Discrete Element Method Results Research Articles

A prediction model of maximum dry density of over coarse-grained soil

The control of maximum dry density is an important issue for over coarse-grained soil. A theoretical formula of the maximum dry density for over coarse-grained soil was derived, which is related to the maximum dry density of the original soil, particle density, and mass fraction of giant particles in the mixed soil. Maximum dry densities of seven groups of mixed soil were calculated by the discrete element method (DEM) and theoretical formula, and the results showed that the maximum dry densities obtained from the DEM were always lower than the results calculated by theoretical formula, and the difference increased with the increase in giant particle proportion. The volume growth coefficient was proposed to describe the increase in pore volume of the mixed soil caused by the embedding of giant particles, and the maximum dry density model for over coarse-grained soil was proposed. The results showed that the model fitted well to the DEM results, validating the high rationality and reliability.

Analysis of cohesive particles mixing behavior in a twin-paddle blender: DEM and machine learning applications

This research paper presents a comprehensive discrete element method (DEM) examination of the mixing behaviors exhibited by cohesive particles within a twin-paddle blender. A comparative analysis between the simulation and experimental results revealed a relative error of 3.47%, demonstrating a strong agreement between the results from the experimental tests and the DEM simulation. The main focus centers on systematically exploring how operational parameters, such as impeller rotational speed, blender's fill level, and particle mass ratio, influence the process. The investigation also illustrates the significant influence of the mixing time on the mixing quality. To gain a deeper understanding of the DEM simulation findings, an analytical tool called multivariate polynomial regression in machine learning is employed. This method uncovers significant connections between the DEM results and the operational parameters, providing a more comprehensive insight into their interrelationships. The multivariate polynomial regression model exhibited robust predictive performance, with a mean absolute percentage error of less than 3% for both the training and validation sets, indicating a slight deviation from actual values. The model's precision was confirmed by low mean absolute error values of 0.0144 (80% of the dataset in the training set) and 0.0183 (20% of the dataset in the validation set). The study offers valuable insights into granular mixing behaviors, with implications for enhancing the efficiency and predictability of the mixing processes in various industrial applications.

An experiment-informed discrete element modelling study of knife milling for flexural biomass feedstocks

A discrete element method (DEM) based approach is used to study the relationships between material attributes (MAs), processing parameters (PPs), and quality attributes (QAs) for the knife milling of maize stalks. An approximate DEM shape model was conceptualized based on real maize stalks and calibrated based on experimental bending test data for flexural properties (elastic bending stiffness, elastic bending angle limit, elastoplastic ratio, etc.). DEM simulations of maize stalk comminution in a Jordan Reduction Solutions (“JRS”) knife mill were performed to investigate the relationships between the MAs (maize stalk size and breakage stress limit), PPs (impeller rotational speed), and QAs (mass throughput and output particle size distribution (PSD)). The DEM results suggest that stalk length has little influence on mass throughput and PSD, whilst stalks with larger cross sections tend to generate larger sizes of milled particles given the same breakage stress limit. Both the DEM and experimental results show that faster impeller rotation (or higher power) does not necessarily generate higher throughput or smaller output PSD, especially for maize stalks of higher breakage stress limit. The correlations between these MAs, PPs and QAs are found highly stochastic, though breakage stress limit dictates mass throughput, regardless of stalk size. The DEM-predicted output particle size tended to match the experimental data with coarse PSDs based on sieve size but showed weakened fidelity with finer material, indicating the potential for further model improvement.

Material characterization and inter/intra-particle validation for DEM simulation of urea coating process

Particle coating is a common operation in the controlled-release fertilizer (CRF) industry, where a coating layer is appliedto the urea cores. A discrete element method (DEM) computer simulation was applied to model the coating process for the urea granules using measured model parameters. The accuracy of DEM input parameters for the coating material properties and the physical and mechanical characterizations of urea granules are crucial to the study of particle coating processes. Thus, to enhance the simulation accuracy, information on the material properties (urea granules) is required. In this study, the elasticity parameters, shear modulus, coefficient of restitution (CoR), and coefficient of rolling and static friction (CoRF) and (CoSF) of urea granules are estimated experimentally in addition to the physical characteristics. The sensitivity of the angle of repose of the urea bed was investigated at different coefficients of friction. According to the obtained experimental value of the angle of repose, values of 0.2 and 0.3 for CoRF and CoSF, respectively, were adjusted to obtain consistent urea granules’ motion for simulation and the experiment. Moreover, the spray droplet sizes and velocity distributions were estimated using the video-imaging process technique. Based on this experimental characterization, elasticity parameters and spray properties were integrated into DEM simulation software as input data to perform numerical analysis of the coating process to compare simulation and experimental results, which show uniformity at three different pan speeds with a maximum deviation of 0.033 at 5 rpm. Also, consistency between DEM and experiment results was achieved in terms of average coating thickness (μm) of the selected samples and average intra-particle thickness variation (CoVintra). These results revealed that the coating film thickness is proportional to the rotation speed for the experiment and simulation. To enhance the inter-particle coating uniformity, the effects of the most significant parameters such as pan speed, filling ratio, particle size, spray rate, and spray angle, were examined. A low spray rate, filling ratio, and high pan speed improved the coating uniformity. The particle size and spray angle show considerable influence on the coating uniformity, where larger particle sizes and spray angles tend to reduce the inter-particle coating uniformity.

Uncertainty propagation in discrete element models using PDEM

The Discrete Element Method (DEM) for the analysis of cohesive granular materials, such as concrete, requires the use of contact interaction models with several parameters. Despite the existence of a number of approaches to estimate these parameters, uncertainty in their values is inevitable. This uncertainty propagates across the system, giving rise to uncertainty in the measured response. It is important to estimate this uncertainty and to check and control dispersion in the results in order to improve the predictive ability of the DEM model. This usually requires running a very large number of simulations to effectively sample the space of random parameters and thereby obtain the probability density function (PDF) of the response. Such an approach is not feasible for DEM-based simulations on account of the inordinate computational cost. The present paper provides an alternate approach based on the Probability Density Evolution Method (PDEM). PDEM, which requires fewer sampling points, is employed for the fast and efficient generation of the PDF. The approach is demonstrated through a DEM model developed for the slump test, widely used to determine the workability of fresh concrete. The simulation results are analysed using PDEM, enabling the PDF of the measured response to be determined from the results of a limited number of trials. Moreover, the dispersion in the DEM results is found to be low, with a relatively small coefficient of variation, thereby attesting to the predictive ability of the DEM model.

Dense granular flow through a flat-bottomed silo: Comparison of the DEM and continuum models with experiments

The gravity flow of granular materials through a flat-bottomed rectangular silo is examined using experiments, the discrete element method (DEM), and continuum models. The flow is bounded by four flat frictional side walls, and the mass flow rate is controlled by an exit slot. The velocity and wall stresses are measured simultaneously in a steady fully developed zone which occurs far from the upper free surface and the exit slot, and they are compared with predictions of the DEM and the continuum models. The scaled velocities measured adjacent to the front face are in excellent agreement with the DEM. The centreline velocity is predicted to within 5%–30% by the DEM for larger widths. The normal and shear stresses measured at the mid-plane of one of the walls agree to within 5%–20% for the DEM. The DEM results are less sensitive to the spring stiffness kn, but are more sensitive to the interparticle coefficient of friction μp. There are larger discrepancies in the predictions of the continuum models when the effect of the front and rear walls are not included. The approximate inclusion of the latter improves the prediction of the shear stress and the slip velocity.

Numerical and experimental investigation of flow and heat transfer in a fixed bed of non‐spherical grains using the DEM‐CFD method

AbstractFixed‐bed drying of grains is a widely used method for determining temperature and humidity changes and pressure drop characteristics during aeration. In this study, an accurate particle‐resolved computational fluid dynamics (CFD) numerical model was developed to investigate the flow and heat transfer in fixed beds of soybean, wheat, and maize grains. A randomly filled non‐spherical grain fixed bed structure was generated using the discrete element method (DEM), and the contact areas between the packed particles are automated. The distribution of grain particles near the wall surface was approximately circular, and the circle became more regular with increase grain sphericity. Compared with the fixed beds of soybean and maize, there was no significant stratification in the fixed beds of wheat, and the difference between the peaks and troughs of porosity oscillations was smaller. Detailed particle‐resolved CFD simulations of the flow and heat transfer in fixed beds with different grains were performed. The results showed that the pressure drop results of the DEM–CFD simulation of different grain shapes were in good agreement with the Nemec–Levec equation and experimental results. The velocity and temperature distribution in the fixed bed have obvious non‐uniform distribution characteristics, which are more visible in the maize pile. Regions with high porosity have a higher average velocity, and the temperature near the wall is higher than that at the center when the heat transfer is unbalanced. The DEM‐CFD model includes the heat conduction process between the fluid and solid phases, which is consistent with the experimental temperature results.Practical ApplicationsIn this work, an accurate particle‐resolved computational fluid dynamics numerical model was developed to investigate the flow and heat transfer in fixed beds of soybean, wheat, and maize grains. The randomly filled non‐spherical grain fixed bed structure is generated by the discrete element method (DEM), and the contact areas between the packed particles are automated treated. The pressure drop and heat transfer processes of different grains were measured using the fixed bed drying experimental platform. The results show that the pressure drop results of DEM–CFD simulation of different grain shapes are in good agreement with Nemec–Levec equation results and experimental results. The DEM‐CFD model includes the heat conduction process between fluid and solid phases, which is in close agreement with the experimental temperature results.

A DEM model to evaluate refill strategies of a twin-screw feeder

Residence time distribution (RTD) modeling has proven to be a valuable tool for material tracking in continuous pharmaceutical processes. Refilling is thoroughly studied in the literature, but the main focus lies on the feed rate disturbances. The impact of the feeders themselves on intermixing of different material batches is often overlooked. Since the experimental methods to measure the RTD feeder discharging processes feeder are complex and material intensive, there is only limited experimental RTD data available in the literature. A DEM (discrete element method) simulation of a discharge of a twin-screw feeder shows that a large fraction of material that is moved and intermixed by the agitator. In addition to the intermixing, there is a tendency to discharge material located above the agitator early. In order to predict the behavior during multiple refill events, three models in order of increasing complexity are presented: (1) A simple exponential RTD assuming perfect intermixing of material batches; (2) a RTD model based on DEM results; (3) particle-level material tracking by extrapolation of the DEM results. All three of these models are able to predict the survival function of old material for late refills at low fill levels, however, earlier refills at high fill levels require more complex models to accurately represent the dynamics inside the hopper of the feeder.

Modeling and validation of coarse-grained computational fluid dynamics–discrete element method for dense gas–solid flow simulation in a bubbling fluidized bed

Computational fluid dynamics (CFD) combined with the discrete element method (DEM) are powerful tools for analyzing dense gas–solid flows. However, the computational cost of CFD–DEM will be unfeasibly great when simulating large-scale engineering applications with billions of particles. Accordingly, the coarse-grained (CG) CFD–DEM method is applied to solve this problem. This investigated method replaces several smaller particles with larger ones called parcels, aiming to reduce the number of particles and fully consider the collision of particles between composition parcels and the collision of particles within composition parcels. First, high-speed photography verifies the numerical simulation's reliability. Then, the CG CFD–DEM was used to analyze the transient spatial distribution, transient average velocity, pressure drop, bed height, and the mixing state of particles in a dense gas–solid fluidized bed. The CG CFD–DEM was also compared with the CFD–DEM results, which showed a good agreement with the calculation results and proved the accuracy and applicability of the method. Finally, the computation time of the CG CFD–DEM was evaluated, showing a significant decrease in computation time with an increasing coarse ratio (k). This investigation can provide theoretical reference for the numerical simulation of the CG CFD–DEM method in dense gas–solid flow.

Effects of non-plastic fines on liquefaction properties of saturated silt using discrete element modeling

This paper presents a numerical investigation using the three-dimensional discrete element method (DEM) to evaluate the macro-scale properties of saturated silt in a cyclic triaxial test. The three-dimensional particle model generated based on scanning electron microscopy (SEM) images of silt was imported into the discrete element program PFC3D and a rigid block model with different fines content was established. The effect of non-plastic fines content on the liquefaction properties of silty rigid block model was studied. The DEM results show that: (1) In the initial stage of dynamic stress loading, the axial strain of the sample develops slowly, and the stress-strain hysteresis loops almost overlap. With the increase of the number of cycles, the axial strain fluctuates violently, and the hysteresis loop becomes larger and develops toward the transverse axis. After the initial liquefaction, the dynamic elastic modulus and strength of the sample decrease significantly. (2) When the fines content increases from 0 to 30%, the liquefaction resistance of silt decrease with the increase of fines content. When the fines content increases from 30% to 100%, the liquefaction resistance of silt increases with the increase of fines content. (3) Under the same cyclic stress ratio, the number of cycles required for silt to liquefy decreases initially and then increases with the increase of fines content, and the critical fines content is 30%. The DEM results were compared with experimental results from the laboratory cyclic triaxial tests to provide insights into the underlying mechanism that governs the effects of fines content value on the liquefaction properties of silt.

Prediction of Single Disc Seeding System Forces, Using a Semi-Analytical and Discrete Element Method (DEM)

There is a rising interest amongst Australian farmers to use disc seeders due to their ability to operate in high residue conditions and at higher speeds, commonly in the range of 12 to 15 km h−1. This paper reports on developing an analytical and discrete element method (DEM) force prediction model suited to a rotating flat disc blade operating at different sweep and tilt angles. To validate the models, field experiments were carried out with a flat disc blade at two tilt angles of 0 and 20° and four sweep angles of 6, 26, 45 and 90° in sandy soil. An analytical approach was developed following an experimental investigation that showed that only the forward portion of the disc blade is actively involved in generating soil failure, while the magnitude of this active portion of the soil-disc interface varied with sweep angle. The predicted active proportions correlated well with the experimental observations. As applying different sweep angles affects the direction of soil movement relative to the disc face, the directions of the friction and resultant forces at different sweep and tilt angles were determined. The equation of soil acceleration force was adapted to account for different sweep angles. Results showed that the predicted force fits relatively well with the measured data at 90, 45 and 26° sweep angle, while the low correlation between predicted and measured force at 6° sweep angle was due to the scrubbing reaction force not accounted for in the model. Results also showed that a better coefficient of determination (R2 = 0.93) was obtained between DEM vs. test results compared to the analytical model predictions (R2 = 0.86), particularly for predicting side forces. It was found from the study that both the developed analytical approach and DEM model enabled the prediction of soil forces at different sweep and tilt angles acting on a flat disc blade, which can assist in optimising disc design to lower the specific resistance.

Meshfree simulation of progressive damage in composite laminates using discrete element analysis

This study presents a Discrete Element Method (DEM) for a meshfree progressive failure analysis of IM7/8552 carbon fibre reinforced polymer laminates at the macroscale. By implementing a cohesive contact model to describe progressive damage, experimental results from over-height compact tension (OCT) tests are used to calibrate the required input parameters in the DEM contact model. The simulation of various open-hole tension (OHT) test specimens validates the presented DEM model with minimal errors in strength predictions of up to 4%. Discrete Element Method results are directly compared to those obtained from finite element (FE) models. It is demonstrated that DEM is able to seemingly incorporate damage evolution leading to realistic macroscopic damage patterns, however the computational cost is increased by a factor of 100 compared to FE simulations.

Modelling residual stress and residual work hardening induced by surface mechanical attrition treatment

Shot peening induced residual stress is strongly related to residual work hardening which relies on both the process and the target material. In this work, residual stress and coupled residual work hardening are studied in a surface mechanical attrition treatment (SMAT) on a cylindrical structure by combining discrete element method (DEM) and finite element method (FEM). Shot, ultrasonic parameters, chamber structure, sonotrode surface, and cylindrical structure of the target are finely modeled in DEM simulation. On the basis of the DEM results, FEM simulation of the SMAT process is performed on a representative area considering the factual impact parameters and elastic-plastic behavior of the target material, and mechanism of residual stress generation is analyzed. It shows that isotropic hardening and kinematic hardening of the target material play a different role in residual stress generation. The obtained maximum compressive residual stress is mainly determined by residual yield stress, while the residual kinematic hardening is nearly unchanged when the coverage is increased. Further analyses indicate that the mechanisms of residual stress generation should be very different for the Johnson-Cook model and the model that includes kinematic hardening, even though these two kinds of models are reported to be able to well predict the shot peening induced residual stress. The gradient of the residual work hardening can be used for further evaluating mechanical properties of the SMATed cylindrical structure.

Investigation on the Influencing Factors of K0 of Granular Materials Using Discrete Element Modelling

Earth pressure coefficient at rest K0 is commonly estimated by empirical equations, which to date has had insufficient accuracy and universality. For better prediction, the investigation on the factors influencing K0 is required. A series of discrete element method (DEM) simulations of oedometer tests are conducted to verify the key factors influencing K0 of granular materials. The influences of initial fabric anisotropy, particle shape, initial void ratio, inter-particle friction angle is investigated. The evolution of microstructure is monitored during the tests to reveal the relationship between the microstructure evolution and K0 values. The results show that the effect of fabric anisotropy exists but is limited. Particle shape, initial void ratio, and inter-particle friction angle all significantly affect the K0 values alone. According to the DEM results, an attempt is made to propose a more reasonable empirical equation in which K0 is a function of relative density, critical state friction angle, and “shape factor”. This new empirical equation has higher accuracy and can consider the effect of particle shape, inspiring the determination of K0 values in practical engineering.

Evidence of a unique critical fabric surface for granular soils

The critical state of granular soils needs to make proper reference to the fabric that develops at critical state. This study substantialises the concept of critical fabric surface (CFS), which attracts the fabric state of granular soils upon continuous shearing. Numerical experiments using discrete-element method (DEM) modelling are conducted under drained and undrained conditions with varying Lode angles. Fabric tensors are defined based on the normals of all contacts and of the strong force contacts only. Both tensors have their spherical component preserved such that the information of coordination number can be carried. A separate series of low-confining-pressure undrained tests is conducted to probe the fabric states of soils in the post-liquefaction regime. Finally, a single CFS spanning across a wide range of coordination numbers is established based on the DEM results. The CFS concept provides an important reference state for soils sheared to large strains, complementary to the traditionally defined critical state. This provides a new perspective to interpret and model the mechanics of granular soils in both pre- and post-liquefied regimes. The evolution of fabric shows that the normalised strong-contact fabric evolves linearly with the stress ratio even for liquefied or anisotropically consolidated soils.

Experimental and numerical modeling study of rock joint mechanical properties and failure mechanisms under cyclic shear

Cyclic loading is a common concept in geotechnical engineering, which has an important effect on rock joints. To study the mechanical properties and failure mechanisms of rock joints under cyclic loading, a kind of rock joint cyclic shear device with a top-to-top configuration is proposed in this work. Cyclic shear tests of rock joints are carried out with the cyclic shear equipment. With three–dimensional texture scanning technology, the cyclic shear failure mode of the rock joint is analyzed. The variation trends of shear strength and stiffness of the rock joint under the combined action of normal stress, joint roughness, shear rate, and cyclic shear number are studied. The results show that the normal stress and the roughness are positively related to the cyclic shear strength of the rock joint. With the increase of shear rate and number of cycles, the shear strength and stiffness of the rock joints decrease gradually. In the first cycle of the shear process, the peak shear strength and stiffness of the rock joints reach the maximum values. After the second cycle, the peak shear strength and stiffness of the rock joints decrease slightly and tend toward a constant value under the same normal stress. Observation of the sheared rock joint surface reveals that sliding and shearing damage to the protrusions occurred during the cyclic shear process. Under the combined action of the normal stress, joint roughness, shear rate, and cyclic shear number, the protrusions would slide while moving up to a different height until they are eventually sheared off. In the next cyclic shear process, the friction and shear damage of the residual protrusions continues to occur. With the increase of the number of cyclic shear processes, the wear on the residual protrusions gradually decreases and stabilizes. Finally, the numerical cyclic shear test of the rock joint is performed with the discrete element method (DEM) code 3DEC. A good comparison is found between laboratory data and DEM results.

Dilated-polyhedron-based DEM analysis of the ice resistance on ship hulls in escort operations in level ice

The ice resistance on ships in escort operations in level ice are investigated using the discrete element method (DEM). A dilated polyhedron—generated by the Minkowski sum of a sphere and a polyhedron—is employed in the DEM; this dilated polyhedron-based DEM (DPDEM) is adopted to simulate the ship–ice interaction, wherein the contact force and bond-failure criterion are considered for the collision and fracture of sea ice, respectively. A three-point bending test was simulated with DPDEM, and a field test was conducted in the Bohai Sea to validate the DEM results. Further, a parametric analysis of flexural strength was conducted to identify the parameters involved in the bond-failure criterion. The ice resistance on icebreakers and cargo ships in level ice are simulated using DPDEM. The simulated ice resistances are compared with the Lindqvist and Riska formulas and the model test, which proves the validity of the DEM simulation. The interaction between ships and level ice is simulated parametrically to investigate the ice resistance on cargo ships with and without the icebreaker escort. Influencing factors such as ship speed, ice thickness, and ship breadth were examined to investigate the ice resistance on the escorted cargo ship. Analysis and change rules of the ice resistance on cargos affected by those factors were given.

Effect of Binder Coatings on the Fracture Behavior of Polymer–Crystal Composite Particles Using the Discrete Element Method

Polymer–crystal composite particles formed by crystals coated with binders are widely used in the fields of medicine, energy, the chemical industry, and civil engineering. Binder content is an important factor in determining the mechanical behavior of composite particles. Therefore, this study aimed to investigate the underlying effect of binder coatings in the fracture micromechanics of polymer–crystal composite particles using the discrete element method (DEM). To achieve this objective, realistic particle and crystal shapes were first obtained and reconstructed based on X-ray micro-computed tomography (μCT) scanning and scanning electron microscope (SEM) images. A series of single particle crushing tests and DEM simulations were conducted on real and reconstructed polymer–crystal composite particles, respectively. Based on the experimental and DEM results, the effect of binder coatings on the crushing strength and crushing patterns of polymer–crystal composite particles was measured. Moreover, the micromechanics of the development and distribution of microcracks was further investigated to reveal the mechanism by which binder coatings affect polymer–crystal composite particles.

Effect of Aggregate Shape Properties on Performance of Porous Asphalt Mixture

Aggregate shape properties can greatly influence the overall performance of asphalt mixtures. In this study, shape-controlled aggregates were fabricated and used in porous asphalt mixture (PAM) to assess the effect of aggregate shape properties on the performance of PAM. Three shapes of the aggregates were fabricated, namely cube, cylinder, and sphere. Two parameters, shape factor and sphericity index, were used to define the aggregate shape properties. Results of discrete-element method (DEM) simulation and laboratory measurements were compared to obtain a better understanding on the effect of aggregate shape on the performance of PAM. The results indicated that aggregate shape properties have notable effect on the overall performance of PAM. Spherical aggregates need only a low compaction effort to achieve the design voids-in-total-mix (VTM). For aggregates with the same shape factor, the overall mechanistic properties of PAM generally decrease with increasing sphericity of the aggregates.

Stress distribution on a soil tillage machine frame segment with a chisel shank simulated using discrete element and finite element methods and validate by experiment

Soil tillage machinery must be highly reliable during its operation. For a limited time following harvest, tillage machines must perform a significant amount of work, and any downtime has a significant negative impact on farm economics. For this reason, manufacturers of agricultural machinery aim to design machines that are both reliable and inexpensive. The development of technologies, especially electronics, has led to completely new possibilities for the design and optimisation of agricultural machinery. During work, a tillage machine may not only be loaded by its own weight, but also by the interaction of any tools interacting with the soil that transmit loads to itsframe. To analyse loads a numerical model of tillage machine was compiled. The stresses acting on the chisel shank during tillage were simulated using a numerical model. The frame of the agricultural machine was also simulated with particular emphasis on the locations where the stresses could be measured. Strain gauges were attached at the same locations in a real-life model, and predicted stress values verified experimentally. A discrete element method (DEM) model was used to create and optimise a comprehensive soil model, and its results were used in a finite element method (FEM) model for a transient analysis. The results from the experimental stress measurements and simulation data from the model showed good agreement. Therefore, the procedures of this study can therefore be used for the design and optimisation of tillage machinery. • Determination of stress on chisel shank and tillage machine frame. • Soil model penetration resistance was modelled using DEM. • Transient task analysis made by ANSYS using FEM. • Calibrated DEM and FEM results were validated by field tests.