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Particle breakage can significantly change the fabric (size and shape of particles and contact network) of a granular material, affecting highly the material's macroscopic response. In this paper, oedometric compression tests are performed on zeolite specimens and x-ray computed micro-tomography is employed, to acquire high resolution 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. 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, there is an initial indication that the spatial distribution of the force network matches the localisation of breakage radially, but it does not seem to localise close to the loading platen. This analysis will enrich our understanding of the mechanisms and evolution of particle breakage.

In the current study, complex-shaped particles are simulated with the Discrete Element Method (DEM) using two different approaches, namely Multi-spheres (MS) and Superquadrics (SQ). Both methods have been used by researchers to represent the shape of real particles. However, despite the growing popularity of utilizing MS and SQ particles in DEM simulations, few insights have been given on the comparison of the macro scale characteristics arising from the two methods. In this respect, initially the characteristics of the two shape representation methods are evaluated in a direct shear test simulation. The results suggest that controlling the sharpness of the edges for SQ particles can lead to a good agreement with the results of MS particles. This way, a set of SQ and MS particles, which are numerically calibrated in the shear tester, are obtained. Furthermore, the macro-scale responses of the numerically calibrated particles are assessed during a slow shearing scenario, which is achieved through simulating quasi-static flow of the particles from a flat-bottom silo. The results for mass discharge, flow profile and wall pressure show a good quantitative agreement. These findings suggest that the numerically calibrated MS and SQ particles in the shear tester can provide similar bulk-scale flow properties. Moreover, the results highlight that surface bumpiness for MS particles and corner sharpness for SQ particles change the characteristics of particles and play a significant role in the shear strength of the material composed of these particles.

During the last two decades, many attempts have been made to develop computational models to represent the behavior of particulate solids in silos. Two methods have been used most commonly: the finite element method and the discrete element method. In an attempt to compare the state of the art in the two methods and to make an unbiased assessment of their capabilities, this project has harnessed expertise from all over the world to compare predictions of a group of standardized problems concerning silos. The first of these problems was deemed the simplest: that of filling a silo or container with solid. The challenge was to determine the stress state in the solid and the resulting wall pressures. The problem description and continuum analysis results were presented and discussed in a companion paper. This paper summarizes the discrete element predictions. A comparison is also made between the discrete and continuum analyses. Finally, comments are made on the outcome of the project.

Realistic yet efficient representation of particle shape is a major challenge for the Discrete Element Method. This paper uses angle-of-repose and direct-shear test simulations to describe the influence of several shape representation methods, and their parameters, on the bulk response of granular assemblies. Three rolling resistance models, with varying coefficient of rolling friction, are considered for spherical particles. For non-spherical particles, superquadrics with varying blockiness and multi-spheres with varying bumpiness are used to model cuboids and cylinders of several aspect ratios. We present extensive quantitative results showing how the various ways used to represent shape affect the bulk response, allowing comparisons between different approaches. Simulations of angle-of-repose tests show that all three rolling friction models can model the avalanching characteristics of cube/cuboid and cylindrical particles. Simulations of direct-shear tests suggest that both the shear strength and the dilative response of the considered non-spherical particles (but not their porosity) can only be predicted by the elasto-plastic rolling resistance model. The quantitative nature of the results allows identifying values of the shape-description parameters that can be used to obtain similar results when using alternative shape representation methods. [ABSTRACT FROM AUTHOR]

Regardless of its origin, in the near future the challenge will not be how to generate data, but rather how to manage big and highly distributed data to make it more easily handled and more accessible by users on their personal devices. VELaSSCo (V isualization for E xtremely La rge- S cale S cientific Co mputing) is a platform developed to provide new visual analysis methods for large-scale simulations serving the petabyte era. The platform adopts Big Data tools/architectures to enable in-situ processing for analytics of engineering and scientific data and hardware-accelerated interactive visualization. In large-scale simulations, the domain is partitioned across several thousand nodes, and the data (mesh and results) are stored on those nodes in a distributed manner. The VELaSSCo platform accesses this distributed information, processes the raw data, and returns the results to the users for local visualization by their specific visualization clients and tools. The global goal of VELaSSCo is to provide Big Data tools for the engineering and scientific community, in order to better manipulate simulations with billions of distributed records. The ability to easily handle large amounts of data will also enable larger, higher resolution simulations, which will allow the scientific and engineering communities to garner new knowledge from simulations previously considered too large to handle. This paper shows, by means of selected Discrete Element Method (DEM) simulation use cases, that the VELaSSCo platform facilitates distributed post-processing and visualization of large engineering datasets. [ABSTRACT FROM AUTHOR]

Current theories and design codes pertaining to storage structures for bulk solids have been developed in the context of rigid-walled silos and may not be applicable for smaller and highly flexible containers that are often used for industrial packaging and intermediate storage. The focus of this study is to investigate the effect of wall flexibility on the bulk stresses and wall pressures during storage using finite element analysis. The results show that when the wall stiffness is low, the computed bulk stresses in the vertical bin section are dominated by plasticity, while the stresses in the hopper section remain in the elastic state. In this situation, the wall pressure in the bin section is heavily influenced by the strength of the stored solid, which controls the extent of plastic flow. Overall, the normal wall pressure in the bin section is found to decrease with wall flexibility leading to a corresponding increase in vertical stress in the stored solid. As a consequence, the stresses in the hopper also increase leading to increasing loads on the hopper walls and potential exacerbation of handling issues for cohesive materials in highly flexible containers. [ABSTRACT FROM AUTHOR]

The bulk properties of powders depend on material characteristics and size of the primary particles. During storage and transportation processes in the powder processing industry, the material undergoes various modes of deformation and stress conditions, e.g., due to compression or shear. In many applications, it is important to know when powders are yielding, i.e. when they start to flow under shear; in other cases it is necessary to know how much stress is needed to keep them flowing. The measurement of powder yield and flow properties is still a challenge and will be addressed in this study. In the framework of the collaborative project T-MAPPP, a large set of shear experiments using different shear devices, namely the Jenike shear tester, the ELE direct shear tester, the Schulze ring shear tester and the FT4 powder rheometer, have been carried out on eight chemically-identical limestone powders of different particle sizes in a wide range of confining stresses. These experiments serve two goals: i) to test the reproducibility/consistency among different shear devices and testing protocols; ii) to relate the bulk behaviour to microscopic particle properties, focusing on the effect of particle size and thus inter-particle cohesion. The experiments show high repeatability for all shear devices, though some of them show more fluctuations than others. All devices provide consistent results, where the FT4 powder rheometer gives lower yield/steady state stress values, due to a different pre-shearing protocol. As expected, the bulk cohesion decreases with increasing particle size (up to 150 µm), due to the decrease of inter-particle cohesion. The bulk friction, characterized in different ways, is following a similar decreasing trend, whereas the bulk density increases with particle size in this range. Interestingly, for samples with particle sizes larger than 150 µm, the bulk cohesion increases slightly, while the bulk friction increases considerably--presumably due to particle interlocking effects--up to magnitudes comparable to those of the finest powders. Furthermore, removing the fines from the coarse powder samples reduces the bulk cohesion and bulk density, but has a negligible effect on the bulk friction. In addition to providing useful insights into the role of microscopically attractive, van der Waals, gravitational and/or compressive forces for the macroscopic bulk powder flow behaviour, the experimental data provide a robust database of cohesive and frictional fine powders for industrially relevant designs such as silos, as well as for calibration and validation of models and computer simulations. [ABSTRACT FROM AUTHOR]

Extrusion is a widely used process for forming suspensions and pastes into designed shapes, and is central to the manufacture of many products. In this article, the extrusion through a square-entry die of non-Brownian spheres suspended in Newtonian fluid is investigated using discrete element simulations, capturing individual particle-particle contacts and hydrodynamic interactions. The simulations reveal inhomogeneous velocity and stress distributions, originating in the inherent microstructure formed by the constituent particles. Such features are shown to be relevant to generic paste extrusion behavior, such as extrudate swell. The pressure drop across the extruder is correlated with the extrudate flow rate, with the empirical fitting parameters being linked directly to particle properties such as surface friction, and processing conditions such as extruder wall roughness. Our model and results bring recent advances in suspension rheology into an industrial setting, laying foundations for future model development, predictive paste formulation and extrusion design. © 2016 American Institute of Chemical Engineers AIChE J, 63: 3069-3082, 2017 [ABSTRACT FROM AUTHOR]

We present a numerical study on the DEM modeling of cohesive solids using a visco-elasto-plastic frictional adhesive contact model [1]. The capabilities of the contact model to capture the mechanical macroscopic behavior of cohesive materials were investigated by means of cone penetration and unconfined compression simulations. The results show that the simulations are able to reproduce qualitatively the typical trend of the penetration resistance profile in cohesive solid characterized by a steady-state at large penetration depths. The contact model is also capable of capturing the dependence of the penetration resistance on the consolidation stress history. Furthermore, the simulations reproduce the relationship between the unconfined strength and the penetration resistance that has been reported in real cohesive materials. Finally, we investigated the scaling laws of the contact model parameters to produce the same load–deformation behavior invariant of the particle size used in the simulations. The results demonstrate the suitability of the proposed model to simulate complex processes involving cohesive solid at large engineering scale scenarios. [ABSTRACT FROM AUTHOR]

One of the major shortcomings of discrete element modelling (DEM) is the computational cost required when the number of particles is huge, especially for fine powders and/or industry scale simulations. This study investigates the scaling of model parameters that is necessary to produce scale independent predictions for cohesionless and cohesive solid under quasi-static simulation of confined compression and unconfined compression to failure in uniaxial test. A bilinear elasto-plastic adhesive frictional contact model was used. The results show that contact stiffness (both normal and tangential) for loading and unloading scales linearly with the particle size and the adhesive force scales very well with the square of the particle size. This scaling law would allow scaled up particle DEM model to exhibit a bulk mechanical loading response in uniaxial test that is similar to a material comprised of much smaller particles. This is a first step towards a mesoscopic representation of a cohesive powder that is phenomenological based to produce the key bulk characteristics of a cohesive solid and has the potential to gain considerable computational advantage for industry scale DEM simulations. [ABSTRACT FROM AUTHOR]

Two main computational methods have been used in recent years to model the behavior of particulate solids in silos. These are the finite element method and the discrete element method. To assess the current state of the art in the two methods applied to silo problems, and to evaluate their capabilities without bias, an international collaborative project was set up to compare predictions of several silo phenomena. The first of these computational challenges was deemed the simplest: that of filling a silo or container with particulate solid. This paper presents an overview of the findings of this first problem, based on a total of 38 independent calculations. Here the collaborative project is briefly outlined and some deduced outcomes from calculations by continuum analysts are compared. The results of discrete element calculations are described in a companion paper, which also compares the two methods and comments on their strengths and weaknesses.

[Display omitted] • Concentrated solar power and asphalt pavement recycling rely on granular heat transfer. • Interstitial gas contribution to heat transfer in dense systems is key for these applications. • A recently proposed model includes the effect of gas for dense, static systems. • We validate this model against experimental data from a heated rotating drum with glass beads. • The model shows excellent agreement with the experiment. Conductive heat transfer in granular material is important in many industrial processes. For dense systems where the materials have low thermal conductivity, much of the heat transfer will occur through interstitial gases. Industry requires flexible and efficient computational methods to capture these phenomena at scale. In this study, a recently proposed particle-based model that includes the contribution of the interstitial gases was validated using an experiment. This model was originally derived from a multi-scale analysis of static, random packings. To test this approach in dense, dynamic systems, the model results were compared to experimental data for glass beads in an indirectly heated rotating drum. Infrared (IR) thermography was used to track the temperature evolution of the glass beads and the drum wall temperature. Discrete element simulations were performed with the experimental wall temperature used as a transient wall boundary condition. Results from the simulation show good agreement with the experimental data both for the bulk average temperature and for the bed profile, demonstrating the model's ability to capture the gas contribution in dynamic systems. [ABSTRACT FROM AUTHOR]

Abstract: In this study, the discrete element method (DEM) was used to investigate the influence of particle shape and interparticle friction on the bulk friction in a Jenike direct shear test. Spherical particle and non-spherical particles using two overlapping sphere giving particle aspect ratio of up to 2 and a full range of interparticle contact friction coefficient were studied numerically. These were compared with physical Jenike shear tests conducted on single glass beads and paired glass beads. To separate the influence of sample packing density from interparticle contact friction on the bulk shearing response, the same initial packing was used for each particle shape in the simulations. The interplay between contact friction and particle interlocking arising from geometric interaction between particles to produce the bulk granular friction in a direct shear test is explored and several key observations are reported. The results also show that particle interlocking has a greater effect than packing density on the bulk friction and for each particle shape; DEM can produce a good quantitative match of the limiting bulk friction as long as similar initial packing density is achieved. [Copyright &y& Elsevier]

Discrete Element Method (DEM) simulations have the potential to provide particle-scale understanding of twin-screw granulators. This is difficult to obtain experimentally because of the closed, tightly confined geometry. An essential prerequisite for successful DEM modelling of a twin-screw granulator is making the simulations tractable, i.e., reducing the significant computational cost while retaining the key physics. Four methods are evaluated in this paper to achieve this goal: (i) develop reduced-scale periodic simulations to reduce the number of particles; (ii) further reduce this number by scaling particle sizes appropriately; (iii) adopt an adhesive, elasto-plastic contact model to capture the effect of the liquid binder rather than fluid coupling; (iv) identify the subset of model parameters that are influential for calibration. All DEM simulations considered a GEA ConsiGma™ 1 twin-screw granulator with a 60° rearward configuration for kneading elements. Periodic simulations yielded similar results to a full-scale simulation at significantly reduced computational cost. If the level of cohesion in the contact model is calibrated using laboratory testing, valid results can be obtained without fluid coupling. Friction between granules and the internal surfaces of the granulator is a very influential parameter because the response of this system is dominated by interactions with the geometry. [ABSTRACT FROM AUTHOR]

The work presented in this paper was carried out as part of the PARDEM project. The overall aim was to quantify the predictive capability of a coupled CFD-DEM approach to simulating the horizontal pneumatic conveying of spherical and low-aspect-ratio non-spherical particles. Carefully controlled experiments were carried out in a 6.5 m long, 0.075 m diameter horizontal conveying line with the aid of the laser Doppler anemometry (LDA). Three different sizes of spherical glass beads, ranging from 0.8 mm to 2 mm and low-aspect-ratio cylindrical shaped particle of size 1 × 1.5 mm were employed. Simulations of the experiments were performed using a two-way coupled computational fluid dynamics and discrete element method (CFD-DEM) implemented in the commercial software FLUENT-EDEM in an Eulerian–Lagrangian framework. Experimental and simulation results of gas and particle velocities for particle laden flow with spherical particles were compared, showing that the CFD-DEM method could capture the experimental trends. However, quantitative discrepancies between simulation and experimental results were observed. Further modelling of low-aspect-ratio cylindrical particles was conducted using a multi-sphere model to represent cylindrical particles in the DEM code. Drag equations were modified in the code to take the effect of particle shape into account. The simulation results of mean axial particle velocity agreed reasonably well with a maximum of 25% discrepancy when compared to experimental measurements using the LDA technique. The discrepancies between simulation and experimental results were attributed to the selected drag model, mesh size and lack of an appropriate mesh interpolation scheme in the selected code. [ABSTRACT FROM AUTHOR]