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 AK: Multiphase Flows I |
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Chair: Pavlos Vlachos, Virginia Polytechnic Institute and State University Room: 101J |
Sunday, November 22, 2009 8:00AM - 8:13AM |
AK.00001: Dynamic Characteristics at the Interface of Underwater Round Gas Jets Chris Weiland, Pavlos Vlachos The gas-liquid interface characteristics of round gas-jets submerged in water was studied across a wide range of Mach numbers (0.4-1.9). High speed shadowphotography was used to image the gas jet and the interface was tracked from the digital images for all points in space and time. The results show how the interface characteristics are governed by buoyancy to momentum driven flow as the Mach number increases. The jet penetration, defined as the maximum length a continuous gas jet occupies 99 percent of the time, increases with the injection Mach number. The penetration is related to the compressible jetting length, defined as the distance from the orifice where the momentum and buoyancy forces are balanced, and signifies a change in the jet behavior spatially from a momentum to buoyancy driven flow. The interface motion is computed as a function of the Mach number and the distance downstream from the orifice. These results indicate the most unsteady jetting process near the orifice occurs at Mach 1, presumably due to the formation of a shock cell structures. [Preview Abstract] |
Sunday, November 22, 2009 8:13AM - 8:26AM |
AK.00002: A numerical study of air layer drag reduction phenomenon on a flat plate Dokyun Kim, Parviz Moin The objective of the present study is to predict and understand the air layer drag reduction (ALDR) phenomenon. Recent experiments (Elbing et al. JFM 2008) have shown large net drag reductions if air is injected beyond a critical rate at the wall. The stability analysis and numerical simulations are performed to investigate mechanisms of ALDR on a flat plate using the same geometry as in the experiment. The linear stability of air-liquid interface is investigated by solving the Orr-Sommerfeld equations, and numerical simulations of two-phase flow have been performed to describe the evolution of air-water interface. The stability analysis shows that the air flow rates, Reynolds number, Weber number, and Froude number are important parameters determining the stability of the air layer. In laminar boundary layer, it is observed from the numerical simulations that the Froude number is the key to the stability of the air layer. The presentation will include a new and very efficient numerical method for two-phase flow calculations used in this study. [Preview Abstract] |
Sunday, November 22, 2009 8:26AM - 8:39AM |
AK.00003: Lagrangian statistics for bubbles in a turbulent boundary layer Michael Mattson, Krishnan Mahesh We are developing the simulation capability for bubbly flows in complex geometries using unstructured grids and an Euler--Lagrangian methodology. In the Lagrangian bubble model, the bubbles are treated as a dispersed phase in the carrier fluid, and individual bubbles are point particles governed by an equation for bubble motion. For this talk, direct numerical simulation is used to solve the Navier--Stokes equations for a spatially-evolving turbulent boundary layer ($Re_{\theta}=1430$) and bubbles are injected into the near-wall region. The bubbly suspension is dilute and one-way coupled equations are used. The temporal evolution of the bubble dispersion and probability density functions of the bubble forces will be presented, with emphasis on the role Stokes number and injection location play in determining bubble behavior. [Preview Abstract] |
Sunday, November 22, 2009 8:39AM - 8:52AM |
AK.00004: A comprehensive subgrid air entrainment model for Reynolds-averaged simulations of free-surface bubbly flows Jingsen Ma, Assad A. Oberai, Donald A. Drew, Richard T. Lahey, Jr, Mark C. Hyman The simulation of free surface bubbly flows using a two-fluid model remains challenging in part due to the lack of a comprehensive air entrainment model that can predict the location and rate of air entrainment for a wide range of flows. In this study we derive one such model and implement it into a computational multiphase fluid dynamics (CMFD) framework that solves the Reynolds-averaged two-fluid equations. The subgrid air entrainment model is derived from a simple argument that the wave action near the air/water interface causes it to ingest air bubbles and they are entrained into the liquid if their downward velocity exceeds that of the interface. This yields a simple expression for the rate of entrainment as a product of the downward gradient of the liquid velocity near the free surface and the turbulent kinetic energy. We have tested the performance of this model and CMFD in simulating the bubbly flow due to a plunging liquid jet, in a hydraulic jump and around a full-scale naval surface ship by comparing with experimental data. It's found that in each case the subgrid air entrainment model and the two-fluid modeling approach yields accurate results. [Preview Abstract] |
Sunday, November 22, 2009 8:52AM - 9:05AM |
AK.00005: On bubble clustering and energy spectra in pseudo-turbulence Julian Martinez Mercado, Daniel Chehata Gomez, Dennis van Gils, Chao Sun, Detlef Lohse We performed 3D-Particle Tracking (3D-PTV) and Phase Sensitive Constant Temperature Anemometry in pseudo-turbulence to investigate bubble clustering and to obtain the mean bubble rise velocity, distributions of bubble velocities, and energy spectra at dilute gas concentrations. To characterize the clustering the pair correlation function $G(r,\theta)$ is calculated. The deformable bubbles with equivalent bubble diameter $d_b=4-5$ mm are found to cluster within a radial distance of a few bubble radii with a preferred vertical orientation. This vertical alignment is present at both small and large scales. The large number of data-points and the non intrusiveness of PTV allowed to obtain well-converged Probability Density Functions (PDFs) of the bubble velocity. The PDFs have a non-Gaussian form for all velocity components and intermittency effects can be observed. The energy spectrum of the liquid fluctuations decays with a power law of $-3.2$, different from the $\approx -5/3$ found for homogeneous isotropic turbulence. [Preview Abstract] |
Sunday, November 22, 2009 9:05AM - 9:18AM |
AK.00006: How does interfacial rheology govern soap bubble cluster dynamics? Sylvie Cohen-Addad, Anne-Laure Biance, Reinhard Hohler Aqueous foams are concentrated dispersions of gas bubbles in a soapy solution. These complex fluids exhibit solid-like or liquid-like mechanical behaviors, depending on the applied shear. When it is increased beyond a yield strain, neighbor switching bubble rearrangements called T1 events are triggered and plastic flow sets in. We study experimentally the dynamics of such strain induced T1s in 3D bubble clusters that we consider as model systems of 3D foams. To determine the hydrodynamics and physico-chemistry that set the duration of T1s, we use foaming solutions of a wide range of well characterized bulk and interfacial rheological properties. At low shear rates, the T1 duration is set by a balance between surface tension and surface viscous forces in qualitative agreement with previous studies of T1s in 2D foams [1] and we present a simple physical model that explains our 3D findings. Moreover, above a characteristic shear rate, rearrangement dynamics are driven by the applied strain. By combining all our results, we link the transition from intermittent to continous flow dynamics in foams to the rheology of the gas-liquid interfaces. \\[4pt] [1] M. Durand, H. A. Stone, \textit{Phys. Rev. Lett.} \textbf{97}, 2226101 (2006). [Preview Abstract] |
Sunday, November 22, 2009 9:18AM - 9:31AM |
AK.00007: Measurements of the fluctuating liquid velocity of a bidisperse suspension of bubbles rising in a vertical channel Juan Carlos Serrano, Santos Mendez, Roberto Zenit Experiments were performed in a vertical channel to study the
behaviour of a
bidisperse suspension of bubbles. Bubbles were produced using
capillaries of
two distinct inner diameters. The capillaries are small enough to
generate
bubbles in the range of 1 to 6 mm in diameter. Using water and
water-glycerin
mixtures, the vertical component of the fluctuating liquid
velocity was
obtained using a flying hot wire anemometer technique. The system is
characterized by the dimensionless Reynolds and Weber numbers in
the range of
$22 |
Sunday, November 22, 2009 9:31AM - 9:44AM |
AK.00008: Determination of Hydrodynamic Parameters on Two--Phase Flow Gas - Liquid in Pipes with Different Inclination Angles Using Image Processing Algorithm Gustavo Montoya, Mar\'Ia Valecillos, Carlos Romero, Dosinda Gonz\'ales In the present research a digital image processing-based automated algorithm was developed in order to determine the phase's height, hold up, and statistical distribution of the drop size in a two-phase system water-air using pipes with 0\r{ }, 10\r{ }, and 90\r{ } of inclination. Digital images were acquired with a high speed camera (up to 4500fps), using an equipment that consist of a system with three acrylic pipes with diameters of 1.905, 3.175, and 4.445 cm. Each pipe is arranged in two sections of 8 m of length. Various flow patterns were visualized for different superficial velocities of water and air. Finally, using the image processing program designed in Matlab/Simulink$^{\textregistered}$, the captured images were processed to establish the parameters previously mentioned. The image processing algorithm is based in the frequency domain analysis of the source pictures, which allows to find the phase as the edge between the water and air, through a Sobel filter that extracts the high frequency components of the image. The drop size was found using the calculation of the Feret diameter. Three flow patterns were observed: Annular, ST, and ST{\&}MI. [Preview Abstract] |
Sunday, November 22, 2009 9:44AM - 9:57AM |
AK.00009: Bubble Size Control Mechanisms and Effect on Flow Regime Thomas Shepard, Paul Strykowski Research has been conducted in an effort to understand the impacts of different control mechanisms on bubble size during air injection into a liquid cross-flow. The motivation for this work is to gain better control during the bubble formation and coalescence processes in order to reliably generate bubbles of different sizes. In this study air is injected through a porous plate into an electrolyte solution flowing through an adjustable geometry channel. The control mechanisms considered include the pore size in the porous plate, the channel height (and thus shear rate) at the injection site, and the electrolyte concentration. The effects of the controls are studied for a range of channel pressures (10-60 psi) and gas to liquid mass flow-rate ratios (0.001-0.005). By varying the different controls, bubbles are generated with a mean diameter of 80-1600 microns while keeping the pressure and gas to liquid mass flow-rate constant. An additional outcome of the research is the demonstrated effect that bubble size has on the transition from bubbly flow to slug flow. [Preview Abstract] |
Sunday, November 22, 2009 9:57AM - 10:10AM |
AK.00010: Disturbances to Air-Layer Skin-Friction Drag Reduction at High Reynolds Numbers David Dowling, Brian Elbing, Simo Makiharju, Andrew Wiggins, Marc Perlin, Steven Ceccio Skin friction drag on a flat surface may be reduced by more than 80{\%} when a layer of air separates the surface from a flowing liquid compared to when such an air layer is absent. Past large-scale experiments utilizing the US Navy's Large Cavitation Channel and a flat-plate test model 3 m wide and 12.9 m long have demonstrated air layer drag reduction (ALDR) on both smooth and rough surfaces at water flow speeds sufficient to reach downstream-distance-based Reynolds numbers exceeding 100 million. For these experiments, the incoming flow conditions, surface orientation, air injection geometry, and buoyancy forces all favored air layer formation. The results presented here extend this prior work to include the effects that vortex generators and free stream flow unsteadiness have on ALDR to assess its robustness for application to ocean-going ships. Measurements include skin friction, static pressure, airflow rate, video of the flow field downstream of the injector, and profiles of the flowing air-water mixture when the injected air forms bubbles, when it is in transition to an air layer, and when the air layer is fully formed. From these, and the prior measurements, ALDR's viability for full-scale applications is assessed. [Preview Abstract] |
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