Discrete Element Method Approach Research Articles

Investigating the effect of flow direction on suffusion and its impacts on gap-graded granular soils

The flow direction is generally different from the gravity direction in geotechnical structures or slopes, the effect of which during suffusion remains unclear. This paper presents a coupled computational fluid dynamics and discrete element method approach to simulate the particle–fluid interaction relevant to this problem. The CFD-DEM approach is first benchmarked by a classic granular system problem, which is then used to investigate the characteristics of suffusion and its impact on the mechanical behavior. Five different angles between gravity and seepage directions for gap-graded soils with two fines contents are examined. Both the macroscopic and microscopic characteristics during suffusion and triaxial loading tests are analyzed. The direction angle is found to play a significant role affecting the erosion process and the mechanical consequence of soils. The results show that the greater the angle is, the harder it is for suffusion to occur and continue.

CFD-DEM simulation of pneumatic conveying in a horizontal pipe

In this paper, the combined approach of computational fluid dynamics (CFD) and discrete element method (DEM) is used to study pneumatic conveying in a horizontal pipe. A new three-dimensional modified wall roughness (WR) model and the discrete random walk (DRW) model are introduced into the CFD-DEM approach to simulate the effect of wall roughness and the random effect of turbulence on particle dispersion respectively. The interaction between particles and fluid of the pneumatic conveying in a horizontal pipe is calculated by both the one-way coupling method and the two-way coupling method. The flow characteristics of the pneumatic conveying in a horizontal pipe, including the particle velocity, the particle mass flux, and the particle trajectory are obtained by the simulations. The particle mass flux predicted by the CFD-DEM with WR-DRW is quite different with that predicted by the traditional CFD-DEM approach although the particle horizontal velocities obtained by two different approaches are similar, and the distribution of particles across the pipe cross section predicted by the CFD-DEM with WR-DRW is more uniform than that predicted by the traditional CFD-DEM approach. The accuracy of the CFD-DEM with WR-DRW is validated by the corresponding experiments from literatures. Comparing to the computational fluid dynamics - discrete phase model (CFD-DPM) without considering collision between particles, the CFD-DEM with WR-DRW developed in this work can be applied to study the pneumatic conveying in a horizontal pipe under both dilute and dense phase conditions.

Numerical investigation of mixed layer effect on permeability in a dynamic blast furnace

AbstractGas flow distribution in a blast furnace (BF) plays a significant role in BF smooth operation, productivity, and thermal efficiency. It is affected by the distribution of burden materials composed of alternating coke and ferrous ore layers. While moving downward coke and ferrous layers of different sizes can mix and form the so‐called mixed layers. Generally, the porosity is lower and hence the pressure drop is higher in the mixed layers. These variations can change the gas flow distribution and BF performance. Previous work tried to quantify the effect of material properties and process conditions on the formation of mixed layers and the resulting local porosity variation. However, these studies were often conducted under simplified conditions. Few were dedicated to the formation of mixed layers in a BF and its effect on BF performance. This work studies the formation of mixed layers in an experimental BF by using a combined computational fluid dynamics and discrete element method approach. The effect on BF performance was evaluated under different operational conditions including different size ratios of coke to iron ore particles, burden distribution, batch weight, and discharge rate. The results obtained were helpful to optimize burden charge for improving BF performance.

A fluid dynamic study of the active phase behavior in a rotary drum with flights of two and three segments

In the rotary dryers with flights, the drying occurs mainly in the active region where the material comes into direct contact with the air stream. The design of these flights is a complex task and studies are necessary to gain an in-depth understanding of the particle dynamics. This work aims to investigate the effects caused by angulations in flights of two and three segments on the particle dynamics, especially in the active region. The prediction of the solids flow was investigated by simulation using a Discrete Element Method (DEM) approach in an open-source software LIGGGHTS®. The simulated results were compared with experimental data using glass beads, with the rotary drum operating with 12 flights. The active region characteristics were analyzed using the responses variables: mass of solids in active phase, percentage of active region area occupied by the particles and the dispersion of the particles in the active region.

DEM Analysis of Track Ballast for Track Ballast–Wheel Interaction Simulation

This study aims to suggest a rational analysis method for a track ballast–wheel interaction that could be further developed to model the interaction in a train-derailment event, based on the discrete-element method (DEM). Track ballast is filled with gravel to form the trackbed. Although finite-element analysis (FEA) is widely applied in structural analysis, track ballast cannot be analyzed using conventional FEA because this approach does not allow separation of elements that share nodes. The DEM has been developed to analyze the dynamic behavior of separable objects, assuming that the objects are rigid. Therefore, track ballast can be modeled as separable rigid pieces of gravel, and its dynamic behavior can be analyzed using a rational contact model. In this study, a rational numerical strategy for track ballast–wheel interaction was investigated using the DEM approach. The suggested analysis method was validated through comparison with the experimental results of a drop test. In addition, case studies were conducted to investigate the effects of the contact-model parameters on the simulation result.

Size effect in heterogeneous materials analyzed through a lattice discrete element method approach

In the Lattice Discrete Element Method (LDEM), different types of mass are considered to be lumped at nodal points and linked by means of one-dimensional elements with arbitrary constitutive relations. In previous studies on the tensile fracture behavior of rock samples, it was verified that numerical predictions of fracture of non-homogeneous materials using LDEM models are feasible and yield results that are consistent with the experimental evidence available so far. In the present paper, a discussion of the results obtained with the LDEM is presented. A set of rock specimens of different sizes, subjected to monotonically increasing simple tensions, are simulated with LDEM. The results were analyzed from the perspective of the brittleness number, proposed by Alberto Carpinteri, to measure the brittleness level of the structure under study. The satisfactory correlation between the experimental results and LDEM results confirms the robustness of this method as a numerical tool to model fracture processes in quasi-brittle materials.

Heat transfer in a pulsed fluidized bed by using coupled CFD-DEM method

Heat transfer behaviors in a pulsed fluidized bed are numerically investigated by means of a combined approach of computational fluid dynamics (CFD) and discrete element method (DEM). The effect of pulsation flow on bed heat transfer characteristics is evaluated. It is found that the pulsed gas flow leads to an intense oscillating flow behavior in the bed, enhancing the the cooling rate of hot particles. Three kinds of heat transfer mechanisms are examined. The results reveal that the particle-fluid convection is dominated. Meanwhile, heat transfer mechanisms in a polydisperse system are also discussed. It is observed that the addition of fine particles promotes the total particle-fluid convection and the fluctuating degree with time becomes evident.

Effect of horizontal vibrations on mass flow rate and segregation during hopper discharge: discrete element method approach

Vibration is often utilized as a means of initiating and/or controlling flow from hopper in industries dealing with powder/granular materials such as packing, conveying, etc. The effect of horizontal vibration on gravity flow of granular material from conical hopper is modeled using the discrete element method. Material considered in this study includes glass beads of diameter ranging between 0.7 mm and 2.0 mm. The flow dynamics and segregation of material are analyzed for different mixtures characterized based on mass percentage of smaller particles (fines) and multi-component mixtures (binary and ternary) at different vibration parameters. The study includes the influence of vibration frequency, acceleration amplitude, fines percentage, diameter ratio and mixture components on segregation and mass flow rate during vibratory hopper discharge. The extent of segregation is calculated by means of mass fraction of fines inside hopper for different operating conditions. The numerical results indicate that the increase in vibration acceleration at a fixed frequency results into increased mass flow rate and there exists acceleration amplitude beyond which segregation is predominant. Mixture components play significant role in segregation behaviour and binary mixture suffers more segregation as compared to ternary mixture. The spatial distribution of the velocity profile indicates that different mixtures behave differently at a particular vibration conditions. The phenomena like sieving or percolation are also observed based on the analysis of top view simulation snap shots.

Multi-physics continuum modelling approaches for metal powder additive manufacturing: a review

PurposeThis paper aims to present a systematic approach in the literature survey related to metal additive manufacturing (AM) processes and its multi-physics continuum modelling approach for its better understanding.Design/methodology/approachA systematic review of the literature available in the area of continuum modelling practices adopted for the powder bed fusion (PBF) AM processes for the deposition of powder layer over the substrate along with quantification of residual stress and distortion. Discrete element method (DEM) and finite element method (FEM) approaches have been reviewed for the deposition of powder layer and thermo-mechanical modelling, respectively. Further, thermo-mechanical modelling adopted for the PBF AM process have been discussed in detail with its constituents. Finally, on the basis of prediction through thermo-mechanical models and experimental validation, distortion mitigation/minimisation techniques applied in PBF AM processes have been reviewed to provide a future direction in the field.FindingsThe findings of this paper are the future directions for the implementation and modification of the continuum modelling approaches applied to PBF AM processes. On the basis of the extensive review in the domain, gaps are recommended for future work for the betterment of modelling approach.Research limitations/implicationsThis paper is limited to review only the modelling approach adopted by the PBF AM processes, i.e. modelling techniques (DEM approach) used for the deposition of powder layer and macro-models at process scale for the prediction of residual stress and distortion in the component. Modelling of microstructure and grain growth has not been included in this paper.Originality/valueThis paper presents an extensive review of the FEM approach adopted for the prediction of residual stress and distortion in the PBF AM processes which sets the platform for the development of distortion mitigation techniques. An extensive review of distortion mitigation techniques has been presented in the last section of the paper, which has not been reviewed yet.

Capturing heat transfer for complex-shaped multibody contact problems, a new FDEM approach

This paper presents a new approach for the modelling of heat transfer in 3D discrete particle systems. Using a combined finite–discrete element (FDEM) method, the surface of contact is numerically computed when two discrete meshes of two solids experience a small overlap. Incoming heat flux and heat conduction inside and between solid bodies are linked. In traditional FEM (finite element method) or DEM (discrete element method) approaches, to model heat transfer across contacting bodies, the surface of contact is not directly reconstructed. The approach adopted here uses the number of surface elements from the penetrating boundary meshes to form a polygon of the intersection, resulting in a significant decrease in the mesh dependency of the method. Moreover, this new method is suitable for any sizes or shapes making up the particle system, and heat distribution across particles is an inherent feature of the model. This FDEM approach is validated against two models: a FEM model and a DEM pipe network model. In addition, a multi-particle heat transfer contact problem of complex-shaped particles is presented.

DECM: A Discrete Element for Multiscale Modeling of Composite Materials Using the Cell Method.

This paper presents a new numerical method for multiscale modeling of composite materials. The new numerical model, called DECM, consists of a DEM (Discrete Element Method) approach of the Cell Method (CM) and combines the main features of both the DEM and the CM. In particular, it offers the same degree of detail as the CM, on the microscale, and manages the discrete elements individually such as the DEM—allowing finite displacements and rotations—on the macroscale. Moreover, the DECM is able to activate crack propagation until complete detachment and automatically recognizes new contacts. Unlike other DEM approaches for modeling failure mechanisms in continuous media, the DECM does not require prior knowledge of the failure position. Furthermore, the DECM solves the problems in the space domain directly. Therefore, it does not require any dynamic relaxation techniques to obtain the static solution. For the sake of example, the paper shows the results offered by the DECM for axial and shear loading of a composite two-dimensional domain with periodic round inclusions. The paper also offers some insights into how the inclusions modify the stress field in composite continua.

Sensitivity Analysis of Semi-Circular Bending Test using Plackett–Burman Matrix

The semi-circular bending (SCB) test is a recently developed test method, adopted by the American Society of Testing and Material (ASTM) as ASTM D8044, to evaluate the cracking resistance of asphalt mixtures. To measure the robustness of the SCB test, a ruggedness test is needed. In this study, the effect of small changes in the key parameters of the test method on the results of the test were numerically investigated. The test method has many variables and set up conditions. Examining all of these parameters would be cumbersome using traditional testing and statistical techniques, as they require a significantly high number of samples. The Plackett–Burman (PB) technique was used to conduct the ruggedness test while reducing the number of tests required. Seven parameters have been examined: notch location, notch depth (low, intermediate and high), air voids, loading rate, and span length. Even using the PB technique, 16 scenarios need to be tested, and each scenario requires three specimens, one for each of the three notch depths. Hence the process requires plenty of time and material. In this study, the SCB test was modeled using a discrete element method (DEM) approach to analyze the fracture behavior of the samples. DEM was used to develop a model that reduced the time and materials required for the SCB test. Results showed that the parameters with most positive effect were intermediate notch depth and notch location, while those with the most negative effect were loading rates and air voids.

Combination of an adaptive remeshing technique with a coupled FEM–DEM approach for analysis of crack propagation problems

This paper presents an enhanced coupled approach between the finite element method (FEM) and the discrete element method (DEM) in which an adaptive remeshing technique has been implemented. The remeshing technique is based on the computation of the Hessian of a selected nodal variable, i.e. the mesh is refined where the curvature of the variable field is greater. Once the Hessian is known, a metric tensor is defined node-wise that serves as input data for the remesher (MmgTools) that creates a new mesh. After remeshing, the mapping of the internal variables and the nodal values is performed and a regeneration of the discrete elements on the crack faces of the new mesh is carried out. Several examples of fracturing problems using the enhanced FEM–DEM formulation are presented. Accurate results in comparison with analytical and experimental solutions are obtained.

Comparison of different drag models in CFD-DEM simulations of spouted beds

Spouted beds are commonly simulated through the Computational Fluid Dynamics – Discrete Element Method approach. The choice of the drag model is still a matter of debate, as they feature peculiar operative conditions. In this work, we simulated two spouted beds containing Geldart-D particles. We tested seven drag models: three are classic models, while four are developed through advanced computational techniques. The results indicate that the key variable is the ratio between the operative and the minimum spouting gas velocity (u/ums). At u = ums only the Gidaspow model can always predict fluidisation, but at low u/ums values the Beetstra model is the best compromise. For higher values, the Rong and Di Felice models behave better, while the others overestimate the particles' velocity. These results can be useful to identify the best performing model and show there is a need for more appropriate models for spouted beds.

CFD-DEM study of air entrainment in falling particle plumes

Dust emission due to air entrainment in falling solids/particle plumes is a common and serious environmental issue in industry. The prediction of the flowrate of air induced by falling particles is pivotal for the design of dust removal equipment in practice but until now there is still limited reliable correlations available. In this work, the process of air entrainment in falling plumes of particles is numerically studied by a combined approach of Computational Fluid Dynamics (CFD) and Discrete Element Method (DEM) (CFD-DEM). A 2D CFD-DEM model is first compared against two 3D models. Then the 2D model is validated against experiments qualitatively at small scale and quantitatively at large scale. Finally the model is used to predict the effect of various important variables including drop height and solids mass flowrate at an industrial scale.

Numerical simulation of local scour around two pipelines in tandem using CFD–DEM method

In the field of offshore oil and gas engineering, the arrangement of multiple pipelines are becoming more common, the spacing between the pipelines and the incoming stream velocity will significantly affect the scouring process around the pipelines. In this study, the effect of space ratio (G/D) and the stream velocity on the scouring process around two pipelines in tandem are investigated using the coupled approach of computational fluid dynamics (CFD) and discrete element method (DEM). Here G is the spacing between the pipelines and D is the diameter of the pipeline. Specifically, the effect of space ratio and the stream velocity are discussed by simulating the gap ratio (G/D) between two pipelines ranging from 1 to 3 with an interval of 1, under the stream velocity U = 0.5,1 and 2 m/s, The results indicate that when G/D ≤ 2, the equilibrium scour depth below the upstream pipeline (S1) is slightly larger than that under the downstream pipeline (S2), S1 and S2 slightly increase as the gap ratio increases. Whereas for G/D > 2, the equilibrium scour depth beneath the upstream pipeline is slightly smaller than that under the downstream pipeline, S1 and S2 slightly decrease as the gap ratio increases. Furthermore, the scour depths are highly dependent on and positively related to the incoming stream velocity, the equilibrium bed profiles are similar under the same incident stream velocity with different gap ratios.

Structural signature of binary sphere mixtures under air impact

Packings of binary spheres formed under air impact were simulated using the coupled computational fluid dynamics (CFD) and discrete element method (DEM) approach. The effects of particle size ratio (PSR) on structural properties of the packings before and after air impact were analysed. The results showed that the air impact condensed the packings. The second peak in the RDF curves became more apparent and the third peak gradually disappeared in the final packings with increasing PSR. The number of contacts between large spheres decreased, while the contacts between large and small spheres increased under the effect of air impact. The orientation of the contacts tended to be more directional for the final packings. Radical tessellation analyses indicated that for both initial and final packings, two peaks were always formed in the distributions of face number per cell, face area and relative volume, while only one peak appeared in the distributions of edge number and perimeter. Furthermore, the average metrical properties for large particles hardly changed with PSR in the final packings, while those for small or all particles decreased after air impact.

Multiscale strength reduction method for heterogeneous slope using hierarchical FEM/DEM modeling

A novel hierarchical multiscale strength reduction method with a coupled finite element method (FEM) and discrete element method (DEM) approach is proposed for a heterogeneous slope. This approach is implemented and validated by employing a soil and rock mixture (SRM) slope. Compared with conventional strength reduction method, this method avoids the expensive cost of large field tests in parameters estimation and provides a better understanding of the failure mechanism with macroscopic and microscopic results. The multiscale simulation approach, which is directly based on particle-level simulation and bypasses conventional constitutive assumptions, provides a novel numerical tool for slope stability analysis of heterogeneous geomaterials.

New discrete element framework for modelling asphalt compaction

During asphalt mixture compaction, loads in the material are mainly transferred through contact between the stones and the interaction between the stones and the binder. The behaviour of such materials is suitable to model using the Discrete Element Method (DEM). In this study, a new DEM modelling approach has been developed for studying the asphalt compaction process, incorporating contact and damage laws based on granular mechanics. In the simulations, aggregate fracture is handled by a recently developed method of incorporating particle fracture in DEM, based on previously performed fracture experiments on granite specimens. The binder phase is modelled by adding a viscoelastic film around each DEM particle. This surface layer has a thickness that obtains the correct volume of the binder phase and has mechanical properties representative for the binder at different temperatures. The ability of the model to capture the influence of mixture parameters on the compactability and the eventual stone damage during compaction is examined for the cases of compaction flow test and gyratory compaction. Explicitly, the influence of different aggregate gradations, mixture temperatures and binder properties are studied. The results show that the proposed DEM approach is able to capture qualitatively and quantitatively responses in both cases and also provide predictions of aggregate damage. One large benefit with the developed modelling approach is that the influence of different asphalt mixture parameters could be studied without re-calibration of model parameters. Furthermore, based on comparative DEM simulations, it is shown that the proposed approach provides more realistic force distribution networks in the material.

CFD–DEM simulation of pneumatic conveying in a horizontal channel

In this paper, the pneumatic conveying in a horizontal channel is simulated by the combined approach of computational fluid dynamics (CFD) and discrete element method (DEM). A modified wall roughness model and the Discrete Random Walk (DRW) model are introduced into the CFD–DEM approach to investigate the effect of the wall roughness and the random effect of the turbulence on particle dispersion respectively, and this combined approach is named as the improved CFD–DEM approach. The interaction between the particles and the fluid is calculated by both the one-way coupling method and the two-way coupling method. The accuracy of the improved CFD–DEM approach is validated by corresponding experiments from the literatures. By using the improved CFD–DEM approach, several dense channel gas–solid flows are simulated. The simulation results indicate that with the increase of mass loading rate, the particle agglomeration is more significant, and the trend of increasing the particle concentration near the upper and lower walls is more obvious. This research shows that the improved CFD–DEM approach developed in this work is an effective tool for studying the particle phase properties under both dilute and dense phase conditions.