Computational Particle Mechanics最新文献

The Brazilian test is probably the most widely accepted method for estimating the tensile strength of brittle materials due to its simplicity in sample preparation and ease of testing. However, despite its widespread use, various shortcomings have been noted since its introduction in the early 1940s. Overlooking these flaws has been shown to result in inaccuracies when determining the tensile strength of a given material. This study employs discrete element numerical simulations to explore some of these shortcomings in detail, with a particular interest in the loading platen curvature and stress distribution within the Brazilian disc and their impacts on the process of crack initiation and propagation. Micro-properties of the bonded particle model were first calibrated for a typical brittle material under different loading conditions. Loading platens with different jaw curvatures (disc-to-jaw curvature ratio in the range of 0 to 0.8) were simulated and the cracking mechanism inside the Brazilian disc was investigated. Results show that for curvature ratios less than 0.67, cracks are almost always generated in the vicinity of loading platens. When the curvature ratio approaches 0.8, tensile cracks initiate from the disc centre possibly due to the longer contact length available. A sensitivity analysis on the effect of disc and loading platen parameters was further conducted and it was found that particle and bond modulus, wall modulus, and particle & bond normal/shear stiffness are the key parameters controlling the transition of the crack initiation point from underneath of loading platens to the disc centre.

Excavation damage zone (EDZ) is an important index to evaluate the damage and failure of surrounding rock. Based on the rock phase field model considering rock nonlinear fracture criteria, the concept of excavation damage zone model is defined, the zoning criterion of excavation damage zone model is proposed. The topological region centered on the contour boundary of the tunnel is defined as damage phase field. The tunnel excavation damage zone based on the modified phase field model, and the inflection point of damage phase field gradient change is taken as the criterion of EDZ. By comparing and analyzing the site monitoring and numerical simulation results of Mine-by tunnel excavation damage zone at −420 m URL, the error is about 0.5%, which verifies the validity of the tunnel EDZ phase field model. The distribution characteristics of EDZ of tunnels with different section shapes are analyzed, the main damage characteristics and key reinforcement areas of tunnels with different shapes are expounded, and the rationality of phase field model of excavation damage zone is further explained.

Using uniaxial compression tests and coupled FDM-DEM numerical simulations, the mechanical and energetic evolutions of unconfined and CFRP-confined coal samples were investigated under varying height-to-diameter ratio (HDR). The results indicate that peak strength decreases, while the elastic modulus increases nonlinearly with an increase in the HDR, and CFRP confinement significantly enhances the mechanical properties. The contact number increases, whereas the contact force decreases, with crack initiation and yield stress points identified through the second-order derivative of the crack number. For unconfined coal samples, damage propagation occurs from the center outward, while for CFRP-confined samples, it progresses from the ends toward the center. As the HDR increases, energy density and axial strain decrease, with geometric size differences and energy conversion emerging as critical factors for instability. CFRP-confined coal samples demonstrate greater energy storage and dissipation capacities compared to unconfined samples. The dissipated energy conversion for unconfined samples peaked at 10.76% at a HDR of 1.5 and was lowest at 5.53% at 3.0, while CFRP-confined samples peaked at 16.34% at 0.5 and dropped to 5.62% at 2.0. These findings reveal that an increasing HDR reduces ductility and raises instability risks, whereas CFRP confinement improves deformation resistance and energy dissipation, offering a theoretical basis for the reinforcement of coal samples.

In underground coal gasification (UCG) projects, the flow of high-temperature gas leads to the simultaneous exposure of surrounding rock in tunnels to high-temperature and cyclic load, posing risks of rock instability. To investigate the failure modes and precursors of rock instability under high-temperature and cyclic loading, uniaxial compression tests and cyclic loading–unloading tests were conducted, utilizing acoustic emission (AE) technology and discrete element method (DEM). In addition, the microstructural damage of red sandstone after heat treatment was analyzed utilizing scanning electron microscopy (SEM) images. The test results reveal that increasing temperature leads to a delayed appearance of change points in AE counts and b value and more pronounced AE signals. Fatigue loading cannot always cause severe damage to red sandstone at the first cycle of the load level, according to the 25 °C group, 400 °C group, and 600 °C group specimens. RA-AF data distribution reveals that shear crack proportion generated by uniaxial compression in red sandstone subjected to the same temperature treatment is significantly greater than that caused by fatigue loading. Furthermore, the shear crack proportion in red sandstone specimens drops and then climbs with increase in temperatures. Based on the critical slowing down (CSD) theory, this paper analyzes the variance curves and autocorrelation coefficient curves of multiple AE parameters. The sudden drop point of the b value was taken as the starting point of rapid crack propagation, and the mutation points of various AE parameters lagging behind this starting point were searched for. It was found that the change point of AE energy variance is closest to the specimen instability point, which can be employed as a precursor for specimen instability. Heat treatments cause the internal structure of the red sandstone to evolve from a pompon-like structure to a flocculated porous structure. In addition, thermally induced cracks gradually develop and connect.

The impact of water content on the dynamic behavior of concrete under the uniaxial compression state at the mesoscale was examined in this study. Extensive two-dimensional (2D) dynamic investigations into the impact of free water on dynamic strength and fracture of concrete of low porosity were performed. The effects of strain rate, fluid saturation and fluid viscosity were investigated in depth. The behavior of fully and partially fluid-saturated concrete was simulated using a mesoscopic pore-scale hydromechanical model based on a unique fully coupled DEM-CFD approach. To generate a fluid movement, the model featured a network of channels in a continuous area between discrete elements. In partially wet concrete, a two-phase laminar fluid flow (air and water) in pores and cracks was proposed. For accurate liquid/gas content tracking, the location and volume of pores and cracks were taken into account. On specimens of a simplified spherical mesostructure that mimicked concrete in both dry and wet conditions, a series of dynamic numerical simulations with varying strain rates were run. The particle fragmentation was disregarded. The dynamic compressive strength increased with the strain rate, fluid saturation and fluid viscosity. The pore fluid pressures slowed a fracture process because of the fluid confinement in pores, which resulted in increased concrete strength.

Tunnels, mining activities and other underground engineering projects are frequently threatened by water and mud inrush accidents when they cross fault zones, which pose challenges to the safety and efficiency of underground engineering. The evolution of particle migration in fault zones under water seepage, which is the primary cause of water and mud inrush, is poorly understood. In this paper, the successive random addition method algorithm is used to generate fault surfaces with different Hurst exponents and employs CFD‒DEM coupled numerical simulation to study the evolution of particle migration and variable-mass seepage characteristics of fault fillings under different fault surface roughnesses and different fault spacings. The results show that a rough fault surface hinders particle migration. In variable-mass seepage through different rough fault surfaces, the loss of fine particles (d < 2.5 mm) exceeds 90%, and with increasing roughness, the contact force chains between skeleton particles decrease, whereas those between fine particles increase. The failure process of variable-mass seepage in rough fault fillings can be divided into three stages: particle migration and reorganization, particle clogging, and instability erosion of skeleton particles, whereas smooth faults (H ≥ 0.75) experience secondary development of particle loss. The expansion of fault spacing reduces the influence of rough fault walls on particle loss, but the rough wall still obstructs the flow of fine particles. These findings provide a scientific basis and technical support for studying and controlling water and mud inrush disasters in fault zones.

Shocked particle-laden flows are important to many natural and industrial processes. When simulating these systems, artificial viscosity is often required to prevent numerical artifacts, such as ringing, from arising in the pressure and density fields. The linear and quadratic coefficients of the artificial viscosity determine the amount of smoothing that occurs in these fields. For particle-laden flows, however, many of the fluid–particle interaction forces, for example, the pressure gradient force and unsteady forces, depend on gradients in the fluid fields. Furthermore, while the shock passes over a particle, these forces can be more dominant than drag. This means that the artificial viscosity coefficients affect how a particle and fluid interact when simulating shocked particle systems. Here this effect is investigated for isolated particles and for a particle curtain using a staggered grid Lagrangian approach. The artificial viscosity coefficients have a significant impact on the maximum force that a fluid imparts to a particle, which is important for determining whether a particle will break up in response to the shock. Furthermore, it is found that the density ratio between the particle and the fluid is important in determining whether the artificial viscosity coefficients have a significant impact on the particle’s motion.

This study demonstrated a simulation of bidispersed granular column collapse using the Discrete Element Method (DEM) and an elastoplastic model based on Smoothed Particle Hydrodynamics (SPH). The present simulation model was developed to solve the deformation of a mixed layer of a small-scale granular material, such as sand, and a large-scale material, such as gravel. In the present model, the behavior of a large granular material was tracked using the DEM, and a small granular material was treated as a continuum on the basis of an elastoplastic constitutive law in an SPH framework. The model was validated by comparing its simulation data with the experimental results of previous studies. First, in the simulation of the collapse of a monodispersed granular column for each granular material size, some parameters were tuned. Thereafter, five simulation cases with varying mixture arrangements of the two granular materials were conducted. The position of the center of gravity of each material in the final deposit after collapse was investigated. The calculated results well agreed with the experimental results.

This article introduces a comprehensive model for simulating the hydrodynamics of a draft-plate spouted bed by employing a coupled CFD–DEM (computational fluid dynamics–discrete element method) approach. In contrast to studies focusing on isolated parameters, the present work applies the DEM approach to analyze the combined influence of all pertinent parameters thoroughly, thus offering significant insights into their collective impact on simulation outcomes. This study investigated five distinct drag force models, evaluated the inclusion and influence of the Saffman lift force, and compared three different formulations for the Magnus lift force. Moreover, five diverse combinations of turbulence models were explored to capture the complex flow dynamics within the spouted bed. Finally, the effects of varying the restitution coefficient on particle collisions were investigated to understand its impact on particle behavior. The simulation results show that the Gidaspow drag model demonstrated superior adaptability, making it appropriate for simulating draft-plate spouted beds. In addition, the study assessed the impact of the Saffman lift force and identified specific regions within the bed where its influence is most pronounced. Moreover, the work established the Robinow–Keller model as the most effective formulation for the Magnus lift force coefficient. Regarding turbulence modeling, the standard k–ε model paired with the dispersed multiphase behavior expression yielded the lowest error, indicating superior accuracy. Moreover, a restitution coefficient of 0.9 was identified as the appropriate value for simulating particle collisions. Furthermore, this study substantially improved accuracy, reducing the simulation root-mean-square error (RMSE) by 55.5% from 0.155 to 0.069.

The longwall top coal caving mining (LTCC) is one of the effective mining methods for thick coal seams in China. With the advancement of LTCC panel, upward mining, downward mining and alternate upward–downward mining always occur in the whole mining process, which has great effect on the top coal drawing law. By summarizing the motion characteristics of top coal and gangue in upward mining or downward mining, the dynamic caving interval technology for the upward–downward transition is proposed in this paper, and the variation of top coal boundary, caving body and top coal recovery at different caving stages in this technology is analyzed by discrete element numerical simulation. The results show that when the LTCC panel adopts the dynamic caving interval technology, the top coal boundary is relatively smooth and the caving body develops preferentially in vertical direction at the upward mining stage with the interval of one-cutting and one-caving. When the interval of three-cutting and one-caving is used at the downward mining stage, the intrusion of gangue can be effectively reduced toward the goaf, and the horizontal development of caving body is obvious. Compared with the conventional fixed caving interval technology, the top coal recovery of the whole panel under the dynamic caving interval technology is improved by about 3.64%. In the excessive caving process, the terminated gangue mixed ratio at downward mining stage and horizontal mining stage is low under the same high top coal recovery, at 10–15%. The research results provide a new approach for improving the top coal recovery at the upward–downward transition stage and obtain a theoretical basis for determining the closing time of caving opening in intelligent LTCC.