Bulletin of the American Physical Society
2006 59th Annual Meeting of the APS Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2006; Tampa Bay, Florida
Session LF: Drops and Bubbles VIII |
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Chair: Howard Stone, Harvard University Room: Tampa Marriott Waterside Hotel and Marina Florida Salon 4 |
Tuesday, November 21, 2006 8:00AM - 8:13AM |
LF.00001: Uncertainty quantification in bubble dynamics Tim Colonius, Rob Hagmeijer We examine models for the statistics of bubble dynamics when the equilibrium radius is a random variable. Such statistics are likely to be in important in continuum (phase-averaged) bubbly flow models, but these models typically assume a single known value for equilibrium radius. For the case of linearized bubble dynamics, the temporal evolution of the moments of the joint probability distribution function of bubble radius, bubble radial velocity, and equilibrium radius is examined. A formula for statistical equilibrium for the case of high Reynolds and Weber numbers is derived. These results are compared to standard reduced-order polynomial chaos expansions of the Rayleigh-Plesset equation and improved models are suggested. Acoustic wave propagation in bubbly media is considered as an application of the linear models. Strategies and preliminary results for the case of nonlinear bubble dynamics will also be presented. [Preview Abstract] |
Tuesday, November 21, 2006 8:13AM - 8:26AM |
LF.00002: Bubbling in a coflow at high Reynolds numbers A. Sevilla, J.M. Gordillo, C. Martinez-Bazan Bubble formation from a needle in a co-flowing liquid environment is studied in detail by means of experiments and boundary-integral numerical simulations. Two different gas injection systems respectively provide constant flow rate and constant pressure conditions. In both cases, a bubbling period can be divided into an expansion stage, which ends with the appearance of a neck, followed by its collapse. Our experimental and numerical results are in excellent agreement, and suggest that the expansion time can be viewed as the time required for an interfacial wave to propagate distances of the order of the injection needle diameter. Two different mechanisms contribute to the collapse stage: on the one hand, the bubble growth induces an overall decrease in pressure inside the bubble and, on the other hand, the region around the collapsing neck experiences an additional pressure drop caused by the Kelvin-Helmholtz-Rayleigh mechanism. Both experimental and numerical bubbling frequencies under constant pressure conditions are smaller than those corresponding to constant flow conditions, what is explained by means of a simple model. Finally, the density of the gas is shown to modify bubbling frequencies \emph{only under constant pressure conditions}. [Preview Abstract] |
Tuesday, November 21, 2006 8:26AM - 8:39AM |
LF.00003: Dynamics and Noise Emission of Vortex Cavitation Bubbles in Single and Multiple Vortex Flow Steven Ceccio, Jaehyug Choi, Natasha Chang, Ryo Yakushiji, David Downling The growth, splitting, and collapse of single vortex cavitation bubbles were examined experimentally for three flow configurations: a single vortex, a vortex that experiences a pressure drop and recovery as it flows through a Venturi, and a vortex that is stretched through interactions with a second, stronger vortex. The underlying vortical flows were characterized and related to the dynamics and noise emission of the cavitation bubbles. Noise was detected during bubble growth, splitting, and collapse. Highly deformed bubbles produced less noise than equivalent spherical bubbles under similar flow conditions during collapse, and, the noise produced during collapse was often much greater that the noise produced during growth and splitting. While the traditional scaling variables of vortex cavitation ($i.e. \Gamma _O$, $r_C $, $\sigma _C $, $r_{b,M} )$ are important parameters that will scale the basic features of the bubble inception, growth, and collapse, the dynamics of vortex cavitation bubbles are not uniquely determined by the non-cavitating vortex properties. Instead, inception, dynamics, and noise production of vortex cavitation bubbles is strongly influenced by the details of the underlying flow field and nuclei distribution. This makes the formulation of general scaling rules problematic. [Preview Abstract] |
Tuesday, November 21, 2006 8:39AM - 8:52AM |
LF.00004: Bubbly flow in a low pressure evaporator and condenser Simo Makiharju, William Schultz, Herman Merte The use of bubbles in a low pressure evaporator and condenser is examined theoretically, experimentally and with simplified computations. A desalination apparatus operating over small temperature differences, approximately 10K, to use waste heat is the motivation for the research. Because of the very small temperature differences and the desire to have compact equipment, use of direct contact condensation or evaporation using bubbles or drops is advantageous. The practical aim is to obtain a simple numerical model to be used in an optimization scheme and hence the model has to be fast to execute. For this reason the heat transfer between the liquid and the dispersed vapor bubbles was to be modeled using simplified correlations. The very low apparatus pressure, 3 to 14 kPa, pressed the need for experimentation as well. Noncondensable gases will be present a further complication. In most of the preliminary research we chose to neglect the effect of these gases, however they will be considered progressively more and the practical requirement is to at least obtain a qualitative correlation counting for their effects. No correlation in the existing literature was found to be valid in the parameter range encountered in the device. Bubbles grow or shrink considerably as a consequence of phase change and the large hydrostatic pressure differences. [Preview Abstract] |
Tuesday, November 21, 2006 8:52AM - 9:05AM |
LF.00005: Bubbles from an Underwater Nozzle: Scaling and Memory at Pinch-Off Nathan C. Keim, Wendy W. Zhang, Sidney R. Nagel The pinch-off of air bubbles underwater is a non-universal singularity that retains a memory of nozzle shape and size.\footnote{N.C.~Keim, P.~M{\o}ller, W.W.~Zhang, S.R.~Nagel. arXiv:cond-mat/0605669 (2006)} Using high-speed video, we have studied the evolution of the pinching neck of air, with both water and ethanol as outer fluids in order to examine the role of surface tension. We find that while the scaling exponent of the radial length scale is independent of nozzle size, its axial scaling has an exponent that varies significantly with nozzle diameter. We have also systematically studied how the cylindrical asymmetry of initial conditions (\emph{e.g.}\ nozzle tilt angle) is remembered at pinch-off, as measured by the number and size of satellite bubbles, and the distortion of the singular neck shape. [Preview Abstract] |
Tuesday, November 21, 2006 9:05AM - 9:18AM |
LF.00006: A boiling heat transfer surface for creating a single stream of vapor bubbles. Zachary W. Douglas, Marc K. Smith, Ari Glezer The high heat transfer rate characteristic of boiling is limited by the vapor-bubble removal rate and a critical transition to film boiling. External forces, such as acoustic waves, can be used to enhance vapor-bubble removal and improve heat transfer. In order to explore such enhanced vapor-bubble removal processes, a boiling heat transfer surface has been designed to control the location, growth, and detachment of a single stream of vapor bubbles. The device consists of an insulating annulus surrounding a thermally conductive pin 1 mm in radius. The upper surfaces of the annulus and the pin have a thin polished electro-plated copper coating. When heated from below, the pin provides a thermal conduit that creates a local hot spot on the copper surface. The majority of all bubble nucleations occur within 3 mm of the center of the pin. A thin hydrophobic coating centered on the hot spot encourages the formation of a single vapor bubble that grows to a size determined by the radius of the hydrophobic coating. When the bubble detaches from the surface, a new vapor bubble forms in the same location. High-speed video and bubble-size and thermal measurements will be presented to document the characteristics and performance of this heat transfer surface. [Preview Abstract] |
Tuesday, November 21, 2006 9:18AM - 9:31AM |
LF.00007: Bubbly wake of surface vessels Fran\c{c}ois Caill\'e, Jacques Magnaudet, Christophe Clanet We study the length of the bubbly wake of surface vessels. This wake is important for the boat security since it can extend to several ship length and thus increases the detectability of the ship by torpedoes. The image analysis of the wake of real scale ships reveals the sensitivity of the length to propellers. We have thus conducted a systematic study in the laboratory of the interaction bubble/propeller, trying to address several questions:\newline - what is the role of cavitation?\newline - is the propeller able to attract the bubbles present along the ship at the sea surface?\newline - if attracted, can these bubble be broken by the propeller? [Preview Abstract] |
Tuesday, November 21, 2006 9:31AM - 9:44AM |
LF.00008: Axial Dispersion in Segmented Gas-Liquid Flow in Curved Channels Metin Muradoglu, Axel Guenther, Howard Stone Segmented gas-liquid flows have been studied for many years because of their applications in chemical analyses, and recently have been studied widely owing to developments in microfluidics. Here axial dispersion of a tracer in liquid flow segmented by gas bubbles is studied computationally in a two-dimensional setting using a finite-volume/front-tracking method. Both straight and curved channels are considered. We find that the dispersion increases as the Peclet number ($Pe$) decreases both in straight and curved channels but there is a significant difference between the straight and curved channel cases at high Peclet numbers. When the Peclet number is sufficiently large so that molecular diffusion is negligible, there is essentially no dispersion in the straight channel case since a lubricating thin liquid layer persists on the wall as observed by Kreutzer et al. (2005). However the lubricating liquid layer is periodically broken in the curved channel case leading to enhanced axial dispersion. Good agreement is found between the computational results and the lumped parameter model of Pedersen and Horvath (1981) when Peclet number is sufficiently low, i.e., the film and bulk regions are mixed well. [Preview Abstract] |
Tuesday, November 21, 2006 9:44AM - 9:57AM |
LF.00009: The shape of bubbles and Drops rising in a Nematic Liquid Crystal Chunfeng Zhou, Pengtao Yue, James J. Feng, Chun Liu, Jie Shen This work is motivated by recent experimental observation of unusual ``inverted-heart" shapes that a bubble assumes when rising in an anisotropic fluid. A possible explanation is in terms of the molecular orientation of the matrix fluid with respect to the bubble surface. In this work, we use numerical simulations to test such a hypothesis. The moving interface problem is formulated in a diffuse-interface framework. The anisotropic fluid is represented by a simplified Leslie-Ericksen theory for nematic liquid crystals, with director anchoring on the surface of an isotropic drop. The simulations are carried out using axisymmetric finite elements. Results show an array of drop shapes, depending on the interplay among inertial, capillary, anchoring and elastic effects. Drops with sufficiently strong planar anchoring and moderate elasticity rising in a medium with vertical far-field orientation assume the inverted-heart observed in experiments. This is shown to be mainly due to the competition between interfacial tension, anchoring energy and bulk elastic energy. Furthermore, two boojum defects appear on the upper and lower poles. The size of the defects plays an significant role in shaping the rising bubble. [Preview Abstract] |
Tuesday, November 21, 2006 9:57AM - 10:10AM |
LF.00010: Bubbles and droplets in magnetic fluids Philip Yecko In this work, the behavior of ferrofluid droplets and of bubbles rising in a ferrofluid is studied using direct numerical simulations based on a volume of fluid (VOF) method. A ferrofluid is a suspension of small (5--15 nm) magnetic particles in a carrier liquid which may be water or a hydrocarbon oil, stabilized against settling by Brownian motion and against agglomeration by coating each particle with a layer of surfactant. Although their main application is the fluid O-ring found in computer hard disk drives, ferrofluids have been more recently recognized for their use in micro- and nano-fluidic pumping, and applications to drug delivery are under investigation. Because ferrofluids are opaque, numerical simulations offer a unique opportunity to visualize flows that cannot be easily visualized experimentally, yet little effort has been directed to numerical simulations of realistic magnetic fluids. In this work, we develop and test a multiphase simulation code, based on Surfer, which can dynamically follow the behavior of small numbers of droplets, bubbles or layers of ferrofluid and ordinary viscous fluid for so-called linear magnetic material. In the rising bubble tests, we quantify the vertical elongation of the bubble and the resulting reduction in drag and rise time. In the falling droplet experiments, we demonstrate the effect of variable magnetic properties on the shape and trajectory of the droplet, including the instability threshold where droplet fission occurs. [Preview Abstract] |
Tuesday, November 21, 2006 10:10AM - 10:23AM |
LF.00011: Study of Bubbly Flows in Turbulent Boundary Layers Sheilla Torres-Nieves, Jose Lebron-Bosques, Francisco Moraga, Luciano Castillo Microbubble injection into a liquid turbulent boundary layer has been proven to effectively reduce frictional drag. Most of the experiments done to date have been conducted on flat plate geometries were bubbles are injected into a nominally zero-pressure-gradient turbulent boundary layer. Numerical simulations, although limited, have been performed to support these experiments. In fact, none of the published bubbly flow simulations deal with the case of non-zero pressure gradients. In this work, Reynolds Averaged Navier Stokes (RANS) simulations are performed to study different boundary layers, containing bubbles, on a horizontal flat plate. The behavior and distribution of these bubbles, and their effect on the mean velocity, Reynolds stresses and turbulent kinetic energy will be considered in this investigation. CFDShipM, a code developed at Rensselaer, will be modified in order to account for both a favorable and an adverse pressure gradient. Simulations will cover a range of void fractions, and bubble sizes. Furthermore, the results will be compared against the LDA data from Cal et al (2006), Brzek et al. (2006) and others. [Preview Abstract] |
Tuesday, November 21, 2006 10:23AM - 10:36AM |
LF.00012: Numerical simulation of upward/downward turbulent bubbly flow with microbubble-fluid interactions. Alfredo Soldati, Cristian Marchioli Direct numerical simulation of low-Reynolds-number turbulence ($Re_\tau = 150$) is coupled to Lagrangian tracking to study the behavior of $220 ~\mu m$ bubbles in a vertical upward/downward turbulent channel flow. Bubble velocity and concentration statistics provide evidence of bubble preferential segregation in high-speed regions for upflow and low-speed regions for downflow. Fluid velocity statistics show that the main effect of bubbles is to increase/decrease the liquid flow rate (and, in turn, the turbulent fluid velocity fluctuations and the Reynoldss stresses) in case of upflow/downflow. Significant flow rate variations were observed, even at void fraction of O(0.0001). In the upflow case, the non-uniformity of bubble distribution along the wall-normal direction limits the flow rate increase; due to the strong bubble accumulation near the walls, the hydrodynamic push induced by bubbles on the fluid occurs mainly in the near-wall region, where it is immediately counterbalanced by an increase in the wall shear. This two competing effects tend to reduce the efficiency of the turbulence modulation mechanisms. In the downflow case, analysis of the fluid velocity profiles indicates that, on average, bubbles do not significantly modify the statistical features of the flow structure. [Preview Abstract] |
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