the present experiment condition. The comparison with existing study revealed that the present lift correlations were not able to predict the values and new models were needed.

Numerical Simulation of Bubbles in Shear Flow

AbstractTo gain a basic understanding of foam flow, as it can be found e.g. in transport of aerated food, simulation tools can help to provide better insight. Shearing of the bubbles appears in different flow geometries and is for a bubble assembly not captured analytically. Also experimentally, those flow fields are hard to observe so that simulations are the method of choice.Our method to simulate foams uses a volume of fluid approach that is based on the free surface algorithm by Körner et al. [1]. Different from classical multiphase methods, only the liquid phase is simulated and special boundary conditions at the liquid‐gas interface account for the gas phase. With this approach high density ratios, e.g. in water‐air systems, are easier to realize than in other methods. High density ratios are even necessary to physically justify the model, where the dynamics of the lighter phase are partially neglected. This method is integrated in the Lattice Boltzmann software framework waLBerla [3] (widely applicable Lattice Boltzmann solver from Erlangen†) that can be used on massively parallel computers and thus allows to simulate even large bubble assemblies.As first validation, single bubbles are sheared with different capillary numbers and the simulation results are compared to literature [2] and show good agreement. The next step is shearing a bubble assembly which is arranged like a dense sphere packing. In order to investigate the geometrical configuration of the assembly and its impact on the behavior during a shear deformation, the bubble assembly is rotated with different angles with respect to the shear direction. (© 2014 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Interaction of equal-size bubbles in shear flow

The inertia-induced forces on two identical spherical bubbles in a simple shear flow at small but finite Reynolds number, for the case when the bubbles are within each other's inner viscous region, are calculated making use of the reciprocal theorem. This interaction force is further employed to model the dynamics of air bubbles injected to a viscous fluid sheared in a Couette device at the first shear flow instability where the bubbles are trapped inside the stable Taylor vortex. It was shown that, during a long time scale, the inertial interaction between the bubbles in the primary shear flow drives them away from each other and, as a result, equal-size bubbles eventually assume an ordered string with equal separation distances between all neighbors. We report on experiments showing the dynamic evolution of various numbers of bubbles. The results of the theory are in good agreement with the experimental observations.

Effects of Surfactant on the Lift Force Acting on a Bubble in a Shear Flow

A bubble motion in aqueous surfactant solutions is investigated with the numerical simulation. It is known that the reduction of the terminal velocity of rising bubbles is explained by the so-called Marangoni effect, which is caused by the surfactant adsorption to the air/water interface. But few studies have been done for the shear-induced lift force acting on a bubble under the presence of the Marangoni effect. Here, we conduct the simulation on the force acting on the contaminated bubble in a linear shear flow. To consider the Marangoni effect on the bubble motion, the balance equation for the concentration in liquid phase and the interfacial concentration at bubble surface are solved on the three-dimensional boundary-fitted grid system. The surfactant adsorption and desorption processes to the air/water interface are considered by the Langmuir's kinetics laws. The present simulation showed that the reduction of the lift force occurs when the surface concentration becomes sufficiently large. They also showed that the azimuthal distribution of surface concentration of surfactant gives a large influence to the lift force, although it doesn't to the drag force.

Determining flow type, shear rate and shear stress in magmas from bubble shapes and orientations

We interpret the shear environments that produced bubble textures in obsidian samples using the results of theoretical, numerical and experimental studies on the deformation of bubbles in shear flows. In particular, we use the shapes and orientations of bubbles (vesicles) in obsidian to estimate shear rates and shear stresses, and assess flow type (simple vs. pure shear). This technique can be used to determine shear rates in volcanic conduits, the origin of pyroclastic obsidian, and the emplacement history and dynamics of obsidian flows. A spherical bubble in a viscous fluid subjected to a low Reynolds number, steady flow field deforms until it reaches a steady shape and orientation. Bubble deformation is governed by the competing stresses from shearing that deforms, and surface tension that rerounds. The ratio of these stresses is the capillary number, Ca. Because the relationships among Ca, bubble orientation and shape differ for pure and simple shear, we can distinguish between these flow types using preserved bubble geometries. Furthermore, because Ca is a function of shear rate, we can use relationships between Ca and the magnitude of deformation to calculate shear rates when melt viscosity and surface tension are known. To demonstrate the potential of the technique, we examine three obsidian samples chosen for diversity of origin and texture. Two of the samples have low crystallinities and banding defined by layers of different vesicularity. Bubble geometries in a spatter-fed obsidian flow sample record deformation by pure shear, whereas a juvenile obsidian clast from a pyroclastic fall deposit records predominantly simple shear. A third sample from an obsidian flow has banding marked by variable concentrations of microlites, and shows bubble deformation most consistent with dominantly simple shear and some bubble relaxation. The highest shear rate and shear stress are recorded by the pyroclastic obsidian (shear rate=0.01 s −1, shear stress=60 kPa). The spatter-fed obsidian flow sample deformed by pure shear records the lowest shear stress (4.9 kPa). The obsidian flow sample, deformed by simple shear, records the lowest shear rates (10 −6.6–10 −6.9 s −1) despite high shear stresses (22–36 kPa) because of the high viscosity of the degassed rhyolite (0.13 wt% water).

Spiraling bubbles: how acoustic and hydrodynamic forces compete.

Experiments to study the effect of acoustic forces on individual bubbles in shear flows have been carried out. In the system that we have used, the competition between acoustic and fluid dynamical forces results in a spiraling bubble trajectory. This dynamics is modeled by expressing the balance between Bjerknes and hydrodynamic forces in terms of an ordinary differential equation model, to which a separation of time scales is applied. The success of this model shows that the simple force-balance approach is still meaningful when bubbles are subjected to sound fields.

The Rise of Bubbles in a Vertical Shear Flow

Full numerical simulations of two- and three-dimensional bubbles in a shear flow, by a finite difference front tracking method, are presented. The effects of inertial, viscous, gravitational, and surface forces on the lift of a deformable bubble rising due to buoyancy in a vertical shear flow, are examined. Bubbles with a large surface tension coefficient migrate toward the downward moving fluid, as predicted analytically for a cylinder or a sphere in a shear flow. Bubbles with smaller surface tension deform, and generally migrate in the opposite direction. The combined effects of the shear flow and the buoyancy deform the bubble in such a way that the circulation around the deformed bubbles is opposite to that of undeformed bubbles.

The low Reynolds number deformation of a gas bubble in shear flow: a general approach via integral equations

The time-dependent deformation of an incompressible or compressible gas bubble in an arbitrary shear flow at low Reynolds number is formulated as a second boundary-value problem for Stokes' equations. This problem is solved via a classical Fredholm's integral equation of second kind. Contrasting with the existing previous works using the integral equation approach for this problem, the integral equation method developed here yields a unique bubble surface velocity, regardless of any flow and bubble axisymmetric property. From those previous works, the uniqueness of the surface velocity has been proved only for a spherical bubble in axisymmetric flows, and it has been conjectured that the surface velocity remains unique as long as the bubble shape and the exterior flow are axisymmetric.

Mass transfer problem for particles, drops and bubbles in a shear flow

A numerical solution of the axisymmetric steady heat and mass transfer problem for spherical particles, drops and bubbles in a linear Stokes shear flow is obtained for the entire range of Peclet numbers. Simple approximate expressions for the average Sherwood number in good agreement with the results of the numerical calculations are proposed.

The mechanism of bubble growth in a simple shear flow.

The growth of bubble in shear flow and uniform flow is studied numerically (with F. E. M.) and experimentally. The calculated results indicate that the growth rate of bubble in shear flow is not influenced by the condition of flow, and in the case where the bubble viscosity ratio is approximately one, the growth rate of bubble is equal to that of rigid gas-liquid interface. The experimental results show that the growth rate of bubble in shear flow is lower than that in uniform flow; and in shear flow and uniform flow the interfacial resistance against gas diffusion increases with decreasing the interfacial area of bubble.

The Deformation of Small Viscous Drops and Bubbles in Shear Flows

The Deformation of Small Viscous Drops and Bubbles in Shear Flows

The Deformation of Small Viscous Drops and Bubbles in Shear Flows

The Deformation of Small Viscous Drops and Bubbles in Shear Flows

The Relationship between the Apparent Viscosity and the Void Fraction in Two-Phase Flow

In this paper, the apparent viscosity which is one of the most important apparent mean physical properties in a two-phase flow, has been measured by a specially designed rotation viscometer. The experiment has been carried out using a small air bubbles-liquid tow-phase mixture. From the experimental results, the relationship between the specific viscosity ηsp and the void fraction φ has been expressed with an experimental formula involving the dimensionless number Ta, which shows the amount of the deformation of bubbles in shear flow.