Bulletin of the American Physical Society
61st Annual Meeting of the APS Division of Fluid Dynamics
Volume 53, Number 15
Sunday–Tuesday, November 23–25, 2008; San Antonio, Texas
Session EH: Bubbles III |
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Chair: Prabir Daripa, Texas A&M University Room: 101B |
Sunday, November 23, 2008 4:10PM - 4:23PM |
EH.00001: Numerical study on the drag coefficient for an ellipsoidal bubble with fore-aft asymmetry Toshiyuki Sanada, Shu Takagi, Takayuki Saito We evaluate the drag coefficient for ellipsoidal clean bubbles rising steadily at high \textit{Re}. Flow fields and bubble shapes are obtained using a numerical simulation. The method is based on a finite-difference solution of the equation s of motion on an orthogonal curvilinear coordinate system [Takagi et al., \textit{Phys. Fluids} (1994), Ryskin {\&} Leal, \textit{J. Fluid Mech.} (1984)]. The degree of fore-aft asymmetric bubble shape is quantitatively evaluated using Legendre polynomials. The numerically obtained drag coefficients are compared with those of experimental results. In addition, by comparing the drag coefficients with those for symmetric ellipsoidal bubble obtained analytically by Moore [\textit{J. Fluid Mech.} (1965)], and via numerical simulation by Blanco {\&} Magnaudet [\textit{Phys. Fluids} (1995)], the effect of fore-aft asymmetry on a drag coefficient is evaluated. Furthermore the formation of the standing eddy at the rear of deformable bubbles is discussed. [Preview Abstract] |
Sunday, November 23, 2008 4:23PM - 4:36PM |
EH.00002: Turbulence characteristics of liquid motion induced by single rising bubble Koichi Morikawa, Toshiyuki Sanada, Takayuki Saito Characteristics of the surrounding liquid motion of a single rising bubble were experimentally investigated. A single bubble with three kinds of diameters was released under complete control of initial diameter, orientation and launch timing using a bubble generator. Especially, we focused on the bubbles with zigzagging motion. Both the surrounding liquid motion and bubble motion were simultaneously visualized via PIV measuremen. Vorticity distribution and standard deviation of liquid-phase velocity were calculated. Moreover, we analyzed the turbulence intensity of liquid-phase motion induced by the bubble within measurement time. We clarify both the distribution of disturbance and the intensity of disturbance. As a result,each bubble formed various vorticity distributions in its vicinity during the bubble rising. On the other hand, the turbulence intensity of the liquid-phase velocity was obviously different. Associated with increase in the bubble radius, the area of disturbance region induced by bubble was spread. The horizontal component of the turbulence intensity was increased by the bubble with significant interface motion. [Preview Abstract] |
Sunday, November 23, 2008 4:36PM - 4:49PM |
EH.00003: Evolution and axial symmetry breaking of the toroidal vortex behind a clean bubble Minori Shirota, Kiyonori Koga, Ayaka Sato, Toshiyuki Sanada, Masao Watanabe We experimentally observed the wake structure behind a rising clean bubble by using silicon oil solution of photochromic dye. Clean bubble condition was realized since both the dye and the silicon oil are non-polar. A single bubble was generated just below a colored region where the dye was activated by UV sheet light illumination. Once the bubble passed the colored part of the liquid, the bubble was accompanied by some portion of activated dye. Hence the flow structure in the rear of the single rising bubble was visualized. In this visualization method, we are able to distinguish the liquid portion trapped behind the bubble from the non-colored surrounding liquid that flows in the colored trapped portion. We precisely controlled the size of the bubble in order to observe how the size of the toroidal vortex behind a bubble evolves and the axial symmetry breaks. The relation between the in-flow of the surrounding liquid into the toroidal vortex and the bubble motion was studied in detail. [Preview Abstract] |
Sunday, November 23, 2008 4:49PM - 5:02PM |
EH.00004: Magnetic Bubbles Xue Zhao, Pedro Quinto-Su, Claus-Dieter Ohl Bubbles in liquids driven by a sound field are used in many disciplines: for example bubbles clean surfaces in ultrasonic water bathes, they catalyze unique chemical reactions in sonochemistry, and under special conditions even create light. However, conventional bubbles have a major limitation when placed in an acoustic field: it is extremely hard to control their position. Here we present a new type of bubble that has permanent magnetization originating from a shell of self-assembled nanoparticles, so that magnetic fields can be used to control the bubble's position independently. We will report on the recipe and the experiment to study bubble oscillations in weak magnetic fields. The magnetic susceptibility of the bubbles is proportional to their surface area,$\chi =(9\pm 3\times 10^{-6}m)r^2$, where $r$ is the radius. Also they are compressible in moderate acoustic fields and induce a microstreaming flow with a toroidal vortex at the upper pole of the bubble. Similar microstreaming flows have been used to transport and rupture cells at small scales. Thus we envision applications in manipulation of biological materials and in microfluidic devices using acoustic and magnetic forces. [Preview Abstract] |
Sunday, November 23, 2008 5:02PM - 5:15PM |
EH.00005: Interaction of gas bubbles with a shock wave near a solid boundary Stephen Shaw, Peter Spelt, Omar Matar The interaction of both single and multiple gas bubbles in water with an initially planar shock in the neighbourhood of a solid boundary is considered. The compressible Euler equations in each phase are solved in axisymmetric and 3-D Cartesian geometry using up to a third-order accurate ENO-Roe scheme for the spatial fluxes in characteristics space; the solutions are evolved temporally using a third-order accurate TVD RK method. The interface between the water and gas phases is tracked with a level set function and interfacial boundary conditions are imposed using the Ghost Fluid method. The solid boundary is captured by employing reflection computational boundary conditions. In the case of a single bubble, the effect of the shock strength and of the initial location of the bubble relative to the boundary on the resultant bubble shapes, liquid jet shapes and velocities is assessed. The importance of the shock wave reflection from the boundary on the resultant dynamics is studied, together with incurred modifications due to bubble multiplicity. This work has applications in shock wave lithotripsy, cavitation-induced damage and surface cleaning. [Preview Abstract] |
Sunday, November 23, 2008 5:15PM - 5:28PM |
EH.00006: On the stabilization of surface nanobubbles Detlef Lohse, Bram Borkent, Michael Brenner, Holger Schoenherr Recent experiments have convincingly demonstrated the existence of surface nanobubbles on submerged hydrophobic surfaces. However, classical theory dictates that small gaseous bubbles quickly dissolve because their large Laplace pressure causes a diffusive outflux of gas. Here we first present atomic force microscopy (AFM) data on the geometric shape of these surface nanobubbles over more than one decade in magnitude, revealing that the contact angle $\theta$ (defined on the gas side) goes to zero for small surface nanobubbles, which leads to a reduction of the Laplace pressure. Based on these data, we then suggest that the surface bubbles are further stabilized by a continuous influx of gas near the contact line, due to the gas attraction towards hydrophobic walls. This influx balances the outflux and allows for a meta-stable equilibrium, which however vanishes in thermodynamic equilibrium. [Preview Abstract] |
Sunday, November 23, 2008 5:28PM - 5:41PM |
EH.00007: Bubble pinch-off in a stagnant liquid pool at high Reynolds numbers Roc\'Io Bola\~nos-Jim\'enez, Marco Rivetti, Alejandro Sevilla, Carlos Mart\'Inez-Baz\'an, Jos\'e Manuel Gordillo We present a theoretical, numerical and experimental study of the \emph{symmetric} pinch-off which takes place when a bubble is grown from a nozzle placed at the bottom of a stagnant pool of liquid. Our experiments show that the initial stages of bubble pinch-off are driven not only by surface tension, but also the by the liquid hydrostatic pressure. Moreover, we obtain a simple scaling law for the global collapse time which is shown to be consistent with the experimental results. Boundary integral numerical simulations are also shown to be in excellent agreement with the experiments. In addition, we discuss the dynamics of the final stages previous to pinch-off, providing with a simple model, based on the \emph{local slenderness} around the neck, which is shown to closely reproduce the time evolution of both the minimum radius and the local axial curvature once surface tension and viscous effects are self-consistently incorporated. This is confirmed by experiments performed with water as well as with different silicone oils. Finally, the dynamics of azimuthal perturbations around the zeroth-order collapse solution is addressed. [Preview Abstract] |
Sunday, November 23, 2008 5:41PM - 5:54PM |
EH.00008: Evolution of Bubbles through Gas Injection from a Micro-Tube into Liquid Cross-Flow Sina Ghaemi, Payam Rahimi, David Nobes Generation of small-size bubbles is of importance in many processes such as chemical, medical and food industries. The most common method of bubble generation is injection of gas from an orifice into the liquid phase. In spite of simplicity of this method, appropriate conditions should exist to avoid bubble growth and obtain required small-size bubbles. Thorough understanding of the bubble formation and growth can reveal the required conditions and ensure detachment of the bubbles from the orifice with desired timing to control their size. In this work, evolution of bubbles from a micro-size gas injection tube into liquid cross-flow is investigated. Special attention has been devoted to optimize the conditions to generate micro-size bubbles. Specifically, the influence of gas injection tube size and location, gas and liquid Reynolds numbers and the geometry of the mixing chamber on the bubbles evolution is studied. High-speed shadowgraphy technique is applied to investigate bubbles size and shape. A Particle Tracking Velocimetry algorithm is also applied to calculate bubbles velocity. The velocity field of the liquid flow surrounding the bubbles is also characterized using a Mirco-Stereo-Particle Image Velocimetry technique. [Preview Abstract] |
Sunday, November 23, 2008 5:54PM - 6:07PM |
EH.00009: The effect of bubbles on flow structure and heat transfer in laminar boundary layers Vladimir Ajaev, David Brutin, Lounes Tadrist We develop a mathematical model of the effect of a vapor or gas bubble trapped between liquid and a flat plate on the laminar boundary-layer-type flow in the liquid. For very thin bubbles the effect amounts to simple modification of the no-slip condition, but as the bubble height increases, there are significant changes in the structure of the boundary layer formed behind the bubble. We investigate how these changes affect the heat transfer in the boundary layer. The model is used to explain some recent surprising experimental findings showing an increase of the wall heat transfer coefficient during boiling under the microgravity conditions compared to its value under normal gravity. [Preview Abstract] |
Sunday, November 23, 2008 6:07PM - 6:20PM |
EH.00010: Direct numerical simulation of single gas bubbles in pure and contaminated liquids Peter Lakshmanan, Peter Ehrhard Disperse gas bubbles play an important role in many industrial applications. Knowing the rising velocity, the interfacial area, or the critical size for break-up or coalescence in different systems can be crucial for the process design. Hence, knowing the fundamental behaviour of a single bubble appears mandatory for the examination of bubble swarms and for the Euler-Lagrange or Euler--Euler modelling of disperse systems. In the present work a level--set--based volume--tracking method is implemented into the CFD--code OpenFOAM to follow the free interface of a single bubble. The volume-tracking method is coupled with a transport model for surfactants on the interface, including adsorption and desorption processes. The dependency of surface tension on the local surfactant concentration on the interface is modelled by a non-linear (Langmuir) equation of state. Marangoni forces, resulting from surface tension gradients, are included. The rise of a single air bubble (i) in pure water and (ii) in the presence of surfactants of different strengths is simulated. The results show good agreement with available (experimental and theoretical) correlations from literature. [Preview Abstract] |
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