Discrete Particle Assembly Research Articles

DEM simulation of effects of particle size distribution on meso-mechanical properties of slip zone soil

Particle Size Distribution (PSD) exerts a substantial influence on the mechanical properties of geological materials such as rocks and soils, which can be viewed at a microscale as an assembly of discrete particles. An exploration into the effects of particle gradation on the properties of these materials provides valuable insights into their nature. In the study, the Discrete Element Method (DEM) was used to conduct numerical shear tests on eight distinct groups of slip zone soil, each characterized by a different particle gradation. The aim was to examine the meso-mechanical properties and shear evolution laws of slip zone soil numerical samples with both optimal and sub-optimal PSDs. Findings underscore the pivotal role that PSD plays in various aspects, including dilatancy, the evolution of the displacement field, the network of contact force chains, the principal stress, and the distribution of normal and tangential contact forces within the slip zone soil. It was observed that the network of contact force chains in the numerical samples with an optimal PSD was more complex than in those samples with a sub-optimal PSD. Additionally, the distribution of principal stresses before and after shear was more uniformly balanced. This particle size-based study offers significant reference value for future investigations into the impact of PSD on the macroscopic and meso-mechanical properties of slip zone soil. By augmenting this knowledge, a more comprehensive understanding of the fundamental behavior of these materials can be attained, leading to improved prediction and management of geological risks.

Three-Dimensional Optothermal Manipulation of Light-Absorbing Particles in Phase-Change Gel Media.

Rational manipulation and assembly of discrete colloidal particles into architected superstructures have enabled several applications in materials science and nanotechnology. Optical manipulation techniques, typically operated in fluid media, facilitate the precise arrangement of colloidal particles into superstructures by using focused laser beams. However, as the optical energy is turned off, the inherent Brownian motion of the particles in fluid media impedes the retention and reconfiguration of such superstructures. Overcoming this fundamental limitation, we present on-demand, three-dimensional (3D) optical manipulation of colloidal particles in a phase-change solid medium made of surfactant bilayers. Unlike liquid crystal media, the lack of fluid flow within the bilayer media enables the assembly and retention of colloids for diverse spatial configurations. By utilizing the optically controlled temperature-dependent interactions between the particles and their surrounding media, we experimentally exhibit the holonomic microscale control of diverse particles for repeatable, reconfigurable, and controlled colloidal arrangements in 3D. Finally, we demonstrate tunable light-matter interactions between the particles and 2D materials by successfully manipulating and retaining these particles at fixed distances from the 2D material layers. Our experimental results demonstrate that the particles can be retained for over 120 days without any change in their relative positions or degradation in the bilayers. With the capability of arranging particles in 3D configurations with long-term stability, our platform pushes the frontiers of optical manipulation for distinct applications such as metamaterial fabrication, information storage, and security.

A numerical model of discrete element – Liquid bridge – Liquid thin film system for wet deforming granular medium at low saturation

A numerical model of discrete element – liquid bridge – liquid thin film system for mesoscopic hydro-mechanical analysis of wet deforming granular medium at low saturation is proposed. A numerical scheme for the model is devised and implemented to construct initial meso-structure of representative volume element (RVE) of wet discrete particle assembly with low saturation degree of liquid, particularly liquid bridge system among neighboring particles and liquid thin films adhered on the surfaces of particles, where no liquid bridges are covered. Three mechanisms governing evolution of the liquid bridge – liquid thin film system and liquid transfer along particle gaps are studied and integrated into the numerical model. A staggered and iterative DEM based solution procedure for mesoscopic hydro-mechanical modelling of wet RVE is formulated and implemented. Meso-hydro-mechanically informed constitutive curves for unsaturated porous continua can be extracted without need of macroscopic phenomenological constitutive relations and associated material parameters.

Modeling and experimental analysis of particle damping phononic crystal plate

A phononic crystal with particle assembly dampers was constructed. A high-speed camera was used to record and observe the vibration phenomenon of the particle assembly under external excitation. By analyzing the vibrations of the particle assembly, a continuous system equivalent to the discrete particle assembly was proposed. The particle damping equivalent model was fitted to data using an artificial neural network. Two particle damping phononic crystal plates were designed. Based on the equivalent model, a particle damping phononic crystal model was established. The results of the simulations and experiments showed that the average damping effect of the particle damping structure was greater than 75%. Particle damping phononic crystals possess excellent low vibration transmission characteristics, narrow bandgap characteristics at low frequencies, and high damping characteristics.

DEM modelling of unsaturated seepage flows through porous media

A new hybrid discrete–continuum numerical approach that explores the key advantages of both discrete and continuum approaches is proposed to model unsaturated seepage flows through porous media. In contrast to existing approaches where a porous medium is often represented by a continuum medium or required a background mesh, the proposed approach explicitly exploits the discrete contact network formed by an assembly of discrete solid particles. Each solid particle is assumed to occupy an equivalent-continuum space, over which the governing equations for unsaturated seepage flow are derived. These governing equations are then discretised and solved on the discrete contact network through a new numerical procedure that links micro-diffusivity to the macro-one. Thanks to this concept, the proposed approach is capable of describing the nature of flow in unsaturated porous media at the microscale level. This unique feature also enables the proposed approach to naturally simulate the water flow through the heterogeneous porous media without any ad hoc treatments. In this paper, the mathematical concept of the proposed approach together with its implementation features and performances for a rigid porous media is presented and discussed. The focus is placed on its application to the discrete element method (DEM), although the proposed concept, in general, can be applied to any other methods possessing similar features.

Effective hydro-mechanical material properties and constitutive behaviors of meso-structured RVE of saturated granular media

In the framework of the second-order concurrent computation homogenization method for saturated granular material, meso-structured RVE (Representative Volume Element) is modeled as a compact assembly of saturated discrete particles. The boundary conditions imposed on the RVE are comprised of the two parts: (1) the boundary displacements and pressures prescribed by downscaled macroscopic strain variables and pore pressure, and (2) constrained fine scale fluctuations of mesoscopic hydro-mechanical variables expressed in terms of periodic boundary conditions.The present paper focuses on the estimation of effective hydro-mechanical material properties and constitutive behaviors of meso-structured RVE of saturated discrete particle assembly via a proposed homogenization procedure. To achieve this target, a novel scheme for determining and imposing periodic boundary conditions on peripheral particles contacting with the RVE boundary of saturated discrete particle assembly is proposed.Numerical results demonstrate performances and capabilities of the proposed scheme in estimating effective hydro-mechanical non-linear material properties and predicting hydro-mechanical constitutive behaviors of saturated granular materials evolving with loading histories. They also demonstrate the significance and necessity of imposing periodic boundary conditions on the RVE in proper prediction of effective hydro-mechanical material properties and constitutive behaviors.

Consideration of maximum impact force design for a rock shed against dry granular flow

This paper focuses on the estimation of the maximum impact force of dry granular flow upon a rock shed using a coupled discrete element method (DEM) and finite element method (FEM). The dry granular flow is modeled as an assembly of discrete particles, and the rock shed is modeled by applying the FEM. The coupled DEM–FEM approach calibrated with a small-scale physical experiment is used to simulate the movement of a dry granular flow impacting the rock shed. Full-scale numerical modeling based on the field model is constructed to estimate the maximum impact force of dry granular flow on a rock shed. Based on the numerical results, three key stages of the impact process are identified: startup streams slippery, impact, and pile-up. The sensitivities of bulk density, impact height, slope angle, cushion thickness, and friction coefficient to the maximum impact force are 1, 0.63, 2.68, 0.09, and 0.73, respectively, in the benchmark model, and the parameters with high sensitivities should be given priority in the design of rock sheds. Moreover, an evaluation formula of maximum impact force is obtained and based on the numerical results and Buckingham’s principle.

A novel scheme for imposing periodic boundary conditions on RVE in second-order computational homogenization for granular material

Purpose This paper aims to present a novel scheme for imposing periodic boundary conditions with downscaled macroscopic strain measures of gradient Cosserat continuum on the representative volume element (RVE) of discrete particle assembly in the frame of the second-order computational homogenization methods for granular materials. Design/methodology/approach The proposed scheme is based on the generalized Hill’s lemma of gradient Cosserat continuum and the incremental non-linear constitutive relation condensed to the peripheral particles of the RVE of discrete particle assembly. The generalized Hill’s lemma conducts to downscale the macroscopic strain or stress measures and to impose the periodic boundary conditions on the RVE boundary so that the Hill-Mandel energy equivalence condition is ensured. Because of the incremental non-linear constitutive relation condensed to the peripheral particles of the RVE, the periodic boundary displacement and traction constraints together with the downscaled macroscopic strains and strain gradients, micro-rotations and curvatures are imposed in the point-wise sense without the need of introducing the Lagrange multipliers for enforcing the periodic boundary displacement and traction constraints in a weak sense. Findings Numerical results demonstrate that the applicability and effectiveness of the proposed scheme in imposing the periodic boundary conditions on the RVE. The results of the RVE subjected to the periodic boundary conditions together with the displacement boundary conditions in the second-order computational homogenization for granular materials provide the desired estimations, which lie between the upper and the lower bounds provided by the displacement and the traction boundary conditions imposed on the RVE respectively. Research limitations/implications Each grain in the particulate system under consideration is assumed to be rigid and circular. Practical implications The proposed scheme for imposing periodic boundary conditions on the RVE can be adopted solely for estimating the effective mechanical properties of granular materials and/or integrated into the frame of the second-order computational homogenization method with a nested finite element method-discrete element method solution procedure for granular materials. It will tend to provide, at least theoretically, more reasonable results for effective material properties and solutions of a macroscopic boundary value problem simulated by the computational homogenization method. Originality/value This paper presents a novel scheme for imposing periodic boundary conditions with downscaled macroscopic strain measures of gradient Cosserat continuum on the RVE of discrete particle assembly for granular materials without need of introducing Lagrange multipliers for enforcing periodic boundary conditions in a weak (integration) sense.

Quantifying the impact of a debris avalanche against a flexible barrier by coupled DEM-FEM analyses

Flexible barriers are an effective type of mitigation measure against debris avalanches in valleys and can affect the kinetic energy and mass of debris avalanches. To investigate the complex dynamic interaction between a debris avalanche and a flexible barrier, a modeling framework was developed based on coupled discrete element method (DEM) and finite element method (FEM). The debris avalanche was modeled as an assembly of discrete particles accounting for collision–friction effect, and the flexible barrier was modeled by applying the FEM accounting for connections and contact in a realistic manner. The coupled numerical model was well validated by comparing the numerical results with experimental data reported in the literature. This model was employed to examine the dynamics of debris avalanche–barrier interaction, such as the interaction process, impact force, energy dissipation mechanism, and evolution of the internal forces of the components, as well as the influences of the bulk density and impact height of the debris avalanche and the slope angle of the channel on the coupled impact force. Based on the numerical results, three interaction stages were identified, namely, frontal impact, run-up, and pile-up. During the interaction process, the effects of run-up and dead zone formation need to be accounted to avoid conservative estimates of the impact force. The analyses of the energy transformation showed that 76.2% of the initial total energy was dissipated by interaction friction, and the flexible barriers may mainly focus on the frontal impact energy of the debris avalanche by using an energy design approach. The sensitivities of the bulk density, impact height, and slope angle on the maximum impact force are 1, 0.186, and 2.084, respectively. The parameters with high sensitivity are the main influencing parameters and should be given priority in the design of flexible barriers.

Criterion for Change in the Particle Settling Regime in Viscous Liquid

The results of numerical modeling are presented for an assembly of discrete particles settling in a viscous liquid. A simple criterion is found for a change in the Stokes settling to sedimentation convection, which as a limit goes to the Rayleigh–Tailor currents.

Multiscale modeling and characterization of coupled damage-healing-plasticity for granular materials in concurrent computational homogenization approach

A multiscale modeling and characterization method for coupled damage-healing-plasticity occurring in granular material is proposed. The characterization is performed on the basis of multiscale modeling of granular material in the frame of concurrent second-order computational homogenization method, in which granular material is modeled as gradient-enhanced Cosserat continuum at the macroscale. The damage-healing-plasticity is characterized in terms of meso-structural evolution of discrete particle assembly within representative volume elements (RVEs) assigned to selected local material points in macroscopic continuum, with no need to specify macroscopic phenomenological constitutive models, failure criteria along with evolution laws, and associated macroscopic material parameters.The proposed modeling and characterization method for coupled damage-healing-plasticity in granular material is comprised of the following three constituents. The incremental non-linear constitutive relation for the discrete particle assembly of RVE is first established. Then the meso-mechanically informed incremental non-linear constitutive relation of macroscopic gradient-enhanced Cosserat continuum is derived from volume averages of the RVE-scale solutions. Finally the thermodynamic framework is set up to define meso-mechanically informed anisotropic damage and healing factors, anisotropic net damage factors combining both damage and healing effects, and plastic strains. Densities of damage, plastic and total dissipative energies as well as non-dissipative healing energy, as scalar internal state variables, are provided to compare the effects of damage, healing and plasticity on material failure and structural collapse. The numerical example of strain localization and softening problem is performed to demonstrate the performance and applicability of the proposed multiscale modeling and characterization method of coupled damage-healing-plasticity for granular materials.

Microscopic numerical modeling of Thermo-Hydro-Mechanical mechanisms in fluid injection process in unconsolidated formation

The temperature difference between injected fluid and surrounding rock can play an important role in the initiation and propagation of fractures in completion and simulation operations. In this work, the thermal effect is investigated using DEM-based micromechanics model implemented in Particle Flow Code (PFC). The rock mass is represented by an assembly of discrete particles that are bonded at contacts. In the developed thermo-hydro-mechanical (THM) module, heat conduction across the solids in the rock matrix and heat convection between the injected fluid and solid particles are coupled. The evolution of the system is driven by a continuous point source of heat. Simulation results show that more cracks tend to be generated in anisotropic thermal conductivity, and the thermal induced stress, temperature distribution and pore pressure predicted by this model are consistent with laboratory observations and simulation results from conventional continuum mechanics models. Furthermore, the Fracability Index is introduced for evaluating the fracturing performance, in the comparison between hydro-injection and hydro-thermal injection (heating and cooling). This study indicates that thermal stresses can greatly affect initiation and propagation of fractures and assist the communication between injection pressure and pore pressure in the rock formation; thermal fracturing can be considered as an effective method to connect fracture network, enhance weak zones and create more fractures.

Meso-hydro-mechanically informed effective stresses and effective pressures for saturated and unsaturated porous media

Abstract Based on the meso-structured Voronoi cell model and the meso-macro homogenization procedure between the discrete particle assembly and the porous continuum for wet granular materials, meso-hydro-mechanically informed effective pressure and effective stress measures for saturated and unsaturated porous media are defined. The meso-hydro-mechanically informed generalized effective stress for saturated porous continua taking into account the volumetric deformation of solid grains due to pore liquid pressure is derived. The Biot coefficient associated to the meso-hydro- mechanically informed generalized effective stress for saturated porous media is formulated. The differences of the definitions for proposed generalized effective stress and Biot coefficient compared with those defined in the generalized Biot theory of saturated porous continua and in averaging theories are discussed. The wet meso-structured Voronoi cell model, consisting of three immiscible and interrelated (i.e. solid grains, interstitial liquid and gas) phases, at low bulk saturation (below about 30%) is proposed. A meso-structural pattern with the binary bond mode of pendular liquid bridges is assumed in particular to derive the meso-hydro- mechanically informed macroscopic anisotropic effective pressure and effective stress tensors for unsaturated porous media. As the isotropic case of the wet meso-structured Voronoi cell model is considered, the meso-hydro-mechanically informed effective pressure tensor degrades to the scalar variable in the same form as in the theory of macroscopic unsaturated porous continua. The proposed meso-hydro-mechanically informed Bishop's parameter is derived and obtained as a function of saturation, porosity, and meso-structural parameters, without need to introduce any macroscopic phenomenological assumptions for the description of hydro-mechanical constitutive behavior.

Mixed FEM–Crushable DEM Nested Scheme in Second-Order Computational Homogenization for Granular Materials

AbstractA mixed FEM–crushable discrete-element method (DEM) nested scheme in the frame of second-order computational homogenization for granular materials was proposed. The particle breakage followed by the discontinuity and dissipative relative movements between each of two immediate neighboring particles were modeled at the mesoscale to perform both the downscaling and upscaling between the mixed FEM at the macroscopic continuum scale and the crushable DEM at the mesoscopic discrete particle assembly scale. To develop the crushable DEM for modeling the mesostructural evolution within representative volume elements (RVEs) assigned to integrating points of the macroscopic Cosserat continuum in the homogenization, two grain breakage models consisting of crushing criteria and the fracture mode for an individual crushable particle were developed. Not only the contact forces, but also contact moments exerted on each individual grain via the contacting points on the grain surface, were taken into account to se...

Two scale modeling of behaviors of granular structure: size effects and displacement fluctuations of discrete particle assembly

This study's primary aim is to check the existence of a representative volume element for granular materials and determine the link between the properties (responses) of macro structures and the size of the discrete particle assembly used to represent a constitutive relation in a two-scale model. In our two-scale method the boundary value problem on the macro level was solved using finite element method, based on the Cosserat continuum; the macro stresses and modulus were obtained using a solution of discrete particle assemblies at certain element integration points. Meanwhile, discrete particle assemblies were solved using discrete element method under boundary conditions provided by the macro deformation. Our investigations focused largely on the size effects of the discrete particle assembly and the radius of the particle on macro properties, such as deformation stiffness, bearing capacity and the residual strength of the granular structure. According to the numerical results, we suggest fitting formulas linking the values of different macro properties (responses) and size of discrete particle assemblies. In addition, this study also concerns the configuration and displacement fluctuation of discrete particle assemblies on the micro level, accompanied with the evolution of bearing capacity and deformation on the macro level.

Multiscale hydro-mechanical analysis of unsaturated granular materials using bridging scale method

Purpose – The purpose of this paper is to extend the bridge scale method (BSM) developed for granular materials with only the solid phase to that taking into account the effects of wetting process in porous continuum. The granular material is modeled as partially saturated porous Cosserat continuum and discrete particle assembly in the coarse and fine scales, respectively. Design/methodology/approach – Based on the mass and momentum conservation laws for the three phases, i.e. the solid skeleton, the pore water and the pore air, the governing equations for the unsaturated porous Biot-Cosserat continuum model in the coarse scale are derived. In light of the passive air pressure assumption, a reduced finite element model for the model is proposed. According to the decoupling of the fine and coarse scale calculations in the BSM, the unsaturated porous Cosserat continuum model using the finite element method and the discrete element model using the discrete element method for granular media are combined. Find...

Thermodynamic framework for damage-healing-plasticity of granular materials and net damage variable

Based on the meso-structured Voronoi cell model for discrete particle assembly and the derived meso-mechanically informed constitutive relations of anisotropic Cosserat continuum, thermodynamic framework of isothermal meso-mechanically informed damage-healing and plastic process for granular materials is presented. The accumulated net (effective) damage factor tensor combining both material damage and healing effects is defined in terms of the initial (undamaged) and current (damaged) elastic moduli tensors of the meso-structured Voronoi cell attributed to the material point. According to the non-negativity of thermodynamic energy dissipations, the net damage variable is separated into the two component internal state variables; i.e. the damage and healing variables, which are accumulated in terms of incremental damage and healing variables, respectively. The meso-mechanically informed macroscopic damage-healing and plastic characterization are achieved without the need to specify macroscopic phenomenological damage, healing and plastic criteria, and their evolution laws. The merit of the proposed tensorial net damage and healing variables in modeling healing effects on initial weakened elastic stiffness (i.e. initial material defects) is demonstrated in terms of their isotropic scalar forms and integrated into the continuum damage-healing mechanics. The numerical results conceptually illustrate the performance of the proposed definitions of meso-mechanically informed net damage, damage, and healing variables. The coupled damage-healing and plastic process in anisotropic Cosserat continuum for granular materials is characterized in terms of densities of thermodynamic dissipations that make effects of the damage-healing and the plastic component processes on the material failure quantitatively comparable.

Simulation of Red Particles in Blood Cell

Red blood cell (RBC) particle detection and counting with characteristics in blood cell systems has been done by computer simulation. A simulation region, including plasma, red blood cells (RBCs) and platelets, was modeled by an assembly of discrete particles. The proposed method has detected the red particle from blood cell systems through different simulations of MATLAB and GAMBIT & FLUENT. After the detection, the number of red particles in a sampled cell has been counted and the characteristics about the red particles for analyzing the Birth-Death growth of each red particle have been found.

Investigation of geomechanical responses of reservoirs induced by carbon dioxide storage

Assessment of the suitability of potential sub-surface storage sites for CO2 storage cuts across several issues, a dominant part being the sustainability in terms of the retention capacity of prospective reservoirs. Questions often raised but not properly investigated border on the stability of underground reservoirs during the injection process and the protracted effect after injection is fully completed. A review of studies on CO2 sequestration reveals several uncovered areas with one significant aspect being the geo-mechanical effect of CO2 injection and storage within the underground formation. A computational framework has been built as part of a series of ongoing investigations to ascertain the susceptibility of underground formations during and after CO2 is introduced. This is made possible by adopting a discrete element modelling methodology as a first step in the sequence of a designed procedure. By applying this technique, the formation materials are idealised as an assembly of discrete particles interacting in a manner which allows for specific descriptions of the morphology and fracturing events. Computational tests conducted on several types of models representative of reservoir formations reveal reservoir geo-mechanical responses highly dependent on factors, such as material property of rocks, pressure build-up and injection pressure. An example of this is observed in the mode of fracturing events which is significantly influenced by the rate of fluid injection. The outcome of this study forms a strong basis towards a better understanding of the behaviour of reservoir formations subjected to CO2 injection and storage. In addition, information from these studies could serve as a reference for enhanced oil recovery processes and enhanced coal bed methane productions.

Micro–macro homogenization of granular materials based on the average-field theory of Cosserat continuum

This paper performs further study on the micro–macro homogenization approach of granular materials (Li et al., 2010) based on the advancement of Hill’s lemma for Cosserat continuum (Liu, 2013). Firstly, the average couple stress tensor, expressed as the volume integration of quantities over the representative volume element (RVE) in the average-field theory of Cosserat continuum, is further deduced and expressed in terms of discrete quantities on the discrete particle assembly RVE of granular materials. The expression is also discussed and compared with other typical definitions of the effective couple stress tensor for granular materials in the literature. Then, rate forms of micromechanically based constitutive models consistent with different types of RVE boundary conditions are derived and discussed. Since the presented micro–macro homogenization approach is used, not only the micro–macro energy equivalence is satisfied, but also the microstructure and its evolution can be taken into account in the constitutive formulation with no need of specifying macroscopic phenomenological constitutive model.