Bulletin of the American Physical Society
2005 58th Annual Meeting of the Division of Fluid Dynamics
Sunday–Tuesday, November 20–22, 2005; Chicago, IL
Session EE: Multiphase Particle-Laden Flows II |
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Chair: Lian-Ping Wang, University of Delaware Room: Hilton Chicago Continental B |
Sunday, November 20, 2005 4:10PM - 4:23PM |
EE.00001: Wall effects on the motion of an approaching solid sphere Fu-Ling Yang, Melany Hunt As a solid sphere approaches a stationary wall, the motion of the sphere is retarded by the presence of the wall. The slowdown is more pronounced for the cases of small particle Stokes numbers where the particle possesses less inertia. Based on our experiment observations for immersed particle-on-wall collisions, this wall effect becomes non-negligible when the gap is roughly of sphere radius. To describe this phenomenon, an equation of motion for a single solid sphere moving towards a solid wall is proposed. In our model, viscous drag, added mass force and history force are considered with proper modifications for the presence of the solid wall. The predicted sphere motion is compared with experiment data for moderate and small particle Stokes numbers. [Preview Abstract] |
Sunday, November 20, 2005 4:23PM - 4:36PM |
EE.00002: Inerial Particle Clusterting John Christos Vassilicos, Lu Chen, Susumu Goto We integrate the motion of many small spherical inertial particles in 2D inverse cascading DNS turbulence assuming that these particles do not affect the fluid flow and do not interact with each other. Their clustering closely reflects the clustering of zero-acceleration points for a broad range of Stokes numbers. For small enough Stokes numbers, we show that inertial particles move with zero-acceleration points and with no other points because of turbulent sweeping of small eddies by large ones. However, Kinematic Simulations of 2D turbulence where this sweeping is absent also cause particle clustering for an equally broad range of Stokes numbers, but as a result of a clear spatial anticorrelation with zero-velocity points where streamline curvature is high. This subtler effect could not be captured by DNS because there are many more zero-acceleration points than zero-velocity points. [Preview Abstract] |
Sunday, November 20, 2005 4:36PM - 4:49PM |
EE.00003: The shear induced motion of a particle over a rough plane F. Charru, E. Larrieu, J.-B. Dupont, R. Zenit The interaction of a spherical particle with a fixed rough bed in a simple shear viscous flow is studied experimentally. The shear flow is produced using an annular Couette cell which has a rectangular cross section and is filled with silicon oil. The rough bed consists of a monolayer of glued particles, randomly positioned on an annular ring placed on the bottom of the channel. By means of digital image particle tracking, the position of the test particle was obtained from a high speed video imaging system. The velocity of the particle was calculated in the stream, cross stream and vertical directions. Values of the mean and fluctuating components were calculated for a wide range of parameters, varying the particle size and density, the fluid viscosity and the mean shear, $\gamma$. It was found that the normalized stream-wise mean particle velocity $U/U_S$, where $U_S$ is the Stokes settling velocity, depends only on the dimensionless shear rate, $\theta=\mu \gamma/(\Delta \rho g d)$, also called Shields number. This is consistent with the fact that the particle Reynolds number was smaller than 1 for most experiments. A simple model is proposed, based on a balance of hydrodynamic forces and a lumped friction force. Good agreement is found between the model predictions and the experiments. [Preview Abstract] |
Sunday, November 20, 2005 4:49PM - 5:02PM |
EE.00004: Development of a consistent Lagrangian--Eulerian approach for particle--laden flows Rahul Garg, Chidambaram Narayanan, Djamel Lakehal, Shankar Subramaniam The Lagrangian--Eulerian (LE) simulation approach for particle-- laden flows represents the fluid phase as an Eulerian field while particles are tracked in a Lagrangian frame. The LE solution must satisfy a consistency condition arising from relating LE quantities to the Eulerian--Eulerian (EE) description. The standard assumption for volumetrically dilute flows ($\alpha_p\ll 1$) is to neglect the effects of volume displaced by the dispersed phase and enforce mass conservation in the fluid phase through a solenoidal fluid velocity field. Recent analysis shows that the use of this fluid--phase mass conservation equation is not justified merely based on dilute flow assumption. The goal of the study is to check whether the velocity field obtained by solving the exact fluid-mass phase conservation equation is considerably different from the current LE appproach. This is important for flows that are dilute but have locally steep gradients in the volume fraction. The test problem chosen is a particle--laden driven--cavity flow. Accurate estimation of the fluid-particle interaction force from Lagrangian particle properties is shown to be essential for ensuring consistency of the LE simulation. Accurate estimation methods to verify the consistency of LE method to EE description also extend the LE approach to dense particle-laden flows. [Preview Abstract] |
Sunday, November 20, 2005 5:02PM - 5:15PM |
EE.00005: History and added mass forces on a spherical particle at finite Reynolds numbers Lulama Wakaba, S. Balachandar The unsteady force on a spherical particle, immersed in an ambient flow, can be expressed as a sum of quasi-steady, added mass, pressure gradient and history forces. Interestingly, at finite $Re$ the history contribution is quite complex even in an unsteady uniform flow, exhibiting a non-monotonic behavior. Furthermore, its behavior for acceleration and deceleration is not symmetric. In the presence of ambient shear, unsteady conditions give rise to history contributions for both drag and lift. The behavior of lift is particularly interesting, since the history-induced complexities are magnified. In this study, the history kernels for drag and lift are extracted and investigated for a range of Reynolds numbers. Unlike drag, the lift kernel is found to scale linearly with Reynolds number and shear rate. An investigation of the added mass for an accelerating/decelerating particle with an existing wake is also presented. Particular emphasis is placed on the interaction between an imposed rapid acceleration/deceleration and a pre-existing finite Reynolds number wake. The results clearly establish the independence of added mass coefficient on acceleration number and the prior state of flow. [Preview Abstract] |
Sunday, November 20, 2005 5:15PM - 5:28PM |
EE.00006: Motion of Non-Axisymmetric Particles in a Simple Shear Flow Andre Benard, Liping Jia, Charles Petty The motion of rigid particles of complex shapes is studied in this work using a new closure model. Each particle is non axisymmetric and its orientation is described with a second order tensor. The geometry such particles (e.g. ellipsoids) and their interactions with the surrounding fluid are described by a third order tensor instead of the single parameter often used for axisymmetric particles (spheroids). The flow-induced alignment of these particles can be computed by solving an evolution equation for the orientation distribution function but such computations are costly. Instead, an evolution equation for the second moment of the distribution function, which forms a fourth order tensor, is solved in order to obtain the average orientation of the particles in homogeneous flows. A closure model is introduced in this work and its performance is studied for a prescribed simple shear flow. [Preview Abstract] |
Sunday, November 20, 2005 5:28PM - 5:41PM |
EE.00007: Motion of Heavy Particles in a Bidisperse Turbulent Suspension Lian-Ping Wang, Orlando Ayala, Wojciech Grabowski We consider the motion of small heavy particles in a turbulent carrier flow, in terms of both particle-laden flow and micro- aerodynamics. The particles are assumed to be at least one order of magnitude smaller than the Kolmogorov length of the undisturbed carrier-flow turbulence, with a volume faction on the order of 10E-5 as in the recent experiments of Alisesa et al. (2002, or ACHL02) and Yang and Shy (2005, or YS05). A hybrid direct numerical simulation (HDNS, Wang et al. 2005, J. Atmos. Sci. 62: 2433) approach is used to simulate the turbulent suspension, in which the disturbance flows due to particles are treated analytically by a superposition method and the undisturbed air turbulence is simulated by a pseudospectral method. First, in order to validate the HDNS approach, we simulate the conditions used in the experiments of ACHL02 and YS05 and compare the HDNS results with the experimental results. We focus on the average settling rate of particles and study how the preferential concentration and local aerodynamic interactions enhance the settling rate. Second, we will explore how the HDNS approach could be further extended to include the effect of particles on the energy spectrum of the carrier flow turbulence (i.e., two-way coupling). [Preview Abstract] |
Sunday, November 20, 2005 5:41PM - 5:54PM |
EE.00008: Finite-sized Particles in Non-Dilute Convecting Suspensions: Efficient Numerical Approach to Sedimentation Tobias H\"{o}ink, J\"{o}rg Schmalzl, Ulrich Hansen The numerical simulation of a non-dilute suspension has to date been infeasible for particles that are smaller than the characteristic length scale of the fluid flow but sufficiently large so that Stokes' settling needs to be considered. We have developed an efficient algorithm that allows the numerical study of non-dilute suspensions in which such particles settle. Our approach considers a consistent settling velocity and the density contribution due to particle mass. We apply this method to 2D and 3D convection models and investigate emergent structures, resulting from the competing effects of convection and particle settling. For appropriately balanced forces we find three styles of motion: a temperature-dominated style where most particles remain suspended, a particle dominated style where the particles separate from the fluid and a style of repetitive motion. [Preview Abstract] |
Sunday, November 20, 2005 5:54PM - 6:07PM |
EE.00009: An instability of a homogeneous sheared gas-solid suspension driven by preferential concentration Donald Koch, Ganesh Subramanian, Ian Eames We have performed a linear stability analysis for an unbounded, initially homogeneous dilute particle-gas suspension subject to a simple shear flow for particle Stokes numbers that are small but non-zero. It is well known that the inertia of aerosol particles causes them to be thrown out of vortices and to accumulate in regions of high strain rate. This leads a decrease in the particle concentration in regions where the perturbation velocity reinforces the vorticity of the imposed shear flow and an increase in particle concentration in regions where the perturbation velocity tends to cancel the vorticity of the imposed flow. The gravitational force acting on this inhomogeneous density field reinforces the perturbation velocity leading to a growth of the perturbation. The shearing motion turns the wave vector of the disturbance flow eventually arresting the growth. However, if the shear is weak compared with the gravitational settling, the perturbation grows exponentially larger than its initial value before this arrest occurs. We suggest that secondary instabilities may continue the growth of particle concentration fluctuations. [Preview Abstract] |
Sunday, November 20, 2005 6:07PM - 6:20PM |
EE.00010: Flow responses of semi-dilute aqueous solutions of Polyethylene Oxide seeded with silica nanoparticles Daniel Joseph, Haoping Yang, Runyuan Bai, Jing Wang The Polyethylene Oxide (PEO) solutions seeded with nanoparticles give rise to greatly enhanced flow responses in tubeless siphons, open siphons, rod climbing and die swell which are described here. We focus on the exceptional properties of these solutions in some of the well known flows which are used to demonstrate the dramatic differences in flow response of Newtonian and highly viscoelastic fluids. Here, we go one step further, and use these prototypical flows to demonstrate the dramatic differences in flow response of semi-dilute PEO solutions and these same solutions seeded with silica nanoparticles in modest concentrations of no more than 3{\%} by weight. [Preview Abstract] |
Sunday, November 20, 2005 6:20PM - 6:33PM |
EE.00011: Aging properties of semi-dilute aqueous solutions of Polyethylene Oxide seeded with silica nanoparticles Jing Wang, Linda Heuer, Daniel Joseph Rheological properties of semi-dilute aqueous solutions of Polyethylene Oxide (PEO) seeded with silica nanoparticles are studied as a function of aging time. The viscosity, dynamic moduli and extensional properties of solutions of 0.5{\%} PEO of 4 and 8 million g/mol molecular weight seeded with 10-20 nm silica particles in concentrations of 1, 2 and 3{\%} by weight are greatly enhanced by the nanoparticles. The aforementioned properties were measured every week over a period of up to 18 weeks. The degradation of rheological properties with aging time in this period is greatly reduced and, in some cases is completely suppressed, by nanoparticles. The evolution of properties of these nanosolutions is not perfectly understood but is possibly determined in a resolution of the competing effects of absorption of PEO onto the silica with cleavage of the polymers due to oxidation and other effects at work in static samples. [Preview Abstract] |
Sunday, November 20, 2005 6:33PM - 6:46PM |
EE.00012: Modeling of elutriation phenomenon in fluidized beds Nan Xie, Francine Battaglia, Kevin Timmer, Robert Brown Elutriation occurs extensively in fluidized processes due to input of a wide range of particles or formation of fines by way of attrition. The entrained solids must be recovered or removed from the gas because of economic or environmental reasons. Thus the total quantities entrained and the concentration of solids in the gas stream must be known in order to design the solids removal systems. In the current research, the elutriation phenomenon is simulated using a multi-fluid finite-volume code. Numerical simulations are used to predict elutriation behavior for two types of solid particles with different diameters and densities fluidized by air in a laboratory-scale bed. The superficial gas velocity is chosen between the terminal velocities of the two solids. The total solids entrained for varying initial mass fractions of fines in the bed are computed and compared with experiments. The elutriation rate constants obtained from simulations are compared with the empirical formulations available in the literature. The concentration of solids distribution in the freeboard along the bed height is also presented. [Preview Abstract] |
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