Bulletin of the American Physical Society
62nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 54, Number 19
Sunday–Tuesday, November 22–24, 2009; Minneapolis, Minnesota
Session GJ: Bubbles IV: Microbubbles, Bubble Motion |
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Chair: Andrea Prosperetti, The Johns Hopkins University Room: 101I |
Monday, November 23, 2009 8:00AM - 8:13AM |
GJ.00001: Microbubbles transfer and segregation mechanisms in turbulent upward/downward channel flow. Dafne Molin, Andrea Giusti, Alfredo Soldati The dispersion of microbubbles in a turbulent channel flow is studied by means of direct numerical simulation (DNS), both in upward and downward flow with a two-way coupling approach. Microbubbles dispersion shows a sharply distinct behavior in the two flow cases: in upward flow bubbles tend to accumulate and segregate near the walls, whereas in the downward flow they tend to segregate in the center of the channel. This different spatial distribution, which is due to the interplay between turbulent wall transfer mechanisms and the local fluid forces acting on bubbles (especially the lift force), is expected to have an influence on the flow field. In this work, we present detailed results from a systematic analysis on the effect of the different forces acting on bubbles and how the flow statistics are modified by the presence of bubbles. [Preview Abstract] |
Monday, November 23, 2009 8:13AM - 8:26AM |
GJ.00002: Study of dynamics of microbubble generation in microchannels Ryoji Miyazaki, Yoshimasa Goshima, Shu Takagi, Yoichiro Matsumoto The novel technique to generate micrometer-order bubbles was developed by using a microchannel with a squeezed T-junction, and the mechanism of bubble generation was investigated by using a high-speed camera with 106 Hz and the microscopy. The experiments were conducted by using three kinds of channels with the different cross-section size, and pure water, ethanol and silicon oil were selected as the liquid phase to examine the effect of the cross-section size of the channels and the physicality of the liquid phase. The liquid velocity at the T-junction and the gas pressure were set at 0.1$\sim $3.0 m/s and 10$\sim $200 kPa, respectively. The experimental results indicate that the proposed technique realizes to generate 10$\sim $30 $\mu $m diameter bubbles, and the diameter of the generated bubble becomes smaller with an increase of the liquid velocity, until limit points of bubble generation. From the experiment near the bubble generated limit, liquid pressure balances with the gas pressure and the Laplace pressure under the bubble generated limit, and the bubble diameter is dominated by Weber number which is defined using an equivalent diameter of the cross-section of the channel and the mean velocity of the liquid phase. [Preview Abstract] |
Monday, November 23, 2009 8:26AM - 8:39AM |
GJ.00003: Numerical study on the behavior of a microbubble encapsulated by hyperelastic membrane in the ultrasound field Yunqiao Liu, Kazuyasu Kazuyasu Sugiyama, Shu Takagi, Yoichiro Matsumoto The surface stability problem of a microbubble encapsulated by a neo-Hookean hyperelastic membrane is numerically addressed. To predict this nonlinear behavior, the continuity equation and the Navier-Stokes equation are directly solved by means of the boundary-fitted finite-volume method on an orthogonal curvilinear coordinate system. The force balances of the membrane are derived from the traction jump condition, coupling with the in-plane tensions and transverse shear tension. The bubble is insonified by an ultrasound pulse at frequency of 1MHz and consisting of a burst of 10 cycles. The strain-softening features are presented referring to a linear model based on the Rayleigh-Plesset equation. For small acoustic amplitude, the result based on the neo-Hookean model is in good agreement with that on the linear model. With the increasing of oscillatory amplitude, the neo-Hookean membrane bubble shows an enhanced strain-softening effect -- larger expansion, smaller contraction and higher harmonics during contraction. In addition, the neo-Hookean membrane bubble presents second-order shape instability. At the same time, this second-order mode shows subharmonics characteristics, which is considered as a potential medical application for ultrasonic imaging. [Preview Abstract] |
Monday, November 23, 2009 8:39AM - 8:52AM |
GJ.00004: Effect of Ultrasound-Induced Bjerknes Force on the Dynamics of Microbubbles. Interaction with Saffman's lift Alberto Aliseda, Cheryn Engebrecht We will discuss results of experiments on the trajectories of Ultrasound Contrast Agents immersed in low Reynolds steady flow in a pipe. The microbubbles are subject to hydrodynamic forces, and are under the effect of external ultrasound forcing propagating normal to the flow direction. High speed visualization of the microbubbles trajectories shows significant deviations in the direction perpendicular to the flow. This displacement is due to the balance of the Bjerknes force and Saffman's lift. The dependency of the value and orientation of the microbubbles trajectories indicates a rich mechanism for the coupling between these two forces. In the absence of ultrasound excitation, Saffman's lift forces the microbubbles towards the wall. The volume oscillations induced on the microbubble by the propagating ultrasonic pressure waves significantly modify the lift, reversing its direction and making it away from the wall. [Preview Abstract] |
Monday, November 23, 2009 8:52AM - 9:05AM |
GJ.00005: Numerical Simulation of a Bubble Bouncing with a Free Surface Toshiyuki Oyama The paper presents a numerical study of a bubble-bouncing with a free surface using a three dimensional front-tracking method. According to our preliminary study, the bubble-free surface interaction is summarized as follows. The bubble becomes slightly oblate as it propels upward, and the bubble starts contacting at the side, rather than the top, to the elevated free surface. Then the liquid in film between the bubble and free surface is gradually drained until the bubble reaches the highest position. Finally, the bubble bounces back from the free surface due to the stored energy on the both of the surfaces and the self-induced flow field. We focus in the rebound depth, and duration time of bubble-free surface contact (contact time, hereafter). The contact time measured from the distance between the bubble center and free surface exhibits -0.5 power of surface tension coefficient, whereas the contact time based on the distance between the bubble top and free surface was found to be insensitive to surface tension coefficient. In the presentation, we also discuss the velocity field within the liquid film and the time-dependency of the film volume. [Preview Abstract] |
Monday, November 23, 2009 9:05AM - 9:18AM |
GJ.00006: Multiscale interactions of bubbles with free vortex flows Justin Finn, Ehsan Shams, Sourabh Apte We simulate bubble and particle interactions with several types of free vortex flows using both a Discrete Element Model (DEM) and a fully resolved approach. In the DEM approach, DNS is used with Lagrangian particle tracking to compute the motion of a subgrid scale dispersed phase. The {\it volumetric} displacement of the fluid by the dispersed phase is modeled along with interphase momentum-exchange for more realistic coupling of the dispersed phase to the flow. In the fully resolved approach, a fictitious domain technique is used with refined grids to directly compute the motion of the dispersed phase to obtain high fidelity solutions. First, both approaches are used to simulate bubble entrainment into a stationary Gaussian vortex [Oweis et al. 2005]. Next, bubble entrainment and interaction with a traveling vortex tube [Sridhar \& Katz 1999] is simulated using the DEM approach. Finally, a viscous falling `blob' of particles is simulated [Walther \& Koumoutsakos 2001, Mitts 1995], where the dispersed phase generates and interacts with a 3D vortex ring. The results show that the less expensive DEM approach with volumetric coupling is able to capture clustering induced flow distortion, while the fully resolved approach gives insight into dispersed phase scale interactions with the flow. [Preview Abstract] |
Monday, November 23, 2009 9:18AM - 9:31AM |
GJ.00007: Power spectral density in mono-dispersed bubbly flows Santos Mendez, Roberto Zenit An experimental study was carried out to determinate the power spectral density functions of mono-dispersed bubbly flows in a vertical channel using flying hot-film anemometry. To improve the phase discrimination technique, an optic fiber was attached to the hot-film sensor. In this manner, it was possible to clearly separate the erroneous signals caused by bubble collision with the sensor. A special array of capillaries was used to produce nearly mono-dispersed flow. Measurements were performed with gas fractions up to 6$\%$. The power spectral density distributions were found to have a good qualitative agreement with those obtained by other authors. Depending on the values of the Reynolds number and gas volume fraction a progressive change from a $-5/3$ to $-3$ decay was observed. [Preview Abstract] |
Monday, November 23, 2009 9:31AM - 9:44AM |
GJ.00008: Self propulsion of bubbles in wedge-shaped geometries Howard Stone, Thibault Scoarnec, Ann Lai, Mathilde Reyssat Self propulsion of bubbles and drops can be created by geometrically forcing capillary pressure gradients. We investigated such self propulsion experimentally by confining long bubbles in flat wedge-shaped geometries that have rectangular cross sections and are closed at both ends. The bubble moves from the narrow end toward the wider end with a speed that monotonically decreases in time. The fluid motion past the bubble occurs through the corners between the bubble and walls of the rectangular cross-section, so that the fluid flow is fully three dimensional. In order to quantitatively describe the motion of the bubble we introduce a one-dimensional model in the spirit of lubrication theory. The predictions of the model are in very good agreement with the experimental measurements and capture the variations with bubble size, wedge angle, and viscosity of the continuous phase. [Preview Abstract] |
Monday, November 23, 2009 9:44AM - 9:57AM |
GJ.00009: Change of \textit{Re} dependency of single bubble 3D motion by surface slip condition in surfactant solution Yoshiyuki Tagawa, Ami Funakubo, Shu Takagi, Yoichiro Matsumoto Path instability of single bubble in water is sensitive to surfactant. One of the key effects of surfactant is to decrease bubble rising velocity (i.e. increase drag) and change bubble slip condition from free-slip to no-slip. This phenomenon is described as Marangoni effect. However, the surfactant effect to path instability is not fully investigated. In this research, we measured bubble 3D trajectories and velocity in dilute surfactant solution to reveal the relation between 3D motion mode and slip condition. Experimental parameters are types of surfactants, concentrations and bubble sizes. Bubble motions categorized as straight, spiral or zigzag are plotted on two-dimensional field of bubble Reynolds number \textit{Re} and normalized drag coefficient $C_{D}^{\ast }$ which is strongly related to surface slip condition. Range of \textit{Re} is from 200 to 1000 and $C_{D}^{\ast }$ is from 0 to 1. Our results show that when $C_{D}^{\ast }$ equals 0 or 1 (free-slip condition or no-slip condition, respectively), bubble motion mode is changed by \textit{Re}. However when $C_{D}^{\ast }$ is 0.5, bubble motion is always spiral. It means that \textit{Re} dependency of bubble motions is strongly affected by slip condition. We will discuss its mechanism in detail in our presentation. [Preview Abstract] |
Monday, November 23, 2009 9:57AM - 10:10AM |
GJ.00010: Slowing down bubbles with sound Cedric Poulain, Remie Dangla, Marion Guinard We present experimental evidence that a bubble moving in a fluid in which a well-chosen acoustic noise is superimposed can be significantly slowed down even for moderate acoustic pressure. Through mean velocity measurements, we show that a condition for this effect to occur is for the acoustic noise spectrum to match or overlap the bubble's fundamental resonant mode. We render the bubble's oscillations and translational movements using high speed video. We show that radial oscillations (Rayleigh-Plesset type) have no effect on the mean velocity, while above a critical pressure, a parametric type instability (Faraday waves) is triggered and gives rise to nonlinear surface oscillations. We evidence that these surface waves are subharmonic and responsible for the bubble's drag increase. When the acoustic intensity is increased, Faraday modes interact and the strongly nonlinear oscillations behave randomly, leading to a random behavior of the bubble's trajectory and consequently to a higher slow down. Our observations may suggest new strategies for bubbly flow control, or two-phase microfluidic devices. It might also be applicable to other elastic objects, such as globules, cells or vesicles, for medical applications such as elasticity-based sorting. [Preview Abstract] |
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