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The formation of shear bands is one of the most remarkable phenomena in the dynamics of granular matter. Several parameters have been so far identified to influence the behavior of the shear bands. We carried out experiments to investigate the evolution of the shear bands in the split-bottom Couette cell in the presence of confining pressure. We employed the Particle Image Velocimetry (PIV) to characterize the shear band both in the absence and presence of external pressure. Our results show that the location and width of the shear band are affected by both the confining pressure and the filling height. The shear zone evolves towards the middle of the cylinder and expands to a broader region with increasing applied pressure or filling height; also the angular velocity decreases relative to the rotation rate of the bottom disk. Our findings are consistent with prior empirical observations on the formation of wide shear bands at free surfaces.

Powders &amp; Grains 2021

Shearing of granular materials in a confined split-bottom Couette cell

[EPJ Web of Conferences, Volume 249 (2021)](https://www.epj-conferences.org/articles/epjconf/abs/2021/03/contents/contents.html)

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Powders & Grains 2021

Powders & Grains 2021 will be held in Buenos Aires (Argentina, VIRTUAL) July 5, 13, 21, 29 and Aug 6, 2021. The aim of the conference is to give an up-to-date picture of the broad research field of granular media, powders and grains. Contributions from experts around the world will focus on research topics that cover a wide range of open problems in the field.

technical paper

Copper mining produces significant amounts of associate waste, leading to the construction of huge mining waste dumps in the highly seismic zone of the Andes Mountains. To analyze the seismic stability of these deposits, it is necessary to characterize the mechanical properties of these materials in laboratory, which is quite difficult due to the large size of particles composing these structures. In this study, the global stability of mine waste dumps is analyzed by means of discrete element simulations. Three analogues upscaled particle size distributions were generated from the original PSD. The particles composing the dump were modeled using regular polygons. The modeled dumps were subject to a seismic loading by applying a velocity signal on the boundary walls, which was integrated directly from accelerometers database. The results show that failure in these structures is produced by infinite slope, for samples scaled by a factor until ten. This methodology is encouraging for the analysis of the global stability of mine waste dumps and for forecasting the runout distance after failure.

Discrete modeling of waste rock dumps stability under seismic loading

Viscoelastic granular materials are present in several disciplines. One example is asphalt mixture employed in road construction. In the last three decades, discrete element modeling has been positioned as a valid tool for the analysis of this multiphase material at the grain-scale. All this despite the simplification of the
shape of the particles used in these studies. In this work, it is proposed a simplified procedure for the generation of viscoelastic granular samples composed of irregular polyhedra. The numerical aggregates were generated by a Poisson-Voronoi tessellation based on the particle size distribution (PSD) and statistic data of aggregates, without using complex imaging technics. This procedure set the porosity of the packing, while controlling the PSD. Using this procedure implies a significant computational-time reduction by skipping several preparation stages for polyhedral samples, such as deposition by gravity and compaction. This approach can be used for the study granular materials as inclusion in a solid matrix as concrete or asphalt mixtures, particle breaking, and fatigue damage of viscoelastic materials.

Contact Dynamics modeling of viscoelastic granular materials using irregular polyhedral particles

To understand the typically heterogeneous flowing behavior of granular materials, it is important to combine flow tests with three-dimensional imaging. To probe the flow behavior of granular materials over a wide range of flow rates, it is imperative to be able to impose such flow rates in a well controlled manner while performing imaging tests that are compatible with all imposed flow rates. Achieving both flow control and bulk imaging capacity is challenging for a number of reasons. Here, we describe the design of a setup in which we are able to do imaging while imposing a constant overall shear rate on a granular material. We characterize the setup in which flow tests will be performed, which consists of a bottom-driven cone-plate or double-cone design. We show that the setup can be integrated in x-ray microtomography devices to aid particle tracking based flow measurements. The design is also compatible with typical rheometer setups. We also perform high
speed imaging of a granular flow in an ultra-fast x-ray scanner, for which we provide proof-of-principle data in a simplified shear setup. The designed flow geometry is also compatible with said high speed imaging facility, where particle image velocimetry can be employed to extract quantitative flow field data.

Improved evaluation of granular media flows using an X-ray scanning compatible cone-plate setup

Granular materials can segregate spontaneously due to differences in particle properties when subjected to vibrations, shear strain or because of the equipment geometries. Although the difference in particle size is the most critical factor that drives segregation, the effects of large density difference may also be detrimental for a lot of industries. In this work, we experimentally investigate density-driven segregation in bi-disperse mixtures of particles having the same size but different density when subjected to non-uniform shear rates. We found that the features of the segregation process are related to the density ratio as well as to the dimensionless loaded mass. The experimental outcomes are then compared with the solution of a simple density-driven segregation model. The model can successfully capture the main features of segregation driven by density for a range of density ratios.

Shear-driven density segregation: an experimental study

Some years ago, Harth et al. experimentally explored the steady state dynamics of a heated granular gas of rod-like particles in microgravity [K. Harth et al. Phys. Rev. Lett. 110, 144102 (2013)]. Here, we report numerical results that quantitatively reproduce their experimental findings and provide additional insight into the process. A system of sphero-cylinders is heated by the vibration of three flat side walls, resulting in one symmetrically heated direction, one non-symmetrically heated direction, and one non-heated direction. In the non-heated direction, the speed distribution follows a stretched exponential distribution p(v) ∝ exp(−(|v|/C)^1.5 ).
In the symmetrically heated direction, the velocity statistics at low speeds is similar but it develops pronounced exponential tails at high speeds. In the non-symmetrically heated direction (not accessed experimentally), the distribution also follows p(v) ∝ exp(−(|v|/C)^1.5 ), but the velocity statistics of rods moving toward the vibrating wall resembles the indirectly excited direction, whereas the velocity statistics of those moving away from the wall resembles the direct excited direction.

Continuously heated granular gas of elongated particles

Poroelasticity theory predicts wave velocities in a saturated porous medium through a coupling between the bulk deformation of the solid skeleton and porous fluid flow. The challenge emerges below the characteristic wavelengths at which hydrodynamic interactions between grains and pore fluid become important. We investigate the pressure and volume fraction dependence of compressional- and shear-wave velocities in fluid-saturated, random, isotropic, frictional granular packings. The lattice Boltzmann method (LBM) and discrete element method (DEM) are two-way coupled to capture the particle-pore fluid interactions; an acoustic source is implemented to insert a traveling wave from the fluid reservoir to the saturated medium. We extract wave velocities from the acoustic branches in the wavenumber-frequency space, for a range of confining pressures and volume fractions. For random isotropic granular media, the pressure-wave velocity data collapse on a single curve when scaled properly by the volume fraction.

Direct numerical simulation of wave propagation in saturated random granular packings using coupled LBM-DEM

Granular matter transported by hydraulic conveying behaves under several transport regimes. The techncal challenge is to predict the dilute or dense regime of transport as well as the pressure drop required in order to design properly a granular hydraulic conveying system. We address this problem with a laboratory scale experimental setup able to reproduce the key features of hydraulic conveying f grains. Pressure drop measurements as well as image analysis have been used to characterize a slug transport regime and its crossover towards a moving bed transport regime. A dimensionless characteristic curves diagram has been established and a reasonably good pressure drop prediction with a generic power law is explained in details. This preliminary work has the potential to open perspectives for the design of a predictive tool for pressure drops in hydraulic conveying systems.

Hydraulic transport regimes of grains in a millifluidic confinement

One striking difference between aggregates of flexible frictional fibres and other granular materials like rigid spheres is the effective cohesion of their assembly. While glue or capillary bridges are needed to shape aggregates of spherical particles and build sandcastles, for fibres, no need for glue to build a nest. Here we study an assembly of mono disperse flexible fibres. We first use X-ray microtomography to characterise the geometry of the initial assembly, the number of contact points and mean curvatures of the fibres. Using forcedisplacement measurements, we characterise the macroscopic cohesive strength of the aggregate by varying the geometry of the fibres, the fibres mechanicals properties and the packing of the preparation. Finally, we relate the macroscopic mechanical behaviour of the assembly with the filament reorganisation at the microscopic scale.

Cohesion of bird nests

We present the results of both experimental and numerical investigations of the silo discharge for a cohesive granular material. In our study, thanks to a cohesion-controlled granular material (CCGM) we propose to investigate the effect of the cohesive length lc, on the discharge of a silo for two different configurations, one axisymmetric, and one quasi-2D rectangular silo. In both configurations, an adjustable bottom is used to control the size of the orifice. As observed for cohesionless granular material by previous studies, the mass flow rate and the density through an orifice are mostly controlled by the diameter of the orifice D. The experimental results of the quasi-2D silo are compared with continuum numerical simulations.

The effect of cohesion on the discharge of a granular material through the orifice of a silo

Structural characteristics of two-dimensional elliptic granular packs with various aspect ratios and intergranular friction coeffcients were studied using the Discrete Element Method (DEM). Isotropic compaction from random unjammed state leads to a jammed state with polycrystals of orientationally ordered clusters (OOC). The OOCs were identified using a cluster labelling algorithm, based on the relative angle \Delta\theta between the major axes of two contacting particles. The threshold value of \Delta\theta was optimised to give the strongest correlation between OOCs and the force chain network. We found that the resulting OOC size distribution decays algebraically with an exponent of -2, independently of grain aspect ratio and material properties.

Shearing of granular materials in a confined split-bottom Couette cell

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Discrete modeling of waste rock dumps stability under seismic loading

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Shearing of granular materials in a confined split-bottom Couette cell

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Next from Powders & Grains 2021

Discrete modeling of waste rock dumps stability under seismic loading

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The effect of cohesion on the discharge of a granular material through the orifice of a silo

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PRESENTATIONS

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