Bulletin of the American Physical Society
61st Annual Meeting of the APS Division of Fluid Dynamics
Volume 53, Number 15
Sunday–Tuesday, November 23–25, 2008; San Antonio, Texas
Session BP: Multiphase Flows II |
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Chair: Alberto Fernandez-Nieves, Georgia Institute of Technology Room: 202A |
Sunday, November 23, 2008 10:30AM - 10:43AM |
BP.00001: Droplet production due to a filming jet in crossflow Timothy Shedd, May Corn, Marco Arienti, Marios Soteriou This work presents the results of a study of a liquid jet atomized by a gas cross-flow in the vicinity of a plane wall normal to the liquid jet axis. Droplets and ligaments with sufficient momentum will impinge on the wall, forming a thin liquid film. A highly time-resolved fluorescent liquid film thickness measurement technique is used together with high speed video to correlate the temporal behavior of jet and droplet impingement with the film thickness. Results of time correlated PDPA and film thickness measurements are also presented, showing the relationship between droplet production and film thickness versus droplet production from the jet alone. [Preview Abstract] |
Sunday, November 23, 2008 10:43AM - 10:56AM |
BP.00002: 3D analysis of liquid jet break-up from high-speed synchronized videos Braden McDermott, Timothy Shedd, May Corn, Marco Arienti, Marios Soteriou The role played by the air stream in selecting the dominant wave mode in liquid jet in crossflow break-up is investigated in an experimental facility where a rectangular air jet is directed orthogonally toward the liquid column. The column is seen to immediately broaden upon impact of the air jet, with surface waves initiating along the windward surface. The ensuing break-up dynamics is captured by two synchronized high-speed cameras with identical lenses and orthogonal fields of view. The side view of the windward surface shows a wavelike structure whose peaks correspond, in the top view of the column, to transverse ligaments that increase in length with distance. The wave troughs correspond to thin sheets which distend, stretch, and eventually perforate into smaller droplets leaving behind the thicker transverse ligaments. The coherent structures of this complex dynamics are revealed by the simultaneous analysis of the two synchronized image sequences in a range of turbulent and non-turbulent flow conditions. Three-dimensional reconstruction aspects will be discussed, particularly how to identify the dominant wavelike modes that appear periodically in the video sequences. [Preview Abstract] |
Sunday, November 23, 2008 10:56AM - 11:09AM |
BP.00003: Dynamics of a liquid jet atomized by gaseous crossflow Arienti Marco, Gregory Hagen, May Corn, Marios Soteriou When captured by high-speed, high-resolution videos, the dynamics of a liquid jet subject to a crossflowing air stream with no external forcing appears to be composed by slow column bending motions and fast traveling surface waves. Sequences of consecutive near field line-of-sight images of the jet acquired for gas Weber numbers between 10 and 300 and momentum flux ratios between 10 and 100 are analyzed with the method of snapshots to decouple this complex liquid interface motion into few fundamental dynamic modes. The exposure time of each snapshot ``freezes'' the flow, thus providing a sharp liquid interface, while the acquisition rate is comparable to the characteristic time of moderate Weber number surface waves. Spectral decomposition analysis reveals broad-band oscillations that can be linked to the amplification of column waves near the point of column break-up. As the Weber number increases at constant momentum flux ratio, the broad band peak shifts toward higher frequencies until the aliasing limit is reached. As the liquid jet velocity increases, and the column becomes turbulent, a cascade of finer temporal and spatial structures are found with increasing pixel brightness variation intensity that affect similarly small scales of the downstream spray. [Preview Abstract] |
Sunday, November 23, 2008 11:09AM - 11:22AM |
BP.00004: Towards robust numerical simulation of air-blast atomization with high density ratios Olivier Desjardins, Vincent Moureau While numerical methods for multiphase flows have progressed significantly in the past few years, simulating realistic flows with high density ratios remains a major hurdle, especially when combined with high shear, as encountered in air-blast atomization devices. In order to alleviate this issue, the Ghost Fluid Method (GFM) is extended to allow for higher accuracy. Additional robustness is achieved through the implementation of shock-capturing schemes. This approach is employed with both structured and unstructured meshes for the simulation of air-blast atomization of liquids with high density ratio. In particular, the atomization of a round water jet by a fast co-axial air stream studied experimentally by Marmottant and Villermaux (JFM 2004) is simulated in details. Advantages and limitations of this technique are discussed for this case, as well as for canonical two-phase configurations. [Preview Abstract] |
Sunday, November 23, 2008 11:22AM - 11:35AM |
BP.00005: Detailed simulation of atomizing liquid jets using a Spectrally Refined Interface (SRI) approach Heinz Pitsch, Olivier Desjardins Simulating primary atomization remains an outstanding challenge due to the presence of turbulence, small scale liquid structures, and singular surface tension forces. A new approach is presented that employs sub-cell quadrature nodes to provide a high order polynomial reconstruction of a level set function. This Spectrally Refined Interface (SRI) description is coupled to semi-Lagrangian transport to alleviate the small time step requirements usually associated with local refinement, and is combined with the Ghost Fluid Method (GFM) to accurately and robustly handle the discontinuous material properties in the two phases, as well as surface tension forces. This technique is validated over a range of test cases and is shown to provide a very accurate description of the interface even at the limit of numerical resolution. Highly detailed simulations of atomizing two-phase jets are conducted, and the physical processes occurring during atomization are discussed. [Preview Abstract] |
Sunday, November 23, 2008 11:35AM - 11:48AM |
BP.00006: Simulation of Pattern Formation in a Rotating Suspension of non-Brownian Settling Particles Tsorng-Whay Pan, Roland Glowinski, Suchung Hou We present numerical results of pattern formation for a settling suspension of non-Brownian spherical particles in a completely filled horizontal rotating cylinder. The experimental results have been recently reported [S.G. Lipson, J. Phys: Condens. Matter 13, 5001 (2001) and W.R.Maston, B.J. Ackerson, and P. Tong, Phys. Rev. E 67, 050301(R) (2003)]. We assume that these phenomena are modeled by the Navier-Stokes equations for incompressible Newtonian viscous fluids coupled to the Euler-Newton equations describing rigid-solid motions. The numerical methodology relies on the combination of a finite element method, operator-splitting, and a Lagrange multiplier based fictitious domain method allowing the flow calculations to take place in a fixed simple shape space region [R. Glowinski, T.W. Pan et al, J. Comp. Phys. 169, 363 (2001)]. We have studied the interactions of up to 640 balls. We found that the drafting, kissing, and tumbling among balls due to the initial forces is one of key factors for forming and maintaining the clusters and the competition between the rotation speed of the cylinder and the gravity acting on the balls is also crucial. [Preview Abstract] |
Sunday, November 23, 2008 11:48AM - 12:01PM |
BP.00007: A Subgrid Model for Predicting Air Entrainment Rates in Bubbly Flows Jingsen Ma, Assad A. Oberai, Donald E. Drew, Richard T. Lahey, Jr., Francisco J. Moraga In this talk we present a fairly simple subgrid air entrainment model that accurately predicts the rate of air entrainment, which is critical in simulating multiphase (air/water) flows. The derivation of this model begins by assuming that a thin sheet of air is carried into the water by the inertia of the liquid at the free surface. A momentum balance on the entrained gas layer results in an expression for the entrained volumetric gas flow rate, in terms of the local liquid velocity, gas viscosity etc., which are readily available from a multiphase RANS-type simulation. This model has been validated against extensive experimental data on both plunging jets and hydraulic jumps over a wide range of liquid velocities. It was implemented in a two-fluid computational fluid dynamics code (CFDShipM) to be used to predict the void fraction distribution underneath a plunging liquid jet at different depths and jet velocities. The results were found to match the experimental observations very well. The application of this model to more challenging problems, including hydraulic jumps and full-scale ship simulations, is currently underway. [Preview Abstract] |
Sunday, November 23, 2008 12:01PM - 12:14PM |
BP.00008: Study of the instabilities induced near the separator plate in atomization processes Daniel Fuster, Stephane Zaleski This work presents current advances in the simulation of the primary atomization zone, paying a special attention to the effect of the separator plate on the flow patterns observed downstream. Gerris, a CFD Open Source code, is used to perform the simulations. The methods implemented on it combining adaptive quad/octree spatial discretisation, geometrical Volume-Of-Fluid interface representation, balanced-force continuum-surface-force surface tension formulation and height-function curvature estimation, have allowed us to carry out accurate simulations near the separator plate. The inclusion of the separator plate in the analysis have been shown to have a capital importance on the instabilities generated just after it. The influence of some operational parameters like the momentum ratio, the gas and liquid Reynolds numbers based on the thickness of the boundary layer, the density and viscosity ratios or the thickness and angle of the separator plate are investigated. The analysis of these phenomena is aimed at shedding some new insight into the physical mechanisms controlling atomization processes and to provide better basis for future theoretical analysis. [Preview Abstract] |
Sunday, November 23, 2008 12:14PM - 12:27PM |
BP.00009: Atomization in an Air-Water Pipe Flow Sylvain Boulesteix, Patricia Ern, Francois Charru The atomization process at the interface of a liquid layer sheared by a high-velocity gas flow plays a crucial role in mass and momentum tranfer between phases. The knowledge of the characteristics of entrained droplets is a first step towards a better understanding of this phenomenon. We have therefore investigated these characteristics for a horizontal air-water pipe flow using a high-speed camera. This revealed that primary atomization mostly occurs through two mechanisms: bag and ligament breakups, like in the case of a single drop or of a liquid jet sheared by a high-velocity gas stream. We also observed that secondary atomization due to collisions between drops is a frequent phenomenon that may have a greater importance in the reduction of droplets sizes than previously considered in the litterature. Finally, digital image processing allowed us to measure the probability density functions of droplets sizes and velocities, which will be discussed here. [Preview Abstract] |
Sunday, November 23, 2008 12:27PM - 12:40PM |
BP.00010: Aerosol particle motion induced by non-linear sound waves Jay Cleckler, Feng Liu, Said Elghobashi Solid or liquid particles with small response times execute periodic motion when exposed to acoustic waves of significant amplitudes. The amplitude of particle velocity oscillations and the degree to which these oscillations lag acoustic velocity oscillations depend on the ratio of the particle response time to the acoustic period. For small amplitudes, experimental data deviate slightly from basic theory. Data are scarce for the motion of aerosol particles in large amplitude acoustic waves. These large amplitude waves display nonlinear steepening among other effects which may affect particle trajectories. These nonlinearities are primarily due to convection and result in modifying the oscillations of particles exposed to these acoustic waves. This presentation summarizes the results of numerical simulations in which a particle is exposed to fully nonlinear acoustic waves. The unsteady two- dimensional compressible Navier-Stokes and energy equations are solved for a laminar flow via the numerical method of Wall et al. (JCP, 2002). The particle motion equation is solved to obtain the particle trajectories. Computed particle trajectories, as a function of acoustic amplitude, frequency and particle response time are compared with basic theory and the experimental data of Gonzalez et al. (JAS, 2000). Particle trajectories are also computed at larger wave amplitudes that cannot be currently achieved in experiments. [Preview Abstract] |
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