Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session R4: Bubbles: Flow Dynamics and Clusters 
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Chair: Carlos MartinezBazan, University of Jaen Room: 3006 
Tuesday, November 25, 2014 1:05PM  1:18PM 
R4.00001: Numerical simulation of rising bubble with chemical reaction Kirti Sahu, Manoj Tripathi, Omar Matar, George Karapetsas The dynamics of a rising bubble under the action of gravity and in the presence of an exothermic chemical reaction at the interface is investigated via direct numerical simulation using VolumeofFluid (VOF) method. The product of the chemical reaction, and temperature rise due to the exothermic chemical reaction influence the local viscosity and surface tension near the interfacial region, which in turn give rise to many interesting dynamics. The flow is governed by continuity, NavierStokes equations along with the convection equation of the volume fraction of the outer fluid and the energy equation. The effects of the Bond, Damkohler, and Reynolds numbers, and of the dimensionless heat of reaction are investigated. The results of this parametric study will be presented at the meeting. [Preview Abstract] 
Tuesday, November 25, 2014 1:18PM  1:31PM 
R4.00002: The dynamics of rising bubble inside a viscoplastic material Manoj Tripathi, Kirti Sahu, George Karapetsas, Omar Matar The axisymmetric dynamics of a bubble rising in a Bingham fluid under the action of buoyancy is investigated. The equations of mass and momentum conservation, coupled to an equation for the volume fraction of the Bingham fluid, are solved used a VolumeofFluid (VOF) approach. A regularised constitutive model is used for the description of the viscoplastic behaviour of the material. We found that for large yield stresses, and for weak surface tension the bubble is highly deformable, and the rise is unsteady and is punctuated by periods of rapid acceleration, which separate stages of quasisteady motion. During the acceleration periods, the bubble aspect ratio exhibits oscillations about unity, whose amplitude and wavelength increase with increasing yield stress and decreasing surface tension. These oscillations are accompanied by the alternating formation and destruction of unyielded zones at the bubble equator, as the bubble appears to ``swim" upwards. [Preview Abstract] 
Tuesday, November 25, 2014 1:31PM  1:44PM 
R4.00003: Linear stability of the wake and path of a rising bubble with a realistic shape Jos\'e Carlos CanoLozano, Joel Tchoufag, Jacques Magnaudet, David Fabre, Carlos Mart\'InezBaz\'an A global linear stability analysis of the flow past a bubble rising in still liquid is carried out using the real bubble shape and the terminal velocity obtained for various sets of Galileo (Ga) and Bond (Bo) numbers in axisymmetric simulations performed with the multiphase software Gerris Flow Solver. Once the bubble shape is known, the axisymmetric, steady base flow is computed by means of an iterative Newton method with the finite element software FreeFem++, and the eigenvalue problem is solved with the shiftinvert Arnoldi technique implemented in the SLEPc library. The critical curve separating stable and unstable regimes is obtained in the (Ga, Bo) and (Reynolds number, aspect ratio) spaces. This allows us to discuss the effect of the bubble shape and aspect ratio on the wake and path instabilities. We observe that the foreandaft asymmetry of the bubble has some influence on the stability since, for a given aspect ratio, bubbles with a realistic shape (i.e. a flatter front and a more rounded rear) are more stable that perfectly spheroidal bubbles. [Preview Abstract] 
Tuesday, November 25, 2014 1:44PM  1:57PM 
R4.00004: Bubble formation dynamics in a planar coflow configuration: Influence of geometric and operating characteristics C\'andido Guti\'errezMontes, Roc\'Io Bola\~nosJim\'enez, Carlos Mart\'InezBaz\'an, Alejandro Sevilla An experimental and numerical study has been performed to explore the influence of different geometric features and operating conditions on the dynamics of a waterairwater planar coflow. Specifically, regarding the nozzle used, the innertoouter thickness ratio of the air injector, $\beta =$H$_{\mathrm{i}}$/H$_{\mathrm{o}}$, the watertoair thickness ratio, h$=$H$_{\mathrm{w}}$/H$_{\mathrm{o}}$, and the shape of the injector tip, have been described. As for the operating conditions, the water exit velocity profile under constant flow rate and constant air feeding pressure has been assessed. The results show that the jettingbubbling transition is promoted for increasing values of $\beta $, decreasing values of h, rounded injector tip, and for uniform water exit velocity profiles. As for the bubble formation frequency, it increases with increasing values of $\beta $, decreasing values of h, rounded injector and parabolicshaped water exit profiles. Furthermore, the bubble formation frequency has been shown to be lower under constant air feeding pressure conditions than at constant gas flow rate conditions. Finally, the effectiveness of a timevariable air feeding stream has been numerically studied, determining the forcing receptivity space in the amplitudefrequency plane. Experimental results corroborate the effectiveness of this control technique. [Preview Abstract] 
Tuesday, November 25, 2014 1:57PM  2:10PM 
R4.00005: Numerical and experimental analyses of the translation of bubbles due to nonspherical interface deformations Elena IgualadaVillodre, Daniel Fuster, Javier Rodr\'IguezRodr\'Iguez, Hugo Dutilleul Bubbles developing strong interface deformations (e.g. jetting) experience a strong net force that influences significantly their translational motion. In this work, the translation of bubbles as a result of nonspherical interface deformations is studied both numerically and experimentally. The Gerris flow solver is used to solve for a simplified model of the oscillation of a gas bubble in an incompressible liquid. In particular, we solve for the 3D conservation equations in both phases in a system where the total volume changes in the gas are imposed. Assuming a uniform pressure within the bubble, the conservation equations inside the bubble can be rewritten as a function of the temporal evolution of the bubble's volume. Thus, using volume change rates experimentally measured, we identify different regimes in which the bubble deformation induces a net translation velocity significantly larger than the one obtained with models assuming spherical symmetry. We explore the effect of three parameters: Weber number, dimensionless intensity of the pressure wave and relative distance of the source of the nonspherical perturbation. We support the conclusions extracted from the numerical analyses with experimental measurements of the bubble translational velocity exposed to shock waves. [Preview Abstract] 
Tuesday, November 25, 2014 2:10PM  2:23PM 
R4.00006: Dynamics of a bubble bouncing at a compound interface Jie Feng, Metin Muradoglu, Howard A. Stone Bubbly flow is extensively involved in a wide range of technological applications, which generate a great demand for understanding of bubble physics. The collision, bouncing and coalescence of moving bubbles with liquid/gas and liquid/solid interfaces, as the first stage for the formation of foams and flotation aggregates, have been the subject of many studies, but there are still unanswered questions related to how the properties of the interface influence the dynamics. For example, Zawala et al. 2013 have tried to investigate how the kinetic energy of the bubble affects the liquid film drainage during the collision with an airwater interface. Inspired by Feng et al. 2014, we study the dynamics of an air bubble bouncing at a liquid/liquid/gas interface, in which a thin layer of oil is put on top of the water. The presence of the oil layer changes the interfacial properties and thus the entire process. Combined with direct numerical simulations, extensive experiments were carried out to investigate the effects of the oil layer thickness, oil viscosity, bubble size and initial impact velocity on the bouncing of the bubble at the compound interface. In addition, a massspring model is proposed to describe the bubble dynamics and interactions with the oil layer. [Preview Abstract] 
Tuesday, November 25, 2014 2:23PM  2:36PM 
R4.00007: Comparison of Detailed BubbleCluster Simulations with Reduced Models Arpit Tiwari, Carlos Pantano, Jonathan B. Freund Reducedphysics models of bubble ensembles depend on lengthscale separation between the characteristic size of the cluster and the comprising bubbles. They have been remarkably successful in reproducing qualitatively the grossscale development of the clusters. Studies based on such models, consistent with the experimental findings, suggest that the cluster collapse propagates inward, with pressure focusing toward the geometrical center (with particularly violent collapse of bubbles at its core). The bubblescale dynamics near the focus are therefore anticipated to be particularly important in the damage of adjacent surfaces. Quantifying these dynamics is the goal of our threedimensional simulations, which explicitly represent the nonspherical dynamics of each bubble within the cluster. We simulate collapse of a hemispherical cluster of 50 bubbles adjacent to a plane rigid wall for different initial configurations. Results show that the qualitative behavior matches predictions from the homogenized and particlebased reduced models. However, the peak pressures show strong dependence on bubblescale dynamics. In the detailed simulations, they are typically only a small fraction of those predicted by the reduced models. A systematic comparison with these models will be presented. [Preview Abstract] 
Tuesday, November 25, 2014 2:36PM  2:49PM 
R4.00008: Bubble size measurements in a bubbly wake Ashish Karn, Jiarong Hong, Christopher Ellis, Roger Arndt Measurements of bubble size distribution are ubiquitous in many industrial applications. Conventional methods using image analysis to measure bubble size are limited in their robustness and applicability in highly turbulent bubbly flows. These flows usually impose significant challenges for image processing such as a wide range of bubble size distribution, spatial and temporal inhomogeneity of image background including infocus and outoffocus bubbles, as well as the excessive presence of bubble clusters. This talk introduces a multilevel image analysis approach to detect a wide size range of bubbles and resolve bubble clusters from images obtained in a turbulent bubbly wake of a ventilated hydrofoil. The proposed approach was implemented to derive bubble size and air ventilation rate from the synthetic images and the experiments, respectively. The results show a great promise in its applicability for online monitoring of bubbly flows in a number of industrial applications. [Preview Abstract] 
Tuesday, November 25, 2014 2:49PM  3:02PM 
R4.00009: Experimental study of interactions between bubbles and bubble wakes, via PIV/LIF Daisuke Shinohara, Takayuki Saito A study of the interactions between bubbles and bubble wakes is essential to improve the efficiency of an industrial application. The knowledge of the bubbles and bubble wakes interactions in a bubble swarm, however, is still few. The purpose of this study is to experimentally investigate these interactions in a bubble column. For this specific purpose, a bubbleswarm generator that controls the formation and launch of the bubbles precisely was employed. Equivalent diameters of the bubbles was about 2.6 mm. The two bubbles were launched side by side and the bubblebubble distance was 7 mm. The center bubble was launched between two leading bubbles 9.75 ms behind the leading bubbles. Using a high speed video camera and PIV, we visualized motion of the bubbles and their surrounding liquid motion. Those bubbles linearly ascended during 0.07 sec after launched. An aspect ratio of the center bubble in the time span from 0.03 to 0.07 sec after launched was smaller than those of the leading bubbles. The wakes of the leading bubbles are considered to enhance dynamic pressure acting on the center bubble. Hence, the bubble wake contribution is important to understand a bubbly flow. [Preview Abstract] 
Tuesday, November 25, 2014 3:02PM  3:15PM 
R4.00010: Multiple steady bubbles in a HeleShaw channel Christopher Green, Giovani Vasconcelos We construct analytical solutions, in the form of conformal mappings, solving the free boundary problem for the shapes of any finite number of steadily translating bubbles in a HeleShaw channel. These solutions can be decomposed into the sum of two analytic functions  corresponding to the complex potentials in the laboratory and cotravelling frames  which conformally map a bounded multiply connected circular domain onto respective degenerate polygonal domains (infinite strips with interior slits of finitelength which are either horizontally or vertically aligned). These functions are obtained using the generalised SchwarzChristoffel formula for multiply connected domains in terms of the SchottkyKlein prime function. The solutions we have found are very general and make no assumptions on the geometrical arrangement of the bubbles. [Preview Abstract] 
Tuesday, November 25, 2014 3:15PM  3:28PM 
R4.00011: A Dynamic Testbed for Supercavitating Vehicles David Sanabria, Gary Balas, Roger Arndt Underwater vehicles that travel inside a gas cavity offer possibilities for highspeed transportation as a result of reduced contact area with the fluid and drag reduction. Validation and testing of mathematical models and control systems for these vehicles is a challenge due to the cost and complexity of experimental facilities and procedures. In particular, planing forces generated when the vehicle back end pierces the supercavity, can lead to instability and are challenging for validation and testing. A cost efficient approach to the experimental validation of control systems for a supercavitating vehicle is presented in this talk. The test method uses a small scale supercavitating vehicle, free to rotate in a highspeed water tunnel, to evaluate control systems designed for stabilization and tracking of attack angle commands. The vehicle is equipped with a disk cavitator and two lateral fins used for control. The key feature of the validation approach is that planing forces and their effects are captured in the highspeed water tunnel. The proposed validation method is uniquely suitable to validate the robustness of control strategies in the presence of realistic flow conditions and planing. [Preview Abstract] 
(Author Not Attending)

R4.00012: The significance of electrically induced shear stress in drainage of thin aqueous films Vladimir Ajaev, Christiaan Ketelaar We develop a model of drainage of a microscale thin aqueous film separating a gas bubble and a solid wall. In contrast to previous studies, the electrostatic effects are accounted for not only in the normal but also in the shear stress balance at the liquidgas interface. We show that the action of the tangential component of the electric field leads to potentially strong spatially variable shear stress at the deforming charged interface. This previously overlooked effect turns out to be essential for correctly estimating the longtime drainage rates. Timedependent fluid interface shapes predicted by our model are in very good agreement with the experimental data. [Preview Abstract] 
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