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 A3: Multiphase Flow I: Bubbly Flows |
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Chair: Gretar Tryggvason, University of Notre Dame Room: 303 |
Sunday, November 20, 2011 8:00AM - 8:13AM |
A3.00001: Bubble Interactions in Multiphase Turbulent Channel Flows Jiacai Lu, Gretar Tryggvason Direct numerical simulations of deformable bubbles in weakly turbulent upflows in vertical channels have shown that the flow has a remarkably simple structure. For upflow, lift forces drive nearly spherical bubbles to the walls, forming a bubble-rich wall-layer. For downflow the bubbles are, on the other hand, driven away from the walls, creating a bubble free wall-layer. In both cases the addition or removal of bubbles to the center continues until the two-phase mixture there is in hydrostatic equilibrium. The lift on deformable bubbles is, however, generally nearly zero and the bubbles do not migrate laterally, on the average. Here we examine the effect of the size of the computational domain, and the number of bubbles, as well as the effect of including bubbles of different sizes. The results show that the distribution of bubbles sliding along the walls in upflow is generally very uneven, with parts of the wall crowded with bubbles and other parts bubble free. The presence of larger bubbles also tends to cause large fluctuations in the flow, including disrupting the wall-layers. [Preview Abstract] |
Sunday, November 20, 2011 8:13AM - 8:26AM |
A3.00002: Effect of Bubble Deformability in Multiphase Turbulent Channel Flows Sadegh Dabiri, Jiacai Lu, Gretar Tryggvason Recent results of direct numerical simulations (DNS) of deformable bubbles in turbulent upflows in vertical channels are discussed. Earlier results have shown that the lift forces leading to a lateral migration of nearly spherical bubbles controls the flow. In weakly turbulent flows, nearly spherical bubbles form a dense bubbly wall-layer but more deformable bubbles stay away from the walls, resulting in very different flow structures and flow rates. The different flow regimes have a very simple structure, but it is necessary to examine the collective motion of many bubbles rather than individual ones to predict the void fraction profile. Here the transition from one flow regime to the other as the deformability of the bubbles is changed is examined. We show that as long as the bubbly wall-layer is maintained, the flow rate increases slowly with the bubble deformability, but once the bubbles are sufficiently deformable and leave the wall, there is a sharp increase in the flow rate. [Preview Abstract] |
Sunday, November 20, 2011 8:26AM - 8:39AM |
A3.00003: A one-way coupled, Euler-Lagrangian simulation of bubble coalescence in a turbulent pipe flow Michael Mattson, Krishnan Mahesh A bubble coalescence model is developed using an Euler-Lagrangian approach for unstructured grids. The Eulerian carrier fluid is solved using large-eddy simulation (LES) and the Lagrangian particle motion is solved using one-way coupled equations relating the turbulent motion of the carrier fluid to the forces on each discrete bubble. The collision process is deterministic; bubble-bubble collisions are assumed to be binary and are modeled using a hard-sphere approach. A stochastic approach is used to model coalescence, with the probability of coalescence being a function of the bubble-bubble interaction timescale and the time to drain fluid between the colliding bubbles. Coalescence in a bubbly, turbulent pipe flow without buoyancy is simulated with conditions similar to a microgravity experiment by Colin, Fabre and Dukler [Int. J. Multiphase Flow (1991) {\bf17}:533--544] and excellent agreement of bubble size distribution is obtained between simulation and experiment. With increasing downstream distance, the number density of bubbles decreases due to coalescence and the average probability of coalescence decreases slightly due to an increase in overall bubble size. [Preview Abstract] |
Sunday, November 20, 2011 8:39AM - 8:52AM |
A3.00004: Bubble Fields in 3D (No Glasses Necessary) Jesse Belden, Sai Ravela, Tadd Truscott, Alexandra Techet Resolution of the three-dimensional (3D) bubble fields induced by a turbulent circular plunging jet is approached using 3D Synthetic Aperture Imaging (SAI). 3D SAI is ideally suited to investigate optically dense multiphase flows due to its ability to reconstruct volumes that contain partial occlusions. Instantaneous bubble sizes and locations in the plunging jet bubble fields are extracted from the volumes and presented for various jet heights. The data are compared with existing literature on bubble penetration depth and size distributions and show excellent agreement. A scaling law for total air concentration as a function of depth below the free-surface is proposed, and is believed to be the first presentation of such a result. Coupled with scaling laws for the maximum air concentration and radial concentration profiles, this new scaling law can be used to determine the entire air concentration profile given a minimal number of single point measurements. [Preview Abstract] |
Sunday, November 20, 2011 8:52AM - 9:05AM |
A3.00005: Shock propagation in a liquid containing bubbly clusters Herve Grandjean, Nicolas Jacques, Stephane Zaleski The propagation of shock waves in a liquid containing spherical bubbly clusters is investigated. A continuum model for the behaviour of such heterogeneous diphasic fluid has been developed, using a scale transition method. Numerical simulations of shock wave propagation reveal that the presence of bubble clusters has a significant influence on the shock dynamics. It is observed that the structure of the shock is dominated by the global response of the clusters instead of the single-bubble dynamics, as in homogeneous bubbly flow. As a consequence, the wavelength of the shock structure in a liquid with bubble clusters may be much larger than for the corresponding homogeneous bubbly liquid. To assess the accuracy of the proposed continuum modelling, axisymmetric simulations in which the position and shape of the clusters are directly specified were carried out. A good agreement between the two approaches was observed. The results of the proposed model were also found to compare well to experiments of the literature. [Preview Abstract] |
Sunday, November 20, 2011 9:05AM - 9:18AM |
A3.00006: A relation between the structures of a void fraction and liquid motion and the dissolved CO2 gas concentration, in a bubbly flow Masahiro Yamada, Takayuki Saito We discuss a relation between the structures of a void fraction and liquid motion and the CO$_{2}$ concentration, using a newly developed photoelectric optical fiber probe (POFP). The POFP is able to simultaneously measure a bubble diameter, velocity, time-series void fraction, and CO$_{2}$ concentration dissolved in the liquid around the bubbles. Moreover, we measured the velocity of the liquid phase at the same point, using LDV. We adopted a similar way of thinking into a consideration of the length scale to the liquid phase motions, void fraction and CO$_{2}$ concentration in order to evaluate spatial-scale. The integral-length-like scale of the liquid phase motions indicated few change against the height. The length scale of the void fraction became small toward the upper zone of the bubble column. On the other hand, the length scale of the CO$_{2}$ concentration exists up to the height of the middle zone. Based on these results, we discuss a relation of spatial-scale of the void fraction and the liquid motion to the CO$_{2}$ concentration. [Preview Abstract] |
Sunday, November 20, 2011 9:18AM - 9:31AM |
A3.00007: Two-Phase Flow Model and Experimental Validation for Bubble Augmented Waterjet Propulsion Nozzle J.-K. Choi, C.-T. Hsiao, X. Wu, S. Singh, A. Jayaprakash, G. Chahine The concept of thrust augmentation through bubble injection into a waterjet has been the subject of many patents and publications over the past several decades, and there are simplified computational and experimental evidence of thrust increase. In this work, we present more rigorous numerical and experimental studies which aim at investigating two-phase water jet propulsion systems. The numerical model is based on a Lagrangian-Eulerian method, which considers the bubbly mixture flow both in the microscopic level where individual bubble dynamics are tracked and in the macroscopic level where bubbles are collectively described by the local void fraction of the mixture. \textsc{Dynaflow}'s unsteady RANS solver, \textsc{3DynaFS}-\textsc{Vis}$^{\copyright }$ is used to solve the macro level variable density mixture medium, and a fully unsteady two-way coupling between this and the bubble dynamics/tracking code \textsc{3DynaFS}-DSM$^{\copyright }$ is utilized. Validation studies using measurements in a half 3-D experimental setup composed of divergent and convergent sections are presented. Visualization of the bubbles, PIV measurements of the flow, bubble size and behavior are observed, and the measured flow field data are used to validate the models. Thrust augmentation as high as 50{\%} could be confirmed both by predictions and by experiments. [Preview Abstract] |
Sunday, November 20, 2011 9:31AM - 9:44AM |
A3.00008: Macroscopic phase-field model of partial wetting: bubbles in a capillary tube Luis Cueto-Felgueroso, Ruben Juanes Drops and bubbles are non-spreading, local, compactly supported features. They are also equilibrium configurations in partial wetting phenomena. Yet macroscopic theories of capillary-dominated flow are often unable to describe these systems. We propose a framework to model multiphase flow in porous media with non-spreading equilibrium solutions. We illustrate our approach with a one-dimensional model of two-phase flow in a capillary tube. Our model allows for the presence of compactons: non-spreading steady-state solutions in the absence of external forces. We show that local rate-dependency is not needed to explain globally rate-dependent displacement patterns, and interpret dynamic wetting transitions as the route from equilibrium, capillary-dominated configurations, towards viscous-dominated flow. Mathematically, these transitions are possible due to non-classical shock solutions and the role of bistability and higher-order terms in our model. [Preview Abstract] |
Sunday, November 20, 2011 9:44AM - 9:57AM |
A3.00009: Critical scaling in the rheology of damped random spring networks Brian Tighe Physical, biological, and engineered materials ranging from foams and emulsions to bioppolymer and bar-joint networks can be modelled as random networks of springs. We study the oscillatory rheology of random networks immersed in a viscous background fluid, and show how their response is intimately tied to the presence or absence of floppy modes in the zero frequency limit. The rheology displays dynamic critical scaling with three different regimes: viscous fluid, elastic solid, and shear thinning power law fluid. We give scaling arguments to explain all of the critical exponents and confirm our predictions with numerics. [Preview Abstract] |
Sunday, November 20, 2011 9:57AM - 10:10AM |
A3.00010: Pseudoturbulence and regime transition in polydisperse flow Santos Mendez-Diaz, Roberto Zenit, Jose Luis Mu\~noz-Cobo, Sergio Chiva, Simon Martinez, Fausto Sanchez Vertical polydisperse liquid-gas flow was experimentally studied to analyze the regime transition from bubbly to slug flows. Laser Doppler Anemometry (LDA) and multi-tip conductivity probes were used to measure local flow parameters as liquid velocity, void fraction and superficial gas velocity in a cylindrical pipe. Particle Reynolds number was ranged from 100 to 10000 and Weber number from near to 0 to 100. The void fraction was progressively increased to obtain bubbly, transition and slug flow regimes. The power spectral density obtained from liquid velocity shows a nearly constant decay energy exponent when radial void fraction distribution shows a wall-peak in bubbly regime, whereas in transition and slug regimes the exponent decay shows a non-constant value when void fraction increases. Critical flow conditions defined by Reynolds and Weber numbers are suggested to identify regime transition from bubbly to slug. Turbulence intensity and other local flow parameters are analyzed to validate the proposed criteria. [Preview Abstract] |
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