Bulletin of the American Physical Society
63rd Annual Meeting of the APS Division of Fluid Dynamics
Volume 55, Number 16
Sunday–Tuesday, November 21–23, 2010; Long Beach, California
Session HK: Granular Flows II |
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Chair: Nicholas Pohlman, Northern Illinois University Room: Long Beach Convention Center 201B |
Monday, November 22, 2010 10:30AM - 10:43AM |
HK.00001: ABSTRACT WITHDRAWN |
Monday, November 22, 2010 10:43AM - 10:56AM |
HK.00002: Shear-driven segregation of dense granular mixtures Yi Fan, Kimberly Hill Shear-driven segregation of dense granular mixtures has been associated with a number of interesting pattern formation problems. We use experimental and computational split-bottom cells to isolate segregation effects associated with shear gradients from those associated with gravity. We find the effect of shear gradients much less dramatic than initial observations of segregation suggest. While a segregation pattern emerges in a circular split-bottom cell that appears coincident with the shear gradient, we find the pattern is orthogonal to the active segregation flux. We measure a toroidal convection roll that, in conjunction with gravity-driven segregation, is likely responsible for the dramatic horizontal segregation pattern. On the other hand, computational results from a parallel split-bottom cell indicate a subtle segregation flux associated with the shear gradient. A current predictive form of kinetic theory based on binary collisions dominating the particle dynamics predicts segregation in the opposite direction from observed trends. This indicates the direction of shear-driven segregation depends on the nature of the flow itself, collisional or frictional. [Preview Abstract] |
Monday, November 22, 2010 10:56AM - 11:09AM |
HK.00003: Local Dynamics of Granular Size Segregation Adam Keith, James Puckett, Karen Daniels We seek to quantify the local mechanisms which drive granular size segregation, using a two-dimensional system. We perform experiments using a bi-disperse mixture of disks floating on a tilted air table, agitated by bumpers at the bottom edge. A layer of large particles initially placed at the bottom of the system mixes with a layer of small particles above it, eventually resegregating to the upper surface. We record the position of each particle and measure the average segregation velocity as a function of local packing fraction $\phi$ for all particles and local concentration $c$ of small particles. The velocity of the large particles is strongly dependent on packing fraction; particles in regions of lower $\phi$ tend to move downward, while those in regions of higher $\phi$ ascend through the material. In contrast, we find that the effect of local concentration $c$ is weak. [Preview Abstract] |
Monday, November 22, 2010 11:09AM - 11:22AM |
HK.00004: Density Fluctuations in Vibrating Granular Monolayers Gustavo Castillo, Nicolas Mujica, Pablo Gutierrez, Loreto Oyarte, Scott Waitukaitis, Rodrigo Soto This study aims to quantify density fluctuations in a fluidized quasi-two-dimensional granular system close to a solid-liquid-like transition. This transition is reached above an acceleration threshold and at sufficiently high density. The system is a shallow square cell built with two square ITO coated glass plates. The cell is filled with approximately 10000 spherical 1 mm stainless steel particles and the filling density is about 85\%. Due to the dissipative nature of grain contacts, energy is injected in the system by vertical vibrations. To characterize the system we measure its Static Structure Factor as well as dynamical correlation functions. [Preview Abstract] |
Monday, November 22, 2010 11:22AM - 11:35AM |
HK.00005: Liquid effect on the vibration of granular media in cylindrical cavities Enrique Guzman, Roberto Zenit The study of the interactions of granular media with liquid phases is important both, from the academic applied points of view. A particularly interesting problem concerns the dispersion of the granular phase into the liquid phase. To this end, a series of experiments are being conducted in order to determine the conditions under which such dispersion takes place. The experimental apparatus consists of a short transparent cylinder (LvD) with its axis oriented in a horizontal position. The cavity is completely filled with liquid and a prescribed number of glass spheres forms a deposit layer at the bottom. The cylinder, which is initially at rest, is set into a vertical vibrating state of motion by means of an external actuator. While the amplitude of the excitation remains fixed, its frequency is swept (continously) from 5Hz to 15Hz. Synchronized high speed imaging is then used to identify the frequency at which the stratified-to-dispersed transition occurs. Preliminary results clearly indicate the essential role played by the properties of the liquid (i.e. density, viscosity and superficial tension) and of the spheres (i.e. size and number) during the process. The objective of the study is to determine the conditions required to produce appropriate dispersions for different combinations of liquids and spheres. [Preview Abstract] |
Monday, November 22, 2010 11:35AM - 11:48AM |
HK.00006: High-speed x-ray tomographic imaging of a ball impacting on sand Tess A.M. Homan, Evert C. Wagner, Rob F. Mudde, Detlef Lohse, Devaraj van der Meer When a ball is dropped in fine, very loose sand, a cavity is formed inside the sand bed which collapses, creating a jet and entraining an air bubble. At a fixed depth below the surface, the shape and dynamics of a horizontal cross section of the cavity are studied by means of high-speed x-ray tomography system. Repeating the procedure at different depths provides a full time-resolved reconstruction of the cavity within the sand bed. Using this reconstruction we test several hypotheses on the process of sand jet formation. [Preview Abstract] |
Monday, November 22, 2010 11:48AM - 12:01PM |
HK.00007: Measuring the effect of air during granular impact events Devaraj van der Meer, Tess Homan, Sylvain Joubaud, Detlef Lohse Air is known to play a crucial role during the impact of a sphere into a bed of fine, loose sand. This can be traced back to a significant increase of the drag the object experiences inside the sand at low ambient pressures, but what remains unclear is the mechanism by which the drag increases. To shed light upon this mechanism we record the pressure changes during impact, both above and below the bed. From this, with the help of Darcy's law, we deduce the magnitude of the air flows inside the sand which are caused by the impacting sphere and relate these to the observed drag increase. [Preview Abstract] |
Monday, November 22, 2010 12:01PM - 12:14PM |
HK.00008: Emergence of fluid-like granular ejectas generated by sphere impact Jeremy Marston, Sigurdur Thoroddsen Experimental data is presented for the speed and shape of the ejecta when a solid sphere impacts onto a granular bed. We use high-speed imaging to provide direct measurement of individual grain velocities and trajectories as well as the overall evolution of the granular ejecta. For larger bed grain sizes, the velocities of the first ejected grains increase with the kinetic energy of the impacting sphere. We also observe that the fastest grains, which can obtain velocities up to 8 times that of the impacting sphere, emerge at the lowest ejection angles. As the grain size is decreased, a more `fluid-like' behavior is observed whereby the ejected material first emerges as a thin sheet between the sphere and the bed surface. In this instance, in contrast to previous studies, we find the evolution of the ejecta radius approaches a $t^{1/2}$ scaling law, as seen for the crown evolution in liquid drop impacts. [Preview Abstract] |
Monday, November 22, 2010 12:14PM - 12:27PM |
HK.00009: Cooperative penetration in a light granular medium J. Carlos Ruiz-Suarez, Felipe Pacheco-Vazquez A projectile impacting against a granular medium exemplifies the interesting nature of granular matter. Whether the projectile is an asteroid striking the crust of a planet or an object thrown against a granular bed in the laboratory, once the intruder makes contact with the medium it inevitably encounters a stopping force. The character of this force underscore several fundamental issues, from geological and biological sciences, to soil research and technological applications. The impact velocity dependence, the final penetration depth as a function of different parameters, the nature of the drag force, are nowadays well understood thanks to the recent work carried out by different groups. Furthermore, the effects of confinement, object symmetry and fragility of the medium have also been considered. However, despite all this effort, we know very little about what occurs when more than one intruder impact simultaneously a granular medium. Here we show and discuss some experimental findings about the penetration dynamics followed by a group of intruders impacting a granular medium. The particles used in our study are much lighter than water, therefore, intruders penetrate deeply into the system depicting intriguing cooperative behaviours that hint to hydrodynamic-like interactions. [Preview Abstract] |
Monday, November 22, 2010 12:27PM - 12:40PM |
HK.00010: Electrostatics in sandstorms and earthquakes Troy Shinbrot, Nirmal Thyagu, Thomas Paehtz, Hans Herrmann We present new data demonstrating (1) that electrostatic charging in sandstorms is a necessary outcome in a class of rapid collisional flows, and (2) that electrostatic precursors to slip events - long reported in earthquakes - can be reproduced in the laboratory. [Preview Abstract] |
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