Slowly sheared dense granular flows: Crystallization and nonunique final states

Tsai, Jih-Chiang; Gollub, Jerry P.

doi:10.1103/physreve.70.031303

Cited by 58 publications

(45 citation statements)

References 43 publications

(75 reference statements)

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“…Such effects are particularly strong in two dimensional studies [43], and this plays a much weaker role in three-dimensional situations, where there is more geometrical freedom. For example, Tsai and Gollub [37] showed that crystallization in 3D monodisperse packings would only occur after many hours of shearing. To check the effect of particle size, a drainage simulation was carried out using model F using particle diameters uniformly distributed over 0.95d to 1.05d.…”

Section: Polydispersitymentioning

confidence: 99%

“…A dye added to the liquid is illuminated by a light sheet of thickness less than 0.1d and imaged from an orthogonal direction using a 512 × 480 pixel resolution CCD camera, where 20 pixels corresponds to one d. The particles in an image appear dark against a bright background with a flat intensity profile across each particle. We then make use of convolution procedure [37] to convert each image into a 2D map consisting of bright sharp peaks of intensity corresponding to particle centers which are then obtained using a centroid algorithm [38]. This procedure yields particle position in every image to within a twentieth of a particle diameter.…”

Section: A Index-matching Experimentsmentioning

confidence: 99%

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Physical test of a particle simulation model in a sheared granular system

2009

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We report a detailed comparison of a slow gravity driven sheared granular flow with a computational model performed with the Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS). To our knowledge, this is the first thorough test of the LAMMPS model with a laboratory granular flow. In the experiments, grains flow inside a silo with a rectangular cross-section, and are sheared by a rough boundary on one side and smooth boundaries on the other sides. Individual grain position and motion are measured using a particle index matching imaging technique where a fluorescent dye is added to the interstitial liquid which has the same refractive index as the glass beads. The boundary imposes a packing order, and the grains are observed to flow in layers which get progressively more disordered with distance from the walls. The computations use a Cundall-Strack contact model between the grains, using contact parameters that have been used in many other previous studies, and ignore the hydrodynamic effects of the interstitial liquid. Computations are performed to understand the effect of particle coefficient of friction, elasticity, contact model, and polydispersity on mean flow properties. After appropriate scaling, we find that the mean velocity of the grains and the number density as a function of flow cross-section observed in the experiments and the simulations are in excellent agreement. The mean flow profile is observed to be unchanged over a broad range of coefficient of friction, except near the smooth wall. We show that the flow profile is not sensitive to at least 10% polydispersity in particle size. Because the grain elasticity used is smaller in the computations as compared with glass grains, wave-like features can be noted over short time scales in the mean velocity and the velocity auto-correlations measured in the simulations. These wave features occur over an intermediate timescale larger than the particle interaction but smaller than the timescale of the macroscopic flow features. The wave features become more prominent as grain elasticity is further reduced. We then perform a detailed comparison of the particle fluctuation properties as measured by the displacement probability distribution function and the mean square displacement. Excellent agreement is observed over a time interval over which particles can be tracked effectively in the experiments. Using the longer tracking intervals possible in the simulations, we find that the diffusion in the layers is greater in the flow direction, than in the perpendicular direction. Further signatures of confinement and hopping between layers is observed. All in all, our study provides strong support for the LAMMPS model of granular flow, and further supports the hypothesis that the interstitial liquid has negligible effects on granular fluctuations provided the flow is slow.

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Section: Polydispersitymentioning

confidence: 99%

Section: A Index-matching Experimentsmentioning

confidence: 99%

Physical test of a particle simulation model in a sheared granular system

2009

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“…Many experiments have been done on drainage flows in quasi-2d silos where particles are tracked accurately at a transparent wall [9,14,15,16,27]. Some three-dimensional particle tracking in granular materials and colloids has also been done with magnetic resonance imaging [28], confocal microscopy [29], index matching with an interstitial fluid [30], and diffusing-wave spectroscopy [31], although these systems are quite different from a pebble-bed reactor core. Experimental studies of more realistic geometries for PBR have mostly focused on the porosity distribution of static packings of spheres [32,33], which affects helium gas flow through the core [34,35,36].…”

Section: Introduction a Backgroundmentioning

confidence: 99%

Analysis of granular flow in a pebble-bed nuclear reactor

et al. 2006

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Pebble-bed nuclear reactor technology, which is currently being revived around the world, raises fundamental questions about dense granular flow in silos. A typical reactor core is composed of graphite fuel pebbles, which drain very slowly in a continuous refueling process. Pebble flow is poorly understood and not easily accessible to experiments, and yet it has a major impact on reactor physics. To address this problem, we perform full-scale, discrete-element simulations in realistic geometries, with up to 440,000 frictional, viscoelastic 6cm-diameter spheres draining in a cylindrical vessel of diameter 3.5m and height 10m with bottom funnels angled at 30• or 60• . We also simulate a bidisperse core with a dynamic central column of smaller graphite moderator pebbles and show that little mixing occurs down to a 1:2 diameter ratio. We analyze the mean velocity, diffusion and mixing, local ordering and porosity (from Voronoi volumes), the residence-time distribution, and the effects of wall friction and discuss implications for reactor design and the basic physics of granular flow.

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“…Granular materials exhibit many complex phenomena [11,12]. The ordering of monodisperse granular materials, when subjected to vibration or shear flow, is one example [13][14][15][16][17][18]. The transition from a disordered state to an ordered state is marked by a significant increase in the solid fraction manifested as a compaction of the material [13,17].…”

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confidence: 99%

Sidewall-friction-driven ordering transition in granular channel flows: Implications for granular rheology

Mandal

Khakhar

2017

Phys. Rev. E

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We report a transition from a disordered state to an ordered state in the flow of nearly monodisperse granular matter flowing in an inclined channel with a bumpy base, in discrete element method simulations. For low particle-sidewall friction coefficients, the particles are disordered and the Bagnold velocity profile is obtained. However, for high sidewall friction, an ordered state is obtained, characterized by a layering of the particles and hexagonal packing of the particles in each layer. The extent of ordering, quantified by the local bond-orientational order parameter, varies in the crosssection of the channel, with the highest ordering near the side walls. The flow transition significantly affects the local rheology -the effective friction coefficient is lower, and the packing fraction is higher, in the ordered state compared to the disordered state. A simple model, incorporating the extent of local ordering, is shown to describe the rheology of the system.Understanding the flow of granular materials is important in the context of natural phenomena and industrial processes [1][2][3][4][5]. Considerable progress has been made in the development of theories for the rheology of granular flows, with the objective of developing continuum models for the analysis and design of large systems [6][7][8][9][10]. Granular materials exhibit many complex phenomena [11,12]. The ordering of monodisperse granular materials, when subjected to vibration or shear flow, is one example [13][14][15][16][17][18]. The transition from a disordered state to an ordered state is marked by a significant increase in the solid fraction manifested as a compaction of the material [13,17]. In sheared systems, which are the focus of the present study, ordered states show a layering of particles with adjacent particle layers sliding past each other without much interchange of particles between the layers [15,16,19]. Shear stresses are consequently lower compared to the disordered state. Order-disorder transitions thus affect system behavior significantly, however, they have not been explicitly incorporated in the analysis of granular rheology.The most detailed studies of order-disorder transitions in granular shear flow are computational, using the discrete element method (DEM) to simulate the flow of monodisperse particles (diameter d) on a rough inclined plane with periodic boundary conditions (no sidewalls) [19][20][21]. Kumaran and Maheshwari [20] carried out a study of the transition using a base comprising particles of diameter, d b , arranged on a square lattice. When the base roughness parameter (d b /d) was less than 0.6, the system underwent a transition to an ordered state, comprising hexagonally close packed particle layers sliding on each other. The shear stress (τ xy ) in the ordered state followed the Bagnold scaling (τ xy ∝γ 2 , whereγ is the shear rate) but with smaller values of the Bagnold coefficients compared to the disordered state. The transition was very sharp, occurring over a 1% change in the roughness parameter. A...

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Slowly sheared dense granular flows: Crystallization and nonunique final states

Cited by 58 publications

References 43 publications

Physical test of a particle simulation model in a sheared granular system

Physical test of a particle simulation model in a sheared granular system

Analysis of granular flow in a pebble-bed nuclear reactor

Sidewall-friction-driven ordering transition in granular channel flows: Implications for granular rheology

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