Discrete Simulation Research Articles

A New Symmetry-Enhanced Simulation Approach Considering Poromechanical Effects and Its Application in the Hydraulic Fracturing of a Carbonate Reservoir

The exploration of fractured-vuggy carbonate reservoirs usually involves hydraulic fracturing to maximize recovery. At present, effectively communicating natural discontinuities is a technical challenge. In this article, we investigated the origin and propagation of cracks in fractured-vuggy reservoirs using discrete element hydraulic fracturing simulations that included poromechanical effects. A particular focus on the microscopic force-displacement symmetry of adjacent pore pressures is introduced. Our results demonstrate that the poromechanical effect significantly increases the strength of overpressurized reservoir formations. Moreover, the effect of injected fluid viscosity on the hydraulic fracturing effectiveness was studied through two simulation tests. The outcomes highlight the critical influence of fluid viscosity on the propagation of micro-cracks in overpressure fractured-vuggy reservoirs.

In-house or outsourcing? The impact of remanufacturing strategies on the dynamics of component remanufacturing systems under lifecycle demand and returns

We consider a component manufacturing and remanufacturing system where, due to the end-of-life warranty, new and after-sales demand must be satisfied. Two kinds of demand exhibit different lifecycle patterns with different scales and a time lag, while a third correlated component return lifecycle with again a different lag and scale, driven by adoption of remanufacturing, is also presented. To achieve supply and demand balance during demand lifecycles, companies need a strategic decision on their remanufacturing: remanufacturing outsourcing strategy (ROS) or remanufacturing in-house strategy (RIS), yet inadequately studied from system dynamics perspective. We developed base-stock system dynamics models and analytically explored the dynamic implications of RIS and ROS remanufacturing strategies under correlated lifecycle demand and returns. Applying z-transform and discrete time simulation, we found that RIS outperforms ROS system including less peak capacity cost, less inventory holding cost and less backlog cost. Also, the bullwhip of the RIS is always less than the ROS system. However, the adoption of the RIS may result longer-lasting manufacturing production and thus lead to a higher cost: an important cost needs to be strategically considered. Thereby, from system dynamics perspective, the component manufacturer needs carefully consider trade-offs between production and inventory costs, as well as their demand lifecycle characteristics to choose the right remanufacturing strategy.

Numerical study of gas–solid transport characteristics within vortex region induced by porous media using lattice-Boltzmann based discrete particle simulation

Porous media, as delivery carriers, holds great potential application in various fields particular in drug delivery. A comprehensive understanding of gas–solid transport characteristics within porous media is significant and regulating retention and release of drug particles. A combined approach of lattice Boltzmann method and time-driven hard-sphere model (LBM-TDHS) was used to investigate particle transport process in porous media. The vortex induced by porous structures was observed. Particle trajectories with different connectivity of the vortex region and inlet flow angles was conducted. This study revealed that fluid velocity between connected regions significantly affects the gas streamlines’ shape and particle trajectories. Different inlet flow angles lead to five modes of particle trajectories in vortex regions. Finally, some modifications were performed in general irregular porous structure to control particle trajectories. LBM-TDHS successfully investigated particle dynamics in vortex regions within porous media, demonstrating potential applicationin optimizing the structure of drug delivery carriers.

Extended kinetic theory applied to pressure-controlled shear flows of frictionless spheres between rigid, bumpy planes.

We numerically investigate, through discrete element simulations, the steady flow of identical, frictionless spheres sheared between two parallel, bumpy planes in the absence of gravity and under a fixed normal load. We measure the spatial distributions of solid volume fraction, mean velocity, intensity of agitation and stresses, and confirm previous results on the validity of the equation of state and the viscosity predicted by the kinetic theory of inelastic granular gases. We also directly measure the spatial distributions of the diffusivity and the rate of collisional dissipation of the fluctuation kinetic energy, and successfully test the associated constitutive relations of the extended kinetic theory, i.e., a kinetic theory which includes the role of velocity correlations. We then phrase and numerically integrate a system of differential equations governing the flow, with suitably modified boundary conditions. We show a remarkable qualitative and quantitative agreement with the results of the discrete simulations. In particular, we study the effect of (i) the coefficient of collisional restitution, (ii) the imposed load and (iii) the bumpiness of the planes on the profiles of the hydrodynamic fields, the ratio of shear stress-to-pressure and the gap between the bumpy planes. Finally, we predict the critical value of the imposed load above which crystallization occurs, based on the value of the solid volume fraction near the boundaries obtained from the numerical solution of the kinetic theory. This notably reproduces what we observe in the discrete simulations.

Structured bubbling in vibrated gas-fluidized beds of binary granular particles: experiments and simulations.

Mixing and segregation of granular particles on the basis of size and density from vertical vibration or upward gas flow is critical to a wide range of industrial, agricultural and natural processes. Recently, combined vibration and gas flow under certain conditions has been shown to create periodically repeating structured bubbling patterns within a fluidized bed of spherical, monodisperse particles. Here, we demonstrate with experiments and simulations that structured bubbling can form in binary mixtures of particles with different size and density, but with similar minimum fluidization velocities. Structured bubbling leads to particles mixing regardless of initial particle configuration, while exciting particles with only gas flow produces smaller unstructured bubbles which act to segregate particles. Discrete particle simulations match the experimental results qualitatively and, in some regards quantitatively, while continuum particle simulations do not predict mixing in the case of structured bubbling, highlighting areas for future model improvement.

Fabric-based jamming phase diagram for frictional granular materials.

A jamming phase diagram maps the phase states of granular materials to their intensive properties such as shear stress and density (or packing fraction). We investigate how different phases in a jamming phase diagram of granular materials are related to their fabric structure via three-dimensional discrete element method simulations. Constant-volume quasi-static simple shear tests ensuring uniform shear strain field are conducted on bi-disperse spherical frictional particles. Specimens with different initial solid fractions are sheared until reaching steady state at a large shear strain (200%). The jamming threshold in terms of stress, non-rattler fraction, and coordination numbers (Z's) of different contact networks is discussed. The evolution of fabric anisotropy (F) of each contact network during shearing is also examined. By plotting the fabric data in the F-Z space, a unique critical fabric surface (CFS) becomes apparent across all specimens, irrespective of their initial phase states. Through the correlation of this CFS with fabric signals corresponding to jamming transitions, we introduce a novel jamming phase diagram in the fabric F-Z space, offering a convenient approach to distinguish the various phases of granular materials solely through the direct observation of geometrical arrangements of particles. This jamming phase diagram underscores the importance of the microstructure underlying the conventional jamming phenomenon and introduces a novel standpoint for interpreting the phase transitions of granular materials that have been exposed to processes such as compaction, shearing, and other complex loading histories.

Концепция верификации работы клеточных автоматов при вариации геометрических решёток для модели диффузионного процесса

The methodology of cellular automata occupies one of the key positions in the discrete dynamic simulations. The adequacy of the results of solving applied problems is largely determined by the choice of the type of geometric lattice of the cellular automaton. The article presents a formalization and numerical verification of the concept of verification of the operation of a cellular automata model of the classical diffusion process for a set of basic two-dimensional geometric lattices. The approach is based on a direct comparison of the space-time solution of the diffusion equation obtained by the finite element method with the solution of a discrete analogue of this equation based on the operation of a cellular automata algorithm. The software implementation of the cell-automation was performed with the C+ using the Unity platform while the finite element solution of the evolutionary problem of mathematical physics was carried out using the Matlab tools. Cellular automaton lattices were constructed using various cell geometric primitives traditionally used in diffusion modeling applications. Obtained findings indicate that the hexagonal cellular automaton demonstrates the minimum error in the numerical implementation.

A soft departure from jamming: the compaction of deformable granular matter under high pressures.

The high-pressure compaction of three dimensional granular packings is simulated using a bonded particle model (BPM) to capture linear elastic deformation. In the model, grains are represented by a collection of point particles connected by bonds. A simple multibody interaction is introduced to control Poisson's ratio and the arrangement of particles on the surface of a grain is varied to model both high- and low-frictional grains. At low pressures, the growth in packing fraction and coordination number follow the expected behavior near jamming and exhibit friction dependence. As the pressure increases, deviations from the low-pressure power-law scaling emerge after the packing fraction grows by approximately 0.1 and results from simulations with different friction coefficients converge. These results are compared to predictions from traditional discrete element method simulations which, depending on the definition of packing fraction and coordination number, may only differ by a factor of two. As grains deform under compaction, the average volumetric strain and asphericity, a measure of the change in the shape of grains, are found to grow as power laws and depend heavily on the Poisson's ratio of the constituent solid. Larger Poisson's ratios are associated with less volumetric strain and more asphericity and the apparent power-law exponent of the asphericity may vary. The elastic properties of the packed grains are also calculated as a function of packing fraction. In particular, we find the Poisson's ratio near jamming is 1/2 but decreases to around 1/4 before rising again as systems densify.

PORE STRUCTURE AND PERMEABILITY EVOLUTION OF POROUS MEDIA UNDER IN SITU STRESS AND PORE PRESSURE: DISCRETE ELEMENT METHOD SIMULATION ON DIGITAL CORE

In stress-sensitive oil and gas reservoirs, formation rock deformation occurs under in situ stress and pore pressure, affecting the rock's porosity and permeability. Pore deformation is the fundamental mechanism. However, the literature on numerical simulation of rock porosity and permeability at the pore scale is rare. In this paper a numerical simulation framework of pore scale is proposed based on the discrete element method. The pore geometry and permeability evolution of the core are quantitatively analyzed by the digital core method. Firstly, the coupled fluid-discrete element method (CFM-DEM) is used to simulate the samples' deformation under different stress and pore pressures. We then reconstruct the digital core using Avizo. Finally, the pore geometric topological structures are analyzed, and the permeability changes are calculated. The results show that stress can reduce porosity, modify pore shape, and lead to poor porosity connectivity and permeability, while pore pressure can weaken such trends.

Experimental and simulation study on compressive failure of rock with pre-Y-shaped cracks.

A large number of joints and fissures are prevalent in the rock mass, which has an important influence on the mechanical properties of the rock mass. To study the failure mechanical characteristics of Y-cracked rocks, the paper analyzes the influence of different angles of prefabricated Y-cracked rocks on the mechanical strength characteristics of the rocks and the crack extension evolution through uniaxial compression indoor tests and discrete element PFC2D numerical simulation. The results indicate that the stress-strain curves of rocks containing prefabricated cracks exhibit five stages: the initial pore-fracture compaction stage, the elastic stage, the crack stable development stage, the crack unstable development stage, and the post-peak rupture. The peak strength of the specimen shows an evolutionary process of decreasing, then increasing, and then decreasing with the increase of the Y-shaped crack angle. The failure of the sample is mainly caused by the shear crack expansion at the crack tip. The different Angle cracks directly affect the mechanical properties of the sample and the generation and evolution of new cracks. The final failure of rock is mainly the result of microcrack propagation, convergence and penetration to form macroscopic damage zone. Finally, combined with PFC numerical simulation, the distribution of micro-cracks and the damage pattern of rock damage are compared and analyzed, and it is found that the two are in good agreement, which reflects the rationality of the model.

Evaluation of waiting time for outpatient services at Respira Hospital Yogyakarta using discrete system simulation

The Ministry of Health of the Republic of Indonesia has established standard rules for the quality of outpatient service in hospitals. One indicator of the quality of outpatient services at a hospital is the patient's waiting time to be served either by a specialist or other services such as a pharmacy. Respira Hospital Yogyakarta is a special pulmonary and respiratory hospital in Yogyakarta that continues to improve the quality of its services. Based on the results of observations and interviews it is known that in terms of waiting time, patients at Respira Hospital Yogyakarta still have to wait to get service. Examples of queues that occur include patients waiting for a specialist doctor's examination for around 75 to 90 minutes. waiting at the pharmacy and cashier for up to 60 minutes or more. This study attempts to evaluate the waiting time for outpatient services at Respira Hospital Yogyakarta using a simulation. Based on the simulation results, it is known that the patient's waiting time in the system is 217.33 minutes and the longest waiting time is in the pediatric polyclinic and pharmacy departments. After the scenario implementation were made, namely in the pediatric polyclinic and pharmacy sections, the waiting time decreased to 177.19 minutes. This means that evaluations carried out using simulations can help hospitals reduce waiting time for outpatients

Simulation and Evaluation of Distributed consensus Network for Multi-Agent Systems for Sybil Attacks

Distributed average consensus represents the amount of computing of inputs held by multiple agents that communicate through peer-to-peer networks. Collaboration among operators is essential for any distributed standard consensus protocol as every specialist needs to contribute to different operators, typically the adjacent (neighbouring) operators. Internet-of-Things (IoT) implementation is challenging because of its heterogeneous, massively distributed nature. The challenges of this challenge can be addressed with blockchain-based platforms and technologies. Testing and evaluation platforms are required for Blockchain deployments in IoT. A realistic and configurable network environment is presented in this paper to evaluate consensus algorithms. Many blockchainevaluation platforms do not provide a configurable and realistic network environment or are tied to a specific consensus protocol. With our simulator, practitioners can evaluate how consensus algorithms affect network events in congested or contested scenarios to determine the best consensus algorithm. It is proposed to achieve this task by generalizing consensus methods. The Blockchain simulator employs Discrete event network simulations for increased fidelity and scalability. In addition to evaluating the time, state block rate (%), estimation error, average throughput, and simulation time, we evaluate the performance of the proposed techniques based on the number of peer nodes. A comparison of the average transaction delivery rate with a traditional protocol is shown. The proposed protocol has a higher throughput average than the traditional one.

Multi-objective optimization of concrete pumping S-pipe based on DEM and NSGA-II algorithm

In the process of fresh concrete pumping, the S-pipe plays a crucial role in the pumping system. The impact and scratching of concrete aggregate on the wall of S-pipe will cause the wear of the wall and lessen its service life. The structural design of the pumping S-pipe typically relies on experience, and the cost for experiments is too expensive. In addition, the influence mechanism of structural parameters on S-pipe wear is still unclear. In view of this, to reduce the wear of S-pipe and investigate the influence of structural parameters on the wear of S-pipe, the structural parameters of S-pipe (curvature radius r1 and r2, horizontal dimension l3, and inclination angle θ) were optimised through discrete element numerical simulation combined with the non-dominated sorting genetic algorithm-II (NSGA-II) in this paper. Firstly, the wear coefficient was determined by friction and wear test. The discrete element method (DEM) model of pumping suction unit was simplified, and the S-pipe was parameterised. Secondly, sample points and their exact values of the optimization objectives were obtained by Latin hypercube sampling (LHS) plan and DEM simulations, respectively. From these, a Kriging surrogate model was constructed. Taking the average wear rate and the maximum wear of the S-pipe as the optimization objectives, a multi-objective optimization design was performed based on the NSGA-II and the Kriging models. After optimization, a series of Pareto optimal S-pipe models were obtained. Comparing the optimised model with the original model, the average wear rate and maximum wear amount of the S-pipe were reduced by 9% and 26%, respectively.

Stress–Structural Failure of a 610 m Crushing Station Left-Side Tunnel Section in Jinchuan II Mine: A Numerical Simulation Study

To address the stress–structural failure phenomenon that can be induced by the excavation of a left-side tunnel section of a 610 m crushing station, an unmanned aerial vehicle was used in this study to collect the geological conditions and rock mass information of the working face, and important geometric information such as the attitude and spacing of rock mass were extracted. Based on the identified attitude and spacing information, a three-dimensional rock mass structure and numerical simulation model of the 610 m crushing station left-side tunnel section were constructed using discrete element numerical simulation software (3DEC) (version 5.0). The results show that the surrounding rock instability of the left-side tunnel section of the 610 m crushing station is controlled by both the stress field in the contact zone between reddish-brown granite stratum and the gray-black-gray gneiss stratum. The cause of stress–structural failure is that the joint sets (JSet #2 and JSet #3) are most likely to form unfavorable blocks with the excavation surface due to unloading triggered by the excavation. Therefore, stress–structural failure disasters in jointed strata sections are one of the key issues for surrounding rock stability during crushing station excavation. It is suggested to adopt ‘optimized excavation parameters + combined support forms’ to systematically control stress–structural failure after unloading due to the excavation from three levels: surface, shallow, and deep. The stress–structural failure mechanism of deep rock mass is generally applicable to a large extent, so the results of this research have reference value for engineering projects facing similar problems around the world.

The force and dynamic response of low-velocity projectile impact into 3D dense wet granular media

The presence of liquid bridges between particles in a wet granular media gives rise to capillary forces at the microscopic scale that dramatically alters the material's microstructure and macroscopic responses. Previous experimental studies show significant differences in the penetration depth of the projectile into assemblies of dry or wet particles. Still, there is a lack of fundamental understanding of the difference induced by the presence of liquid bridges in dynamic regimes. In this study, the contact model parameters of wet glass beads are calibrated employing the angle of repose tests and cylinder lifting tests, and a series of three-dimensional discrete element method simulations of spheres impact into wet granular packings are conducted. The dynamic characteristics of the projectile are obtained numerically and found to be in good agreement with the experimental data. The analyses indicate that the final penetration depth in the wet granular media is linearly related to the initial velocity and has a power function relationship with the impact energy. The generalized Poncelet law is suitable for wet granular materials, but the presence of liquid significantly affects the values of the parameters. There is a velocity “stagnation zone” at the initial stage of the wet granular impact, and the final penetration depth and the duration interaction time are smaller than that of the dry case. The difference can be attributed to the resistance evolution of the projectile exerted by particles and gives the parameter scaling law of the peak impact force. Further, a macro-micro transition technique is employed to characterize the velocity field, pressure field, and stress field inside the wet granular media and the spatial distribution is quantified based on stress theory.

The interpretation of discrete dislocation dynamics simulation data: verification and validation (V&V) with application to size/scale effects and free surface effects

The interpretation of discrete dislocation dynamics simulation data: verification and validation (V&V) with application to size/scale effects and free surface effects

Photophysical Outcomes of Water-Solvated Heterocycles: Single-Conformation Ultraviolet and Infrared Spectroscopy of Microsolvated 2-Phenylpyrrole.

The molecular chromophores within brown carbon (BrC) aerosols absorb solar radiation at visible and near-ultraviolet wavelengths. This contributes to the overall warming of the troposphere and the photochemical aging of aerosols. In this investigation, we combine a suite of experimental and theoretical methods to reveal the conformation-specific ultraviolet and infrared spectroscopy of 2-phenylpyrrole (2PhPy)─an extended π-conjugated pyrrole derivative and a model BrC chromophore─along with its water microsolvated molecular complexes (2PhPy:nH2O, n = 1-3). Using resonant two-photon ionization and double-resonance holeburning techniques alongside MP3 (ground state) and ADC(3) (excited state) torsional potential energy surfaces and discrete variable representation simulations, we characterized the ultraviolet spectra of 2PhPy and 2PhPy:1H2O. This analysis revealed evidence for Herzberg-Teller vibronic coupling along the CH wagging and NH stretching coordinates of the aromatic rings. Conformation-specific infrared spectroscopy revealed extended hydrogen-bonding networks of the 2PhPy:nH2O complexes. Upon stepwise addition of H2O solvation, the nearest H2O acceptor forms a strong, noncovalent interaction with the pyrrole NH donor, while the second and third H2O partners interface with the phenyl and pyrrole aromatic rings through growing van der Waals π/H atom stabilization. A local-mode Hamiltonian approach was employed for comparison with the experimental spectra, thus identifying the vibrational spectral signatures to specific 2PhPy:nH2O oscillators.

Discrete element method simulation of rice grains impact fracture characteristics

The fracture of rice grains and other cereals is a common phenomenon during various processes such as seeding, harvesting, and processing. With the objective of elucidating the underlying fracture mechanisms involved, this paper aimed at simulating the fracture of rice grain caused by impacts at different locations on the surface using the discrete element method (DEM) and a bonded particle model (BPM). The aim was to establish an accurate BPM of rice grain using an impact bench test combined with the DEM. To uncover the mechanisms underlying the fracture of rice grains, the fracture process of rice grains at different locations was visualised using impact simulation. The findings demonstrated that this BPM can replicate the fracture behaviour of rice grains. The impact fracture velocity of rice grains varied across different locations, with the highest fracture velocity observed at the centre of the grain, reaching a maximum of 39.5 m s−1. The propagation of cracks played a crucial role in the occurrence of fractures in rice grains. The impact force and kinetic energy were concentrated in the impact zone, resulting in the formation of fracture surfaces in rice grains. The impact toughness of the impact zone in rice grains exhibited a direct correlation with the impact fracture velocity of the rice grains. This study elucidated the fracture mechanism of rice grains and determined the impact fracture velocities at different locations, thereby establishing a reliable foundation for minimising losses in various agricultural stages including seeding, harvesting, and processing.

Improved ergonomic layout design of metro control center based on virtual simulation technology and genetic algorithm

For the existing control center layout methods do not have targeted measures to implement human factors requirements, this paper puts forward a specific human factors design method for metro control center layout. This method covers the overall process of ergonomic standards and actual engineering requirements in personal space determination, overall location design, and personnel post layout. It introduces discrete personnel simulation to assist in physical control center layout design for the controllers’ safety. To make the posts between the unified lines and the connections between the lines more reasonable, an improved genetic algorithm is used to solve the Systematic Layout Planning. This design flow and algorithm can be quickly adapted according to the project's actual needs and the application of human factors guidelines. This method is also suitable for the layout planning and design of similar large spaces, large numbers of people, and complex associated control centers.

A framework for modeling, generating, simulating, and predicting carbon dioxide dispersion indoors using cell-DEVS and deep learning

Carbon dioxide concentration in enclosed spaces is an air quality indicator that affects occupants’ well-being. To maintain healthy carbon dioxide levels indoors, enclosed space settings must be adjusted to maximize air quality while minimizing energy consumption. Studying the effect of these settings on carbon dioxide concentration levels is not feasible through physical experimentation and data collection. This problem can be solved by using validated simulation models, generating indoor settings scenarios, simulating those scenarios, and studying results. In previous work, we presented a formal Cellular Discrete Event System Specifications simulation model for studying carbon dioxide dispersion in rooms with various settings. However, designers may need to predict the results of altering large combinations of settings on air quality. Generating and simulating multiple scenarios with different combinations of space settings to test their effect on indoor air quality is time-consuming. In this research, we solve the two problems of the lack of ground truth data and the inefficiency of producing and studying simulation results for many combinations of settings by proposing a novel framework. The framework utilizes a Cellular Discrete Event System Specifications model, simulates different scenarios of enclosed spaces with various settings, and collects simulation results to form a data set to train a deep neural network. Without needing to generate all possible scenarios, the trained deep neural network is used to predict unknown settings of the closed space when other settings are altered. The framework facilitates configuring enclosed spaces to enhance air quality. We illustrate the framework uses through a case study.