Particle Discrete Element Method Research Articles

An unresolved SPH-DEM model for simulation of ductile and brittle surface erosion by abrasive water-jet (AWJ) impact

The abrasive water-jet (AWJ) erosion process involves the complex interaction between fluid medium, abrasive particles and solid material, which brings great challenges to the establishment of numerical model. Because traditional grid-based methods are not suitable for the problems of local deformation and material removal, the meshfree method smoothed particle hydrodynamics (SPH), based on the unresolved coupling and the discrete element method (DEM), is adopted to establish the model for AWJ study. The fluid medium is treated as a weakly compressible viscous liquid, the solid material is treated as an elastic-plastic material, and the abrasives are treated as rigid bodies. The fluid and solid phases are discretized with SPH particles, and the abrasives are described with DEM particles. The Johnson-Cook (J-C) and Johnson-Holmquist-II (JH-2) constitutive models are used to describe the stress-strain behavior of ductile and brittle materials, respectively. The effectiveness of the numerical model is further verified by AWJ impact experiments. The plastic deformation and cumulative failure characteristics of ductile materials, and the crack formation and propagation characteristics of brittle materials are systematically analyzed. The results provide insight for the AWJ research and lay a foundation for investigation of other complex fluid-particle flow in a numerical way.

A Discrete Element Method Calibration Approach for Large Ore-flow Applications

The major constraint for large discrete element method (DEM) simulations is the high demand for computation power. To keep the model in a feasible range of simulation time and to conduct the numerical calculations efficiently, the implemented material can be simplified and upscaled. After this, a calibration of the material is inevitable and sets the link between the simulation model and reality. Therefore, an alternative calibration approach is established in this contribution to mimic the flow behaviour of the original material with multi-spherical DEM particles. The calibration approach utilises flow zone dimensions, which were observed in block-caving operations. Additionally, a passive draw-point is used to calibrate the angle of repose. Applying the calibration approach for Kiruna iron ore showed that, with the help of the calibration approach, the material in the simulation could successfully reproduce the targeted flow zone and angle of repose.

Two-scale concurrent simulations for crack propagation using FEM–DEM bridging coupling

The Discrete element method (DEM) is a robust numerical tool for simulating crack propagation and wear in granular materials. However, the computational cost associated with DEM hinders its applicability to large domains. To address this limitation, we employ DEM to model regions experiencing crack propagation and wear, and utilize the finite element method (FEM) to model regions experiencing small deformation, thus reducing the computational burden. The two domains are linked using a FEM–DEM coupling, which considers an overlapping region where the deformation of the two domains is reconciled. We employ a “strong coupling” formulation, in which each DEM particle in the overlapping region is constrained to an equivalent position obtained by nodal interpolation in the finite element. While the coupling method has been proved capable of handling propagation of small-amplitude waves between domains, we examine in this paper its accuracy to efficiently model for material failure events. We investigate two cases of material failure in the DEM region: the first one involves mode I crack propagation, and the second one focuses on rough surfaces’ shearing leading to debris creation. For each, we consider several DEM domain sizes, representing different distances between the coupling region and the DEM undergoing inelasticity and fracture. The accuracy of the coupling approach is evaluated by comparing it with a pure DEM simulation, and the results demonstrate its effectiveness in accurately capturing the behavior of the pure DEM, regardless of the placement of the coupling region.

Simulation-based personal fatality risk assessment due to the fragmentation hazard

In the military and chemical industry, modeling the fragmentation hazard field is of great significance in conducting the fatality risk assessment and calculating the safety distance for person. Although the ballistic methodology achieves the simulation of fragmentation flight trajectory, the acquisition of accurate initial projection data of fragmentation is a challenge. By coupling continuum-discontinuum element method and particle discrete element method, the fragmentation power algorithm is established, which achieves the integrated simulation of the initial projection data of fragmentation and the flight process of fragmentation. First, continuum-discontinuum element method is adopted to simulate the detonation products-driven fragmentation acceleration process by introducing the explosive detonation model, which achieves the acquisition of initial projection data of fragmentation. Then, the particle discrete element method is adopted to simulate the flight trajectory and power data of fragmentation. Based on the numerical and experimental result, the accuracy of fragmentation power algorithm is verified. In conjunction with the personal vulnerability model, a systematic numerical simulation framework is established to conduct the simulation-based personal fatality risk assessment when the metal-cased munition detonates accidentally, and the results indicate that the personal fatality risk due to the fragmentation becomes severer with the increase of munition curvature.

Spherical harmonic–based DEM in LAMMPS: Implementation, verification and performance assessment

Particle shape plays a major role in the behaviour of most granular systems. This has led to increasing interest in the representation of arbitrarily shaped particles in discrete element method (DEM) simulations. In this paper, we present a simulation approach based on the representation of particle shapes using spherical harmonics where their radii can be calculated in spherical coordinates. An energy-conserving contact model is adopted which is based on the volume of overlap between interacting particles. Contact detection makes use of the bounding spheres of the interacting particles, simplifying its incorporation within a conventional sphere-based DEM code. The volume of overlap and other required quantities are calculated using Gaussian quadrature integration of the spherical cap formed by the bounding spheres. Both the accuracy and the computational cost increase with the number of quadrature points. The algorithm has been implemented as a LAMMPS user package, and verified by means of energy conservation. The performance and parallel scaling of the approach are illustrated, and an observed scaling limitation owing to load imbalance arising from the evaluation of the overlap volume is discussed. Program summaryProgram Title: SH-DEM LAMMPS packageCPC Library link to program files:https://doi.org/10.17632/vk6fj6yjtf.1Developer's repository link:https://github.com/EPCCed/lammps/tree/feature-sh-demLicensing provisions: GPLv2Programming language: C++Nature of problem: Particles are often highly non-spherical. Spherical harmonics provide a natural way to represent complex particle shapes within a discrete element method (DEM) simulation. However, there is no publicly available DEM code which allows particle shapes to be represented using spherical harmonics.Solution method: The SH-DEM package extends the capabilities of LAMMPS so that irregularly shaped particles can be represented using spherical harmonics. The package includes the definition of a new ‘shdem’ atom style for spherical harmonic particles, a time integration scheme for these particles based on the Velocity Verlet algorithm, algorithms for detecting and evaluating contacts between spherical harmonic particles, evaluation of the contact forces between these particles and rigid walls, and two energy computes for groups of spherical harmonic particles.Additional comments including restrictions and unusual features: The SH-DEM package is applicable only to 3D simulations. In order for a particle to be defined by spherical harmonics, it is required that any line segment drawn from an origin inside the particle crosses the contour of the particle's three-dimensional surface only once. If the ‘shdem’ atom style is used, the current implementation requires all particles to be defined using this atom style, e.g., mixtures of sphere and shdem atom styles are not permitted.

Mesoscopic simulations of a fracture process in reinforced concrete beam in bending using a 2D coupled DEM/micro-CT approach

In this study, a complex fracture process in a short rectangular concrete (RC) beam reinforced by one longitudinal bar (without vertical reinforcement) and subjected to quasi-static three-point bending was numerically explored in 2D conditions. A critical diagonal shear crack in the beam caused it to fail during the experiment. The numerical simulations were conducted with a classical particle discrete element method (DEM). A three-phase concrete description (aggregate, mortar, and interfacial transitional zones (ITZs) around aggregates) accounted for the concrete heterogeneity. In mesoscopic DEM calculations based on a 2D X-ray CT scan, the actual shape and placement of aggregate particles in concrete were taken for granted. In the study, the steel bar with ribs was replicated. ITZ was also assumed between the bar and mortar. Without imposing any bond-slip law, a geometrical bar/concrete interface condition was explicitly considered. The focus was on the force–deflection diagram, fracture process, contact forces, and stresses along the bar. A good level of agreement about the evolution of the vertical force versus the deflection and failure mechanism was attained between DEM analyses and in-house laboratory tests despite simplified 2D conditions. A strong effect of concrete mesostructure on the crack pattern was found.

Applications of DEM particle breakage models in mineral industrial

Modeling processes are carried out in the mineral industry as well as in many areas depending on the development of computer technologies and software. Discrete Element Method (DEM) is used in modeling studies to explain the interaction of particles with other particles and communication equipment. The DEM provides the capability to simulate the movement of the granular media in a series of computational processes of each individual particle that consists of the granular media. It is becoming increasingly widely used to predict energy consumption, wear, particle breakage and particle size distribution in crushing and grinding processes that can be described in terms of granular materials using DEM. The selection of particle breakage models used by commercial software for modeling DEM particle breakage is important. In this study, it is summarized the studies have been carried out to understand the performance of particle breakage methods, which are Bonded Particle Model (BPM), Fast Breakage Model (FBM) and Particle Replacement Model (PRM), in the modeling of comminution equipment. In addition, the relationship between particle and breakage energies and theory of applied forces are described in detail for three breakage models existing in commercial DEM simulators.

Study on generation and evolution of base voids in heavy-haul railway tunnels using particle flow method

Most existing freight railway tunnels have experienced basement cracking, damage, and even mud-pumping after a certain period of operation. For heavy-haul railway tunnels with high traffic densities, large capacities, and large axle loads, this situation will be more serious. This paper establishes a simplified model of a single-track heavy-haul railway tunnel using the two-dimensional particle discrete element method. The meso-evolution process of the concrete structure and surrounding rock void at the bottom of the tunnel is studied in the later stages of repeated loading by heavy-haul trains. The difference in the base void between waterless and water-saturated bases is compared. The cracks in the surrounding rock are first generated at the lowest position of the invert center, and run from the middle to the sides and from the top to the bottom. The terminal cracks return to the free surface, exhibiting a parabolic shape that opens upward. There are more cracks in the middle and fewer at the sides. The asynchronous deformation of the surrounding rock and the inverted arch is positively correlated with the number of cracks. The voiding process under water-saturated conditions is similar to that without water, but the depth and width over which the rock mass is crushed and the range of voiding increase exponentially in the water-saturated case, and the void appears earlier. It is anticipated that these research results will contribute to the structural design and operational safety of heavy-haul railway tunnels.

Development and Implementation of a Damage Model for Potato Tuber Blackspot in Discrete Element Method to Analyze Harvesting and Handling Processes

Highlights Method for determining the discoloration of potato tissue samples induced by mechanical stress. Model of the discoloration susceptibility of specific potato tissue. Particle segmentation algorithm in the Discrete Element Method for contact localization in post-processing. Development of the "Discrete Element Method Blackspot Potato Tuber Model" capable of predicting internal blackspot damage in potato tubers. Abstract. Blackspot damage caused by mechanical load during the harvesting and processing stages of potato tubers remains difficult to mitigate. Field tests are inadequate as it takes around two days to develop blackspot discoloration, making it unclear when and where the initial damage occurs. Particle simulation, particularly with the Discrete Element Method (DEM), offers opportunities to improve machine performance and reduce crop losses. However, the DEM cannot predict local blackspot damage, making it unsuitable for reducing potato tuber damage. To address this, the “DEM blackspot potato tuber model” presented in this contribution integrates a discoloration function and a segmentation algorithm of DEM particles to predict and analyze blackspot damage during harvesting and processing. Based on mechanical experiments on tuber tissues with different loading conditions, a discoloration function was proposed dependent on strain and strain rate. The segmentation algorithm adapts to the varying potato tuber particle size by varying the number of segments to generate segments of average blackspot size. Two different validation tests are used to identify the single and multiple impact capabilities of the model. The preliminary results show that the discoloration induced by single impact events is well predicted in terms of the number of damaged tubers as well as the average damage per tuber. Damage of multiple impacts is less accurately predicted since the model does not take into account the accumulation of damage. Keywords: Blackspot, Damage model, Discrete Element Method, Potato handling, Simulation.

Discrete Element Modelling of a Bulk Cohesive Material Discharging from a Conveyor Belt onto an Impact Plate

The discrete element method (DEM) has become the numerical method of choice for analysing and predicting the behaviour of granular materials in bulk handling systems. Wet-and-sticky materials (WSM) are especially problematic, resulting in build-up and blockages. Furthermore, due to the large number of particles in industrial-scale applications, it is essential to decrease the number of particles in the model by increasing their size (upscaling or coarse graining). In this study, the accuracy with which upscaled DEM particles can model the discharge of a cohesive material from a belt conveyor onto an inclined impact plate was investigated. Experimentally, three sand grades (particle size distributions, PSDs) were used, each in a dry (non-cohesive) state and with three levels of moisture-induced cohesion. The effects of the modelled PSDs on the material flow, build-up on the plate, the peak impact force and the residual weight were investigated. Although a linear cohesion contact model was mostly used, the results were also compared to that of the Johnson–Kendall–Roberts (JKR) and simplified JKR (SJKR) models. It was found that the general profile of the pile (build-up) could be accurately modelled, but using a more accurate (but still upscaled) PSD improved the results. The impact force and the residual weight on the plate could be accurately modelled (error <15%) if the particle size was not excessively scaled. The maximum acceptable scaling factor was found to be a geometric factor of the bulk measure of interest, and not a factor of the physical particle size. Furthermore, with an increase in cohesion, the bulk measures such as the thickness of the discharge stream and the height of the material build-up increased, which meant that the maximum acceptable scale factor also increased. The results are valuable for future accurate and efficient modelling of large industrial scale applications of WSMs.

Shape-changing particles for locally-resolved particle geometry in DEM simulations

Taking into account the complex shape of particles in discrete element method (DEM) simulations of large-scale granular systems is computationally demanding due to the time-consuming contact detection algorithms for polyhedral particles. In this short communication, a novel approach that locally resolves the particle shapes where needed and uses a simplified representation elsewhere, to accelerate simulations without compromising accuracy, is presented. For this purpose, a method employing a smooth transition of the particle shape representation from analytical spheres to shape-resolving polyhedra is introduced in DEM. The feasibility and correct implementation of this approach are demonstrated through simulations of hopper discharge involving spherical and dodecahedral particles from a flat bottom silo or shaft kiln. The model capabilities, in terms of accuracy as well as reduction in computational effort, are quantified for a moving bed with continuous outflow.

Study on mechanical properties and microscopic damage mechanism of tight sandstone reservoir under uniaxial compression

Due to the characteristics of low porosity, low permeability and serious anisotropy in tight reservoirs, it is difficult for conventional hydraulic fracturing theory to accurately guide the efficient exploitation of tight reservoirs. It has been shown that the reservoir rock mechanical properties are the key factor impacting the fracturing effect, but the current research on the damage properties of tight reservoir rocks is not comprehensive enough. Therefore, in order to improve the fracturing theory of tight reservoirs, this paper first explores the evolution mechanism of rock fractures through uniaxial compression experiments. Secondly, based on the particle discrete element method, the damage and failure process of tight sandstone under uniaxial compression is simulated from the microscopic scale. The test results show that the rock failure mainly includes tensile failure, shear failure, and tensile-shear failure; Internal micro-fractures will interconnect during rock destruction to form primary fractures through the rock mass, while secondary micro-fractures will also be generated. The numerical simulation results show that when the rock is subjected to tensile-shear failure, with the increase of load, tensile micro-fractures are mainly produced in the specimen, accompanied by a few shear fractures. Under the joint action of shear failure and tensile failure, V-shaped cracks are easily formed in rock. The tensile strength of rock is mainly affected by the microscopic tensile strength, and the cohesive force, modulus, stiffness ratio, friction coefficient and friction angle have significant effects on the compressive strength of rock. Therefore, a reasonable choice of microscopic parameters can realistically simulate the compression-tensile strength ratio of the rock. The research results of this paper can provide the theoretical basis of rock mechanics for the efficient exploitation of tight reservoirs.

Multi-GPUs DEM algorithm and its application in the simulation of granular materials

The Discrete Element Method (DEM) is one of the most common numerical approach for modeling the physical and mechanical properties of granular materials. However, its computational efficiency can be a major bottleneck, which significantly hinders its application in engineering fields. To overcome this challenge, a multi-GPU framework with OpenMP has been developed. In this framework, the DEM particles in 3D space and their corresponding contact information are sorted in a given direction in the simulation, and each GPU device handles a subdomain of particles and relative contacts. Additionally, the contact information is densely arranged to minimize memory usage. To further enhance efficiency, sorting and packing algorithms for the contact information have been introduced. During calculating loop, the number of contacts on each device is monitored, and the spatial division of devices is adjusted to balance memory distribution. The efficacy of the multi-GPU algorithm is verified through a chute flow test. Using numerical results, the segregation process and mechanics of granular materials during sliding have been studied in detail. Furthermore, the simulations demonstrate that the developed multi-GPU algorithm greatly enhances computational efficiency and reduces memory usage per device.

Numerical investigation of scour around the monopile using CFD-DEM coupling method

The risks posed by local scour around offshore monopile foundations are substantial and threaten the structural safety of these installations. It is therefore essential to study the mechanisms of local scour to mitigate these risks. Numerical simulation is an effective tool to help us understand the mechanisms of local scour. However, the accurate modeling of the local scour process remains to be challenging due to the complexity of sediment transportation, in which the continuity threatment of sediment phase is typically used, the discrete behavior of the sediment is hard to be described. Therefore, in this paper, in order to conduct the deepin investigation into the mechanism of the local scour around monopile, a three-dimensional CFD-DEM model was established by coupling the computational fluid dynamic method (CFD) and discrete element method (DEM). To increase the efficiency and accuracy of the CFD-DEM model, the coarse grain method (CGM) was used to decrease the number of required DEM particles in the simulation, and the angle of repose was used to calibrate the contact parameters between DEM particles. During the validation process of the numerical model, it was found that the development of the scour depth and the morphology of scour pit obtained by the CFD-DEM model showed good agreement with the previously published results. The CFD-DEM simulation revealed that seepage-induced vertical drag force plays a crucial role in the initial stage of local scour. Two sediment transportation processes, "Pile toe erosion-Slope avalanche" and "Push-Accumulation-Wash", were observed and the relationship between bedload sediment flux and the Shields number was quantified. In the end of this paper, the critical evaluation of the CFD-DEM method was conducted.

Vibratory compaction properties of off-shore gravity quays rubble bed based on rigid blocks

This paper intends to identify the safety and stability issues within quay structures, including scouring and hollowing, in the infrastructure for off-shore rubble bed. The three main research approaches are to combine 3D laser scanning technology, particle discrete element method and on-site experiments. The rubble bed model is established using the rigid block (Rblock) element in discrete element particle flow. Combined with the gradation generation method and the calibration of on-site tests, the influencing factors and evolution characteristics of vibratory compaction technique of rubble bed are then numerically investigated. First, the shape of the on-site stones is characterized using 3D mesoscopic extraction by 3D laser scanning technology.Statistics are used to examine the shape charactrization parameters of stone particles, and 3D mesoscopic characterization description and reconstruction techniques are developed for stone particles. Second, the 3D model of rubble bed is then built using the Rblock element in discrete particle flow, and the necessary model parameters are calibrated through a on-site vibratory compaction test. Additionally, the PFC3D program is used to study two types of influencing factors, namely the particle characteristics and vibration conditions during the vibratory compaction process of rubble bed. The results show how the riprap thickness, particle size, grading effect, vibration time, vibration frequency, and effective excitation amplitude affect the vibratory compactory properties of rubble bed. The functional link between various influencing factors with density degree and deformation modulus is proposed after combining experimental tests to examine the relevance between compaction parameters with density degree after vibration and deformation modulus. The suggested discrete element method can serve as a guide for vibratory rubble bed compaction.

Numerical simulation study on erosion collapse failure mechanism of cemented conglomerate accumulation

PurposeCemented conglomerate accumulation is a weak and heterogeneous medium that occurs in western China. It consists mainly of argillaceous cement that loses strength rapidly upon contact with water, leading to collapse instability failure. Its deformation failure mechanism is complex and poorly understood. In this paper, the erosion failure mechanism of cemented conglomerate accumulation is investigated.Design/methodology/approachThe collapse failure process after erosion of the slope foot for typical cemented conglomerate accumulation is studied based on field investigation using the particle discrete element method. And how the medium composition, slope angle and cementation degree influence the failure mode and process of the cemented conglomerate accumulation is examined.FindingsThe foot erosion of slope induces a tensile failure that typically manifests as “erosion at the foot of slope – tensile cracking at the back edge of slope top – integral collapse.” The collapse failure is more likely to occur when the cemented conglomerate accumulation has a higher rock content, a steeper slope angle or a weaker cementation degree.Originality/valueA model based on rigid blocks and disk particles to simulate the cemented conglomerate accumulation is developed. It shows that the hydraulic erosion at the foot of the slope resulted in a different failure mechanism than that of general slopes. The results can inform the stability management, disaster prevention and mitigation of similar slopes.

A coupled MPM-DEM method for modelling soil-rock mixtures

Aiming at modelling the mechanical behaviour of soil-rock mixtures accurately and efficiently, a coupled MPM-DEM formulation combining the material point method (MPM) and the discrete element method (DEM) is proposed. It is solved concurrently via the contact force linking the two individual methods. Specifically, the soil is modelled with MPM as continuums to avoid handling the contacts between fine particles. The rocks are modelled by DEM to capture the contact characteristics of rocks. This method is validated with ball impacting and block sliding tests first for the contact between material points and DEM particles. Its capability in describing the mechanics of soil-rock mixtures is thereafter proved by comparing the simulation results with pure DEM simulations of binary mixtures and laboratory tests of soil-rock mixtures. It is demonstrated that MPM-DEM can reproduce the stress–strain response of soil-rock mixtures and capture the influence of rock contents and rock sizes. In addition, a coarse-graining modelling scheme is implemented, i.e., representing the soil particles with fewer material points, which significantly increases the efficiency compared with pure DEM. Our proposed method provides a novel way to model soil-rock mixtures with reasonable computational efforts, which sheds light on simulating large-scale soil-rock mixtures in nature or engineering.

DEM breakage calibration for single particle fracture of maize kernels under a particle replacement approach

The Discrete Element Method (DEM) has become increasingly popular for simulating breakage operations and grinding equipment performance. Two main breakage approaches can be found in literature: bonded particle models and particle replacement models. This work is focused on fitting the parameters of a DEM particle replacement breakage model for maize, implemented in software Rocky DEM. Once all required experimental and calibration procedures were developed for maize specific DEM properties and parameters, single particle compression tests were carried out to fit the parameters of the breakage model, which was extensively analyzed for validating its capability of representing the material. Maize properties showed to be in agreement with literature sources. Compression tests simulated in DEM led to the same results as their experimental counterparts, validating the model’s capability of representing maize breakage. These results are the basis for further simulation of a full milling process in a hammer mill.

Reloading Mechanical Properties and Particle Flow Simulation of Pre-Peak Confining Pressure Unloading Sandstone

The excavation-unloading damage effects of western high-geostress slopes on rock were explored by testing the pre-peak confining pressure unloading sandstone reloading mechanical properties. The deformation and failure mechanisms were studied from a mesoscopic perspective using the particle discrete-element method. (1) Approaching the unloading failure, confining pressure increased the specimen bearing capacity attenuation. (2) The confining pressure unloading promoted microdefect propagation and development; the specimens increased rapidly to the damage stress value after reaching the initiation stress value. The penetration fracture zone was more evident and expansive in the model, and the distribution of the dense crack areas was more concentrated in the fracture zone and area. (3) The average interval of the tangential contact force was the largest in the direction of crack expansion and propagation. The strong force chains were shown to primarily bear external loads, whereas the weak force chains played a key auxiliary role in maintaining stability. (4) The number of cracks developing in the confining pressure unloading damage process indicated that the loading process did not cause damage to the specimens. The fracture zones further propagated and formed on the dominant fractures based on the damage caused by the confining pressure unloading disturbance.

Thermal Modeling of Polyamide 12 Powder in the Selective Laser Sintering Process Using the Discrete Element Method.

Selective laser sintering (SLS) is one of the key additive manufacturing technologies that can build any complex three-dimensional structure without the use of any special tools. Thermal modeling of this process is required to anticipate the quality of the manufactured parts by assessing the microstructure, residual stresses, and structural deformations of the finished product. This paper proposes a framework for the thermal simulation of the SLS process based on the discrete element method (DEM) and numerically generated in Python. This framework simulates a polyamide 12 (PA12) particle domain to describe the temperature evolution in this domain using simple interaction laws between the DEM particles and considering the exchange of these particles with the boundary planes. The results obtained and the comparison with the literature show that the DEM frame accurately captures the temperature distribution in the domain scanned by the laser. The effect of laser power and projection time on the temperature of PA12 particles is investigated and validated with experimental settings to show the reliability of DEM in simulating powder-based additive manufacturing processes.