Bulletin of the American Physical Society
2006 59th Annual Meeting of the APS Division of Fluid Dynamics
Sunday–Tuesday, November 19–21, 2006; Tampa Bay, Florida
Session KE: Multiphase and Particle-Laden Flows V |
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Chair: Satish Kumar, University of Minnesota Room: Tampa Marriott Waterside Hotel and Marina Florida Salon 123 |
Monday, November 20, 2006 5:15PM - 5:28PM |
KE.00001: Particle rotation in shear flows Johanna Bluemink, Leen Van Wijngaarden, Andrea Prosperetti, Detlef Lohse For a \textit{linear shear flow} it is known that a particle rotates \textit{slower} than the surrounding flow when inertial effects are included. Experiments performed for a sphere fixed (but free to rotate) in a flow in \textit{solid body rotation} indicate that the rotation rate of a sphere can be \textit {faster} than the rotation rate of the flow. Numerical simulations at moderate $Re$ confirm this observation. To gain understanding of the phenomenon the effects of stream-wise and cross-stream shear on the rotation rate of a fixed sphere in a flow are numerically investigated. Moreover, the change of the flow due to the sphere is recorded. The results indicate that for moderate $Re$ the two types of shear have completely different effects on the particle rotation rate. Moreover, the cross-stream effects appear to be dominant to the stream-wise effects. When the two types of shears are combined to create a strain field, the particle starts to rotate, although the undisturbed flow is non-rotational. [Preview Abstract] |
Monday, November 20, 2006 5:28PM - 5:41PM |
KE.00002: Forces and moments on a rigid rolling sphere in wall-bounded linear shear flows Fady Najjar, Lanying Zeng, Lin Zhang, S. Balachandar, Paul Fischer Understanding drag and lift forces as well as moments exerted on a finite-sized particle in the wall vicinity is a problem of key interest in various engineering applications including aerosol transport, and lift-off of sand particles on sea/ocean shores.Simulations based on high-order parallel spectral element method (Nek5000) are being pursued to investigate the flow past a finite-sized particle. The particle is located in the vicinity of a solid wall and embedded in a linear shear flow. A range of particle location away from the wall has been considered, ranging from almost touching the wall to farther away. We consider a range of Reynolds (Re) numbers varying from $10$ to $200$, where Re is defined based particle diameter and local velocity at particle center. The lift and drag forces as well as the moments are calculated and show a substantial increase in the lift coefficient ($C_L$) as the particle moves closer to the wall and in the limit the particle sits on the wall. The contributions of the viscous and pressure terms to the lift coefficient, $C_L$, will be presented along with comparison to experimental measurements. We then consider the case of rolling particle on a wall for varying speeds to understand the lift and moments mechanisms generated. Results obtained from these simulations will be compared with experiments as well as low Re theory. [Preview Abstract] |
Monday, November 20, 2006 5:41PM - 5:54PM |
KE.00003: Surface Deformation in a Liquid Environment Resulting from Single Particle-Wall Collisions A. Ruiz-Angulo, M.L. Hunt For dry systems (gas-particle) prior studies have investigated the effects of inelasticity on particle rebound and deformation. In wet systems (liquid-particle) the viscous forces can complicate the collision process. For elastic conditions, a range of particle Stokes number where lubrication forces are non-negligible has been previously found. The present work presents experiments on particle-wall collisions where small deformations are allowed, attempting to find the limits of viscous effects by comparing dry and wet systems. Under dry and wet conditions (varying the fluid viscosity), a particle strikes a specimen attached to a long rod. The results show a significant decrease on the coefficient of restitution for both systems and a slight decrease in deformation depth for wet systems. [Preview Abstract] |
Monday, November 20, 2006 5:54PM - 6:07PM |
KE.00004: Velocity fluctuations and hydrodynamic dispersion in a settling suspension of solid particles with finite Reynolds numbers Xiaolong Yin, Donald Koch It is well known that, in simulations of settling particles with periodic boundary conditions, the long-range nature of Stokes flow hydrodynamic interactions leads to an algebraic divergence of particles' velocity variance and hydrodynamic diffusivity with increasing domain size. This paper explores the impact that fluid inertia has on these scaling behaviors. Our lattice-Boltzmann simulations of settling particles with Reynolds numbers of 1-10 indicate that the particle and fluid velocity variance grow logarithmically with system size as has been predicted theoretically by Koch (1993) for randomly distributed particles with Oseen velocity disturbances. However, the coefficient of the logarithmic function is reduced by partial buoyancy screening associated with a deficit of particles in the wake of a test particle. The hydrodynamic diffusivity is proportional to the product of the root-mean-square velocity and the system size, consistent with a scaling theory in which the decorrelation of particle velocities results from fluctuation-induced sampling of the fluid velocity disturbance caused by the other particles [Preview Abstract] |
Monday, November 20, 2006 6:07PM - 6:20PM |
KE.00005: Settling of small heavy particles in flows with wide scale separation Javier Davila Many theoretical and numerical studies found in literature describe how the settling of small heavy particles is enhanced by most vortex flows, being this enhancement optimal for Stokes and flow Froude numbers of the order unity. In this work we study the dynamics of particles in a vortex cellular flow with two length scales: the radius of the vortices $R$ and the distance between them $L$. Considering an artificial life time of the vortices we have found that when $L/R \sim 10^2$ the results are qualitatively similar to those found in DNS. However when $L/R > 10^3$ the scaling of the maximum average settling velocity changes. This result can be explained in terms of the time and velocity scales appearing in the problem (Davila \& Hunt, JFM 2001). Apart from the inertial particle response time and the fluid residence time in this case it is important to consider the particle residence times associated with both scales, the life time of the vortices, and the time that small heavy particles need to escape from the vortices. The average settling velocity can be more than 80\% larger than the terminal velocity. As a result of this strong difference in settling velocities between particles the collision efficiency may reach very high values. [Preview Abstract] |
Monday, November 20, 2006 6:20PM - 6:33PM |
KE.00006: Phase-field simulation of gravitational settling of a growing dendritic crystal. Minh Do-Quang, Gustav Amberg When solidifying a melt, for instance in casting of metals, the melt is typically cooled until its bulk temperature is below the equilibrium freezing temperature. Then any free nuclei in the melt will grow, often creating dendritic `equiaxed' crystals. These growing solid particles typically have a slightly different density from the melt, and will thus begin to settle due to gravity. In the present talk an individual nucleus is simulated as it grows into a dendritic crystal. The melt flow and the convective heat transfer around the crystal are simulated as it settles due to gravity. There is an intricate coupling between the settling and the evolution of the crystal, as the relative flow will influence the growth, which in its turn will affect the subsequent settling motion. Simulations are done in two dimensions using a semi-sharp phase-field model. Inside the solidified part, the flow is constrained to a rigid body motion by using Lagrange multipliers. The model is formulated using two different meshes. One is a fixed background mesh, which covers the whole domain, the other is an adaptive mesh that is translated and rotated with the movement of the solid particle. [Preview Abstract] |
Monday, November 20, 2006 6:33PM - 6:46PM |
KE.00007: Separation of micron-scale particles using helium II Sylvie Fuzier, Steven Van Sciver, Neal Kalechofsky Micron scale particles immersed in helium II will rapidly achieve a terminal velocity based on a balance between gravity and viscous drag with the normal fluid component. This settling velocity is proportional to the density difference between the particle and helium II and the square of the particle diameter. If one applies a vertical upward heat flux, the resulting normal fluid component velocity can counteract the settling velocity, suspending the particles or even giving them a net upward motion. Using this principle, we have built an experimental helium II counterflow channel consisting of segments of varying width such that the heat flux decreases along the channel axis. This configuration allows particles of a particular size range to be suspended and collected in different areas of the counterflow channel. First results of this separation process are presented. [Preview Abstract] |
Monday, November 20, 2006 6:46PM - 6:59PM |
KE.00008: An experimental study of particle-laden thin film flow down an inclined plane Chi Wey, Thomas Ward, Andrea Bertozzi, Anette Hosoi We experimentally investigate a flowing slurry solution with particle density higher than the background fluid density. The slurry exhibits observable behavioral differences as we approach in the limit of maximum packing. To study such differences, a finite volume of slurry is flowed down a walled incline plane of dimensions 1 m x 14 cm. By analyzing the average front position as a function of time and varying parameters such as the inclination angle, particle concentrations, and volume, we observe trends in the effective viscosity for film thicknesses O(500 $\mu$m - 2000 $\mu$m) where the poly-disperse particles are in the range of 200 $\mu$m - 400 $\mu$m. At low concentration, the data follows trends predicted by the classical Huppert [Nature 300({\bf{2}}), 1982] solution for a homogeneous fluid flowing down an inclined plane. At intermediate and high concentrations, the average front speed scales with the exponent predicted by the Huppert, but there are interesting observations in the measured viscosity as a function of inclination angle and volume. [Preview Abstract] |
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