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 AP: Multiphase Flows I |
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Chair: Francine Battaglia, Virginia Polytechnic Institute and State University Room: 202A |
Sunday, November 23, 2008 8:00AM - 8:13AM |
AP.00001: Role of slip between a probe particle and a gel in microrheology Henry Fu, Vivek Shenoy, Thomas Powers In the technique of microrheology, rheological information is deduced from the behavior of microscopic probe particles under thermal or active forcing. Microrheology requires knowledge of the force felt by a probe particle in response to displacements, which we investigate for a spherical particle using the two-fluid model. The gel is represented by a polymer network coupled to a surrounding solvent via a drag force. We obtain an analytic solution for the response function in the limit of small volume fraction of the polymer network, and neglecting inertial effects. We use no-slip boundary conditions for the solvent at the surface of the sphere. The boundary condition for the network at the surface of the sphere is a kinetic friction law specifying the tangential stress. We show that the far field motion and the force on the sphere are controlled by the solvent boundary conditions at high frequency and by the network boundary conditions at low frequency. At low frequencies compression of the network can also affect the force on the sphere. We find the crossover frequencies at which the effects of sliding of the sphere past the polymer network and compression of the gel become important. [Preview Abstract] |
Sunday, November 23, 2008 8:13AM - 8:26AM |
AP.00002: Quasi-2D foam: Rheology and fracture Shehla Arif, J.-C. Tsai, Sascha Hilgenfeldt A single layer of acqueous foam bubbles captured between parallel plates in a Hele-Shaw rectangular-channel geometry is a system with many desirable qualities: the foam can be observed over the entire width and length of the channel and interpreted as effective medium, but at the same time all information about every single bubble (size, shape, deformation) is readily available. We show that a proper modification of Bretherton's single-bubble theory allows for a quantitative description of foam rheology in this quasi-2D set-up, both for the general flow resistance of the foam and for the motion of individual bubbles. For the latter, the viscous flow resistance can be related to pressure and stress distributions in the foam when it is driven by imposed external pressure. Such a foam ultimately undergoes fracture in two modes aptly described as ``brittle'' and ``ductile,'' and the rheology measurements yield a criterion for the transition point between these two behaviors. [Preview Abstract] |
Sunday, November 23, 2008 8:26AM - 8:39AM |
AP.00003: Numerical Simulations of a Biomass Fluidizing Bed with Side Port Air Injection Francine Battaglia, Mirka Deza, Theodore J. Heindel Fluidized beds can be used to gasify biomass in the production of producer gas, a flammable gas that can replace natural gas in process heating. As part of the reactor design, side air ports strategically placed along the reactor column can help promote and improve mixing. Modeling these reactors using computational fluid dynamics is advantageous when performing parametric studies for design and scale-up. From a computational point of view, two-dimensional simulations are easier to perform than three-dimensional simulations, but they may not capture the proper physics. Comparisons of two- and three-dimensional simulations in a 10.2 cm diameter cold-flow fluidized bed with side air injection are used to determine when two-dimensional simulations are adequate to capture the bed hydrodynamics. The medium used to represent biomass is ground walnut shell, which has been shown to have desirable fluidization characteristics. The simulations will be quantitatively compared with X-ray computed tomography experiments for pressure drop through the bed, particle distribution and bed expansion height. [Preview Abstract] |
Sunday, November 23, 2008 8:39AM - 8:52AM |
AP.00004: Measurement and Modeling of Channel Wall Vibration Subjected to Internal Bubbly Flow Mingming Zhang, Joseph Katz, Andrea Prosperetti The effect of a bubbly flow, injected into both a square and rectangle channels, on channel wall vibrations is studied experimentally and theoretically. The vibrations are measured under various gas void fractions, bubble diameters and channel dimensions. A theoretical model, based on a waveguide theory and bubble dynamics, is developed to predict the dominant frequencies in vibration spectra, the corresponding decay rates and propagation phase speeds. Results show that, compared with no bubble case, the presence of bubbles substantially enhances the power spectral density of vibrations, by up to 27 dB in a square channel and 37 dB in a rectangular channel. The origin of enhanced vibrations is attributed to the excitation of the streamwise propagating pressure waves, created by an initial acoustic energy generated during bubble formation. The model predicts very well the magnitudes and trends of the dominant spectral frequencies, the corresponding decay rates and phase speeds. The frequency, attenuation and phase speed decrease substantially with increasing void fraction but slightly with increasing diameter. [Preview Abstract] |
Sunday, November 23, 2008 8:52AM - 9:05AM |
AP.00005: Optical measurements of phase concentration and velocity distributions of a horizontal gas jet in a 2D bubbling fluidized bed Alexander Mychkovsky, Steven Ceccio, Volker Sick Optical measurement techniques are used for spatially and temporally resolved measurements of phase concentrations and velocities in a horizontal gas jet injected in a 2D bubbling fluidized bed. A fiber optic probe has been developed to measure Laser Induced Fluorescence (LIF) signals from an acetone tracer to quantify jet gas concentration and elastic Mie scatter from the bed particles to determine the solids fraction in a localized region. These two optical signals are spectrally separated and therefore enable simultaneous measurements of the two phases. In addition, jet gas and particles velocities are obtained with a Laser Doppler Velocimetry (LDV) system. These measurements yield phase concentration and velocity profiles necessary to characterize the dynamic behavior of gas jets in fluidized beds. [Preview Abstract] |
Sunday, November 23, 2008 9:05AM - 9:18AM |
AP.00006: Assembly of particles at fluid-fluid interfaces using electric fields Nadine Aubry, Pushpendra Singh, Muhammad Janjua, Sai Nudurupati In this talk, we present a new technique to assemble micro- and nano-sized particles into monolayers (two-dimensional arrays). For this, we sprinkle particles at a fluid-fluid interface (or at the free surface of a liquid) and apply a uniform electric field normal to the interface. The electric field generates horizontal electrostatic forces on the particles due to dipole-dipole forces which, together with the capillary forces, put the particles into motion until their reach an equilibrium position where the two forces balance each other. In the final arrangement, particles are placed at a certain distance of one another, a distance which can be controlled by varying the electric field strength. The technique, which works on a variety of particles, including micro/nano-sized and neutral particles, is investigated both experimentally and theoretically. A good quantitative agreement between the two approaches is found. [Preview Abstract] |
Sunday, November 23, 2008 9:18AM - 9:31AM |
AP.00007: Redistribution and removal of particles from drops surfaces Sai Nudurupati, Muhammad Janjua, Pushpendra Singh, Nadine Aubry It was recently shown by us that particles distributed on the surface of a drop can be concentrated at its poles or the equator by subjecting it to a uniform electric field. In this talk we show that the method can be used to separate particles experiencing positive dielectrophoresis on the surface a drop from those experiencing negative dielectrophoresis. This, in fact, can be used to form a composite (Janus) drop by aggregating particles of one type near the poles and of the second type near the equator. We also show that when the ratio of the distance between the electrodes to the drop diameter is smaller than a critical value the drop bridges the gap between the electrodes and then breaks into two or more major droplets. For the larger values of this ratio the drop undergoes tip-streaming. The former case is used to remove particles concentrated near the drop's equator and the latter for removing particles at the poles. [Preview Abstract] |
Sunday, November 23, 2008 9:31AM - 9:44AM |
AP.00008: Unsteady Forces on Particles in Viscous Compressible Flow Manoj Parmar, Andreas Haselbacher, S. Balachandar The primary objective of our work is the study of unsteady forces on particles in compressible flow. In incompressible flow, unsteady inviscid and viscous forces arise from the no-penetration and no-slip conditions on the surface of a particle. In prior work, building on results by Miles and Longhorn, we have investigated the unsteady force on particles in inviscid compressible flow at finite Mach numbers. The results indicate that the unsteady force can become about twice as large as the incompressible value even for subcritical Mach numbers. The contributions of this work are twofold. First, based on the kernel for the unsteady inviscid force in compressible flow, we present a simple model for the unsteady force on particles arising from shock-wave impact, and assess it by comparison with experimental and computational results. Second, we construct a unified model based on an unified kernel for the unsteady inviscid and viscous forces in compressible flow. [Preview Abstract] |
Sunday, November 23, 2008 9:44AM - 9:57AM |
AP.00009: Blocking effects of a sphere or spheroid immersed in linear shear flows in the Stoke's regime Longhua Zhao, Roberto Camassa, Richard McLaughlin Building on work by Wu and Chwang who developed closed form exact solutions of the Stokes for the case of a sphere or spheroid embedded in a linear shear layer, we study the behavior of fluid particles in such flows and document rigorously that the blocking behavior which was observed by Wu and Chwang for the two dimensional case occurs in the fully 3D case well. We compute explicitly the volume of the blocking region, which is seen to be infinite, and present the explicit and analytic solution for the particle trajectories for this fully 3D flow. Time permitting, we explore cases when the sphere or spheroid have centers displaced from the background shear symmetry line. We document an interesting bifurcation in the particle trajectories using numerical techniques. [Preview Abstract] |
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