Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session F56: Granular Materials and Flows |
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Sponsoring Units: GSNP Chair: Mark Shattuck, City College of New York Room: BCEC 255 |
Tuesday, March 5, 2019 11:15AM - 11:27AM |
F56.00001: The influence of pressure on segregation and diffusion in a shear flow Paul Umbanhowar, Alexander M Fry, Julio Mario Ottino, Richard Lueptow The effect of confining pressure on segregation and diffusion of granular material is studied in Discrete Element Method (DEM) simulations of horizontal planar shear flow, where a feedback scheme maintains a constant shear rate for varying pressure. Both the segregation rate and the ultimate degree of segregation in initially mixed size-bidisperse and density-bidisperse beds decrease with increasing pressure. In contrast, the collisional diffusion is pressure independent. Consequently, segregation is reduced relative to diffusive mixing with increasing pressure. To test these findings, we compare DEM results to predictions of a continuum model that includes a pressure dependent segregation velocity and a pressure independent diffusion term. The model accurately predicts the steady-state segregation for both size and density bidisperse mixtures over a wide range of flow conditions. Additional simulations with segregated initial conditions demonstrate that a high enough overburden pressure reduces segregation sufficiently that significant mixing occurs, implying that manipulation of the shear-pressure state in granular flows could be used to drive particle mixtures to either mixed or segregated states as desired. |
Tuesday, March 5, 2019 11:27AM - 11:39AM |
F56.00002: Kinetic Theory Approach for Modeling Granular Segregation Richard Lueptow, Yifei Duan, Paul B Umbanhowar Momentum conservation equations for individual constituents have been used to derive expressions for the percolation velocities of bidisperse segregating species in a granular flow. Many gravity-driven segregation models assume that the inter-species drag takes the form of Darcy's law, which gives a linear relation between the drag forces and the percolation velocities. However, a simple linear drag law is not sufficient to describe the segregation behavior. To address this, we propose a relation based on the kinetic theory of granular flow that includes terms to account for the effects of the particle surface friction, the coefficient of restitution, and the local inertial number. The percolation velocity derived from the momentum balance equation with this drag model agrees well with DEM simulations of uniform shear flows of density bidisperse particles, accurately predicting the difference between upward species velocity and downward species velocity through the depth of the flowing layer for different density ratios and relative constituent concentrations. |
Tuesday, March 5, 2019 11:39AM - 11:51AM |
F56.00003: Size segregation in driven granular media Philip Wang, Abe Clark, Nicholas Ouellette, Mark Shattuck, Corey Shane O'Hern Granular flows involve grains of different sizes and mass densities, which either remain well-mixed or segregate. We perform discrete element simulations of model granular systems composed of frictionless, bidisperse disks (with diameter D_{l} and D_{s}, mass density ρ_{l} and ρ_{s}, for large and small disks) under gravity and driven by either simple shear or vibration to understand parameters that control mixing and segregation. We have shown that sheared granular system possess 1) a geometrically-segregated regime, where the system becomes increasingly segregated with decreasing D_{s}/D_{l}, 2) a weight-segregated regime, where the system becomes less segregated as D_{s}/D_{l} decreases further, and 3) well-mixed states. We identified the boundaries between these three regimes as a function of D_{s}/D_{l }and ρ_{s}/ρ_{l}. We performed similar studies of vibrated systems to determine if the boundaries between mixed and segregated states depend on the driving method. We show that the large particles rise to the top only if ρ_{l}<ρ_{s}, i.e. we only find weight-segregation in vibrated systems. We also show that by driving only large particles in vibrated systems, large particles will rise for intermediate packing fractions (0.7>φ>0.5). |
Tuesday, March 5, 2019 11:51AM - 12:03PM |
F56.00004: Lift force in size and density segregation Lu Jing, Adithya Shankar, Paul Umbanhowar, Richard Lueptow We computationally study the forces experienced by a large intruder in a sheared particle bed. The intruder is attached to a virtual spring, which allows measurement of segregation forces when the intruder reaches an equilibrium position. By subtracting a buoyancy-like contribution from the overall segregation force, the lift force on the intruder is measured for a variety of size and density ratios, and local shear rates. We find that lift force depends only on size ratio and shear rate, but not on density ratio. At a given shear rate, the lift force scales with the square of the size ratio. We propose a lift force model that successfully predicts the combination of size and density ratios at which the intruder rises or sinks. In the future, the lift force model will be used to predict segregation velocity in size and density disperse mixtures. |
Tuesday, March 5, 2019 12:03PM - 12:15PM |
F56.00005: Mesoscopic features of a granular dynamics under cyclic compression Zackery Benson, Anton Peshkov, Michelle Girvan, Derek C. Richardson, Wolfgang Losert We study the reversibility of rotational motion of spherical grains via molecular dynamics simulations. From the simulations, we are able to capture the motion, position, and force distribution of grains during cyclical compression and dilation and find excellent agreement with our experimental data. Additionally, we analyze the network structure formed from the contact forces between individual grains and show how the network dynamics are affected by compression amplitude as well as the evolution of the network from a transient to a steady state. Further, we use a simple machine learning approach combined with experimental data to characterize different states of our granular system. |
Tuesday, March 5, 2019 12:15PM - 12:27PM |
F56.00006: Avalanche precursors in a frictionnal model Axelle Amon, Baptiste Blanc, Jean-Christophe Geminard An experimental approach to study precursors to avalanches is to progressively tilt a box filled with sand and to monitor the events that take place below the avalanche angle. Such experiments have shown the existence of two types of events: localized rearrangements implying only a few grains and large coherent events implying an increasing part of the sample. Those micro-ruptures occur with an angular periodicity, starting from about half of the avalanche angle until the avalanche takes place. |
Tuesday, March 5, 2019 12:27PM - 12:39PM |
F56.00007: Modeling avalanches in granular matter Pierre Soulard, Denis Dumont, Paul Rambach, Thomas Salez, Elie Raphael, Pascal Damman Sand is a complex material. At rest, a sand pile is a solid. But as soon as the external stress exceeds a certain threshold, it starts flowing like a liquid. During an avalanche, the two states coexist: a liquid phase rolls on the top of a solid phase, the latter is the result of the sedimentation of the liquid phase. The global description of the rheology of the granular material is still a open question and an active field of research. |
Tuesday, March 5, 2019 12:39PM - 12:51PM |
F56.00008: Collapse of Granular Column : an Adequate Tool to Characterize Granular Matter Denis Dumont, Pierre Soulard, Paul Rambach, Thomas Salez, Elie Raphael, Pascal Damman In spite of a large effort of the scientific community, a global description of the rheology of granular assembly remains largely elusive. To gain a better insight into the mechanical behavior of granular matter, we focus on a canonical set-up: the collapse of columns under gravity. We carried on 3D DEM numerical simulations and experiments for various granular columns on a frictional plane. Interestingly, this flow configuration is dictated by only two parameters : the aspect ratio of the initial column, a, and the friction. The dynamics and final shape of the pile can be rationalized through a modified BCRE scheme. The model only includes two parameters, the aspect ratio of the initial column and the dynamic or neutral angle. Interestingly, all the friction coefficients, grain-grain and grain-wall, are encoded in this single value. |
Tuesday, March 5, 2019 12:51PM - 1:03PM |
F56.00009: Inertial Phenomena and Resistive Force Theory in Wheeled Locomotion on Dry Granular Media Andras Karsai, Shashank Agarwal, Ken Kamrin, Daniel Goldman We use an automated testbed to systematically conduct single-wheel (20 cm diam.) locomotion experiments in dry granular media (~1 mm poppy seeds) at angular velocities ω (0.7-6.4 rad/s) where substrate-based inertial effects emerge. In contrast to Resistive Force Theory based predictions for which translation speed is proportional to angular velocity, as ω increases, the translation speed plateaus to a speed dependent on wheel geometry and the system’s force distributions. A frictional plasticity model shows similar phenomena despite lacking a rate-dependent constitutive model. This effect can be explained through a force-momentum balance which accounts for the inertial effects caused by changes in substrate inflow/outflow in the system’s local volume. This force balance creates a net force loss that increases with translational velocity, creating a material-enforced speed limit for wheeled locomotion. Current RFT models describe quasistatic kinematic bodies, but an inertial correction to the RFT forces shows promise in extending RFT into capturing speed-dependent scenarios as well. |
Tuesday, March 5, 2019 1:03PM - 1:15PM |
F56.00010: Understanding slipping of wheels in granular media locomotion Shashank Agarwal, Andras Karsai, Daniel Goldman, Ken Kamrin Slipping of circular wheels at high rotation rates on granular media like sand is a commonly experienced phenomenon. At the same time, granular materials themselves are known to be rate-insensitive for a fairly large range of strain rates. In order to identify the fundamental phenomena responsible for the sudden increase in the slipping of wheels above a certain rotational velocity, a plasticity-based continuum modelling study is performed. Lugged wheel locomotion simulations, validated against experimental findings, are performed for a wide range of rotation rates. A momentum conservation-based argument is proposed to explain and predict the onset of increased slipping. An empirical form of optimal rotation rates for wheel locomotion in terms of various associated system parameters is also suggested. |
Tuesday, March 5, 2019 1:15PM - 1:27PM |
F56.00011: Clogging of soft particles in vibrating 2D hoppers Mia Morrell, Eric Weeks We study the outflow of soft, low-friction hydrogel beads from a quasi-2D hopper, examining the probability of clog formation as a function of hopper exit width. By tilting the hopper chamber, we vary the force of gravity driving the flowing bead system. We find that clogging of soft beads requires the hopper aperture to be only slightly larger than the bead diameter, and increasing the force driving the beads towards the exit results in a decreased probability of clogging, holding exit width constant. We then investigate the effects of vibration of the entire hopper system, working in a high frequency limit. Vertical vibrations decrease the clogging probability, due to a disruption of clogging arches. Horizontal vibrations increase the clogging probability, due to an effectively smaller slit size. |
Tuesday, March 5, 2019 1:27PM - 1:39PM |
F56.00012: Measuring the yield stress of charged granular media: how a net charge leads to cohesive powders C. Mark Lewis, Jeremy M. Laprade, Brandon M. Hoover, Abdoul R. Ayouba, Freeman S. Dong, Oscar S. Hernandez-Daguer, Anthony Dinsmore Charged powders are common in nature and industry yet still exhibit many surprising behaviors that are poorly understood. Here we report that piles of grains that all carry the same sign of net charge can, surprisingly, behave like a brittle, cohesive solid. Our experiments probe slab-shaped piles placed on an insulating substrate atop a grounded plate. Sample were irradiated with ions from a corona discharge device and the final surface voltage, V_{S}, was measured. For ordinary sand, removing the pile from the ground plate caused the charged grains to fly off. By contrast, sand grains with hydrophobic coating formed piles that were stable, cohesive and rigid. We measured the rigidity of these piles in two ways: the minimum tilt angle θ that led to failure, and the yield shear stress, σ_{Y}. We found that both quantities increased as V_{S}^{2}. θ can exceed 90^{○} and σ_{Y} can reach three times its zero-charge value. We also measured the decay of V_{S} over time and found that a stretched exponential function fit the data over timescales of many days. These results may lead to new ways to manipulate charged granular materials and provide new insights into the properties of lunar or Martian soils. |
Tuesday, March 5, 2019 1:39PM - 1:51PM |
F56.00013: Jamming and Tiling in Aggregation of Rectangles Eli Ben-Naim, Daniel Ben-Naim, Paul Krapivsky We study a random aggregation process involving rectangular clusters. In each aggregation event, two rectangles are chosen at random and if they have a compatible side, either vertical or horizontal, they merge along that side to form a larger rectangle. Starting with N identical squares, this elementary event is repeated until the system reaches a jammed state where each rectangle has two unique sides. The average number of frozen rectangles scales as N^{α} in the large-N limit. The growth exponent α=0.229±0.002 characterizes statistical properties of the jammed state and the time-dependent evolution. We also study an aggregation process where rectangles are embedded in a plane and interact only with nearest neighbors. In the jammed state, neighboring rectangles are incompatible, and these frozen rectangles form a tiling of the two-dimensional domain. In this case, the final number of rectangles scales linearly with system size. |
Tuesday, March 5, 2019 1:51PM - 2:03PM |
F56.00014: Analyzing The Flow of a System of Spheres Using Shape-Anisotropic Particles Justin Roberts, Shubha Tewari We report here on a simulation conducted using LAMMPS [https://lammps.sandia.gov] of a small concentration of dimers in a gravity-driven flow of spheres. The simulation box is a quasi-2D vertical hopper with a rectangular outlet containing a 1% concentration of dimers in an equal mixture of frictional spheres of diameter ratio 1.2. Each dimer consists of two contacting spheres glued together, and all spheres undergo the same Hertzian contact interactions. We track the positions, velocities and orientations of the dimers relative to the flowing spheres and find that while the flow is collisional, the dimers do not reorient significantly except near the walls and near the outlet, where the fluctuations in the sphere velocities are higher. However, as outlet size changes, the region with the largest reorientation per dimer moves from near the edges for large openings, to near the center for small openings, thus appearing to track where velocity fluctuations are large. |
Tuesday, March 5, 2019 2:03PM - 2:15PM |
F56.00015: Confined Packing of Granular Rods Julian Freeman, Cong Cao, Sean F Peterson, Yujie Wang, Scott Franklin, Eric Weeks We conduct a series of experiments and simulations to observe the effects surfaces have on the packing structure of randomly packed rods. Our experiments use cylindrical containers of different diameters, and rods of aspect ratios ranging from 4 to 32. We find that the rods packed into smaller cylindrical containers yielded lower volume fractions than in larger containers. Our results are extrapolated to an infinite container size, and the subsequent volume fraction decreases with increasing aspect ratios, in agreement with previous simulations. We also do x-ray tomography experiments and study simulated rod packings, which reveal boundary layers where the packing differs from the bulk. At the bottom, rods form layers, although their average volume fraction is similar to that of the bulk. The sides and top layers are both packed with lower volume fractions than the bulk. In particular, the top boundary layer has the strongest perturbation to the overall volume fraction. |
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