3-7 July 2017
> Montpellier France

Video Abstracts

All contributors to the scientific program, including keynote speakers, speakers, and poster presenters are invited to increase the visibility of their scientific work by means of video abstracts. At the conference, each video abstract can be introduced by a 3 minutes oral presentation in a dedicated session. We will ask you to deliver a PowerPoint file prior to the conference.

How to prepare a video abstract

Instructions and templates can be found here.
Contact: video@pg2017.org

Video contributions

Can soil friability parameterize particle linkage?

Luis Alfredo Pires Barbosa (1)*, Thomas Keller (2,3), and Antonio Carlos de Oliveira Ferraz (1),
(1) University of Campinas, School of Agricultural Engineering, 13083-875, Av. Candido Rondon, 501, Campinas, Brazil
(2) Agroscope, Department of Natural Resources and Agriculture, CH 8046 Reckenholzstrasse, 191, Zurich, Switzerland
(3) Swedish University of Agricultural Sciences, Department of Soil and Environment, Box 7014, Uppsala, Sweden
*Corresponding author: luis.barbosa@feagri.unicamp.br

Soil aggregates play an essential role in soil functioning and hence impact crop yield. Soil friability is a measure of the ease of soil crumbling and quantifies to what extent tensile strength decreases with increasing aggregate size. This provides valuable information for modelling aggregate fragmentation due to applied stresses (e.g. seedbed preparation). This research aimed at delineating the relationship between bond strength and friability from crushing simulations of three different aggregate sizes. Aggregates were created as a pack of particles ruled by a cohesive frictional contact law, and internal flaws were simulated by removing a randomized number of particles at random positions. We performed 30 crushing simulations for each aggregate size to define the mean tensile strength (s) by the Weibull distribution, and run simulations for seven different bond strengths. Initial results show that differences in s due to bond strength were most pronounced for the smallest aggregates, which is consistent with literature data, and that the model was able to reproduce the friability concept (i.e. decreasing s with increasing aggregate size). However, bond strength only had little effect on soil friability, suggesting that bond strength or internal flaws should be modelled as a function of aggregate size as well.

An experimental method to measure the strain field inside a granular material: application to highly compressed granular systems

Jonathan Barés, Thi-Lo Vu and Serge Mora,
Laboratoire de Mécanique et Génie Civil, Université de Montpellier, France

In many situations granular materials are highly stressed and undergo high plastic or hyper-elastic deformations: gauge fault, sintered powders, globules in veins... However, most of the experiments stay in the small deformation regime for theoretical convenience and are not able to properly catch the behaviour of these materials. By mean of a novel experimental approach we overcome the limitations of the photo-elastic method to measure local strain in highly deformed particles.
We built an apparatus capable of compressing a densely packed set of 2D bidisperse discs made of silicone while imaging them very accurately after each loading step. This accurate imaging process permits first to follow precisely the evolution of the particle positions and geometries. Then a homemade Digital Image Correlation code catches the evolution of the displacement fields in each particle.
All non-linear elasticity tensors (deformation, Cauchy-Green, strain...) are deduced and open a new field of experimental analysis for 2D granular matter as well as the opportunity to compute the stress tensor.
We believe this new measurement method, not limited to a specific material, shape or loading mechanism opens a wide panel of new experimental possibility...

Structure of hexapod 3D packings: understanding the global stability from the local organization

Jonathan Barés (1,2), Yuchen Zhao (2), Mathieu Renouf (1), Karola Dierichs (3), and Robert Behringer (2),
(1) Laboratoire de Mécanique et Génie Civil, Université de Montpellier, CNRS, Montpellier, France
(2) Department of Physics & Center for Nonlinear and Complex Systems, Duke University, Durham, North Carolina 27708, USA
(3) Institute for Computational Design, University of Stuttgart, Stuttgart, Germany

Aggregates of non-convex particles have shown to be particularly stable which makes them good candidates to design new lightweight and reversible structures. However, few is known about the fundamental reason of their stability. We presents a X-ray computed tomography method to investigate the local structure of piles made of hexapod particles. It permits to get the position and orientation of each particle as well as to detect their contact points. Measurement of the coordination numbers, statistics of the contact positions and local density evaluation for different packing configurations show a good agreement with the previous studies carried out at the global scale and permits to explain the main local mechanisms leading to stable structures.

Collisional Model of the Drag Force of Granular Impact

Cacey Stevens Bester and Robert Behringer,
Duke University

Impact of granular targets by solid projectiles is an experimental approach to understanding force transmission in soft matter. A granular target can cause a free-falling projectile to come to an abrupt stop as its momentum is dissipated to the grains. A complete interpretation of the stopping force, incorporating grain-scale interactions during impact, remains unresolved. We use direct force measurements and high-speed imaging to determine the forces acting on a projectile as it decelerates through a dense granular medium. These impact studies probe the unique response and properties of granular media.

Publication: “Collisional model of energy dissipation in three-dimensional granular impact”, C. S. Bester and R. Behringer, Physical Review E 95, 032906 (2017).

Jamming transition evinced by Voronoi Tessellation

Eduardo Boaventura (1,2), Fernando Ducha (1), and A.P.F Atman (2,3),
(1) Graduate program in Mathematical and Computational Modeling - Centro Federal de Educação Tecnológica de Minas Gerais (CEFET-MG). Av. Amazonas, 7675 - Nova Gameleira, Belo Horizonte, MG, Brazil. Cep 30510-000.
(2) Physics and Mathematics Department - Centro Federal de Educação Tecnológica de Minas Gerais (CEFET-MG). Av. Amazonas, 7675 - Nova Gameleira, Belo Horizonte, MG, Brazil. Cep 30510-000.
(3) Instituto Nacional de Ciência e Tecnologia de Sistemas Complexos (INCT SC).

Our work is based on two-dimensional simulation of certain amount of grain inside a closed box containing an intruder of fixed coordinates on its longitudinal axis. This intruder is a grain of larger dimension than the other grains. The system is at the horizontal plane.
At some point the box begins to move against the intruder. This movement causes grain accumulation in front of the intruder (causing a drag force on it) and, depending on the packaging fraction of the system, i.e., depending on the grain density, an empty space will appear behind this intruder. This empty space is called cavity.
Of course, the higher the grain density of the system, the greater the drag force experienced by the intruder. We can also imagine that the larger the grain density, the smaller the area of the cavity, behind the intruder.
The video shows a top view of the evolving system, that is, we show the view of an observer standing over the box, placed on a table. The boundaries of the box are not shown. Initially the box is static and the size of the grains are increased to achieve the desired granular density. The video shows the evolution of systems with packing fraction of 77%, 79% and 81%, respectively.
By calculating the Voronoi tessellation of the granular system and analyzing the variations of the polygons of the cavity and intruder, that is, the variations of the areas and the number of sides of these polygons, we can infer information about the jamming phase transition of the system.

​Calibration of micromechanical parameters for DEM simulations by using the particle filter

Hongyang Cheng (1,*), Takayuki Shuku (2), Klaus Thoeni (3) and Haruyuki Yamamoto (4),
(1) Multi Scale Mechanics (MSM), Faculty of Engineering Technology, MESA+, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands,
(2) Graduate School of Environmental and Life Science, Okayama University, 3-1-1 Tsushima naka, Kita-ku, Okayama 700-8530, Japan,
(3) Centre for Geotechnical Science and Engineering, The University of Newcastle, Callaghan, NSW 2308, Australia,
(4) Graduate School for International Development and Cooperation, Hiroshima University, 1-5-1, Kagamiyama, Higashi-hiroshima 739-8529, Japan

The calibration of discrete element method (DEM) simulations is typically accomplished in a trial-and-error manner which generally lacks in objectivity and is filled with uncertainties. To deal with these issues, the particle filter is employed as a novel approach to calibrating the DEM models of granular soils. Within the sequential Bayesian framework, the posterior probability density functions (PDFs) of micromechanical parameters are approximated by independent model trajectories, referred as "particles", given the triaxial response of granular soils. In this work, two different contact laws are employed in DEM simulations and a granular soil specimen is modeled as polydisperse packing with various numbers of spherical grains. Knowing the evolution of physical states of the material, the proposed probabilistic calibration method can incrementally update the posterior probabilities in a five-dimensional parameter space. Both the identified parameters and posterior PDFs are analyzed to understand the effect of grain configuration and loading conditions. Numerical predictions using "particles" with highest weights agree well with the experimental results. The advantage of the particle filter lies in the estimation of posterior PDFs, from which the robustness of the selected contact laws, the uncertainties of the micromechanical parameters and their interactions are analyzed. The micro-macro correlations, which are byproducts of the probabilistic calibration, are extracted to provide insights into the multiscale mechanics of dense granular materials.

A calibration framework of DEM variables using genetic algorithms

Huy Q. Do, Alejandro M. Aragón, and Dingena L. Schott,
Faculty of Mechanical, Maritime and Materials Engineering, Delft University of Technology

The discrete element method (DEM) is a widely accepted method for modeling the behavior of granular materials. However, a major barrier to the effective use of DEM is calibration. It is time consuming and a challenge to select appropriate microscopic variables and their values so that simulations can accurately reproduce the behavior of real systems. In this research, a procedure for fast and effective automated calibration using genetic algorithms is proposed. This approach is successfully demonstrated for single- and multi-objective optimization calibration problems. Five unknown variables, i.e., rolling and sliding friction coefficients, size, density, and Young's modulus of particles are determined based on the known bulk properties of density, AoR, and discharging time. The Pareto-optimal front visualizes the best compromise between the two conflicting objectives: the model error and the simulation time. For large scale applications, this is important to obtain efficient DEM variables with minimum model errors subject to specific simulation time constraints.

Self-Structuring of granular material under capillary bulldozing

Guillaume Dumazer (1), Bjornar Sandnes (2), Monem Ayaz (1), Knut Jorgen Maloy (1), and Eirik Grude Flekkoy (1),
(1) University of Oslo, Physics Department
(2) University of Swansea, College of Engineering

An horizontal glass tube is filled with a sedimented mixture of water and glass beads. A syringe pump is withdrawing the water at a constant flow rate, which drives the gas/liquid meniscus towards the sedimented granular material. The granular material is being compacted by the progress of the meniscus and the frictional interactions between grains and confining walls increase. The pressure imposed by the pumping increases as well until the capillary pore pressure is reached, so that the gas/liquid interface penetrates and quickly percolates inside the granular packing. The rapid increase of flow rate during percolation fluidizes the still immersed packed granular material which leads to split the percolated section which remains as a static plug.
More details on G. Dumazer et al., Phys. Rev. Lett., 117, 028002 (2016).

Granular silo flow around various obstacles

K. Endo and H. Katsuragi,
Nagoya University, Japan

The two-dimensional granular silos with a horizontal bottom (exit width is 25 mm) and a circular, triangular, or inverted-triangular obstacle are filled with steel particles (6.35 mm diameter). By opening the exit, gravity-driven granular flows are induced. The granular flow fields are affected by the shape of obstacle. Although the discharge flows are steady and their flow rates are constant, strong spatiotemporal inhomogeneity can be observed in the silo flows. This flow-field difference affects the clogging condition and the drag force exerted on the obstacle.
For the details of analyzed results, please see http://arxiv.org/abs/1611.03927 (PG2017 proceedings) and http://arxiv.org/abs/1706.04791.

The sand stalagmite

Laetitia Fontaine,
amaco and CRAterre-ENSAG

Dry sand flows verticaly in two shallow plates, one of them being filled with water. In the dry plate, a normal pile of sand is formed. In the plate with water, where one would expect the sand to dispers, a slender column builds up rapidly instead. The presence of water in the plate produces a more vertical structure which is the result of the combined action of rising water and the resulting capillary cohesion. A simple application of granular media physics or sand castle physics.

How to make a super sand pile?

Laetitia Fontaine,
amaco and CRAterre-ENSAG

By compacting thin layers of sand in a cup, between which small metal grids have been inserted, one obtains an extremely resistant sandpile : it easily tolerates the weight of a man. The grids in fact withold the horizontal thrust of the grains, greatly increasing the resistance of the sandpile, and thus preventing its collaps.

Sand-drop tower

Laetitia Fontaine,

Drops of sand filled with water fall on a bed of dry sand. On contact with the dry sand, the water contained in the drops of wet sand is instantly aspirated by capillary action in the dry sand. The drops of sand solidify and form small balls : the majority of the water has been absorbed, the remaining little water is trapped between the grains and forms capillary bridges that ensure the cohesion of the sand balls. On the waterproof support, the water is not absorbed by capillarity. It is therefore impossible to make the tower of sand drops. If a drop does not fall perfectly in the axis of the tower, an alternation of the orientation of the balls occurs and the tower adopts a zig-zag structure.
This film is inspired by research carried out by Julien Chopin and Arshad Kudrolli.

Effect of grain shape on the jamming of two-dimensional silos

Ezequiel Goldberg (1), C. Manuel Carlevaro (1,2), Luis A. Pugnaloni (3),
(1) UDB Fisica, Universidad Tecnologica Nacional - FRBA, Argentina.
(2) Instituto de Fisica de Liquidos y Sistemas Biologicos (CONICET La Plata, UNLP), Argentina.
(3) Dpto. Ing. Mecánica, Universidad Tecnologica Nacional - FRLP, CONICET, Argentina.

Examples of clogging events when particles of different shapes (disks and regular polygons) flow though a narrow constriction.

Fractal analysis of grain contour

Giulia Guida (1,*), Francesca Casini (2), and Giulia M.B. Viggiani (2),
(1) Università Niccolò Cusano, 00166 Rome, Italy
(2) Università degli Studi di Roma “Tor Vergata”, Dipartimento di Ingegneria Civile e Informatica, 00133 Rome, Italy

Fractal analysis is an easy, fast and convenient method used in image processing to characterise the shape and the asperities of objects. In this work the fractal method is applied to images of grains in order to describe particle morphology: overall form (macro-scale), local features (meso-scale) and roughness (micro-scale). It consists to compute automatically the perimeter through sets of sticks of decreasing lengths and to plot results in term of perimeter versus the corresponding stick length in a bi-logarithmic plane. Features associated to the trend of results allow to define three morphology descriptors associated at the three scales of observation.

Transverse Mixing of Ellipsoidal Particles in a Rotating Drum

Siyuan He (1), Jieqing Gan (1), David Pinson (2), and Zongyan Zhou (1,*),
(1) Department of Chemical Engineering, Monash University, Melbourne, VIC 3800, Australia,
(2) BlueScope Steel, Port Kembla, NSW 2505, Australia

This video shows the temporal evolution of mixing patterns for monodisperse ellipsoids of different aspect ratios at 15 rpm. Apparently, both oblate and prolate spheroids mix faster than spheres at the same rotational speed. This results from the stronger effects of convective and diffusive mixing for ellipsoids, which has been be highlighted in the video. In addition, the development of mixing index for ellipsoids with different aspect ratios is presented. In this way, the mixing speed of ellipsoids could be compared with the one of spheres quantitatively.

Insights into the mechanics of wheat fractionation by numerical simulations

Karsta Heinze (1,2), Xavier Frank (2), Valérie Lullien-Pellerin (2), Matthieu George (1), Farhang Radjai (3,4), and Jean-Yves Delenne (2),
(1) Laboratoire Charles Coulomb, UMR 5221, CNRS-Université de Montpellier, 163 Rue Auguste Broussonet, Montpellier, France
(2) UMR IATE, CIRAD, INRA, Montpellier SupAgro, Université de Montpellier, Montpellier, France
(3) LMGC, UMR 5508, CNRS-Université de Montpellier, 163 Rue Auguste Broussonet, Montpellier, France
(4) MSE2, UMI 3466, CNRS-MIT, DCEE, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, USA

The central part of wheat grains is a natural cemented granular material composed of starch granules glued by a protein matrix (gluten). They are milled under high forces to produce food products such as flour. Numerical simulations are of great value to better understand the fragmentation behavior of this biological material. Relying on peridynamics simulations we studied the effect of granule size distribution on the tensile strength of 2D macroscopic assemblies.

Bottom-up model for understanding the effects of wheat endosperm microstructure on its mechanical strength: http://www.sciencedirect.com/science/article/pii/S0260877416302175
Changes in the starch-protein interface depending on common wheat grain hardness revealed using atomic force microscopy: http://www.sciencedirect.com/science/article/pii/S0168945215300121

Dune formation in dilute phase pneumatic transport system

S Jhalani, A Patankar, A Makawana, M Bose,
Department of Energy Science and Engineering, IIT Bombay; India - 400076

Dunes are ripples on the surface of desert sands formed by grazing winds. They are also observed in a flow of a dilute gas-solid mixture through horizontal pipes, channels and pneumatic conveying systems. They form even in a clean pipe, as a result of particle segregation, unlike in desert dunes which are formed on a surface of a bed of the particles. While dunes have been observed in clean pipes, a detailed and close examination of the initiation and growth is not available in literature. Here we investigate the formation of dunes using high speed imaging on a laboratory scale pneumatic transport set-up with a transparent test section. Glass particles with 166 μm Sauter mean diameter are used for the experiments. We find multiple distinct isolated dune shaped clusters at the bottom of the pipe even at a very low bulk solid volume fraction. A closer look reveals that the dunes are closely packed and the transition layer is only a few particle diameters thick. The clusters are largely very stable but they sometimes become unstable. The particle velocity at the dune surface is much lower than that in the bulk.

We thank Sanket, Mohit, Harshal and Prof. Sudesh Balan, IDC, IIT Bombay for helping us with experiments and editing the video.

Evolution of particle breakage studied using x-ray tomography and the discrete element method

Zeynep Karatza (1,2,*), Edward Andò (2), Stefanos-Aldo Papanicolopulos (1), Gioacchino Viggiani (2), and Jin Y. Ooi (1),
(1) School of Engineering, The University of Edinburgh, Edinburgh EH9 3JL, U.K.,
(2) Univ. Grenoble Alpes, CNRS, Grenoble INP*, 3SR, F-38000 Grenoble, France,
(*) Institute of Engineering Univ. Grenoble Alpes

Particles exist in great abundance in nature, such as in sands and clays, and they also constitute 75% of the materials used in industry (e.g., mineral ores, medicine, paint, detergent powders). When a load is applied the response of a granular material at the bulk (macro) scale originates from the changes at the particle scale. If particle breakage occurs, the grading of the material and the shape and size of its particles will change and these induce changes in the contact network from where forces are transmitted. As a result, changes at the micro-scale can significantly affect the mechanical behaviour of a granular material and this explains why the mechanisms leading to particle breakage have been a common subject among several fields, including geomechanics. Here, oedometric compression tests are performed on zeolite granules specimens and x-ray computed micro-tomography (XCT) is employed, to acquire high resolution (pixel size = 0.01mm) 3D images of the specimens throughout the test. The images are processed, to describe breakage spatially and quantify it throughout the test and gain information about the mechanisms leading to particle breakage. In addition to the image processing, the discrete element method (DEM) is used to study the initiation and likelihood of particle breakage, by simulating the experimental test during the early stages of loading and using quantitative results from the images to inform and validate the DEM model. A discrete digital image correlation is used, in order to incrementally identify intact grains and simultaneously get results about the strain field within the specimen, as well as the kinematics of individual grains and fragments. In the initial stages of breakage, there is a clear boundary effect on the spatial distribution of breakage, as it is concentrated at the moving boundary (more than 90% of total breakage) and circumferentially (more than 70% of total breakage) close to the apparatus cell. Three main breakage patterns are observed and related to the coordination number; lower coordination numbers lead to intricate breakage patterns, whereas intact particles have consistently a higher number of contacts. The DEM model can reproduce the bulk response of the material until the point where substantial breakage governs the macroscopic response and it starts to soften. Additionally, the spatial distribution of the force network matches the localisation of breakage radially (which also explains the increase of the K0), but it does not seem to localise close to the loading platen. When particles start to break, in the DEM there is an increase to the friction ratio and significant amount of particles are experiencing slipping. Additionally, the amount of particles breaking (as measured from XCT) agrees with the number of particles experiencing the breakage force in DEM at the same loading increments. This analysis will enrich our understanding of the mechanisms and evolution of particle breakage, as from the XCT we can investigate contacts and breakage patterns and to complete this analysis we can get information from the DEM regarding the amount and distribution of contact forces.

Analysis of plate drag in granular materials using large-scale DEM simulation

Murino Kobayakawa*, Shinichiro Miyai, Takuya Tsuji, and Toshitsugu Tanaka,
Komatsu MIRAI Construction Equipment Cooperative Research Center, Osaka University, Japan

The response of granular materials to plate drag is numerically studied using a large-scale DEM simulation. The effect of initial packing volume fraction of the materials on the horizontal force acting on the plate is examined. The results show that at lower initial fraction, the force reaches approximately constant as the plate advances, while at higher initial fraction, the force profile has a larger oscillation. The analysis of local volume fraction in the materials during the drag shows that at the higher initial fraction, a clear shear band, reaching from the plate tip to the free surface, is observed but not at the lower initial fraction. The force oscillations are attributed to the consecutive appearing and disappearing of the shear bands. In addition, the rake angle of the plate has a significant influence on the evolution of the shear bands.

Mechanics of granular column collapse in fluid: Effect of initial packing density

K. Kumar (1), J-Y. Delenne (2), K. Soga ​(3),
(1) Department of Engineering, University of Cambridge, UK,
(2) IATE, ​ UMR 1208 INRA-CIRAD-Montpellier Supagro-UM2,France,
(3) Department of Civil and Environmental Engineering, University of California, Berkeley, USA

Two-dimensional sub-grain scale numerical simulations are performed to understand the flow dynamics of granular collapse in fluid. The Discrete Element (DEM) technique is coupled with the Lattice Boltzmann Method (LBM), for fluid-grain interactions, to understand the evolution of submerged granular flows. The fluid phase is simulated using Multiple-Relaxation-Time LBM (LBM-MRT) for numerical stability. In order to simulate interconnected pore space in 2D, a reduction in the radius of the grains (hydrodynamic radius) is assumed during LBM computations. A parametric analysis is performed to assess the influence of the granular characteristics (initial packing) on the evolution of flow and run-out distances for varying slope angles. The granular flow dynamics is investigated by analysing the effect of hydroplaning, water entrainment and viscous drag on the granular mass.
LBM-DEM simulation results show that a dense granular column behavior was initially affected by the development of negative pore pressure, which resulted in increased interparticle shear resistance. This caused the delay in initiating the mass flow and reduced the kinetic energy to move laterally. During the runout phase, the flow front had an acute angle creating more vortices and drag forces along the top of the granular mass, which resisted the movement of the granular mass. This in turn resulted in shorter runout distances when compared to the dry granular column collapse cases.
In the loose granular column collapse cases, positive pore pressures were observed inside the granular body as it sheared during the initiation phase. This allowed the body to initiate the movement faster than the dense column cases. During the runout phase, water was entrained at the flow front due to its particular parabolic shape and the packing density decreased, causing reduction in the interparticle forces at the flow front and creating some lubrication-like effect. The friction between the moving body and the bottom surface was also reduced to zero as there was no effective stress acting on the sliding plane (i.e. hydroplaning). The combined effect of positive pore pressure development, water entrainment in the front body and the hydroplaning resulted in longer runout distance compared to the dry granular column collapse cases that had the same loose packing density. An opposite behaviour was observed in the dense granular column cases, which shows the importance of initial packing density in the development of runout mechanisms.
In both dense and loose granular column cases, the final packing density was the same, indicating some critical state like behaviour, in which the initial packing memory is lost after large shearing of granular material.

Clogging of granular media in vertical pipes discharged at constant velocity

Diego López, Iker Zuriguel, Diego Maza, Luis Fernando Urrea,
Universidad de Navarra, Pamplona, Spain

In this video we introduce the topic of granular clogging in narrow vertical pipes. Despite being scarcely studied, this is rather important by its practical implications in mining. Indeed, the implementation of ore passes (even in open mines) is becoming an effective solution as it save production costs at the same time that in diminishes the environmental impact of traditional exploitation procedures. Recent experimental results are presented and compared with the case of clogging in silos.

Arching in three dimansional clogging

Sára Lévay and János Török,
Department of Theoretical Physics, Budapest University of Technology and Economics, Budafoki út 8, H-1111 Budapest, Hungary

Arching in dry granular material is a long established concept, however it remains still an open question how three dimensional orifices clog. We investigate by means of numerical simulations how the outflow creates a stable structure able to stop the flow. The average shape of the dome is almost a semi-ellipsoid but individual ones have large holes in it indicating a dome composed of two-dimensional force chains rather than three-dimensional structure.

Freely cooling granular gas

Stefan Luding,
Multi Scale Mechanics, University Of Twente, Enschede, Netherlands

Snapshots and animations from an event-driven (ED) simulation with N=99856 particles in 2D and periodic boundary conditions. The system has volume fraction v=0.25 and it cools due to dissipation so that 10 per-cent of the relative velocity is lost per collision, i.e., the restitution coefficient is r=0.9.
Remarks: Red, green and blue correspond to large, medium and small values, respectively.

Further information can be found here.

On the torsional loading of elastoplastic spheres in contact

Sadegh Nadimi and Joana Fonseca,
Dept. of Civil Engineering, City University of London

This study confirms the applicability of Deresiewicz’s solution for elastoplastic torsional interaction of spheres in contact.

Dynamic structural changes in unsaturated granular matter

Prapanch Nair and Thorsten Pöschel,
Institute for Multiscale Simulation, Friedrich-Alexander-Universität Erlangen-Nürnberg, Germany

The crown instability after a droplet's impact on a thin film, is often explained using Rayleigh-Plateau, Rayleigh-Taylor or Richtmyer-Meshkov instability or a combination of these. However, a free surface simulation predicting this instability suggests Rayleigh-Plateau instability as the dominant mechanism. Next, the dynamics of breaking of a liquid bridge is seen suggesting an underprediction of criterion for detachment of wet particles, given by analytical models. Thirdly, liquid bridge formation at a Ca=10^(-5) reveals complex hydrodynamics resulting in non-linear free surface phenomena.

Compaction of granular materials composed of deformable particles

Thanh Hai Nguyen(1,2), Saeid Nezamabadi(2), Jean-Yves Delenne(3) and Farhang Radjai(2,4),
(1)The University of Danang - University of Science and Technology, Danang, Vietnam,
(2)LMGC, UMR 5508 CNRS, Université de Montpellier, Montpellier, France,
(3)IATE, UMR1208, INRA, Université de Montpellier, Cirad, SupAgro, Montpellier, France,
(4)(MSE)2 , UMI 3466 CNRS-MIT, CEE,MIT, Cambridge, USA

In soft particle materials such as metallic powders the particles can undergo large deformations without rupture. The large elastic or plastic deformations of the particles are expected to strongly affect the mechanical properties of these materials compared to hard particle materials more often considered in research on granular materials. In this video, we present simulations of highly deformable particles under compression and shear loading by means of two different numerical methods. These simulations are used to analyze the effects of particle shape change on the packing fraction and connectivity of particles.

Avoiding Powder Lumps: Effect of Interfacial Properties and Liquid Flow

Xin Yi Ong (1), Spencer Taylor (2), Marco Ramaioli (1,*),
(1) Department of Chemical and Process Engineering, University of Surrey, Guildford GU2 7XH, UK, (2) Department of Chemistry, University of Surrey, Guildford GU2 7XH, UK

Reconstitution of the food by rehydration of the powder can lead to unsatisfactory product quality due to the formation of lumps, which is a common issue in industrial processes and in the household. This study aims at understanding the interplay between the properties of the powder grains (particle size, density and contact angle) and the characteristics of the liquid flow used to disperse them, in order to obtain a uniform dispersion and avoiding lump formation. The dispersion of insoluble and non-cohesive grains was investigated on both static and moving air-liquid interfaces.

Entrainment of liquid in the powder by “wicking” can lead to complete sinking of the grains. It was found that there exists a critical contact angle, below which powder islands do not form and grains sink. The effect of grain size on powder sinking, or on the depth of the powder island formed, was studied quantitatively to understand the impact of agglomeration on powder dispersion. Importantly, it was observed that introducing flow in the liquid by agitation does not necessarily improve the dispersion process as it can destabilise the whole powder island. Therefore, the detachment of the island from the interface is considered to be the precursor to powder lump formation.

Fragmentation of grains under impact

Luisa Fernanda Orozco (1,4), Jean-Yves Delenne (3), Philippe Sornay (4) and Farhang Radjai (1,2),
(1) Laboratoire de Mécanique et Génie Civil (LMGC), Université de Montpellier, CNRS, Montpellier, France,
(2) UMI 3466 CNRS-MIT, CEE, Massachusetts Institute of Technology (MIT), 77 Massachusetts Avenue, Cambridge CA 02139, USA,
(3) INRA, UMR IATE Montpellier, France,
(4) CEA, DEN, DEC, SFER, LCU, 13108 Saint Paul les Durance, France

We investigate the fracture of a single grain due to impact using three-dimensional Contact Dynamics simulations. The grains are assumed to be perfectly rigid but modeled as an assembly of bonded polyhedral Voronoï cells. The strength of the bonds represents the internal cohesion of the grain. A series of simulations for a range of different values of: number of cells, cohesion, impact velocity, were performed. It was found that the fragmentation efficiency increases with the number of cells, and, it is inversely proportional to the internal cohesion. The impact velocity maximizing the efficiency is found to be 0.08 m/s.

Real-time magnetic resonance imaging of granular flows

Alexander Penn (1,2,*), Klaas P.Pruessmann (2), and Christoph Müller (1),
(1) Laboratory of Energy Science and Engineering, ETH Zurich, Leonhardstrasse 27, 8092 Zürich, Switzerland
(2) Institute for Biomedical Engineering, University of Zurich and ETH Zurich, Gloriastrasse 35, 8092 Zürich, Switzerland
*Corresponding author: apenn@ethz.ch

Experimental measurements of the internal dynamics of 3D granular systems are essential for the development and verification of new models and numerical simulations. However, granular materials are challenging to probe since they are mostly opaque. Thus, optical techniques that commonly provide high temporal and spatial resolution measurements and are routinely used to probe the dynamics of fluids can only image the outer boundary of 3D granular systems. Instead, different tomographic techniques including position emission tomography (PEPT), X-ray computed tomography, electric capacitance tomography (ECT) and magnetic resonance imaging (MRI) have be applied to image the interior of granular systems. MRI comes with the particular advantage that it allows to measure not only particle density, but can directly quantify particle velocity through the implementation of motion-sensitive magnetic gradient pulse sequences [1]. However, MRI of granular materials suffers from inherently low temporal resolutions; the fastest reported acquisition time for a velocity measurement in a 2D slice has been 5 minutes [2, 3]. This lack of speed makes it impossible to study transient dynamic phenomena that are happening at timescales of the order of 10 – 100 ms, e.g. the formation or coalescence of bubbles in fluidized beds. In this work, we overcome speed limits of MRI in granular systems. Our acceleration strategy enables MRI at unprecedented speeds, paving the way for a plethora of dynamic granular studies. We illustrate the capabilities of our method by imaging bubble dynamics in a large cylindrical fluidized bed (diameter 190 mm, height 250 mm).

[1] E., Fukushima, Annu. Rev. Fluid Mech. 31, 95–123 (1999).
[2] A. C. Rees, J. F. Davidson, J. S. Dennis, P. S Fennell, L. F. Gladden, A. N. Hayhurst, M. D. Mantle, C. R. Müller, and A. J. Sederman, Chem. Eng. Sci. 61, 6002 (2006)
[3] M. Pore, D. J. Holland, T. C. Chandrasekera, C. R. Müller, A. J. Sederman, J. S. Dennis, L. F. Gladden, and J. F. Davidson, Particuology 8, 617 (2010).

Segregation during Transport of Granular Material

Bernhard Peters,
Universite du Luxembourg, Faculte des Sciences, de la Technologie et de la Communication, Campus Kirchberg, Luxembourg

Transport of granular materials exists in a lot of industrial processes. Particle segregation arises in transport processes due to differences in particle properties such as particle size, and consequently affects the received quality of the granular matter. However, our understanding of particle segregation and its underlying physics is limited and more research effort is required. Experimental studies [1, 2] usually adopt tracer particles to obtain internal flow information and to unveil particle kinematic behaviour. The particle scale information obtained from this approach is usually very limited for quantitatively assessing particle mixing, and additional numerical model was introduced for explaining particle mixing mechanisms as shown in [1]. As an alternative, Discrete Element Method (DEM) simulations could reproduce particle flows in all kinds of systems, and provide adequate particle scale information for assessing particle mixing and segregation. In this research, the DEM approach is adopted to investigate particle size segregation in a system of two hoppers connected by a chute.
To evaluate particle size segregation in the system, a particle transport process is designed. At initial state, the top hopper contains randomly positioned particles of three different sizes. In phase one, the exit of top hopper is opened and the particles are discharged from the top hopper into the bottom one over a chute under the influence of gravity. In phase two, the particles are discharged from the bottom hopper onto the ground. In order to assess particle size evolution during the filling and the discharge of the bottom hopper, a harmonic diameter Dh(t) is defined.
Results show that size segregation occurs on the chute by comparing the initial particle state on the chute with the particle state near the exit of the chute. It is observed that smaller particles fall through the gaps formed by larger particles and the larger particles flow on top of the smaller particles near the exit of the chute. During the discharge of the bottom hopper, a significant size segregation is indicated from the evolution of the harmonic diameter. At first, the value of the harmonic diameter increases with time until a maximum value is reached. A drop of the harmonic diameter is followed, which indicates the start of discharge of particles sliding on the side wall of the hopper, and finally the value increases again. At the end of the discharge process, a pile is formed on the ground with an angle of repose measured around 39o which corresponds to a friction coefficient of 0.85.

[1] Lim, C. N., Goh, Y. R., Nasserzadeh, V., Swithenbank, J., and Riccius, O. (2001). The modelling of solid mixing in municipal waste incinerators. Powder technology, 114(1), 89-95.
[2] Hassanpour, A., Tan, H., Bayly, A., Gopalkrishnan, P., Ng, B., and Ghadiri, M. (2011). Analysis of particle motion in a paddle mixer using Discrete Element Method (DEM). Powder Technology, 206(1), 189-194.

Micromechanical study of multiphase flow based on the lattice Boltzmann method

Eduard Puig i Montellà (1), Bruno Chareyre (1), Simon Salager (1), Chao Yuan (1), and Antonio Gens (2),
(1) University Grenoble Alpes (UGA),
(2) Technical University of Catalonia (UPC)

Hydrostatic properties of unsaturated granular materials at the pore scale are evaluated using lattice Boltzmann method (LBM) simulations. Numerical simulations are presented by using the multi­component multiphase Shan­Chen LB model implemented in the open source software Palabos.
Practical situations involving multiphase flow include infiltrated rainwater into soil by displacing air and industrial processes such as riser reactors, fluidized beds, dryers, etc.
Drainage simulations are conducted in elementary microstructures (static assemblies of 2 or 3). The meniscus shape, the volume of the liquid bridge and the capillary pressure have been analyzed and proven to match theoretical solutions for configurations of 2 and 3 spheres.

Simulation of the condensation of water in a granular material

Vincent Richefeu (1), Jean-Yves Delenne (2), and Farhang Radjaï (3),
(1) 3SR, UMR 5521, Univ. Grenoble Alpes, 3SR, F-38000 Grenoble, France,
(2) IATE, UMR 1208, Université de Montpellier-INRA-CIRAD-SupAgro, 2 place Pierre Viala - 34060 Montpellier, France,
(3) LMGC, UMR 5508, Université de Montpellier - CNRS, 163 rue Auguste Broussonnet, 34090 Montpellier, France

In this video we use a multiphase Lattice Boltzmann model as a means to generate liquid clusters in a 2D granular material. Liquid droplets condense from the vapor phase and they transform into capillary liquid clusters. As the amount of condensed liquid is increased, liquid clusters of increasing connectivity are formed. From the pressure field inside the clusters we derived the tensile stress carried by the grains. This latter increases as a function of the amount of condensed liquid up to a peak in the funicular state beyond which the stress falls off as a result of pressure drop inside the merging clusters.

Ricochet and Oblique Impact in Granular Material

Leah K. Roth and Heinrich M. Jaeger,
The University of Chicago

Oblique impact of a high-speed projectile into granular material can lead to ricochet. The conditions eliciting this behavior are, however, not well understood. We use molecular dynamics simulations to map out the phase space, quantifying the dependence on impact speed and angle as well as material properties. Our initial findings indicate that the maximum impact velocity is directly related to the speed of sound in the material, while the shape of the resulting compression front hints at the origin of a maximum ricochet angle. We are currently investigating the effect of granular shock waves on projectile behavior in supersonic impacts.

A generalized local rheology for wet granular materials

Sudeshna Roy,
University of Twente, Enschede, Netherlands

Slide 1: Wet granular materials show distinct behavior in nature depending on the flow rate, amount of interstitial fluid and the softness of the materials. However, field studies of such flows under varied condition are difficult. Discrete Particle simulations can be useful to establish basic principles qualitatively and as guidance for improving rheological models of such materials. In our simulations, we vary the shear rate to study flows from slow quasi-static to rapid granular shear flows using Discrete Element Method (DEM). Additionally, we introduce interstitial liquid at contacts and varying amount of cohesion due to liquid bridges to study the transition from dry to wet. Likewise, we modify softness to study the flows from soft snow to hard rocks. Here we show some examples of real life applications of our research.
Slide 2: We study the rheology from our simulation in a split-bottom shear cell set-up. Here we show an experimental set-up of the shear cell and the surface velocity profile measured from the experiment and post-processed using Mercury coarse graining. Different colors indicate blue (low), green (medium) to red (high) magnitude of the velocity.
Slide 3: Depending on the surrounding conditions, granular flows are affected by appropriate time scales namely, time scales related to pressure, shear rate, particle stiffness, gravity and cohesion. Each of the dimensionless numbers are defined as ratio of these time scales. While traditionally the inertial number I, describes the flow rheology under quasi-static to inertial conditions, we observe that other dimensionless numbers are also needed to describe the flow, namely (i) The local compressibility p* (ii) the inverse of relative pressure gradient pg*, and (iii) the Bond number Bo. The macroscopic friction coefficient, defined as the ratio of the shear stress to the normal stress, is expressed in terms of these dimensionless numbers. Thus, we study the rheology of granular materials in terms of these dimensionless numbers, combine the trends and show that they collectively contribute to the rheology as multiplicative functions that constitute a general constitutive law that can capture the generic, system independent flow behavior.
Slide 4: Furthermore, granular materials are known to have non-Newtonian flow behavior for larger shear stress while they remain elastic like solids below their yield stress. More specifically, they behave like a shear thinning fluid. Here, we study the apparent viscosity as a function of inertial number for granular fluids of varying cohesive strength. The dotted cyan line and the solid green line are the predictions obtained from the proposed rheology and the analytical solution respectively at small pressure. The dash-dotted red line and the solid blue line are the fittings and the prediction from the analytical solution respectively at large pressure. We predict the apparent shear viscosity for different intensities of cohesion of wet granular materials. The increase in viscosity and the variable shear thinning behavior of granular materials under low stress, close to a free surface, is predicted well from the proposed rheology models and the analytical method.

Weight of an Hourglass - Theory and Experiment in Comparison

Achim Sack and Thorsten Pöschel,
Institute for Multiscale Simulation, Friedrich-Alexander-Universität Erlangen-Nürnberg, Germany

As sand is released from the top chamber of an hour glass, the weight of the hour-glass indicated by a scale initially decreases because some of the sand is in free fall. Once the sand reaches the bottom of the hour glass the delivery of momentum will act to increase the weight weight of the hour-glass. These competing effects have been traditionally described as cancelling out during the epoch of steady flow and that the experiment can be performed without too much difficulty.
[1] A. Sack and T. Pöschel, Am. J. Phys. (2017)

DEM study of granular flow around blocks attached to inclined walls

Joel Samsu (1,3), Zongyan Zhou (1,3), David Pinson (2,3) and Sheng Chew (2,3),
(1) Laboratory for Simulation and Modeling of Particulate Systems, Department of Chemical Engineering, Monash University, Clayton, VIC 3800, Australia
(2) Iron and Steelmaking Technology, BlueScope Ltd., P.O. Box 202, Port Kembla, NSW 2505, Australia
(3) ARC Research Hub for Australian Steel Manufacturing

In this video, we investigate granular flow in a hopper-like geometry with blocks attached to the inside wall. We compare the results of three simulation cases, with varying angles of the inclined wall, to study the effect of wall angle on particle flow patterns, velocities, total particle forces, and wall stress resulting from the particle-wall interaction. The simulations were carried out using a discrete element method (DEM) modelling framework. Each case is represented by a 3D slot model with periodic boundary conditions applied to the front and back. Particles are monosized and spherical in shape. We believe that the outcomes of this research will be useful for predicting and preventing damage due to intense particle-wall contact in industrial applications with geometrical discontinuities. The issues associated with these discontinuities have not yet been properly addressed. Studying the flow dynamics and forces on containing walls can provide valuable feedback for equipment design and optimising operations to prolong their lifetime. Therefore, solids flow-wall interaction phenomena should be well understood.

Dynamics of particle-laden liquid sheets

Alban Sauret, Pascal Raux, Anthony Troger, Pierre Jop,
SVI (CNRS/Saint-Gobain)

When the thickness of a liquid film becomes comparable to the particle size, unexpected dynamics is observed. The classical constitutive rheological law cannot be applied as the continuum approximation is no longer valid. Here, we consider experimentally a transient free liquid sheet that expands radially and illustrate the influence of the particles on the shape of the liquid film as a function of time and the atomization process. Our study suggests that the influence of particles through capillary effects can modify significantly the dynamics of processes that involve suspensions and particles confined in liquid films.

A CFD DEM study of single bubble formation in gas fluidization of spherical and non spherical particles

Siddhartha Shrestha and Zongyan Zhou*,
Department of Chemical Engineering, Monash UniversityLaboratory for Simulation and Modelling of Particulate Systems, Department of Chemical Engineering, Monash University, Victoria 3800, Australia

Bubble dynamics significantly affect the hydrodynamics of gas-solid fluidized bed since they influence the gas-solid mixing. In this study, simulations using CFD-DEM were carried out to characterize the bubble size and shape for a bubble formed at a single orifice in gas-solid fluidized bed. Impact of parameters such as jet velocity, orifice size and particle shape on bubble equivalent diameter and bubble aspect ratio were analysed and discussed. Bubble equivalent diameter was found to increase with increasing jet velocity and orifice size and changing particle shape. The bubble shape illustrated by aspect ratio, was found to elongate more as it rises through the bed and then commence to expand horizontally after it was detached from the orifice. Aspect ratio was found to be closer to a circle for the bubble at higher jet velocity and orifice size and for non-spherical particles.

Multi Scale Mechanics

Rohit Kumar Shrivastava and Stefan Luding,
Multi Scale Mechanics, University Of Twente, Enschede, Netherlands

Propagation of sound waves through disordered granular materials have been utilized for various purposes like oil/gas exploration or non-destructive testing. As viewed in the video, when a propagating sound wave encounters a discontinuity or disorder, it generates a back-wave and the back-wave is dependent on the type of discontinuity/disorder, for studying these sound waves a model granular chain can be used where the model can be controlled and the dependency of the sound waves on disorder/discontinuity can be analyzed. The results from the analysis can be used for improving signal to noise ratio of seismic signals and hence, assist in improvement of the techniques used during oil/gas exploration, thereby, making the exploration process more economically viable.

Elastic waves in particulate glass-rubber mixture: experimental and numerical investigations/studies

Kianoosh Taghizadeh (1), Holger Steeb (2), Vanessa Magnanimo (1), and Stefan Luding (1),
(1) Multi-Scale Mechanics, Faculty of Engineering Technology (ET), MESA+, University of Twente, Enschede, The Netherlands,
(2) Institute of Mechanics, University of Stuttgart, Stuttgart, Germany

This video shows by wave propagation the elastic response of granular mixtures made of soft and stiff particles subjected under hydrostatic pressure/stress. This allows inferring fundamental properties of granular materials such as elastic moduli and dissipation mechanisms. We compare physical experiments in a triaxial cell equipped with piezoelectric wave transducers and Discrete Element Method simulations (DEM). The behavior of mixtures with high glass content is very well captured by the simulations, without need of any additional calibration, whereas the complex interaction between rubber and glass leave open questions for further study.

Analysis of AoR with 3D scan and self developed GUI

Yuan Tan, Bowen Xue, Willibald A. Günthner, Johannes Fottner, Stephan Kessler,
Institute for Materials Handling, Material Flow and Logistics, Technical University of Munich, Boltzmannstraße 15, 85748 Garching, Germany

As a fundamental material property, particle-particle friction coefficient is usually calculated based on angle of repose which can be obtained experimentally. For those extremely symmetrical slope, measurement of the angle becomes less applicable and decisive, for example, three kinds of biomass shown in this video, i.e. olive stones, poplar chips as well as willow chips. In 2007 three types of truncations were discussed for these kind of undesirable cones by Fraczek [1]. But in the previous studies only several sections of one uneven slopes is chosen in most cases, even so standard methods in definition of a representable section are barely found.
We presented an efficient and reliable method from the new technology, 3D scan, which was used to digitize the surface of heaps and generate its point cloud. Then, two tangential lines of any selected section were calculated through the linear least-squares regression (LLSR), such that the left and right angle of repose of a pile could be derived. As the next step, a certain sum of sections were stochastic selected, and calculations were repeated correspondingly in order to achieve sample of angles. Subsequently, different samples were acquired through various selections of sections. By applying similarities and difference analysis of these samples, the reliability of this proposed method was verified.
Based on this method a corresponding graphical user interface (GUI), enhances the efficiency and ease of use for the underlying logical design of stored program. Instead of text-based programming, such as typed command labels and text navigation, the automatic measurement analysis only requires data input from Kinect and simple icons. This video is supposed to show the whole process, from experiment to data processing. Besides second method is also provided for analysis of varying heap forms. Furthermore, coordinate date of heaps generated by DEM simulation could also be imported, so that the experimental and simulation results will be compared with an identical method. With these efforts deviation between experiment and simulation induced by objective and subjective factors, like random selection of a single angle, could be considerably reduced.

[1] J. Frączek, A. Złobecki, J. Zemanek, Journal of food engineering, 83, 17. (2007)

Viscosity of particle laden films

Yousra Timounay and Florence Rouyer,
Université Paris-Est Marne-la-Vallée

This movie illustrates the retraction of particulate films where large particles bridge both interfaces. Local velocities are measured by PIV during the unstationnary regime. The velocity variation in time and space can be described by a continuous fluid model from which effective viscosity (shear and dilatational) of particulate films is measured. The 2D effective viscosity of particulate films 2D increases with particle surface fraction and it diverges at the critical particle surface fraction equal to 0.84 that corresponds to the jamming surface fraction of a 2D granular media. This property is the manifestation of the granular character of particulate films.

Experimental characterization of biomass powder flow

Clement Vanneste-Ibarcq,
Laboratory for the preparation of bioresources, Univ. Grenoble Alpes, Grenoble, France

Biomass valorisation processes may require the biomass to be finely ground. The obtained powder has a low density. Its particles are elongated and their sizes are small (<1mm) and polydispersed. This leads to poor flowing properties. For example, a biomass powder may form arches during the emptying of a hopper. To avoid process interruptions, it is necessary to characterise the flow of these powders. The objective of this work is to find easy to measure parameters to describe the flow of biomass powders.

Agglomeration of Wet Granular Material in Rotating Drum

Thanh-Trung Vo (1,2), Saeid Nezamabadi (2), Jean-Yves Delenne (3) and Farhang Radjai (2,4),
(1) Bridge and Road Department, Danang Architecture University, Da Nang, Vietnam,
(2) LMGC, UMR 5508 CNRS, Université de Montpellier, Montpellier, France,
(3) IATE, UMR1208, INRA, Université de Montpellier, Cirad, SupAgro, Montpellier, France,
(4) (MSE)2 , UMI 3466 CNRS-MIT, CEE,MIT, Cambridge, USA

We investigate the agglomeration of wet particles in a rotating drum. The capillary cohesion force is modeled as an explicite function of the gap and volume of liquid bridges between particles. In this video, we present examples of simulations of the agglomeration process with results on the growth of agglomerates as a function of the Froude number.

A Granular System of Ellipses under Linear Shear

Dong Wang (1), Hu Zheng (1,2), Robert P. Behringer (1),
(1) Department of Physics, Duke University, Durham, NC USA,
(2) School of Earth Science and Engineering, Hohai University, Nanjing, Jiangsu,China

We use elliptical disks made of photoelastic materials, as a model granular system, to study its response under linear shear. The optical property of the materials enables us to obtain grain scale force information, as shown in the video. The system undergoes a shear jamming transition, i.e., from a stress-free state to a state with force network percolating over the system, with the packing fraction remaining the same. A preferred orientation also emerges as shear strain increases, which indicates a nematic order in the system.

Jamming transition: Heptagons, Pentagons and Discs

Yuanyuan Xu (1), Jonathan Barés (2), Yiqiu Zhao (3), Robert P. Behringer (3),
(1) Taishan College, Shandong University, Jinan, Shandong, China
(2) Laboratoire de Mécanique et Génie Civil, Université de Montpellier, Montpellier, France
(3) Department of Physics, Duke University, Durham, NC, USA

We experimentally study the jamming transition of granular system with photo-elastic non-spherical particles under compression and decompression. The videos are taken under polarizer so that the forces and contacts can be tracked. Three kinds of particles are presented in sequence: heptagons, pentagons and discs.

Avalanches in a Granular Stick-Slip Experiment: Detection Using Wavelets

Aghil Abed Zadeh (1), Jonathan Barés (2), Robert Behringer (1),
(1) Duke University,
(2) Laboratoire de Mécanique et Génie Civil, Université de Montpellier, CNRS

Avalanches have been experimentally investigated in a wide range of physical systems from granular physics to friction. In the corresponding paper of this video, we measure and detect avalanches in a 2D granular stick-slip experiment. We discuss the conventional way of signal processing for avalanche extraction and how statistics depend on several parameters that are chosen in the analysis process. Then, we introduce another way of detecting avalanches using wavelet transformations that can be applied in many other systems. We show that by using this method and measuring Lipschitz exponents, we can intelligently detect noise in a signal, which leads to a better avalanche extraction and more reliable avalanche statistics.

DEM Simulation of Particle Stratification and Segregation in Stockpile Formation

Dizhe Zhang (1), Zongyan Zhou (1,*), and David Pinson (2),
(1) Laboratory for Simulation and Modelling of Particulate Systems (SIMPAS), Department of Chemical Engineering, Monash University, VIC 3800, Australia
(2) BlueScope Steel, Port Kembla, NSW 2505, Australia

Granular stockpiles are commonly observed in nature and industry, and their formation has been extensively investigated experimentally and mathematically in the literature. One of the striking features affecting properties of stockpiles are the internal patterns formed by the stratification and segregation processes. In this work, we conduct a numerical study based on DEM (discrete element method) model to study the influencing factors and triggering mechanisms of these two phenomena. We found that it is a void-filling mechanism that differentiates the motions of particles with different sizes. This mechanism drives the large particles to flow over the pile surface and segregate at the pile bottom, while it also pushes small particles to fill the voids between large particles, giving rise to separate layers. Consequently, this difference in motion will result in the observed stratification and segregation phenomena.

Pull-out experiment in granular material

Yue Zhang, Robert Behringer,
Duke University

This video is recording the pull out process of an intruder in the photoelastic granular material. The video is taken by a fast camera with frame speed 2500fps in about one second. When we add a certain force on the other side of the string, the circular intruder connected to the string goes out immediately. The visible force chains (bright part) build up and break down during this fast process.

Tuning strain of granular matter by basal assisted Couette shear

Yiqiu Zhao (1), Jonathan Barés (2), Hu Zheng (1,3), Robert P. Behringer (1),
(1) Department of Physics, Duke University, Durham, NC, USA
(2) Laboratoire de Mécanique et Génie Civil, Université de Montpellier, Montpellier, France
(3) School of Earth and Engineering, Hohai University, Nanjing, Jiangsu, China

We present a novel experimental setup that can generate programmable azimuthal strain inside 2D granular matter under Couette shear. The setup consists of 21 independently movable concentric rings and 2 boundary wheels with frictional racks. This makes it possible to quasistatically shear the granular matter not only from the boundaries but also from the bottom. In particular, the video shows a linear shear profile that does not generate shear band (no obvious density separation and strain localization even after a huge boundary strain for not very big density). This experiment mimics a uniform shear with a linear velocity profile without a strain limit. We show this by applying a conformal mapping to map the Couette geometry into a rectangular geometry. Bi-disperse photo-elastic particles are used to study the stress distribution.

Vibrational Collapse of Hexapod Packings

Yuchen Zhao (1), Jingqiu Ding (2) , Jonathan Barés (3) , Hu Zheng (1) , Karola Dierichs (4) , Achim Menges (4) , and Robert Behringer (1),
(1) Department of Physics and Center for Nonlinear and Complex Systems, Duke University, Durham, NC, USA,
(2) Department of Physics, Nanjing University, Nanjing, China,
(3) Université de Montpellier, Montpellier, France,
(4) Institute for Computational Design, University of Stuttgart, Stuttgart, Germany

Columns made of convex noncohesive grains like sand collapse after being released from a confining container. However, structures built from non-convex grains can be stable without external support. In the current experiments, we investigate the effect of vibration on destroying such columns. The change of column height during vertical vibration, can be well characterized by stretched exponential relaxation when the column is short, which is in agreement with previous work, while a faster collapse happens when the column is tall. We investigate the collapse after the fast process including its dependence on column geometry, and on interparticle and basal friction.

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