Discrete Numerical Model Research Articles

Human Mastication Analysis-A DEM Based Numerical Approach.

Mastication is an essential and preliminary step of the digestion process involving fragmentation and mixing of food. Controlled muscle movement of jaws with teeth executes crushing, leading towards fragmentation of food particles. Understanding various parameters involved with the process is essential to solve any biomedical complication in the area of interest. However, exploring and analyzing such process flow through an experimental route is challenging and inefficient. Computational techniques such as discrete element numerical modeling can effectively address such problems. The current work employs the Discrete Element Method (DEM) as a numerical modeling technique to simulate the human mastication process. Tavares and Ab-T10 breakage models coupled with Gaudin Schumann and Incomplete Beta fragment distribution models have been implemented to analyze the fragmental distribution of food particles. The effect of particle shape (spherical, polyhedron, and faceted cylinder), size (aspect ratio), and orientation (vertical and horizontal) on breakage and fragment distribution is analyzed. To account for the elastic-plastic behavior and moisture content in food particles, modifications has been made in breakage models by incorporating numerical softening factor and adhesion force. The study demonstrates how numerical modeling techniques can be utilized to analyze the mastication process involving multiple process parameters.

Initiation mechanism of landslides in cold regions: Role of freeze-thaw cycles

Freeze-thaw cycles are recognized as one of the key triggers for some major landslides in cold regions around the world. Though the effects of freeze-thaw cycles on the rock strength degradation have been studied extensively, little effort has been made to qualitatively evaluate how it contributes to the evolution from a stable rock slope to a large-scale mass movement. In this study, we use a discrete element-based numerical model to simulate the entire process of the initiation of landslide under the action of freeze-thaw cycles in a slope with randomly distributed initial cracks. The main goal of this work is to quantitatively describe the landslide evolution process regarding the slope displacement, crack propagation, stress chain and load-bearing structure. Our results show the essence of the displacement evolution of a landslide subjected to freeze-thaw cycles; namely frost heave pressure induces the generation of new cracks, leading to the failure and reconstruction of the load-bearing structure of the slope. Deep-seated landslides can occur when the slope is crossed by a fault; otherwise, the slope is prone to surface erosion or shallow landslides.

Evaluation of Key Development Factors of a Buried Hill Reservoir in the Eastern South China Sea: Nonlinear Component Seepage Model Coupled with EDFM

The HZ 26-B buried hill reservoir is located in the eastern part of the South China Sea. This reservoir is characterized by the development of natural fractures, a high density, and a complex geological structure, featuring an upper condensate gas layer and a lower volatile oil layer. These characteristics present significant challenges for oilfield exploration. To address these challenges, this study employed advanced embedded discrete fracture methods to conduct comprehensive numerical simulations of the fractured buried hill reservoirs. By meticulously characterizing the flow mechanisms within these reservoirs, the study not only reveals their unique characteristics but also establishes an embedded discrete fracture numerical model at the oilfield scale. Furthermore, a combination of single-factor sensitivity analysis and the Pearson correlation coefficient method was used to identify the primary controlling factors affecting the development of complex condensate reservoirs in ancient buried hills. The results indicate that the main factors influencing the production capacity are the matrix permeability, geomechanical effects, and natural fracture length. In contrast, the impact of the threshold pressure gradient and bottomhole flow pressure is relatively weak. This study’s findings provide a scientific basis for the efficient development of the HZ 26-B oilfield and offer valuable references and insights for the exploration and development of similar fractured buried hill reservoirs.

Effects of drilling parameters on stress relief performance during mining

Abstract This study investigated the effects of borehole diameter and borehole interval on the stress relief performance of coal seams during mining. A discrete element numerical model was established based on the Voronoi and block models, and the stress relief performance was investigated in terms of axial stress, vertical displacement, and plastic zone development using the control variates method. The results showed that a decrease in the distance from the workface was accompanied by an increase in the effects of mining, the stress concentration, and the vertical displacement of the roof. Coal mining had positive effects on fracture propagation in front of the workface. As the borehole interval increased, the density of boreholes in the coal seams decreased, reducing the stress concentration and fracture propagation in front of the workface. The stress relief performance of the boreholes in the coal seams was improved at an borehole interval of 8 m. However, as the borehole diameter increased, the stress concentration near the boreholes decreased, reducing the effects of the workface mining on stress relief through the boreholes. Meanwhile, increasing the borehole diameter decreased the vertical displacement of the roof but exacerbated fracture propagation in front of the workface, which improved the stress relief performance of the boreholes in the coal seams. This study provides a reference for arranging boreholes in coal seams during mining.

Reconstruction method for a three-demensional discrete element numerical model of landslides using an integrated multi-electrode resistivity tomography method and an unmanned aerial vehicle survey

To analyze the hazard-causing modes of landslides, this paper proposes a three-dimensional discrete element model reconstruction method that employs an unmanned aerial vehicle survey and multi-electrode resistivity tomography method. To convert the resistivity profile into a material profile, we adopt the peak of the probability density method for material classification and utilize the Haar wavelet transform for image denoising. Subsequently, inverse distance weighting interpolation and the curtain-point method are used to transform two-dimensional profiles into a 3D visualization model. Similarly, the triangular mesh boundary can be extracted from the 3D visualization model using the curtain-point method. A mapping function f including the macroscopic parameters, was defined to populate the particles within the boundaries. Using the iterative method and defining the loss function L for parameter calibration, the targeted 3D discrete element model was constructed after setting the velocity threshold. This method was applied to the Changhe landslide (September 14, 2019) in Gansu Province, China, which had a typical damaged soil layer due to earthquake and rainfall factors. The results indicate that the lower part first exhibits significant displacement, followed by the upper and middle parts, which is consistent with the on-site inspections and UAV findings.

Asymmetric analogous hyperbola model of overburden movement and its verification

The extraction of underground coal resources induces the fracture and movement of overlying strata, leading to geological hazards such as surface deformation, cracks, and even subsidence. Utilizing the analogous hyperbola model of overlying strata movement, we conducted a mechanical analysis to examine the asymmetric fracture mechanism resulting from coal seam mining in thick loose strata. An asymmetric analogous hyperbola model was established by introducing distinct virtual half-axis lengths (b). The thickness impact of thick loose layers (H) and bedrock layer (h) on the asymmetric movement of overlying rock during mining was also discussed. Similarity model tests were conducted to research the migration characteristics and surface subsidence patterns of overburdened rock and thick loose layers at different mining stages and validate the hypothesis of asymmetric overburdened rock migration. Additionally, the discrete element numerical model for thick and loose layers mining was established by using UDEC and discussed the asymmetric analogous hyperbola behaviour of overburden movement and surface subsidence. The comparison results show that the established asymmetric hyperbolic model can effectively predict the movement law of overlying strata and surface subsidence characteristics. Therefore, the proposed model can provide valuable theoretical support for predicting the movement patterns of overburden under thick loose layers and mitigating surface subsidence disasters.

Three-Dimensional Discrete Element Analysis of Bearing Characteristics of Concrete–Cored Sand–Gravel Pile Composite Foundation under Cyclic Dynamic Load

Concrete-cored sand–gravel piles are a kind of composite pile formed by wrapping a concrete-cored pile with a sand–gravel shell, which has the advantages of both a rigid pile and bulk-material pile. The bearing characteristics of the concrete-cored sand–gravel pile composite foundation were investigated by establishing a three-dimensional discrete element numerical model for a cyclic dynamic loading test. The results show that the vertical stress of the core pile body fluctuates greatly at the beginning of loading, and the fluctuation amplitude decreases with the depth, and gradually tends to be stable in the middle and late stages, and the vertical-stress distribution is relatively uniform. The radial stress in the upper part of the core pile body fluctuates greatly, the fluctuation in the lower part is small, and the radial stress in each part of the core pile body gradually tends to be stable in the late-loading period. The radial stress factor of the core pile body reaches the stable speed with the foundation depth decreasing; the fluctuation amplitude of the pile-soil stress ratio decreases with the foundation depth and gradually tends to be stable with the increase in loading. The results of this study can provide a reference for the design and construction of a core sand pile composite foundation.

Effects of unit cell parameters on the thermal shock resistance of auxetic honeycomb sandwich structures: Combining discrete and continuum model

The influence of unit cell geometric parameters on the thermal fracture behavior and thermal shock resistance of honeycomb ceramics sandwich structures is revealed for the first time. The present study establishes a continuum model for the thermal fracture of sandwich structures, and a discrete numerical representation model for the equivalent fracture toughness of honeycombs, respectively. A novel evaluation approach for the thermal shock resistance of porous material structures has been provided by combining the two models. The effects of temperature dependence, face sheet thickness, honeycomb orientation and honeycomb geometrical parameters on thermal shock resistance are studied. Results indicate that changing the inclined angle of honeycomb can effectively increase the critical thermal shock temperature rise. Auxetic honeycomb sandwich structures with the inclined angle of −9° can reach the peak of the critical temperature rise. The results would contribute to the application of honeycomb sandwich structures in thermal protection systems.

Triaxial Test and Discrete Element Numerical Simulation of Geogrid-Reinforced Clay Soil

Indoor triaxial tests on geogrid-reinforced clay elucidate the macroscopic changes in soil strength indices post-reinforcement, yet the underlying mechanisms of strength enhancement require further investigation. By conducting indoor triaxial tests and establishing a corresponding discrete element numerical model, we can delve into the fine-scale mechanisms of geogrid-reinforced soil. This includes analyzing changes in fine-scale parameters such as porosity, the coordination number, and contact stress between soil particles. The findings suggest that an increase in the number of geogrid reinforcement layers leads to a more pronounced improvement in peak strength and cohesion, albeit with minimal impact on the internal friction angle of the specimens. Furthermore, analysis of the triaxial test curves of reinforced soils indicates that the stress–strain relationship adheres to the Duncan–Chang model. Parameters derived from this model have been validated against experimental data, confirming their accuracy. The discrete element model was used to analyze the variations in fine-scale parameters such as porosity and coordination number. It revealed that reinforcement reduces the fluctuation amplitude of porosity and significantly increases the number of particle contacts, resulting in a denser soil structure. Further analysis of the change in contact stress between particles in the discrete element model revealed that the contact force between particles increased significantly after reinforcement and that the reinforcement played a role in restraining the soil particles and dispersing the reinforcement stress, which explains the increase in the strength of the mesh-reinforced clays from another perspective. This further elucidates the strength enhancement mechanism in geogrid-reinforced clay, offering a new perspective on the mechanical behavior and strength development of such materials.

Triaxial behavior of unreinforced and geogrid-reinforced calcareous gravelly sand: Experiment and discrete element modelling

Geogrid reinforcement is a feasible technique for improving the mechanical and deformation behaviors of calcareous gravelly sand that is predominantly distributed in reclaimed foundations as construction materials. However, the shear behavior of geogrid-reinforced calcareous gravelly sand still remains unclear. In this paper, a series of large-scale drained consolidated triaxial compression tests (the specimen 300 mm in diameter by 600 mm in height) were conducted to investigate the effects of confining pressure, moisture content and geogrid inclusion on the mechanical and deformation behaviors of calcareous gravelly sand. The dilation effect, strain-softening behavior, reinforcing effect and particle breakage characteristics of unreinforced and geogrid-reinforced calcareous gravelly sand were comprehensively analyzed and discussed. Additional analysis was also performed from a microscopic perspective using the three-dimensional discrete numerical modelling. The experimental results indicate that calcareous gravelly sand exhibited a greater resistance to strain-softening tendency, and produced a higher susceptibility to crushing under the same input energy than calcareous coarse sand. Geogrid inclusion can effectively mitigate the susceptibility of calcareous gravelly sand to volumetric dilation through constraining the lateral expansion within the reinforced zone, and thus substantially increased the peak shear strength. Moreover, particle breakage appeared to be insensitive to geogrid reinforcement and water content, but increased significantly with increasing confining pressures. The numerical results further indicate that the mechanical response of geogrids was regulated by axial strain and confining pressure. As axial strain was greater than 10%, increased confining pressure resulted in a wider area of stress concentration within the geogrid meshes, and the upper geogrid sustained a greater contact force and tensile strain. The findings in this study are of practical significance for the design and construction of structures in calcareous gravelly sand reinforced with geogrids.

Catch bond kinetics are instrumental to cohesion of fire ant rafts under load

Dynamic networks composed of constituents that break and reform bonds reversibly are ubiquitous in nature owing to their modular architectures that enable functions like energy dissipation, self-healing, and even activity. While bond breaking depends only on the current configuration of attachment in these networks, reattachment depends also on the proximity of constituents. Therefore, dynamic networks composed of macroscale constituents (not benefited by the secondary interactions cohering analogous networks composed of molecular-scale constituents) must rely on primary bonds for cohesion and self-repair. Toward understanding how such macroscale networks might adaptively achieve this, we explore the uniaxial tensile response of 2D rafts composed of interlinked fire ants (S. invicta). Through experiments and discrete numerical modeling, we find that ant rafts adaptively stabilize their bonded ant-to-ant interactions in response to tensile strains, indicating catch bond dynamics. Consequently, low-strain rates that should theoretically induce creep mechanics of these rafts instead induce elastic-like response. Our results suggest that this force-stabilization delays dissolution of the rafts and improves toughness. Nevertheless, above 35[Formula: see text] strain low cohesion and stress localization cause nucleation and growth of voids whose coalescence patterns result from force-stabilization. These voids mitigate structural repair until initial raft densities are restored and ants can reconnect across defects. However mechanical recovery of ant rafts during cyclic loading suggests that-even upon reinstatement of initial densities-ants exhibit slower repair kinetics if they were recently loaded at faster strain rates. These results exemplify fire ants' status as active agents capable of memory-driven, stimuli-response for potential inspiration of adaptive structural materials.

3D printing-aided numerical study of particle shape effect on behaviours of dry granular flows interacting with rigid barriers

Particle shape plays a crucial role in determining the mobility and impact characteristics of granular flows, yet analyses of the effects of particle shape are rare. In this study, three-dimensional (3D) printing technology is used to create rock-like materials with varying shapes. This approach enables the precise control of mechanical properties other than particle shape, eliminating potential confounding factors. Then, we construct particle flow samples with distinct shape characteristics, quantitatively assessing their shape variations according to their overall regularity (OR). Flume experiments are conducted, with the findings used to calibrate a discrete element numerical model. By integrating the numerical simulations and experimental data, we systematically investigate the flow, impact, and deposition behaviours of granular flows under diverse flume inclinations considering the influence of particle shape. The results demonstrate that the mobility and destructiveness of granular flows increases with higher OR values and steeper flume inclinations. The transition from pile-up to runup deposition is primarily controlled by the flume inclination, while within specific inclination ranges, changes in OR can also induce deposition mechanism transition. This study provides substantial insights into interactions between granular flows and protection structures, emphasizing the significance of particle shape and terrain characteristics within a source area.

Hydrodynamic modeling study on a sector of the Timis River

To determine the water flow regime through the minor and major bed of the Timiș River, in the area associated with the studied sector, (levels, flow rates, hydrodynamic slopes, flood zones), as well as determining the discharge capacity of the bed on this analysis section, it was necessary to carry out a discrete numerical modeling in different variants of water flow transit. The assessment of the existing situation was based on the current topographic elevations materialized on the situation plan, the cross-sections and the maximum flows corresponding to the arranged regime, by the Waters National Administration. The analysis sector of the Timiș River was divided into segments (40) limited at the ends by (41) cross- sections obtained in accordance with the actual topographic elevations. The numerical modelling was carried out with the specialized program HECRAS version 4.1 and version 6.3.1. The calibration of the numerical model was carried out at the transit of the historical hydrograph where the maximum level reaches the recorded value Hmax and corresponds to a maximum value of the flow rate Qmax. Next, the transit of the synthetic flood hydrograph was carried out resulting in the identification of the flooded areas on the discrete numerical model as well as where the flow reaches the maximum value.

Prediction ability of discrete element model of loose granular media subjected to complex loadings

The discrete numerical method is recognized today as a powerful method to describe the constitutive behaviour of granular media at both the scale of particles and the scale of the representative elementary volume. This method is also more and more used to address boundary value problems in particular when large deformations may occur or when fluid-solid interactions are involved as, for instance, for internal soil erosion problems. Such discrete numerical models are often calibrated and validated with respect to direct shear tests or drained triaxial compressions. However, even if the models provide satisfying results for these loading paths compared to experimental measurements, their validation with respect to more complex loading paths as the isochoric compression or the path at constant stress deviator still present difficulties, in particular for initially loose granular assemblies. This article investigates the importance of the description of the anisotropy of the initial fabric and of the inter-particle friction law in the simulated responses of loose granular assembly to different kinds of loading paths. It shows how the combination of both can modify importantly the simulated responses to some kinds of loading paths. This investigation is carried out for a numerical discrete model made of spheres by comparison with experimental results on sand. The real shape of particles is considered by introducing rolling resistance at contacts into the model, in order to keep a relatively light model and the possibility to tackle further boundary value problems with a limited computational cost.

DEM-based 2D numerical simulation of the rock cutting process using a conical pick under confining stress

Many factors influence rock breakage during cutting processes, including cutting parameters (attack angle, cutting depth and cutting spacing), rock properties and stress conditions. The influence of confining stress on the linear cutting process is currently not well understood. In this study, a 2D discrete element numerical model is developed utilizing DEM software to simulate the linear cutting process of rock using a conical pick. The model accounts for different levels of confining stress to investigate their effect on the cutting process. The numerical model was first validated to ensure its rationality. Subsequently, the rock breakage process under different confining stresses was analysed by observing variations in cutter force, the development of crack extension, particle displacement, velocity variations, and alterations in internal stress. According to the results, the presence of confining stress facilitated the propagation and expansion of cracks within the original rock, particularly along the region where the free surface was present. Subsequently, during the cutting process under different stress conditions, it was observed that as the confining stress increased, the length of radial crack expansion decreased gradually along the tip of the cutter. Under higher stress conditions, a uniform distribution of mostly tensile cracks appeared inside the rock during the cutting process. Additionally, the regression analyses show that average cutting force and specific energy for rock breakage both exhibit an initial increase, followed by a decrease, with increasing confining stress. This indicates that the rock breakage efficiency was high when the confining stress was low and that a high confining stress can instead improve rock cutting. Through these analyses, insights were gained into the mechanisms underlying rock breakage during the linear cutting process using a conical pick under different levels of confining stress. Ultimately, these findings provide valuable knowledge for predicting and optimizing rock cutting processes under different stress conditions.

Effect of the gradation of binary mixed particle materials on compressive strength and permeability

The gradation of binary mixed particulate materials has a considerable impact on their compressive strength and permeability. To investigate the effect of various gradations on compressive strength and permeability. Uniaxial compression tests and permeability tests were conducted, and simulation-based three-dimensional discrete element numerical models of five distinct binary mixed particulate materials with varying levels were developed. From the standpoint of mesoscopic contact force composition, the effect of coal gangue particle content (ωc) on the strength formation mechanism of binary mixed particle materials was studied. The results indicate that the compressive strength of binary mixed particles increases rapidly with increasing ωc, but the increase is not evident when ωc > 40 %. Moreover, the permeability increases as ωc increases under the same conditions. The grading of recommendation 2# (ωc = 40 %, ωb = 60 %) is optimal. Simulation yields a prescribed grade that is consistent with the test.

Micromechanics-based model of rock considering compaction deformation: Analysis of microcrack influence from discrete and continuous perspectives

This paper proposed a micromechanics based constitutive model of rock considering compaction deformation, incorporating the intricate failure mechanisms of microcracks (i.e., opening, closing, friction, and slip) along with the overall deformation processes of rock (i.e., compaction, elasticity, plasticity, and damage). By combining the proposed continuum constitutive model with a discrete element numerical model featuring gapped flat joint contacts, the influence of initial cracks and crack volumes on rock compaction deformation was comprehensively analyzed from both discrete and continuous perspectives. Our findings reveal that the mechanical properties of the rock can be influenced by both the number and volume of cracks. The product of these two factors determines the overall void ratio, while the number of cracks exhibits an exponential relationship with the deformation resistance of the void under fixed void ratios. Furthermore, we observe that higher confining pressures result in reduced compaction deformation, and the compaction process is independent of force directionality. The comprehensive analysis from both continuous and discrete approaches offers a profound understanding of compaction deformation mechanisms, improves the identification efficiency of discrete compaction parameters, and enables the analysis of increased compaction deformation due to changes in crack states under external conditions (e.g., high temperature).

Development of 1D bounding surface model for lattice spring model on predicting rock failure under cyclic loading

A sound interpretation of the deformation and failure of rock material subjected to cyclic loadings is of great importance when predicting the long-term stability of rock structures. The bounding surface model is generally utilized to depict the mechanical behaviour of rock subjected to cyclic loadings. In this work, a simplified 1D bounding surface model is developed into a 4D lattice spring model (4D-LSM) to predict rock failure under cyclic loading. To better represent the nonlinear behaviour of rock material, a microcrack model is also introduced in the simplified 1D bounding surface model, together with a macro strength criterion and a cohesive zone model to represent the failure process of the rock. GPU-based parallel computing is realized to accelerate the time-consuming numerical simulation. The numerical results indicate that the proposed model reproduces the deformation and failure process of rock material under different cyclic loading paths. Moreover, the fatigue life of the rock under different conditions is predicted, including different stress levels, confining stresses, and preexisting macro joints. These numerical results reveal that the simplified 1D bounding surface model not only satisfactorily describes the mechanical response of rock under cyclic loading (like in classical continuum-based constitutive models) but also shares the advantage of the discrete-based numerical model in modelling the fracture and failure processes of rock under cyclic loading. This approach could also be used in other discrete numerical models, such as the discrete element model (DEM).

Microscopic mechanical properties and fabric anisotropic evolution law of open graded gravel permeable base under dynamic loading

Under traffic dynamic load, the microscopic mechanical properties and the fabric anisotropic evolution of open graded gravel permeable base directly determine the ability of open graded gravel permeable base to resist external loads and maintain stability in service. Firstly, this article constructed a discrete element numerical model for open graded gravel permeable base, and verified the accuracy of the model through experiments. Then, the effects of gradation, nominal maximum particle size, friction coefficient between aggregate particles, and effective elastic modulus of aggregate on the microscopic mechanical properties, migration behavior, and the fabric anisotropic evolution of open graded gravel aggregate particles were studied in sequence. The research results indicate that the proposed image moving target tracking algorithm based on the three branches twin network can achieve precise tracking and measurement of particle microscopic migration behavior. The method proposed in this study for constructing a three-dimensional surface model of gravel particles based on the principle of multi view averaging and the scale invariant feature transformation algorithm can achieve rapid numerical reconstruction of the three-dimensional morphology of gravel particles at a low cost. The internal aggregate particles of intermediate grade gravel are more likely to form a stable network force chain topology structure that can effectively resist external loads. As the maximum nominal particle size increases, the topological structure of the contact force of gravel particles becomes sparse, and the proportion of strong contact force increases. Increasing the friction coefficient of the contact interface between aggregate particles can to some extent hinder the migration behavior of aggregate particles inside the graded gravel. As the effective elastic modulus of the aggregate increases, the internal network strong contact force chain topology of open graded gravel gradually forms. Under external loads, the migration speed and displacement of the aggregate particles inside open graded gravel decrease, and the degree of internal normal contact force and branch vector anisotropy also decrease.

Investigation on Bearing Characteristics of Gravity Wharf Rubble-Mound Foundation in Different Influencing Factors

The use of the heavy hammer compaction method can enhance the bearing characteristics of underwater rubble-mound foundations. This is crucial to ensure the safety and stability of port and coastal engineering projects. In the present work, a combination of image-recognition technology, 3D laser scanning technology, a discrete element numerical simulation, and a field test was first utilized to establish riprap particles and reconstruct a discrete element numerical model of a rubble-mound foundation, and then the effects of various influencing factors on the bearing characteristics of the rubble-mound foundation were studied. The main conclusions are as follows. (1) The load–settlement curve of the rubble-mound foundation can be divided into three stages: rapid growth stage, slow growth stage, and failure stage. (2) The ultimate bearing capacity of the rubble-mound foundation is positively correlated with the vibration time and vibration amplitude. The riprap particle size and the foundation thickness have negligible effects on the ultimate bearing capacity of the rubble-mound foundation. (3) When adjusting the vibration time, vibration amplitude, and foundation thickness, the settlement value of the rubble-mound foundation tends to increase as the compactness increases. On the other hand, the effects of factors such as the riprap particle size, riprap particle gradation, and vibration frequency on the compactness and settlement value of the rubble-mound foundation are less significant.