Bulletin of the American Physical Society
65th Annual Meeting of the APS Division of Fluid Dynamics
Volume 57, Number 17
Sunday–Tuesday, November 18–20, 2012; San Diego, California
Session E8: Particles: Collisions |
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Chair: Jane Wang, Cornell University Room: 25A |
Sunday, November 18, 2012 4:45PM - 4:58PM |
E8.00001: Computing Particle Collisions in Fluids by Incorporating the Lubrication Theory in the Immersed Interface Method Acmae El Yacoubi, Sheng Xu, Z. Jane Wang The interactions of particles in fluids are key to understanding collective behavior of particle suspensions. To compute the dynamics of these systems in the high particle-density limit, one has to treat the collision of particles. There has been experimental and theoretical studies to understand the dynamics of particle collisions in fluids. However, direct numerical simulation remains a challenge. The small gap introduces difficulties in spatial resolution, as it would require successive local refinements of the grid. A scheme with a fixed grid resolution would break down when the gap falls below a threshold. There have been various {\it ad hoc} methods using a repulsive force or modified dry collision equations. However, they can lead to unrealistic dynamics such as the rebound of particles. In this talk, we will present a computational method which applies the lubrication theory in the interstitial region between particles. We will describe the numerical implementation in the immersed interface method framework. The fluid velocity and pressure gradient in the gap are solved for analytically, and are used in the expression of the singular forces. We test our computational scheme by checking against analytical solutions of interactions between a falling cylinder and a wall. [Preview Abstract] |
Sunday, November 18, 2012 4:58PM - 5:11PM |
E8.00002: The Importance of Collisions in the Simulation of Lunar Soil Ejection during Spacecraft Landing Kyle Berger, Philip Metzger, Christine Hrenya When a spacecraft lands on the Moon, the rocket exhaust causes lunar soil to be ejected. Due to the lack of atmospheric drag and reduced gravity, the ejected soil can be extremely hazardous to equipment and/or persons both close and far from the landing point. Current models for the ejection are based on single-particle trajectories. Here we critically assess the impact of collisions on erosion. Specifically, the discrete element method (DEM), which incorporates collisions directly, is used. The system examined is located 6m from the impingement point of the rocket and includes the lift and drag forces from the exhaust plume, as well as lunar gravity. A one-way coupling is utilized to describe how the plume affects the particles. Two versions of the DEM are used: one which resolves collisions using a soft-sphere model and another which ignores the collisions (and is thus similar to the single-particle trajectory calculations). In addition, both non-dissipative and dissipative collisions are considered in the collisional model. Somewhat surprisingly, the erosion rate of the collision-less case lies between that of the dissipative and non-dissipative collisional cases. In addition, a sensitivity analysis to collisional input parameters is also performed. [Preview Abstract] |
Sunday, November 18, 2012 5:11PM - 5:24PM |
E8.00003: Rheology of colloidal suspensions measured by dragging a probe Xin Du, Piotr Habas, Roseanna Zia, Eric Weeks We use active microrheology to study the rheological properties of colloidal suspensions at moderate volume fractions. Traditionally, the rheology of complex fluids is experimentally studied using macroscopic mechanical rheometers. Alternatively, single-particle tracking---microrheology---can be utilized to measure material properties. Microrheology involves the tracking the motion of a probe particle embedded in a complex fluid. In passive microrheology, the motion of the probe particle is driven by thermal fluctuations. Here we study non-equilibrium systems via active microrheology, in which a magnetic probe particle is dragged by a constant external magnetic force through a suspension of colloidal particles. By tracking the mean and mean-square probe motion, the viscosity, diffusivity, and normal stresses are obtained. The effective viscosity of the suspension is determined from the mean velocity of the probe particle. The velocity fluctuations of the probe which are parallel and perpendicular to the mean velocity direction produce force-induced probe diffusion, are measured by the mean-square displacement of the probe. By applying recent theory, the two measurements are combined to understand other rheological properties of the complex fluid such as normal stresses. Our results are in good agreement with macroscopic rheology of similar suspensions, demonstrating that the microscopic technique may be useful for cases when only small sample quantities are available. [Preview Abstract] |
Sunday, November 18, 2012 5:24PM - 5:37PM |
E8.00004: Stability of bed particles near the critical threshold of motion Julian Simeonov, Joseph Calantoni The unsteady flow above a rough bed of mobile spherical particles is investigated with Direct Numerical Simulations. The velocity and pressure are resolved at sub-particle scales using a Cartesian grid numerical method based on a discontinuous extension of the pressure Poisson equation across particle boundaries. The hydrodynamics is fully resolved everywhere except in the gap between colliding particles when the latter becomes smaller than the grid step. To correctly predict momentum dissipation due to the viscous flow in the unresolved gap between colliding particles, we add analytical lubrication forces to the numerically resolved hydrodynamic force. The normal and tangential forces due to mechanical contact are modeled using a linear elastic-plastic law (soft-sphere) and a history dependent friction law, respectively. The collision model is validated against experimental data for normal and oblique immersed collisions of spherical particles. The lubrication effects during weak collisions are essential for damping out the flow-induced vibrations of bed particles confined in surface pockets. The results from our numerical simulations for the initiation of motion are compared with existing laboratory data. [Preview Abstract] |
Sunday, November 18, 2012 5:37PM - 5:50PM |
E8.00005: Observation of the Sling Effect Gregory Bewley, Ewe Wei Saw, Eberhard Bodenschatz We report the first experimental observations of the sling effect, by which fluid turbulence increases the rate of collisions between suspended droplets. We put liquid water-alcohol droplets in a turbulent air flow, and followed their motions in three dimensions with two cameras. The turbulence was approximately isotropic with a Taylor Reynolds number of about 200. The resulting droplet Stokes numbers were between 0.1 and 0.6, depending on the intensity of the turbulence and the size of the droplets. We used two-droplet statistics to characterize the droplet velocity field and its gradient. The droplet velocity field contained gradients that were large enough relative to the droplet response time for slings to occur, according to the theory. The large negative gradients steepened just as was predicted for slings. During this steepening, the droplet gradients decoupled from the background fluid gradients. [Preview Abstract] |
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