Discrete Element Model Simulations Research Articles

USING THE DISCRETE ELEMENT METHOD TO ANALYZE AND CALIBRATE A MODEL FOR THE INTERACTION BETWEEN A PLANTING DEVICE AND SOIL PARTICLES

Dynamic soil behaviour at the contact interface during transplanting makes it difficult to ensure transplanting quality. To solve this problem, the Hertz-Mindlin with bonding contact model was used to calibrate the parameters of soils in Inner Mongolia. Based on the response surface design principle, four-factor and three-level tests were performed using the repose angle as an evaluation index, and the following influence factors were considered: the soil-soil restoration coefficient, the soil-steel restoration coeficient, the soil-steel static friction coefficient and the soil-steel static friction coefficient. A regression model was analysed, and an optimization procedure yielded the following optimum combination of parameters: a soil-soil restoration coefficient of 0.45, a soil-steel restoration coefficient of 0.35, a soil-steel static friction coefficient of 0.85 and a soil-steel rolling friction coefficient of 0.13. This optimal combination was used to simulate the soil at the contact interface. The particle dynamic behaviour and soil particle mass flow were used to analyse the soil dynamic behaviour, showing that the average mass flow during the gradual opening of the duckbilled planter tends to increase over time; when the duckbilled planter gradually leaves soil, the contact interface of soil particles in the corner of the duckbilled planter unit causes a reduction in the fluctuation range of the soil mass flow, which exhibits a wave-like change. After the duckbilled planter has left soil, the contact interface of the soil changes tends to stabilize. The duckbilled planter-soil discrete element simulation model was verified. The results of this study provide a reference for the optimal design of a duckbilled planter structure.

Design, DEM Simulation, and Field Experiments of a Novel Precision Seeder for Dry Direct-Seeded Rice with Film Mulching

Existing devices for dry direct-seeded rice with film mulching in northern China have limitations such as imprecise sowing, unadjustable sowing depth, and seeding device blocking. In this regard, this study proposes a combined seeding method of ‘mini shovel + telescopic pipe’ for dry direct-seeded rice with film mulching. A precision seeder for dry direct-seeded rice with film mulching was developed through theoretical calculations, discrete element modelling (DEM) simulations, and field experiments. The configuration and diameter of the rollers were obtained. Twelve telescopic pipes were evenly distributed on the circumference of the roller, with a contact ratio exceeding one. This ratio reduced the slip rate of the roller effectively. Subsequently, DEM was used to develop a 33 central composite design. The response surface was established with the sowing depth as the response value. According to agronomic requirements, the sowing depth was set to 20 mm. The optimal combination of working parameters was obtained by optimizing the regression equation. The field experiments showed that the performance of the precision seeder for dry direct-seeded rice with film mulching satisfied the requirements of agricultural production, working stably and reliably. The developed device represents a useful solution for dry direct-seeded rice with film mulching.

Bridging the gap between particle-scale forces and continuum modelling of size segregation: application to bedload transport

Abstract

Investigation on asphalt-screed interaction during pre-compaction: Improving paving effect via numerical simulation

Pavement pre-compaction is vital for asphalt pavement construction due to its influence on the quality and service life of asphalt pavement. Conventional methods of evaluating and researching the pavement pre-compaction are based on mixture density with empirical theories, which cannot provide an understanding of asphalt-screed interaction during pre-compaction. An innovative approach is developed in this paper to evaluate pavement pre-compaction through Discrete Element Model (DEM) simulation. The contact model and input parameters were discussed and defined separately based on the real condition of paving compaction. A field test about paving compaction was conducted as well for the model validation, in the test track, the operations of paving machine can be adjusted such as paving speed, paving angle, and paving thickness. In addition, the optimized working operation was recommended in this paper based on the results from DEM simulation as well as filed test. Finally, a method to evaluate the pre-compaction of bulk materials via the average aggregate angular velocity was proposed in this study and validated through the test track, and the index Compaction Increase Ratio (CIR) was defined to compare and describe the quality of pre-compaction.

DEM simulation of the thermo-geomechanical effect of recycled concrete aggregate assemblies in geothermal pavement bases

This paper studies the discrete element modeling (DEM) of the thermal-geomechanical effect of recycled concrete aggregate (RCA) assemblies when harnessing renewable geothermal energy from pavement bases. This research study aims to simulate the behavior of RCA under repeated load triaxial test (RLT) at temperatures varying from 20 °C to 80 °C, to simulate temperature effects when harnessing geothermal energy from pavements. The DEM approach was used to simulate unsaturated soil considering interparticle water meniscus and the variation in its force due to the temperature fluctuation. In this research study, RCA samples with different particle size distributions were simulated under RLT test condition for various temperature conditions. According to the macro-geomechanical results, the resilient modulus (MR), porosity, and accumulated strain of samples showed different trends under varying temperatures. Comparing the micro-geomechanical behavior of the samples indicated that matric suction decreased with increasing temperature. Furthermore, the particle scale matric suction was found to be affected by the size ratio of the contacted particles. It was observed that varying the size ratio between 1 and 4 led to a higher average matric suction. The evolution of coordination number (CN), contact normal and contact normal force indicated that samples with higher sand content experienced decrease in CN and magnitude of contact normal by increasing temperature. Meanwhile, samples with lower sand content tended to have an increased CN and magnitude of contact normal in the direction of anisotropy at increased temperatures. An equation was also proposed to predict MR based on matric suction and the ratio of fine over coarse content, based on the RCA particle sizes.

On the evolution law of a contact normal-based fabric tensor for granular materials

This paper presents a theoretical study on a hybrid fabric evolution law for modelling anisotropic behaviour of granular media. In the hybrid evolution law, the rate of a contact normal-based fabric tensor is related to the rates of both stress ratio tensor and plastic strain. Assumptions and principles that were adopted for the development of the fabric evolution law are presented and discussed at first. Its accuracy is then examined by comparing with discrete element modelling (DEM) results under proportional loading and experimental data under complex loading and unloading processes. It is found that fabric evolution at low stress ratios is closely related to the stress-rate driven term of the hybrid law, while the strain-rate driven term dominates at high stress ratios. The hybrid evolution law satisfies the uniqueness requirement of fabric at the critical state by introducing an ‘attractor’ concept. Overall, fabric evolutions predicted by the hybrid law show a close agreement with DEM simulation results and experimental data.

Surface growth, coagulation and oxidation of soot by a monodisperse population balance model

A monodisperse population balance model (MPBM) is developed here that capitalizes on the rapid attainment of the self-preserving size distribution and asymptotic fractal-like structure of agglomerates by coagulation to simulate their evolution with only three equations. Total agglomerate carbon molar, C, number (N) and area (A) concentrations are tracked. The model accounts for the polydispersity of agglomerates by enhancing their collision frequency by that of their self-preserving size distribution based on the radius of gyration in the free molecular regime. Scaling laws from detailed discrete element modeling (DEM) simulations are used to describe the fractal-like morphology of the agglomerates. The MPBM predicts the evolution of soot fv, N and average mobility and primary particle diameters during surface growth and agglomeration in laminar premixed ethylene flames as well as soot oxidation in a tube reactor within 30% of detailed DEM, sectional population balance simulations and measurements. Thus, when self-preserving size distribution and asymptotic structure of agglomerates are attained, this simple MPBM has unprecedented accuracy and can be readily interfaced with computational fluid dynamic (CFD) to model soot formation in combustion devices or process design and optimization for the synthesis of carbonaceous agglomerate nanoparticles.

Numerical Study of Stability and Connectivity of Vertical Goaf Drainage Holes

During underground longwall coal extraction, overburden strata deformation may result in vertical goaf drainage holes, which are drilled in advance of mining for tail gate gas management, to fail. The performance of these vertical goaf drainage holes is controlled by mine design parameters and local geomechanical properties. This paper investigates the use of advanced 3D finite element modelling and 2D discrete element modelling simulation techniques to understand the fundamentals of vertical goaf drainage hole failure mechanism due to strata shear at a currently operating gassy Australian mine site. Finite element modelling is used to investigate the location of high shear in the overburden strata at the vertical goaf gas drainage hole region during longwall mining and the discrete element modelling is used to examine connectivity from the goaf region to the goaf-gas drainage system.

Discrete Element-Based Optimization Parameters of an Experimental Corn Silage Crushing and Throwing Device

HighlightsA discrete element simulation model was used to improve the performance of a corn silage crushing and throwing device.Feed rate, crushing speed, and dial speed were used as the test factors, and the average cutting force and average energy loss were used as the evaluation indexes in orthogonal testing.The order of significance of the factors was crushing speed > feed rate > dial speed for average cutting force and crushing speed > dial speed > feed rate for average energy loss.Abstract. To improve the performance of a corn silage crushing and throwing device and address the problems of low crushing quality and high power consumption, a discrete element simulation model of a corn silage crushing and throwing device and granular straw was established based on discrete element theory using EDEM, a general-purpose CAE software program designed with modern discrete element model technology to simulate and analyze particle processing and production operations. The average cutting force and average energy loss of the particles were the evaluation indexes, and the influence of feed rate, crushing speed, and dial speed on the evaluation indexes was analyzed using single-factor simulation tests. The order of significance was crushing speed > feed rate > dial speed for the average cutting force and crushing speed > dial speed > feed rate for the average energy loss. Using multi-objective optimization, the optimal combination of feed rate, crushing speed, and dial speed was 3.52 kg s-1, 892.06 rpm, and 1502 rpm, respectively. With the optimal parameters, the average cutting force was 58.20 N and the average energy loss was 0.85 J. To verify the feasibility of the EDEM simulation, field tests were conducted using a trial-produced device, with the acceptability of straw crushing and power consumption as the test indicators. During the field tests, the feed rate, crushing speed, and dial speed were set to 3.52 kg s-1, 890 rpm, and 1500 rpm, respectively. The field tests showed that the acceptability of straw crushing and the power consumption were 93.60% and 6.73 kW·h, respectively, with the optimal parameters, which satisfied the corn silage crushing standard and provides a theoretical and scientific basis for the design and optimization of the device. Keywords: Corn silage, Crushing and throwing device, Discrete element simulation, Motion simulation, Multi-objective optimization method.

Anisotropic nature of the capillary stress tensor

The paper describes a micromechanical approach that explores the anisotropic nature of the capillary stress tensor and its evolution in pendular granular materials via Discrete Element Modeling (DEM) simulations. Dimensionless parameters are used to address the conditions under which the contribution of capillarity (or cohesive interparticle forces) to the stress transmission within a Representative Elementary Volume (REV) is expected to be considerable. From a series of suction-controlled conventional triaxial tests, numerical results show that the significance of the capillary stress and the relative magnitude of its mean to deviatoric components is directly connected to the characteristic particle size and applied stress. In addition, it is shown that the anisotropic character of the capillary stress tensor intensifies with increasing suction. Furthermore, a simple shear test is conducted at constant mean stress to reveal the development of deviatoric capillary stresses in the absence of any change in mean stress, which cannot be captured by the commonly used Bishop’s stress expression.

Study of filtered interphase heat transfer using highly resolved CFD–DEM simulations

AbstractAccurately predicting the complex inhomogeneous heat transfer behavior in gas–solid fluidized beds is of fundamental importance. In this work, we constitute an enhanced filtered interphase heat transfer coefficient (IHTC) closure by systematically filtering the dataset from highly resolved three‐dimensional (3D) computational fluid dynamics–discrete element model simulations. Particularly, effects of several potential filtered variable markers on filtered IHTC predictions are examined by statistical analysis. We reveal the formulated filtered IHTC correction closure manifests a systematic dependence on filtered interphase temperature difference as an additional marker. The proposed closure shows good agreement with the filtered fine‐grid simulation data in an a priori analysis. Moreover, the difference of filtered IHTC corrections deduced from 3D Euler–Euler and Euler–Lagrange simulations is quantified. Finally, the comparative analysis between our proposed filtered IHTC formulation and those in literature is implemented. This work holds a potential to facilitate the development of thermal gas–solid flow modeling.

十字槽开沟器横刀后掠角对土壤扰动的影响

The cross-slot opener is a new type of seeding opener. The sweep angle, which is a key structural parameter of the cross knife, has an important influence on soil disturbance. In this paper, discrete element method and tracer method are used to study the conditions of working speed of 0.83m/s and ditching depth of 130mm, the sweep angles of the transverse knife are 0°, 10°, 20°, 30°, 40°, respectively. The influence of time opener on soil disturbance and working resistance. The simulation analysis of the operation process of the cross groove opener shows that the trenching process under different sweep angles of the transverse knife is basically the same, and the standing knife is the main cause of soil disturbance; both the tracer method and the simulation results show the surface and shallow soil particle disturbance. The proportional and horizontal working resistance first decrease and then increase with the increase of the sweep angle of the cross-slot opener. When the sweep angle of the cross-slot opener is 20°, the micro-disturbance of soil particles and the horizontal work resistance are the least; The relative error of the soil disturbance ratio obtained by the tracer method and the discrete element method are both within 15%, and the relative error of the horizontal working resistance is 6.5%. The established discrete element simulation model can more accurately simulate the soil disturbance process of the cross-slot opener.

Characterizing the intrinsic properties of powder – A combined discrete element analysis and Hall flowmeter testing study

Discrete element modelling (DEM) and experimental characterization were carried out for Hall flowmeter tests, in which Inconel718 powders were poured down through a funnel into a density cup to form a heap. By measuring the flow rate, angle of repose and packing fraction from the experiments, and combining with DEM simulations, we were able to extract the coefficients of friction for the powder-powder and powder-wall interactions. We further studied the effect of powder-powder adhesion and found that the adhesion could greatly affect the flow rate and flow pattern. In particular, by increasing the adhesion, we observed a transition from a continuous to discontinuous discharging flow, and revealed an underlying mechanism for the transition. We also studied the effects of powder size and density cup diameter, and found that at their application size scale, the influences of these parameters on the measured results are negligible. The present work presents a simple approach to study the intrinsic properties of powders, and their influences on flowability and packing property.

Evaluation of internal pore structure of porous asphalt concrete based on laboratory testing and discrete-element modeling

Permeable pavement constructed with porous asphalt concrete has been more widely used due to its environmental significance (reducing stormwater runoff, recuing traffic noise) and traffic safety advantages. Research regarding permeability and pore structural characteristics of PAC is one of the front topics and yet gaps of knowledge still exist in this area. Pore structure is the primary factor in permeability of porous asphalt concrete (PAC) pavement. The composition and distribution of different pore components of PAC and their impact on permeability and influence factors are worth exploring. This paper evaluates pore structure of PAC using both laboratory tests and discrete element modeling (DEM) simulations. PAC specimens with varying nominal maximum aggregate size (NMAS) and varying gradations were prepared and tested in the laboratory to obtain their total void contents and effective void contents. Permeability tests were conducted to evaluate relationship between pore structure and permeability. A relative indicator of percent effective voids was proposed to evaluate void structure of PAC and was proven to be better indicator than absolute void contents. Laboratory test results revealed that t/NMAS, which is lift thickness divided by NMAS shows better correlation with percent effective voids than NMAS. Interfering size (D2.36) aggregate has a negative impact on percent effective voids and the impact is more significant on smaller size mixes. Discrete-element method (DEM) was utilized to further explore the micro-meso structural characteristics of pore structure of PAC. It was revealed that higher interconnected void content creates more open channel for water to flow and is essentially the cause of increasing permeability. Contributions of interconnected voids in terms of creating void volume is significantly higher than dead-end voids and isolated voids.

Normal stress differences in the consolidation of strong colloidal gels

Macroscopic models for the uniaxial consolidation of strong colloidal gels typically characterise the compressive strength of the particulate network in terms of the compressive yield stress Py(ϕ) or uniaxial elastic modulus K ′ (ϕ). Almost all industrial applications involve multi-dimensional (MD) configurations with arbitrary tensorial stress states, and it is unclear how to generalise these 1D constitutive models to MD consolidation. Several studies have attempted to extend these 1D constitutive models to MD by assuming either isotropic consolidation or zero Poisson’s ratio, but the validity of these assumptions is currently unknown. Lacking is a validated tensorial rheology for the consolidation of strong colloidal gels that is capable of predicting the consolidation of these materials. One step toward the development of such tensorial rheology is the consideration of normal stress differences (NSDs) during uniaxial consolidation. Thus, a tensorial constitutive model for the consolidation of colloidal gels cannot be developed without accounting for these NSDs. We address this problem by performing discrete element model (DEM) simulations of the uniaxial consolidation of a two-dimensional (2D) strong colloidal gel and investigate evolution of the tensorial stress state during consolidation. We show that during consolidation, the Poisson ratio increases from zero near the gel point to almost unity near close-packing and uncover the particle-scale mechanisms that underpin these observations. These results provide the first steps toward a complete tensorial rheology of colloidal gels that is capable of resolving the evolution of these complex materials under superposed differential compression and shear. Graphical abstract

Failure processes of cemented granular materials.

The mechanics of cohesive or cemented granular materials is complex, combining the heterogeneous responses of granular media, like force chains, with clearly defined material properties. Here we use a discrete element model simulation, consisting of an assemblage of elastic particles connected by softer but breakable elastic bonds, to explore how this class of material deforms and fails under uniaxial compression. We are particularly interested in the connection between the microscopic interactions among the grains or particles and the macroscopic material response. To this end, the properties of the particles and the stiffness of the bonds are matched to experimental measurements of a cohesive granular medium with tunable elasticity. The criterion for breaking a bond is also based on an explicit Griffith energy balance, with realistic surface energies. By varying the initial volume fraction of the particle assembles we show that this simple model reproduces a wide range of experimental behaviors, both in the elastic limit and beyond it. These include quantitative details of the distinct failure modes of shear-banding, ductile failure, and compaction banding or anticracks, as well as the transitions between these modes. The present work, therefore, provides a unified framework for understanding the failure of porous materials such as sandstone, marble, powder aggregates, snow, and foam.

DEM simulations of polydisperse media: efficient contact detection applied to investigate the quasi-static limit

Discrete element modeling (DEM) of polydisperse granular materials is significantly more computationally expensive than modeling of monodisperse materials as a larger number of particles are required to obtain a representative elementary volume, and standard contact detection algorithms become progressively less efficient with polydispersity. This paper presents modified contact detection and inter-processor communication schemes implemented in LAMMPS which account for particles of different sizes separately, greatly improving efficiency. This new scheme is applied to the inertial number (I), which quantifies the ratio of inertial to confining forces. This has been used to identify the quasi-static limit for shearing of granular materials, which is often taken to be I = 10^{ - 3} . However, the expression for the inertial number contains a particle diameter term and therefore it is unclear how to apply this for polydisperse media. Results of DEM shearing tests on polydisperse granular media are presented in order to determine whether I provides a unique quasi-static limit regardless of polydispersity and which particle diameter term should be used to calculate I . The results show that the commonly used value of I = 10^{ - 3} can successfully locate the quasi-static limit for monodisperse media but not for polydisperse media, for which significant variations of macroscopic stress ratio and microscopic force and contact networks are apparent down to at least I = 10^{ - 6} . The quasi-static limit could not be conclusively determined for the polydisperse samples. Based on these results, the quasi-staticity of polydisperse samples should not be inferred from a low inertial number as currently formulated, irrespective of the particle diameter used in its calculation.

Granular packings with sliding, rolling, and twisting friction.

Intuition tells us that a rolling or spinning sphere will eventually stop due to the presence of friction and other dissipative interactions. The resistance to rolling and spinning or twisting torque that stops a sphere also changes the microstructure of a granular packing of frictional spheres by increasing the number of constraints on the degrees of freedom of motion. We perform discrete element modeling simulations to construct sphere packings implementing a range of frictional constraints under a pressure-controlled protocol. Mechanically stable packings are achievable at volume fractions and average coordination numbers as low as 0.53 and 2.5, respectively, when the particles experience high resistance to sliding, rolling, and twisting. Only when the particle model includes rolling and twisting friction were experimental volume fractions reproduced.

Singular behavior of the stresses in the limit of random close packing in collisional, simple shearing flows of frictionless spheres

As random close packing is approached in a granular medium of frictionless spheres, the pressure, shear stress, and second normal stress become singular with exponents 5/2, 5/2, and 7/4, as predicted by a kinetic theory and confirmed by prior discrete element modeling simulations.

Discrete-element modelling of the trapdoor arching effect in sand and rubberised sand

The onset and evolution of soil arching in a trapdoor apparatus was investigated using discrete-element modelling (DEM). The effects of wall friction, particle size and material type (sand and rubber–sand) on the micro-behaviour of the granular material during arching was monitored. It was observed that when the particles are large with respect to the trapdoor width, they form a stable arch over the trapdoor element. An ‘intermediate zone’ within the granular materials is then introduced. This is the zone where inter-particle contact forces increase in the early stage of active arching and then decrease as the trapdoor displacement continues. The DEM simulation results showed that the granular particles rearranged in reaction to arching, causing uneven densities throughout the backfills. The distributions of two-dimensional stresses in the cross-section of the modelled sand and rubber–sand backfills were obtained using the kriging method and compared.