Bulletin of the American Physical Society
62nd Annual Meeting of the APS Division of Fluid Dynamics
Volume 54, Number 19
Sunday–Tuesday, November 22–24, 2009; Minneapolis, Minnesota
Session BU: Granular II: Jamming II |
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Chair: Daniel Goldman, Georgia Institute of Technology Room: 200I |
Sunday, November 22, 2009 10:30AM - 10:43AM |
BU.00001: Shearing and compression of elliptical particles Somayeh Farhadi, Robert Behringer We have performed 2D biaxial shearing and compression experiments for elliptical photoelastic particles in order to understand the effect of particle shape on microscopic and macroscopic properties of a granular system. The shearing experiment was conducted via a series of small forward and reverse steps using pure shear. We study the evolution of particle orientations and the average number of contacts following each step of shear or compression. Using photoelastic particles enables us to visualize the stress state of the system at the particle scale level. The ongoing analysis addresses the statistical properties of jammed state, including jamming that is reached through compression or through shear. [Preview Abstract] |
Sunday, November 22, 2009 10:43AM - 10:56AM |
BU.00002: Scaling of boundary stresses in a Couette flow of granular mixtures Bereket Yohannes, Kimberly Hill Most constitutive laws for dense granular flows that predict stresses based on particle size and shear rates are developed based on experiments and simulations of uniformly sized spherical particle. These types of constitutive laws have not yet been formulated for granular mixtures. In order to investigate the applicability of certain constitutive laws for dense granular mixtures, we study the boundary stresses due to a Couette flow of binary mixtures of different sized particles using a 3D discrete element method (DEM). For a given mixture, as in a uniform particle size distribution, the stresses scale with the square of the shear rate. However, the stress appears to have a more complicated dependence on both the relative sizes of the particles in the mixture and the relative concentration of the different species. In these simulations the coordination number is found to be a better quantitative parameter than average size to describe the stresses. We present the relationship between boundary stresses, particle size distribution, and coordination number. [Preview Abstract] |
Sunday, November 22, 2009 10:56AM - 11:09AM |
BU.00003: Scaling of boundary stresses in granular mixtures: free surface gravity-driven flows Kimberly Hill, Bereket Yohannes, William Dietrich, Leslie Hsu The ability to predict the stresses particulate mixtures exert on their boundaries is important for many critical natural and industrial applications. For example, predicting boundary stresses due to bouldery debris flows is important for understanding landscape morphology and associated hazard mitigation. Typically, models developed to predict internal and external stresses in these flows contain reference to a particle size and sometimes a maximum packing fraction, neither of which in known for a granular mixture. We find that, while for a mixture sheared in a Couette cell substitutions may be found for both terms, the situation is somewhat more complicated in a free surface flow relevant to debris flows. In the latter case the particle size distribution and the phase of flow is non-uniform in the direction of flow, and both neeed to be considered for determining the local and global stresses in the system. [Preview Abstract] |
Sunday, November 22, 2009 11:09AM - 11:22AM |
BU.00004: Shapes of elastic cylindrical tubes filled with liquids and granular media G. Juliana Gutierrez, Abraham Medina In this work we discuss theoretically and experimentally the deformation equilibrium shapes of the elastic walls of a vertical tube when it was filled with water and sand. The use of elastic soft walls has been motivated to note, through the deformation of the walls, the role of the pressures induced by each material. By using the linear elastic theory it is possible to estimate analytically the shapes of the deformed walls induced by the respective hydrostatic pressures of water and sand. [Preview Abstract] |
Sunday, November 22, 2009 11:22AM - 11:35AM |
BU.00005: Enhanced penetration forces with simultaneous granular intruders Paul Umbanhowar, Lionel London, Yang Ding, Daniel Goldman To better understand how the geometry and actuation of biological and mechanical feet affect locomotor performance on flowable ground, we examine the constant velocity insertion into granular media of two horizontal, parallel rods as a function of rod separation $s$. Our experiments and simulations show that while the force $F$ required to maintain a constant velocity increases linearly with the penetration depth $d$ in all cases, the slope $F/d$ is a non-trivial function of rod separation. As $s$ is increased from zero, $F/d$ initially increases, reaches a maximum value at $\approx2$ grain diameters, and then slowly decreases to twice the value of $F/d$ for a single rod at large separation. Examining force correlations and flow intermittency between the rods, we show that a model of cooperative arching and jamming explains the salient features of our results. [Preview Abstract] |
Sunday, November 22, 2009 11:35AM - 11:48AM |
BU.00006: The design, testing, and performance of RoboClam, a robot inspired by the burrowing mechanisms of Atlantic razor clams (Ensis directus) Amos Winter, Anette Hosoi, Alexander Slocum In this work we present the design, testing, and performance of RoboClam, a robot that mimics digging methods employed by the Atlantic razor clam (Ensis directus). Ensis is one of nature's most adept burrowing organisms, able to dig to 70cm at nearly 1cm/s using only 0.21J/cm. We have found that Ensis reduces burrowing drag by using motions of its shell to fluidize a thin layer of substrate around its body. Although these shell motions have an energetic cost, moving through fluidized rather than packed soil results in dramatically lower overall energy consumption. RoboClam was constructed to understand the limits of razor clam-inspired burrowing, how the relevant parameters scale for different environments and conditions, and how this understanding can be transferred into engineering applications. Through experimental data gathered in idealized granular, as well as real ocean substrates, we show that RoboClam exploits localized fluidization to attain nearly the same burrowing energy savings as Ensis. [Preview Abstract] |
Sunday, November 22, 2009 11:48AM - 12:01PM |
BU.00007: Measuring the phase diagram of granular media J-F. Metayer, D.J. Suntrup III, C. Radin, H.L. Swinney, M. Schroeter The jamming transition in granular media has been the subject of several studies in the last years. However, an experimental observation of phase diagram of granular media in three dimensions is still lacking. The goal of this study is to obtain such a diagram as a function of the shear stress, the packing fraction, $\phi$, and the pressure in a granular bed. Shear stress is obtained by measuring the force, $F$, needed to pull-up a paddle immersed in a granular bed as a function of its packing fraction and the depth of immersion. We find that the value of this force is strongly depending the packing fraction: for low packing fraction ($\phi<0.59$) $F$ is constant while it increases strongly with $\phi$ for higher packing fraction. [Preview Abstract] |
Sunday, November 22, 2009 12:01PM - 12:14PM |
BU.00008: Stick-slip transition at the granular critical state Nick Gravish, Paul Umbanhowar, Daniel I. Goldman We study the force on a flat plate (3.8~cm width, 7.0~cm depth) dragged at constant velocity $v$ through the surface of a granular medium (250~$\mu$m glass beads) as a function of volume fraction $0.57<\phi<0.63$. The dynamics of the drag force $F_d$ are sensitive to $\phi$: we find a sharp transition in the form of $F_d$ at a critical volume fraction $\phi_c=0.605$. For $\phi<\phi_c$, $F_d$ increases with time and saturates, while for $\phi>\phi_c$ $F_d$ exhibits an initial peak followed by periodic oscillations at frequency $f$ about a constant mean. The standard deviation in force (a measure of the fluctuations) shows a sharp transition at $\phi_c$. The force oscillations suggest that the granular media periodically jams and flows as the plate is horizontally translated. Examining the bed surface we observe a spatially periodic scalloped feature of length $\lambda$ which is equal to $v/f$, independent of $v$, and increases linearly with $\phi$ for $\phi>\phi_c$. By measuring the displaced volume after the drag $\Delta V$, we observe a transition from media compaction ($\Delta V<0$) for $\phi<\phi_c$ to dilation ($\Delta V>0$) for $\phi>\phi_c$. [Preview Abstract] |
Sunday, November 22, 2009 12:14PM - 12:27PM |
BU.00009: Velocity field within a \emph{stagnant zone} in a granular flow Enrique Sandoval, Roberto Zenit It is known that in granular flows stagnant zones may appear. It has been usually thought that these regions remain immobile; however creep-like motion has been observed. The objective of this study is to measure the velocity field within the stagnant zone. Are the flow properties of such zone determined by the material (particle size, maximum solid fraction, roughness) or the flow (velocity)? We hope to answer this question. A gravity-driven granular flow was produced with a vertical channel filled with glass beads. The mean speed was controlled with discharge hopper. A solid plate is positioned in the center of the chute; its interaction with the flow produces a stagnant region in its upstream side. Using an image correlation technique (normally used in the PIV method), velocity profiles were measured for different angles, mean discharge velocities and plate sizes. For the case in which plate is perpendicular to flow, the velocity was found to decay exponentially within stagnant region. The exponential decay ratio is proportional to the distance from the plate. In this talk the nature of this behavior and its implications for granular media rheology will be discussed. [Preview Abstract] |
Sunday, November 22, 2009 12:27PM - 12:40PM |
BU.00010: Column Collapse of Rod-like Granular Materials Melissa Trepanier, Scott Franklin We study the collapse of piles of rod-like granular materials, in particular how the particle aspect ratio (length/width) and coefficient of friction affect the runoff. Rod particles can maintain the shape of their container, something round particles cannot, and we find transitional pile heights that determine the onset of collapse. For low aspect ratios, pile heights of less than a particle length do not collapse, implying that vertically oriented rods are anchoring the pile and providing stability. There is a broad transition range of pile heights in which the probability of collapse grows linearly from 0 to 1. The scaling of the runoff distance in and above this region is independent of aspect ratio and friction, depending only on the initial pile geometry. This work could have significant implications for construction of stable structures and understanding avalanches of needle-like snow crystals (hoar). [Preview Abstract] |
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