Bulletin of the American Physical Society
64th Annual Meeting of the APS Division of Fluid Dynamics
Volume 56, Number 18
Sunday–Tuesday, November 20–22, 2011; Baltimore, Maryland
Session H14: Bubbles II |
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Chair: Tim Colonius, California Institute of Technology Room: 317 |
Monday, November 21, 2011 10:30AM - 10:43AM |
H14.00001: Two Fates in Bubble Breakup against Azimuthal Perturbations Lipeng Lai, Wendy W. Zhang Cylindrically-symmetric breakup of an air bubble underwater is unstable against azimuthal perturbations. As perturbations in the initial shape of the bubble neck excite vibrations that persist over time, the breakup dynamics always becomes nonlinear. As a consequence, experiments find various breakups with small variation in control parameters. Here we analyze a simplified process where the dynamics is perturbed by an n=2 single mode and the only variation is the initial phase of perturbation. Using boundary integral method, our simulation classifies qualitatively different breakups into two categories. One is characterized by a topological change of the bubble neck via smooth contact between different parts of air-water interface. This dynamics is dominated by lower fundamental vibrational modes. The other is characterized by the formation of near cusp (high local curvature) along the interface which requires contribution from higher modes. Switching from one outcome (smooth contact) to the other (highly curved surface) as initial phase varies coincides with transition from constructive to destructive interference between wave modes 2 and 4. [Preview Abstract] |
Monday, November 21, 2011 10:43AM - 10:56AM |
H14.00002: Description of the instability transition modes of a bubble rising in still liquids J.C. Cano-Lozano, P. Bohorquez, C. Mart\'Inez-Baz\'an In this research, we have investigated numerically the transition from straight to zigzag motion during the rise of a single gas bubble of diameter $D$ in a pure-clear stagnant liquid, for the limiting case $\rho_g/\rho_l\ll1$ and $\mu_g/\mu_l\ll1$, where $\rho$ is density, $\mu$ is dynamic viscosity, and subindices $g$ and $l$ denote gas and liquid phases, respectively. The transition is determined in terms of the Reynolds, $Re=\rho_l\, g^{1/2}\,D^{3/2}/\mu_l$, and Bond, $Bo=\rho_l\,g\,D^2/\sigma$, numbers as set of nondimensional, independent parameters governing the flow dynamics, in which $g$ is the acceleration due to gravity and $\sigma$ is the surface tension. Subsequently, the neutral curve for the onset of zigzag motion is characterized in the $\{Re,\,Bo\}$-plane.The transition curve is determined computing the real shape of the ascending bubble. Thus, we discuss the effect of the bubble shape and aspect ratio on the instability characteristics of the bubble wake. In particular, we observed that the onset of the zigzag instability begins with the loss of axisymmetry of the wake, developing two counter-rotating vortices, which exhibit a symmetry plane. [Preview Abstract] |
Monday, November 21, 2011 10:56AM - 11:09AM |
H14.00003: Numerical Simulation of Bubble Dynamics in Deformable Vessels Vedran Coralic, Tim Colonius The growth and collapse of cavitation bubbles has been implicated as a potential damage mechanism leading to the rupture of blood vessels in shock wave lithotripsy (SWL). While this phenomenon has been investigated numerically, the resulting simulations have often assumed some degree of symmetry and have often failed to include a large number of influential physics, such as viscosity, compressibility, surface tension, phase change and fluid-structure interactions. We present here our efforts to explore the role that cavitation bubbles play in the rupture of blood vessels in SWL and to improve upon the current state of the numerical approach. We have developed a three- dimensional, high-order accurate, shock- and interface- capturing, multicomponent flow algorithm that accounts for the effects of viscosity and surface tension. At this time, we omit any effects due to elasticity and instead, as a first step, model tissue as a viscous and stiffened gas. We discuss preliminary results for the Rayleigh and shock-induced collapse of a gas bubble within a blood vessel and characterize the increase in vessel deformation with increasing bubble confinement and proximity to the vessel wall. [Preview Abstract] |
Monday, November 21, 2011 11:09AM - 11:22AM |
H14.00004: Toroidal Bubble Rings Interactions in Viscous Fluids Jing Lou, Ming Cheng, T.T. Lim We investigate rising toroidal bubbles in viscous fluids by using a 3-D Lattice Boltzmann model simulation, as well as experimental measurement. The bubble ring behavior is systematically characterized with effects of bubble vortex strength, buoyancy force, interface tension and viscosity dissipation. The bubble rise velocity, bubble spread/diffusion and instability is also modeled and compared with experimental observations. Further on, we modeled two co-centered rising toroidal bubbles interaction in a transient manner. In particular, the complex vortex interaction and its impact on bubble pair and flow wakes are discussed. [Preview Abstract] |
Monday, November 21, 2011 11:22AM - 11:35AM |
H14.00005: Dynamics of air bubbles passing through a liquid-liquid interface Romain Bonhomme, Jacques Magnaudet, Bruno Piar The passage of rising air bubbles through an initially flat horizontal liquid-liquid interface is studied using both laboratory experiments and Direct Numerical Simulation. The dynamics of spheroidal, spherical cap and toroidal bubbles near the liquid-liquid interface and subsequently through the upper liquid are investigated by coupling high-speed shadowgraph visualizations and Particle Image Velocimetry techniques. Axisymmetric computations are also carried out to assess the validity of presently available computational approaches in three-phase flows. These computations are based on two distinct approaches, namely a Volume Of Fluid approach without interface reconstruction and a Cahn-Hilliard model coupled with the incompressible Navier-Stokes equations. Experimental and computational results are compared in various configurations, including cases where the bubble is trapped at the liquid-liquid interface or rises in the upper phase while towing a column of the lower liquid that eventually breaks into droplets of various sizes. [Preview Abstract] |
Monday, November 21, 2011 11:35AM - 11:48AM |
H14.00006: Bubble Oscillations under Forced Vibrations Mohammad Movassat, Nasser Ashgriz, Markus Bussmann Dynamics of a gas bubble in a liquid container in response to forced vibrations is studied. A 3D two-fluid solver is employed to study the bubble behavior. Forced vibration induces an oscillatory buoyancy force on the bubble. In response to the forcing, bubble undergoes both oscillatory translational motion as well as shape deformation. As the amplitude and frequency of oscillations increase, the bubble response goes from a regular and linear behavior to a chaotic and nonlinear region in which large deformations occur and different shape modes are excited. As the forcing increases, the inertia force, due to the momentum of the surrounding liquid, starts to form a liquid jet within the bubble core. Surface tension force may not be strong enough to prevent the formation and penetration of the jet and the liquid jet may pierce the bubble forming a toroidal bubble shape with a liquid core in the middle. [Preview Abstract] |
Monday, November 21, 2011 11:48AM - 12:01PM |
H14.00007: Thermal Lattice Boltzmann Simulations for Vapor-Liquid Two-Phase Flows in Two Dimensions Yikun Wei, YueHong Qian A lattice Boltzmann model with double distribution functions is developed to simulate thermal vapor-liquid two-phase flows. In this model, the so-called mesoscopic inter-particle pseudo-potential for the single component multi-phase lattice Boltzmann model is used to simulate the fluid dynamics and the internal energy field is simulated by using a energy distribution function. Theoretical results for large-scale dynamics including the internal energy equation can be derived and numerical results for the coexistence curve of vapor-liquid systems are in good agreement with the theoretical predictions. It is shown from numerical simulations that the model has the ability to mimic phase transitions, bubbly flows and slugging flows. [Preview Abstract] |
Monday, November 21, 2011 12:01PM - 12:14PM |
H14.00008: An asymptotically consistent diffuse interface method for simulating bubble dynamics in inhomogeneous environments A. Tiwari, C. Pantano, J.B. Freund Theoretical models are effective for describing the physics of spherically symmetric bubble collapse, rebound and oscillation. However, for multi-bubble problems (bubble clouds) or near a solid wall, linear Bjerknes theory can only explain weak bubble interactions that do not lead to significant deviations from spherical bubble. Full-domain-based numerical methods are, in principle, capable of describing the nonlinear dynamics of bubble interactions in these inhomogeneous environments, but simulation of even a single bubble collapse poses significant computational challenges resulting from the inherent dynamical instability of such bubbles. We present an efficient and geometrically flexible numerical method to study bubble interactions. The method is build around a five-equation Eulerian diffuse-interface flow model, asymptotically reduced from Baer--Nunziato's compressible multi-fluid model within the 3D finite-volume framework. A consistent compression technique is developed to prevent the smearing of volume fraction and density across the interface. Upon being rigorously tested for spherically symmetric bubble collapses, we use it to simulate near-wall collapses. We show the distinctive formation of a torus bubble as the involuting jet from the distal surface penetrates the proximal surface. Preliminary results for multi-bubble interactions are also presented. [Preview Abstract] |
Monday, November 21, 2011 12:14PM - 12:27PM |
H14.00009: Numerical Study on Focusing of Ultrasounds in Microbubble-enhanced HIFU Yoichiro Matsumoto, Kohei Okita, Shu Takagi The injection of microbubbles into the target tissue enhances tissue heating in High-Intensity Focused Ultrasound therapy, via inertial cavitation. The control of the inertial cavitation is required to achieve the efficient tissue ablation. Microbubbles between a transducer and a target disturb the ultrasound propagation depending on the conditions. A method to clear such microbubbles has been proposed by Kajiyama et al. [Physics Procedia 3 (2010) 305-314]. In the method, the irradiation of intense ultrasounds with a burst waveform fragmentize microbubbles in the pathways before the irradiation of ultrasounds for tissue heating. The vitro experiment using a gel containing microbubbles has showed that the method enables to heat the target correctly by controlling the microbubble distribution. Following the experiment, we simulate the focusing of ultrasounds through a mixture containing microbubbles with considering the size and number density distributions in space. The numerical simulation shows that the movement of the heating region from the transducer side to the target by controlling the microbubble distributions. The numerical results elucidate well the experimental ones. [Preview Abstract] |
Monday, November 21, 2011 12:27PM - 12:40PM |
H14.00010: A train of rising Bretherton bubbles Michael J. Davis, Peter S. Stewart, Stephen H. Davis Abstract: A closely fitting gas bubble in a vertically aligned capillary tube will rise due to buoyancy when the Bond number exceeds a critical value in the limit of low Capillary number, as shown by Bretherton [{\it J. Fluid Mech.} {\bf 10}, 1961]. We consider a steadily propagating train of such bubbles at various separation distances, and examine the additional influence of a temperature gradient along the walls of the tube. This problem is applicable to processes for manufacturing porous metal solids, where molten foams with low liquid fraction are solidified by an applied temperature drop. We seek to show that gravity can act as a means of control of the porosity of the foam as the liquid is cooled to its melting point. [Preview Abstract] |
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