Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session G11: Bubbles II |
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Chair: Sadegh Dabiri, University of Notre Dame Room: 26A |
Monday, November 19, 2012 8:00AM - 8:13AM |
G11.00001: Motion of a bubble ring in a viscous fluid Jing Lou, Ming Cheng, T.T. Lim A Lattice Boltzmann Method and limited experiments were undertaken to study the dynamic of a vortex ring bubble (or bubble ring) in a viscous incompressible fluid. The study is motivated partly by our desire to assess whether a bubble ring keeps increasing its radius and decreasing its propagation velocity as it rises through fluid was predicted by Turner (1957) and Pedley (1968) or does the ring eventually reach a steady state where its radius and velocity remain constant as was predicted by Joseph et al (2008). The parameters investigated included ring circulation, Reynolds number, density ratio and Bond number. Our experimental and numerical results show that a rising bubble ring increases its radius and decreases its velocity, but the process is interrupted by ring instability that eventually causes it to break up into smaller bubbles. For the range of flow conditions investigated, there is no evidence that a bubble ring has attained a constant speed and constant radius before breaking up. Furthermore, it is found that increasing initial circulation has a stabilizing effect on a bubble ring while increasing Reynolds number or Bond number hastens ring instability, resulting in an earlier break up into smaller bubbles. [Preview Abstract] |
Monday, November 19, 2012 8:13AM - 8:26AM |
G11.00002: Rising of Taylor Bubble through a Liquid-Liquid Interface Tee Tai Lim, Rajeev Jaiman, Huang Sha A gas bubble moving through liquids in a round tube can exhibit various behaviors of both theoretical and practical interest. When the tube diameter is not too large, the large bubbles (i.e., Taylor bubbles) are smooth and glossy, with a bullet-shaped nose, and they rise at a constant velocity along the axis of the tube. The present study aims at investigating the rising of a Taylor bubble through a liquid-liquid interface both experimentally by using high-speed video camera and numerically through the volume-of-fluid (VOF) approach based on the adaptive-refinement. When a bubble risers in a liquid and eventually through a liquid-liquid interface, the interfacial force is transferred to the buoyancy and viscous forces. The layer of the heavy surrounding liquid between the bubble and the interface (termed as thin-film) prevents the bubble from reaching the light phase instantaneously. After some time, significant viscous forces, the pressure gradient within the thin-film acts to drain and eventually the film becomes exceedingly small; and bubble achieves a higher terminal rise velocity and the bubble is elongated. The preliminary results obtained by the VOF approach are in qualitative agreement with the experiment. [Preview Abstract] |
Monday, November 19, 2012 8:26AM - 8:39AM |
G11.00003: Modeling the drainage of viscous bubbles Casey Bartlett, Matthieu Santin, James Bird The lifespan of viscous thin film bubbles are largely dictated by the drainage dynamics of the film. For large enough bubbles, these dynamics are driven by gravity and regulated by viscosity. Past models have assumed that these forces lead to a drainage velocity that increases monotonically with increasing angle from the center axis. Here we show alternative solutions more consistent with experimental data where drainage velocity is not monotonic. We use a combination of numeric and analytic approaches to determine the evolution of the film drainage and investigate if such evolution can be approximated with a self-similar profile. Finally we compare our model results to recent experimental data. [Preview Abstract] |
Monday, November 19, 2012 8:39AM - 8:52AM |
G11.00004: Measuring axisymmetric drainage of large viscous bubbles James Bird, Casey Bartlett, Matthieu Santin Large bubbles on the surface of a viscous liquid can be stable for many minutes, even in the absence of surfactants. Over time the thickness of these bubbles evolve as the liquid film drains under the influence of gravity. Past interferometry measurements have shown that the film thickness at the top of a viscous bubble decays exponentially -- which is consistent with current theories. However these models rely on drainage assumptions away from the centerline, assumptions that have been yet to be validated experimentally. In this talk we present measurements for both the film thickness and drainage velocity along viscous bubbles. Our results demonstrate that current models dramatically under-predict the film thickness away from the centerline. We demonstrate why the dynamics are more subtle than previously assumed, and we offer a model that is consistent with our measurements. [Preview Abstract] |
Monday, November 19, 2012 8:52AM - 9:05AM |
G11.00005: DNS of rising bubbles in a vertical homogeneous shear flow Sadegh Dabiri The bubbly flow occurs in many natural and industrial situations such as boilers and bubble column reactors. In many of these flows, bubbles rise inside a shear layer. Interaction between bubbles and the shear creates a lateral lift force on the bubbles and affects their distribution in the domain which in turn will affect the drag force on the flow and the flow rate. Here, the rising motion of buoyant bubbles in a homogeneous shear flow in vertical direction is studied. In order to create a homogeneous shear flow, periodic boundary condition in all three directions is implemented. A finite difference method with front tracking is developed that satisfies the periodic-shear boundary condition. The effect of the deformability of bubbles on the magnitude and direction of the lateral lift force is discussed. [Preview Abstract] |
Monday, November 19, 2012 9:05AM - 9:18AM |
G11.00006: Size-differentiated lateral migration of bubbles in Couette flow of two-dimensional foam Hadi Mohammadigoushki, James J. Feng In this Talk, we report experiments on lateral migration of bubbles in a two-dimensional foam sheared in a narrow-gap Couette device. A larger bubble in an otherwise monodisperse bubble raft migrates toward the center of the gap as long as the bubble size ratio and the shear rate are each above a threshold. The migration speed is roughly two orders of magnitude higher than that of a single bubble, and increases with the shear rate and the size ratio. The bubble also deforms much more than an isolated one at the same shear rate. Modifying the Chan-Leal solution for the migration of a single submerged bubble or drop, we derive a formula that successfully predicts all the migration trajectories recorded in the experiment. The threshold for migration corresponds to the wall repulsion force overcoming the capillary force in the 2D foam. The size-differentiated bubble migration provides an explanation for previously observed size segregation in sheared 3D polydisperse foams. [Preview Abstract] |
Monday, November 19, 2012 9:18AM - 9:31AM |
G11.00007: Numerical simulations of bubble dynamics at high Reynolds numbers Saul Piedra, Eduardo Ramos We present a three-dimensional numerical simulation of air bubbles rising in water. The analysis is based on the solution of the conservation equations combined with a front tracking method to represent an interface between two immiscible fluids. The interfacial forces incorporate the effect of the surface tension and the material properties of the fluids are calculated in the entire integration domain. In order to follow the bubbles along thousands of diameters in its ascending motion, a moving reference frame technique is used. The shape of the bubbles, the pressure and the velocity fields at different flow conditions calculated with our model are in agreement with experimental observations reported in the literature. Also, the qualitative change in the trajectory of the bubbles from rectilinear to zig-zag to helical motion is reproduced by the model. Dominant physical effects in each mode of displacement are described. [Preview Abstract] |
Monday, November 19, 2012 9:31AM - 9:44AM |
G11.00008: Bubble Transport through Micropillar Arrays Kenneth Lee, Omer Savas In current energy research, artificial photosynthetic devices are being designed to split water and harvest hydrogen gas using energy from the sun. In one such design, hydrogen gas bubbles evolve on the catalytic surfaces of arrayed micropillars. If these bubbles are not promptly removed from the surface, they can adversely affect gas evolution rates, water flow rates, sunlight capture, and heat management of the system. Therefore, an efficient method of collecting the evolved gas bubbles is crucial. Preliminary flow visualization has been conducted of bubbles advecting through dense arrays of pillars. Bubbles moving through square and hexagonal arrays are tracked, and the results are qualitatively described. Initial attempts to correlate bubble motion with relevant lengthscales and forces are also presented. These observations suggest how bubble transport within such pillar arrays can be managed, as well as guide subsequent experiments that investigate bubble evolution and collection. [Preview Abstract] |
Monday, November 19, 2012 9:44AM - 9:57AM |
G11.00009: Bubbles dancing in a vortex: trapping air at a T-junction Daniele Vigolo, Nathan Tyrell, Stefan Radl, Howard Stone We present an unusual phenomenon that occurs to low density material, and in particular air bubbles, entrained in a fluid when flowing through a T-junction. For a range of Reynolds numbers, the flow develops two symmetric vortices. Air bubbles are forced to the center of the vortex due to the centrifugal force and, for Reynolds number, $Re$, greater than $\approx 220$, are then ``trapped'', i.e. they accumulate inside the vortex. Bubbles eventually oscillate (i.e. ``dance'') in the vortex when the flow becomes unsteady for $Re>550$. Experiments were conducted by generating H$_2$, O$_2$ or simply air bubbles in the range $Re = 100$ to $\approx 6,000$ in a variety of T-junction devices. We have also observed a size dependence of the trapping phenomenon. In addition, our 3D numerical simulations have revealed a gradient of pressure, similar to vortex breakdown, that drives the flow towards the center of the T-junction creating two recirculating zones, which trap air bubbles. The presence of light material or air trapped in a flow could be relevant to industrial systems and biological flows, such as blood vessels, and may contribute to unexpected complications and/or failures in these systems. [Preview Abstract] |
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