Discrete element modeling (DEM) of triboelectrically charged particles: Revised experiments

Abstract. Grain augers are primary grain conveying equipment in agriculture. Quantitative prediction of dynamic grain flow in grain augers using discrete element modeling (DEM) has potential to support simulation-based engineering design of grain handling equipment. The objective of this study was to develop a DEM corn model using a multi-response calibration methodology and validation of combine-harvested corn flow in a commercial grain auger. Using a Latin hypercube design of experiment (DOE) sampling from four particle interaction DEM parameters values, 27 DEM simulations were generated for four DEM corn shape approximations (1-sphere, 2-spheres, 5-spheres, and 13-spheres) to create virtual DEM experiments of bucket-discharged and anchor-lifted angle of repose (AOR) tests. A surface meta-model was developed using the DEM interaction parameters as predictor variables, and normalized AOR expressed as a mean square error (MSE), i.e., the sum of square differences between DEM simulations and laboratory-measured AOR. Analysis of the MSE percentiles with lower error differences between DEM simulations and laboratory AOR and the computational effort required per simulation (h per simulation) showed that the 2-spheres DEM model had better performance than the 1-sphere, 5-spheres, and 13-spheres models. Using the best stepwise linear regression models of bucket AOR MSE (R2 of 0.9423 and RMSE of 94.56) and anchor AOR MSE (R2 of 0.5412 and RMSE of 78.02) and a surface profiler optimization technique, an optimized 2-spheres DEM corn model was generated. The DEM predicted AOR with relative errors of 8.5% for bucket AOR and 7.0% for anchor AOR. A DEM grain auger simulation used as a validation step also showed good agreement with the laboratory-measured steady-state mass flow rate (kg s-1) and static AOR (degrees) of corn piled on a flat surface, with DEM prediction relative error ranging from 2.8% to 9.6% and from 8.55% to 1.26%, respectively. Keywords: Corn, DEM, Discharge angle of repose, Discrete element modeling, Grain auger, Lift angle of repose.

Discrete element modeling of acoustic emission in rock fracture

In order to accurately establish a discrete element model for the whole plant flexibility of upright rice during the harvesting period, several physical characteristics, such as geometric features, moisture content, and density, of the entire rice plant were measured, along with frictional properties, such as the static and rolling friction coefficients, and mechanical properties, including the elastic modulus and restitution coefficient. A flexible and upright discrete element model of the rice plant was established using the DEM method based on the Hertz–Mindlin (no slip) and Hertz–Mindlin with bonding mechanical models. The parameters were optimized through Plackett–Burman screening experiments, steepest ascent experiments, and Box–Behnken optimization experiments to accurately determine the discrete element model parameters of each component of the rice plant. The calibration process of the contact parameters between rice grains and steel was analyzed in detail as an example, resulting in a calibration error of 0.68% for the natural repose angle. Taking the calibration of the contact parameters between the main stem and steel as an example, a detailed analysis of the calibration process was conducted. The calibration resulted in a calibration error of 2.76% for the natural repose angle and 2.33% for deflection. This study lays the foundation for understanding the mechanical response of rice and machinery when they are coupled together. Additionally, it provides valuable references for establishing discrete element models of plant species other than rice.

Model of calf muscle tear during a simulated eccentric contraction, comparison between ex-vivo experiments and discrete element model

In this work the behavior of masonry arches, without reinforcement and with partial reinforcement, is investigated by means of three different numerical models. The first one is a Discrete Element model based on rigid blocks, and elastic-plastic interfaces; the second one is a standard heterogeneous Finite Element Model, which is adopted for a detailed micro-modelling of arch voussoirs, joints, and reinforcements. The third model is analytic-numerical, and it is adopted for validating the other numerical results. The aim of the work is the comparison and validation of the numerical Finite and Discrete Element models for the correct simulation of masonry arch behavior, together with the evaluation of the effectiveness of these models in simulating the behavior of the partially reinforced arch.

Discrete Element Modeling for fouled railroad ballast

The development of commercially available discrete element modeling software packages such as Particle Flow Code combined with the improved computational power of desktop PCs have created opportunities to apply discrete element models to the modeling of oil sand behaviour. Discrete element models can simulate micro-mechanical phenomena and can provide fundamental insight into the physical behaviour of oil sand that cannot be captured in finite element or boundary element models. Particle Flow Code (PFC) is being used to study the geomechanical behaviour of oil sands. This paper presents results from initial research conducted to establish the input parameters for PFC to simulate triaxial tests on reconstituted oil sands.

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

A method combining experimental and simulation optimization was used to calibrate parameters to enhance the accuracy of discrete element model parameters during kiwifruit stem separation. First, physical experiments were conducted to determine the intrinsic and contact parameters of kiwifruit stalk. Second, the mechanical parameters of the kiwifruit stalks were determined using three-point bending and shear tests. On this basis, simulation tests were conducted on kiwifruit stalks by combining the Hertz-Mindlin model with a bonding model, and the optimal combination of bonding parameters was confirmed using the bending strength and maximum shear force. Finally, a discrete element model of the kiwifruit was built with the determined bonding parameters and simulated, and the reliability of the model was verified through mechanical tests. The results showed that the density was 867.5 kg/m3, Poisson's ratio was 0.26, the modulus of elasticity was 3.25 × 108 Pa, the recovery coefficient between the fruit stalks and steel parts was 0.365, and the average values of the static and dynamic friction coefficients between the kiwifruit stalks and steel parts were 0.268 and 0.152, respectively. The kiwifruit stem bonding parameters were normal stiffness per unit area kn=7.201×1011 N/m3, shear stiffness per unit area kt=2.379×1011 N/m3, critical normal stress σmax=5.937×108 Pa, critical shear stress tmax = 2.354×109 Pa, and bonded disc radius Rj=0.164 mm. Compared with the results of the mechanical tests, the relative errors of the bending strength and maximum shear of the discrete element model were 2.13% and 2.84%, respectively. The results showed that the discrete element model improves the simulation of the bending and shearing processes of kiwifruit stalks and is capable of characterizing the physical properties of kiwifruit stalks. The results of this study provide a theoretical foundation for the optimal design of end effectors.

Discrete element modeling is beingused increasingly to simulateflow in fluidized beds. These models require complex measurement techniquesto provide validation for the approximations inherent in the model.This paper introduces the idea of modeling the experiment to ensurethat the validation is accurate. Specifically, a 3D, cylindrical gas-fluidizedbed was simulated using a discrete element model (DEM) for particlemotion coupled with computational fluid dynamics (CFD) to describethe flow of gas. The results for time-averaged, axial velocity duringbubbling fluidization were compared with those from magnetic resonance(MR) experiments made on the bed. The DEM-CFD data were postprocessedwith various methods to produce time-averaged velocity maps for comparisonwith the MR results, including a method which closely matched thepulse sequence and data processing procedure used in the MR experiments.The DEM-CFD results processed with the MR-type time-averaging closelymatched experimental MR results, validating the DEM-CFD model. Analysisof different averaging procedures confirmed that MR time-averagesof dynamic systems correspond to particle-weighted averaging, ratherthan frame-weighted averaging, and also demonstrated that the useof Gaussian slices in MR imaging of dynamic systems is valid.

This paper describes an aggregate imaging based Discrete Element Modeling (DEM) methodology successfully employed to model both clean and coal dust fouled railroad ballast behavior. Laboratory direct shear box tests were conducted on granite type clean and fouled ballast aggregate samples. The size, shape and angularity properties of the aggregate particles were obtained through image analysis. Accordingly, direct shear box DEM simulations were conducted for different aggregate contact friction conditions. Both the clean ballast behavior and the heavily fouled case of coal dust filling completely all the voids in ballast were adequately modeled using the DEM approach by assigning a lower contact friction angle as the controlling discrete element model parameter in the latter case. In accordance with the direct shear test results, much lower shear stress-shear deformation curves, i.e., lower shear strengths, were predicted the coal dust fouled case using the DEM simulations.

Current climate models simulate sea ice as a continuous medium despite the discontinuities arising from fracturing and floe-floe interactions. Discrete element models (DEMs) can directly resolve these discontinuities, making them valuable tools for understanding subgrid-scale sea ice dynamics. However, DEMs are typically limited by their high computational cost and large number of unconstrained microscale parameters, which hinder their validation and interpretability. In this work, we develop, calibrate, and evaluate the performance of a low-complexity, two-dimensional bonded DEM for process-based studies of ice flow and fracture. We modify the linear bonded particle model implemented in the molecular dynamics software, LAMMPS, to prevent failure in compression because unfractured sea ice is significantly stronger under compression than tension. Unlike most DEMs, our model does not explicitly integrate angular momentum, which halves the number of computations required for each particle. With this simplification, particles are frictionless and bonds do not break under torques. From simple shear experiments, we show that bonded elements behave elastically prior to failure, with an effective elastic modulus that scales linearly with the inter-particle bond stiffness. These experiments also illustrate that the ice deformation is localized in both space and time, in agreement with observations. Using a canonical geometry idealizing sea ice flow through the Nares Strait, we demonstrate that the model can reproduce ice arch formation and collapse previously observed in higher-complexity models. The small input parameter space can be explored with ensemble runs that would be infeasible for a higher-complexity model. We find that our model can represent four possible outcomes: viscous flow, ice arch formation and collapse, stable ice arch formation, and no fracturing. These regimes collapse onto a single control parameter given by the product of bond stiffness and critical strain. Despite neglecting key processes such as ridging and friction, our model reasonably represents short-timescale, discrete sea ice dynamics with fewer parameters to calibrate and lower computational cost than higher-complexity DEMs.

This work presents the study of a three-dimensional (3D) simulation of the concrete behaviour in a uni-axial compressive test and flexural test using discrete element modelling (DEM). The proposed numerical models are namely, unreinforced cylindrical concrete under a uni-axial compressive test, unreinforced concrete beam under three-point flexural test and lastly, steel reinforced concrete beam under four-point flexural test. Those models were built up with fish programming language and python programming language (see Appendix A1 for the code created) and run into a computer program namely Particle flow code (PFC 3D). The main aim of this paper is to validate those numerical models developed and to study the cracking initiation and failure process in order to understand the fracture behaviour of concrete. The particles were distributed using an algorithm that is based on the sieve test analysis. The parameters were set up in order to validate the numerical model with the experimental result. It was observed that all the three models developed show a strong correlation with the laboratory experiment in term of stress-strain response, load-displacement response, crack pattern and macroscopic cracks development. Once, the bond between the spheres is broken, it leads to the formation of microscopic cracks which is not visible in laboratory experiment. DEM can help to identify which part is more prone to the evolution of microscopic cracks to macroscopic cracks under the discrete fracture network. In addition to, the rosette plot allows identifying the orientation that leads to a significant amount of micro cracks which is essential for designing structures. From the observation recorded in this research, it was observed that DEM is capable to reproduce concrete behaviour both quantitatively and qualitatively. It is also possible to measure the strain energy stored in the linear contact bond and parallel bond. At yield point which corresponds to the maximum amount of microcracks recorded, that strain energy is released in the form of kinetic energy, frictional slip energy, energy of dashpot, local damping. This can be extended further to compute fracture energy in the future work. Hence, it can be concluded DEM can be used to study the heterogeneous nature of concrete and as well as randomness nature of the fracturing of concrete structure.

We develop a discrete element model (DEM) simulation of mixed regular rounded polyhedra and spheres in simple shear with walls and periodic boundaries in 3-dimensions. The results show reasonably realistic behaviour developing shear and dilation or compaction depending on whether the initial state is dense or loose. Similarly non-coaxiality of principal stress direction and strain rate direction are shown. Polyhedra show more general realistic behaviour than spheres but take significantly longer to run. Particle forces include normal elastic, damping, and tangential friction and rolling friction. No cohesion or interstitial fluid is modelled. A separate simplified dynamic implicit finite difference Eulerian continuum model is developed and its parameters are used to fit the DEM results. This uses mass and momentum balances, a non-linear constitutive model and Mohr–Coulomb failure criterion. It runs in 2D with periodic boundaries effectively making it pseudo-1D. The model can reproduce the general trend of the DEM results and is a good basis for further development and understanding the physics.

Three-dimensional discrete element modelling of tillage: Determination of a suitable contact model and parameters for a cohesionless soil