Large-scale Discrete Element Method Research Articles

The Cumulation of Debris Clouds around a Fast-rotating Asteroid

Abstract The rotational mass loss has been realized to be a prevalent mechanism to produce low-speed debris near the asteroid, and the size composition of the asteroid’s surface regolith has been closely measured by in situ explorations. However, the full-scale evolution of the shedding debris has not been examined using the observed particle sizes, which may hold vital clues to the initial growth of an asteroid moonlet and help us to understand the general mechanisms that dominate the formation of asteroid systems. This paper presents our study on the cumulative evolution of the debris cloud formed by a rotationally unstable asteroid. A semianalytical model is developed to characterize the spatiotemporal evolution of the debris cloud posterior to a shedding event. Large-scale discrete element method (DEM) simulations are performed to quantify the clustering behavior of the debris particles in the mechanical environment near the asteroid. As a result, we found that the cumulation of a steady debris cloud is dominated by large pieces of debris, and the shedding particles follow a common migration trend, which fundamentally determines the mass distribution of the debris cloud. For the accretion analysis, we sketched the life cycle of a debris cluster and showed its dependency on particle size. The DEM simulations adopt physical parameters estimated from observations and asteroid missions. The results confirm that porous fluffy cluster structures can form shortly after a shedding event with magnitudes the same as the observed shedding activities. Measurements of these structures show that they themselves possess a certain strength and have the capacity to collisions to absorb dissociative particles that collide with them.

Development of a calibration program for large-scale DEM simulation of hot mix asphalt

This paper presents a comprehensive Discrete Element Method (DEM) calibration program designed for optimal calibration of hot mix asphalt (HMA) in full-scale paving applications. The program allows for the calibration of various asphalt mixes and allows for studies such as how tamper stroke affects pre-compaction based on the calibrated HMA. The study identifies key pavement properties essential to the paving process, including pre-compaction resistance, flow behavior, and friction behavior between HMA and steel or asphalt surfaces, and details their laboratory-based evaluation methods. A contact model that incorporates realistic plastic deformation, adhesive forces, and multiple load cycles is presented in this research. Novel features include a two-clump particle shape system and a two-grain distribution for the grading curve. These parameters, combined with the contact model, facilitate the observation of particle orientations and segregations, ensuring an economical yet robust full-scale simulation. Validation included testing and calibration of three different asphalt mixes (Asphalt Concrete (AC) 11 DS, Stone Mastic Asphalt (SMA) 11 DS, and AC 16 BS). The results demonstrate the successful calibration of all relevant pavement properties using the prescribed methodology and settings. This study advances the understanding of HMA pavement properties and provides a practical framework for optimal calibration in full-scale paving scenarios.

Coarse-graining algorithms for the Eulerian-Lagrangian simulation of particle-laden flows

In the present article, novel Coarse-Graining (CG) algorithms for the Eulerian-Lagrangian (EL) simulation of particle-laden flows are proposed. These include different variants of Reproducing Kernel Particle Methods (RKPM) and an extended Diffusion Two-Step Method (DTSM) for highly polydisperse flows. Owing to the dynamic nature of the kernel function in RKPMs, CG algorithms with high-order consistency properties are constructed and the extra physics of the fluid-particle interaction torque effect on the two-way coupling force distributed to the fluid is taken into consideration. To increase the robustness of RKPMs, they are hybridized with other simple CG algorithms in such a way that each model is activated in the range of its validity. The performance of the new CG algorithms is carefully assessed in comparison to other widely-used algorithms by devising four benchmarks and several grid-independence tests. Based on the analyses, the first-order RKPM demonstrates the best performance in terms of 8 attributes, including conservativity, grid-independency, smoothness, etc., among all algorithms and is recommended when the accuracy is of prime importance. However, due to its higher computational cost compared to the extended DTSM, the latter model would be an affordable alternative for large-scale discrete element method problems when the computational cost is critical.

A machine learning-based particle-particle collision model for non-spherical particles with arbitrary shape

In this study, contacts between identical non-spherical particles are investigated by machine learning approaches. Artificial neural network models are developed to correlate relative positions and orientations of the pair with contact occurrence and properties (mean contact point, mean norm, and inter-penetration depth). To evaluate the idea, eight different particles are generated by super-ellipsoid and spherical harmonic expansion. With these methods, pairs of non-spherical particles (regular or irregular shaped) are generated, and the contact properties are computed through vertices-based calculation. Using random translation and rotation, 80000 datasets are collected for each case and are utilized for training, validating, and evaluating the models. After training the proposed models, they enable accurate and rapid predictions of the contact occurrence and properties of non-spherical particles with arbitrary shape. It requires a smaller amount of data than discrete element method (DEM) applications. This study provides collision models that can be applied to large-scale DEM simulations.

Large-Scale DEM Simulation of Plate Drag in Dry Granular Materials

Plate drag in granular materials is a simple but important because it provides an understanding of how tools interact with soil in soil cutting and tillage. We numerically study the response of dry granular materials to plate drag as a function of initial volume fraction of the materials using a large-scale discrete element method (DEM) simulation. In the simulation, a flat plate is translated horizontally through initially homogeneous materials with different volume fraction and the drag force acting on the plate is examined. The results show that a volume fraction-dependent bifurcation occurs in the force: in an initially loose granular bed, the force reaches an approximately constant value as the plate advances, while in an initially dense bed, the force oscillates with a large amplitude. The force oscillation is attributed to the periodic evolution of a shear band formed only in the dense bed. The behaviors of the drag force and shear band are in close agreement with those obtained experimentally in previous studies. Further analysis shows that the formation of the shear band is explained by the local dilation and compaction of the granular materials induced by the plate drag. In addition, the relationship between the evolution of the shear band and the drag force can be explained quantitatively by using a three-dimensional wedge model considering the variation of the local volume fraction inside the shear band.

Post-processing and visualization of large-scale DEM simulation data with the open-source VELaSSCo platform

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 (Visualization for Extremely Large-Scale Scientific Computing) 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.

Interaction between dry granular materials and an inclined plate (comparison between large-scale DEM simulation and three-dimensional wedge model)

The interaction between dry granular materials and an inclined plate is numerically studied using a three-dimensional discrete element method (DEM) simulation. In the simulation, a plate is dragged horizontally through densely packed dry granular materials. To examine the effect of the rake angle α of the plate on the drag force acting on the plate, three cases with α = 50°, 70°, and 90° are compared (α = 90° for a vertical plate). The results show that for all cases, the force oscillates as the plate advances. As α decreases, the amplitude and frequency of the force oscillation decrease and increase, respectively. The force oscillation is attributed to the periodic evolution of a shear band formed in the materials. The relationships between the rake angle, evolution of the shear band, and drag force can be explained quantitatively by using a three-dimensional wedge model considering the variation of the local volume fraction inside the shear band.

Local dilation and compaction of granular materials induced by plate drag

The response of granular materials to plate drag is numerically studied using a large-scale discrete element method (DEM) simulation. The effect of the initial volume fraction of the materials on the drag force acting on the plate is examined. The results show that a volume-fraction-dependent bifurcation occurs in the force; in an initially loose granular bed, the force reaches an approximately constant value as the plate advances, while in an initially dense bed, the force oscillates with a large amplitude. The force oscillation is attributed to the periodic evolution of a shear band formed only in the dense bed. The behaviors of the drag force and shear band, which depend on the initial volume fraction in the DEM simulation, are in close agreement with those obtained experimentally in previous studies [N. Gravish et al., Phys. Rev. Lett. 105, 128301 (2010); Phys. Rev. E 89, 042202 (2014)]. Further analysis using the DEM simulation shows that the formation of the shear band is explained by the local dilation and compaction of the granular materials induced by the plate drag. Independent of the volume fraction, materials dilate in a wedge-shaped flow region that formed in front of the plate. In the loose bed, a compacted front builds up ahead of the flow region. Because the compacted front advances into a weaker undisturbed region, the flow region behind the front can constantly advance. On the other hand, in the dense bed, the materials largely dilate in a disturbed flow region formed in front of the plate. Because a denser undisturbed region is more stable compared to the flow region, the flow region is strongly confined. As a result, the shear strain is localized along a flow boundary between these regions, and the shear band develops.

Numerical investigation of a coarse-grain discrete element method in solid mixing in a spouted bed

We describe the effectiveness of the coarse-grain model of the discrete element method (DEM) in solid mixing in a spouted bed. The coarse-grain model was developed originally to simulate large-scale DEM simulations efficiently, where the coarse-grain particle represents a group of original particles. In previous studies, the adequacy of the coarse-grain model was proven in a pneumatic conveying system and bubbling fluidized beds through verification and validation tests. Namely, In this study, the coarse-grain model is applied to solid mixing in a spouted bed. Agreement of the mixing state is shown between an original particle system and coarse grain model systems. Subsequently, correlation between solid mixing and macroscopic behavior of the solid particles is examined in the present study. The macroscopic properties such as solid-particle spatial distribution, pressure drop and velocity distribution of the solid phase are confirmed to correspond quantitatively in the coarse-grain model and an original particle system. Finally, the calculation efficiency of the coarse-grain model is evaluated. Consequently, the coarse-grain model is shown to be able to efficiently investigate the solid mixing in a spouted bed.

Periodic cells for large-scale problem initialization

In geotechnical applications the success of the discrete element method (DEM) in simulating fundamental aspects of soil behaviour has increased the interest in applications for direct simulation of engineering scale boundary value problems (BVP’s). The main problem is that the method remains relatively expensive in terms of computational cost. A non-negligible part of that cost is related to specimen creation and initialization. As the response of soil is strongly dependant on its initial state (stress and porosity), attaining a specified initial state is a crucial part of a DEM model. Different procedures for controlled sample generation are available. However, applying the existing REV-oriented initialization procedures to such models is inefficient in terms of computational cost and challenging in terms of sample homogeneity. In this work a simple but efficient procedure to initialize large-scale DEM models is presented. Periodic cells are first generated with a sufficient number of particles matching a desired particle size distribution (PSD). The cells are then equilibrated at low-level isotropic stress at target porosity. Once the cell is in equilibrium, it is replicated in space in order to fill the model domain. After the domain is thus filled a few mechanical cycles are needed to re-equilibrate the large domain. The result is a large, homogeneous sample, equilibrated under prescribed stress at the desired porosity. The method is applicable to both isotropic and anisotropic initial stress states, with stress magnitude varying in space.

Massively Parallel Discrete Element Method Simulations on Graphics Processing Units

This paper outlines the development and implementation of large-scale discrete element method (DEM) simulations on graphics processing hardware. These simulations, as well as the topic of general-purpose graphics processing unit (GPGPU) computing, are introduced and discussed. We proceed to cover the general software design choices and architecture used to realize a GPGPU-enabled DEM simulation, driven primarily by the massively parallel nature of this computing technology. Further enhancements to this simulation, namely, a more advanced sliding friction model and a thermal conduction model, are then addressed. This discussion also highlights some of the finer points and issues associated with GPGPU computing, particularly surrounding the issues of parallelization, synchronization, and approximation. Qualitative comparison studies between simple and advanced sliding friction models demonstrate the effectiveness of the friction model. A test problem and an application problem in the area of wind turbine blade icing demonstrate the capabilities of the thermal model. We conclude with remarks regarding the simulations developed, future work needed, and the general suitability of GPGPU architectures for DEM computations.

Verification and validation of a coarse grain model of the DEM in a bubbling fluidized bed

Dense granular flows are encountered in various engineering fields. The discrete element method (DEM) is used extensively for the numerical simulation of these flow types. Improvements in computer specifications have made it possible to apply the DEM in various systems. Although the DEM is well-established, it has a fatal problem. The problem is that the number of calculated particles is substantially restricted when the simulation needs to be finished within practical time using a single personal computer. On the other hand, many industries require application of the DEM to large scale systems on a single personal computer. We therefore developed the DEM coarse grain model in our previous studies. In the coarse grain model, a group of original particles is simulated using a large-sized particle termed a coarse grain particle, where the total energy is modeled to agree between the coarse grain and original particles. The coarse grain model therefore makes it feasible to perform large scale DEM simulations by using a smaller number of particles than the actual number. The coarse grain model thus far has been verified in two-dimensional fluidized beds. In this study, adequacy of the coarse grain model is verified and validated in a three-dimensional fluidized bed, where scaling ratio is low and high respectively. Consequently, the coarse grain model is shown to simulate the macroscopic behavior of solid particles in three-dimensional dense gas–solid flows.

ＤＥＭ粗視化モデルによる気泡流動層の数値解析

Discrete Element Method (DEM) is widely used in many powder systems. The DEM makes possible to figure out the complicated phenomena related to particle flowability. On the other side, the DEM has a fatal problem, that is, the number of calculated particles is restricted due to the excessive calculation cost. Thereat, we have developed a large scale model of the DEM which is called coarse grain model. The coarse grain represents a group of the original particles so that a large-scale DEM simulation can be performed by using extremely small number of the calculated particles. In our previous studies, the coarse grain model was used in not only gas-solid flow simulations like a pneumatic conveying system but also a fundamental study of solid liquid flows. The coarse grain model is expected to be used in various powder systems. Therefore, in the current study, the coarse grain model was applied to a two-dimensional bubbling fluidized bed. The results by the coarse grain model were in good agreement with that of the original system. The coarse grain model was shown to be available in the bubbling fluidized bed. Moreover, it is exhibited that the calculation speed with the coarse grain model is much faster than that of the original one.

Large-scale discrete element modeling in pneumatic conveying

Discrete element method (DEM) is an effective approach to evaluate granular flows, whereas it is hardly used in investigating the design and the operational conditions in industries. This is due to the fact that the number of calculated particles is restricted by the limit of computer memories. In this study, a coarse grain model for large-scale DEM simulations is proposed, where a modeled particle whose size is larger than the original particle is used instead of a crowd of original particles. The coarse grain model is applied to a three-dimensional plug flow in a horizontal pipeline. The plug length, the cycle and the stationary layer area occupation are compared between the coarse grain particle system and the original particle one. The results show that the coarse grain model can simulate the original particle behavior adequately.

電子写真システム設計のための粉体シミュレーション

Powder simulation which supports the design of electro photographic system is described. The simulation of the charging behavior of toner particles is performed by using the charge transfer equation obtained by impact experiments between plastic spheres and plastic or metal plate. This simulation method, which gives the relation between the charging properties and the chemical components of toner particles, can be applied to the design of toner particles with the optimum charging behavior.The development and flow behavior of a two-component developer are simulated by large scale discrete element method taking into account the magnetic and electrostatic interactions between developing particles. The proposed simulation gives very useful information for the design of a two-component development system. New simulation methods of melting behavior of toner particles in fixing process and blade cleaning behavior are proposed.

Modeling of Solid Particle Flow in Blast Furnace Considering Actual Operation by Large-scale Discrete Element Method

The objective of this paper is to analyze the solid particle flow in a blast furnace having bell-type charging system by using large-scale Discrete Element Method (DEM). About 500000 particles were calculated in this work. The particle discharging behavior of laboratory-scale blast furnace was compared to confirm the material properties used in the simulation work, and the simulated trajectories of tracer particle correlated with those of experimental very well. The melting behavior of iron ore and combustion of coke in the actual blast furnace were modeled by shrinking particles. The simplified bell-type charging system in this simulation mimicked the actual blast furnace, the collapse of coke layer at the top was observed, and the time change of stock level was quite similar. The particle pulsating flow was observed at the upper area of blast furnace, and the descending velocity near side wall was much larger than that of center in this calculation. The melting position of ore was mapped and the most of iron ore were melting above the raceway. This area should be cohesive zone. This modeling is the first step of the analysis of blast furnace by using Discrete Element Method. Although only contact force was considered and simplified melting zone or raceway were introduced, this large-scale simulation has a high potential to analyze the solid flow in the blast furnace, and the abnormal phenomena or serious problem in blast furnace operation will be analyzed in the future by considering gas flow or heat transfer.

Estimation of developed toner mass in two-component electrophotographic system by large-scale Discrete Element Method

The developing behavior of toner particles in a twocomponent development system was simulated to find out the correlation between experimental and simulated results by using large-scale Discrete Element Method (DEM). The effects of developing bias frequency or circumferential speed of photoreceptor on the developed toner mass per area (DMA) was discussed both in experimental and simulation works. The toners were adhered on the outside of latent images under the low frequency, and its mass decreased with increasing the frequency. DMA decreased with increasing circumferential speed of photoreceptor, because the toners had less chances to contact the latent image. The image was also damaged by the magnetic brush under the high speed due to an increase of the shear force. The similar tendency was observed under all conditions, and it was found that the simulation results correlate with experimental ones very well. Therefore, this simulation method is useful for the analysis of the developing behavior in electrophotographic system and the optimization or designing of this system will be possible.

Cell optimization for fast contact detection in the discrete element method algorithm

The objective of this paper is to find out a rule of cell optimization for fast contact detection in the discrete element method (DEM) algorithm. The contact detection process was executed by using the conventional cell model or distinct cell model (DCM) under several particle conditions having different particle size ratio and volume ratio, and its time was measured. The conventional cell model is suitable for the conditions that particle size ratio, d large/ d small, is small and its volume ratio, V small/ V 0, is also small. The optimum cell condition of the conventional cell model is that the mean number of particle in a cell is 1.0–5.0, regardless of the particle size ratio and its volume ratio. Contact detection using DCM, which has several grids for different sized particles, is much faster than that of the conventional model when particles have a large size ratio. The size of the larger cell does not affect the contact detection speed; however, the size of the smaller cell is important for the fast contact detection and its size is determined to be about 3.0 times the particle radius. Therefore, the optimum cell condition is estimated from the particle size ratio and its volume ratio, and a large-scale DEM will be possible.

Analysis of Developing Behavior in Two-Component Development System by Large-Scale Discrete Element Method

The developing behavior of toners (maximum 150000 particles) in a two-component development system was simulated to find out the correlation between experimental results and simulated ones by using the large-scale Discrete Element Method (DEM). The effect of bias voltage, development gap or circumferential speed of a photoreceptor on the developed toner mass per area (DMA) was discussed both in experimental and simulation works. DMA increased with increasing offset voltage due to an increase in the development field, and it decreased with increasing the development gap because of the weakening of the electric field or reduction of a development nip. It also decreased with increasing circumferential speed of the photoreceptor, because the toners had less chances to contact the latent image, and the image was damaged by the magnetic brush under the high speed. The similar tendency was observed both in experimental and simulation works, and the simulation results correlates with experimental ones. Therefore, this simulation method is useful for the analysis of the developing behavior in electrophotographic system and the optimization or designing of this system will be possible.