Granular Matter最新文献

Underground grain squat silos offer a series of advantages, including energy saving, low carbon emissions, and environmental protection, which are of great significance in the context of food security. To investigate the behavior of particle dynamics and the dynamic response between the silo and particles in the underground grain squat silo during eccentric discharge, a combination of numerical simulation and theoretical analysis was employed. A mathematical model was constructed using the discrete element method, and a comparison with Janssen’s theory corroborated its validity. The particle dynamics behavior of eccentric discharge was examined through the established numerical model, with a view to elucidating the variations in dynamic pressure, particle contact stress, velocity, and angular velocity that occur under eccentric discharge conditions. The results demonstrate that: (1) The dynamic pressure and overpressure coefficients exhibit variation in different directions during eccentric discharge, with a gradual decrease observed with increasing discharge hole. (2) The incorporation of a central cylinder introduces a more intricate flow path for the particles during the discharge process. As the number of discharge hole increases, the flow path of the particles extends in length, and the velocity of the particles gradually increases, while the angular velocity and contact stress gradually decrease. (3) The displacement of the silo is relatively minor and diminishes as the quantity of discharge apertures rises. This paper offers a theoretical reference for the optimization of the structural design, grain discharge process, and overall performance of underground grain squat silos.

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The aim of this work is to contribute to a better understanding of the physical aspects involved in the coefficient of lateral earth pressure at rest ((K_0)), focusing in particular on the effect of loading history on this coefficient. For this purpose, a 2D discrete element modeling of a cyclic oedometer test on a granular sample is carried out. It is shown that the evolution of (K_0) with loading is in good agreement with that of empirical formulas derived from experience. Analysis of the variation of certain micromechanical parameters, namely the fabric tensor, the number of contacts and the forces acting in contact with the load, has shown that for a normally consolidated granular sample, horizontal contacts are dominant, this dominance decreases with increasing degree of overconsolidation. The variation in the dominant direction of contacts is due to the differential change in the number of contacts per direction as the load varies. The orientation of the contacts may give some indication of the tendency of (K_0), however, the value of (K_0) is more influenced by the forces acting on these contacts. The evolution of (K_0) according to a loop for a loading-unloading cycle in the oedometer test, is ultimately governed by intergranular friction.

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Effect of stress history on lateral earth pressure coefficient at rest. A discrete elements analysis

For a jet to effectively penetrate a soil-covered target, the depth of penetration (DOP) into the soil must exceed a critical threshold, while the cavity diameter produced by the precursor shaped charge of a tandem warhead should be large enough to allow the unimpeded passage of the main projectile. In light of the prevalent application of protective structures with overburden layers, this study examines the penetration behavior of shaped charge jets into soil targets. Additionally, the impact of soil moisture content—considering seasonal variations—on jet penetration performance is investigated. The analysis addresses the roles of shock waves and compressibility during penetration, proposes a novel four-stage penetration process, and formulates an axial penetration equation. Based on this framework, a jet penetration model for soil targets is developed by incorporating the HELD cavity growth model. Four experimental tests were carried out in which shaped charge jets penetrated soil with different moisture contents. The results indicate that the proposed model provides accurate predictions for both radial cavity growth and DOP, showing good agreement with experimental data. Furthermore, it is observed that radial cavity dimensions increase markedly with rising moisture content, whereas axial penetration depth remains largely unaffected.

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This study utilizes a coupled computational fluid dynamics and distinct element method (CFD-DEM) to analyze the suffusion behaviors of the grain-cementing type methane hydrate-bearing sediments (MHBS) under different methane hydrate (MH) saturations and environmental conditions. MH bonds existing in the MHBS sample were synthetize by using thermo-hydro-mechanical-chemical (THMC) contact bond model, which accounts for the influences of ambient temperature, pore water pressure, and salinity on the MH behavior. A comprehensive series of suffusion tests is performed under various hydraulic heads to investigate the impact of MH saturation and environmental conditions on the suffusion behaviors of MHBS. The results demonstrate that the breakage of MH bonds between fine and coarse particles is a fundamental prerequisite for the migration of fine particles. Both the cumulative fine mass loss ratio and the bond breakage ratio are two important factors in determining the critical hydraulic gradient of MHBS, which increases as MH saturation or the value of condition parameter increases. Besides, the void ratio variation during suffusion of MHBS is influenced by both the particle migration and the hydraulic gradient. Finally, this study presents the permeability coefficient equation and erosion law for MHBS suffusion, which incorporate important parameters related to MH, such as MH saturation, bond breakage ratio, and the condition parameter.

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The arrangement of dry, mono- and bidisperse granular systems composed of spherical particles, dynamically introduced from a central inlet under the effect of gravity, is being investigated within a cylinder with frictionless walls. The analysis, performed using 3D discrete element modeling, investigated the arrangement and structure of granular systems with an 1:3 particle diameter ratio and varying mass fractions. The analysis focuses on the temporal and spatial evolution of kinetic segregation, the ordering of the systems, and the development of different types of interactions (particle–particle and particle–wall). Additionally, the 2D arc of the upper surface of the particle system is described using for a quick determination of the interstitial air volume and the void fraction within the granular systems. For the analysis of the effect of dynamic filling, static, space-filling samples were created. A detailed research plan was prepared to thoroughly document the computational methods of the study. Based on the incoming mass fraction into the same system, 6 segregation 3D zones can be distinguished. Fractures (cracks) form in the framework of the mono-disperse particle system early in its arrangement within the container. The resulting concave surface can be well approximated with linear and quadratic curves. The average normal force acting on the volume units of the smaller particles is 1.7–2.8 times greater than that of the larger ones. The temporal segregation of particles barely depends on size. The entry of small particles at a 10% mass fraction into the system results in segregation and the rearrangement of the particle framework. The relationship between the number of contacts of the granular system’s particles and the cylinder’s bounding elements is exponential in quasi–static states.

The mechanical properties of rockfill materials degrade markedly under dry–wet cycles, leading to uneven settlement of rockfill dams over time. To elucidate the underlying mechanisms, this study utilizes the particle discrete element method (DEM) to conduct numerical simulations of triaxial compression on crushable rockfill samples under various dry–wet cycle scenarios. This computational approach facilitates a comprehensive analysis of the material's macroscopic mechanical behavior while simultaneously revealing its microscopic degradation processes. The research findings are presented in detail below: (1) Macroscopic law: The results indicate that an increase in the number of dry–wet cycles (N) corresponds with a decrease in peak stress and an increase in volume strain. However, these parameters tend to stabilize, showing minimal change once N exceeds a certain threshold. Furthermore, the analysis reveals an exponential relationship between N and the key shear strength parameters: the apparent cohesion (c) and the internal friction angle (φ). Concurrently, as the cycles progress, both the rate of particle breakage (B_r) and the net increase in particle count progressively diminish. (2) Microscopic mechanism: An increase in N leads to the expansion of force chain networks, a wider distribution of particle fractures, and a more extensive displacement field within the sample, accompanied by a significant rise in the average coordination number. Mirroring the macroscopic behavior, these microscopic changes also become less pronounced after N surpasses the established threshold, suggesting the sample is approaching a state of mechanical stability. (3) Damage evolution: Analysis of the modified Duncan-Chang model reveals that its constitutive parameters (A, B, and C) demonstrate an exponential relationship with the number of dry–wet cycles (N). These parameters undergo a phase of rapid exponential growth during the initial cycles, which subsequently transitions to a stable state as the cycling progresses.

To systematically investigate the mechanical behaviors of rockfill materials under the combined influence of particle shape and sample size, a stochastic algorithm that incorporates three-dimensional shape parameters such as elongation, flatness, sphericity, and convexity, was employed to generate irregularly shaped rockfill particles. A triaxial numerical simulation method was proposed for rockfill materials, which comprehensively accounts for both particle shape and sample size. Discrete element numerical simulations of rockfill materials with varying particle shapes and sample sizes were conducted to investigate the effects of these two factors on the mechanical properties and nonlinear constitutive models from both macroscopic and microscopic perspectives. The evolution of microscopic characteristics including coordination number, Euler angles, and fabric anisotropy of rockfill bodies were examined, and the stress–strain and volumetric strain responses of various particle samples were analyzed, revealing the correlation mechanisms of particle shape and sample size on their macro- micro properties of rockfill materials. Based on these findings, a novel constitutive model that integrates both particle shape and sample size was presented, aiming to offer a more accurate and comprehensive theoretical framework for understanding the mechanical behavior of rockfill materials.

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This study examines the interaction between granular flows and barrier systems in landslides, employing both a landslide model test and PFC^3D discrete element simulations. The investigation focuses on the temporal evolution of the granular flow front velocity and changes in flow states, with an emphasis on slope angles and lateral blockage ratios. The results demonstrate that the front velocity of granular flows exhibits a generally increasing trend. More pronounced acceleration occurs on steeper slopes with moderate blockage ratios, while lower slopes or higher blockage ratios lead to slower acceleration. Despite this, an overall increase in velocity is consistently observed across all scenarios. A critical transition is identified at the 0.2-s mark, where slope angle plays a pivotal role, and interactions with barriers cause the flow to split and deflect, resulting in energy dissipation. Furthermore, under controlled single-variable conditions, the study reveals that the average flow velocity is inversely proportional to the lateral blockage ratio and directly proportional to the slope angle. The barrier system significantly influences the flow velocity: for the first row of barriers, the total average velocity attenuation increases with steeper slopes and higher blockage ratios, while for the second row, the attenuation shows a proportional relationship with the lateral blockage ratio. These findings contribute to the understanding of granular flow dynamics in landslides, providing valuable insights for debris flow mitigation, landslide risk management, and engineering applications involving barrier systems.

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Granular flows are widely present in engineering and natural phenomena, and their interactions with obstacles represent a critical research direction in flow dynamics, with significant implications for disaster prediction and engineering optimization. This study focuses on the shock wave interference phenomenon in gravity-driven granular flows interacting with parallel cylindrical obstacles, aiming to explore its physical mechanisms and influencing factors. The experimental design incorporates varying obstacle spacings and slope angles to systematically investigate the impact of shock wave interference on flow structure, geometric characteristics, and velocity field distribution. The results indicate that decreasing obstacle spacing significantly enhances shock wave interactions, leading to an increase in the standoff distance (D_standoff) of the shock wave front and greater upstream flow obstruction. Conversely, increasing slope angle accelerates the free-stream velocity, further intensifying the shock wave strength. Analysis using a convection–diffusion model reveals that diffusion intensity has a critical influence on the curvature radius of the shock wave, with larger spacings reducing convective effects and thereby increasing the shock wave curvature. Additionally, velocity field profile analysis highlights a dual mechanism of particle velocity reduction and deflection, where the number of deflection peaks and total deflection area increase with obstacle spacing. These findings provide new insights into the dynamic mechanisms underlying granular flow-obstacle interactions and offer theoretical support for controlling granular flows in related engineering applications.

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To acquire the ability to numerically study the rheology of particulate two-phase flows that lack scale separation, we present a general method to average or coarse-grain the equations of motion of a mixture of a continuous fluid of arbitrary rheology and non-Brownian particles, interacting via contacts, of arbitrary shapes and compositions. It universally covers ensemble and typical spatio-temporal averaging procedures and overcomes two shortcomings of existing methods. First, the derived micromechanical expressions for the coarse-grained fields are mathematically exact and formulated in a manner that allows a computationally cheap extraction from Direct Numerical Simulation–Discrete Element Method (DNS–DEM) simulations, avoiding the unlimited-order derivatives appearing in previous exact formulations. Second, the microscopic volume fraction of each particle is its corresponding indicator function, rather than the traditional volume-weighted delta distribution at its center of mass, to ensure that the resulting macroscopic fluid and solid volume fractions add precisely to unity. This leads to an additional contact stress contribution not seen in standard coarse-grained expressions for granular matter, and, for non-spherical particles, to particle-rotational contributions to translational solid phase balance equations. Many implementations of DNS–DEM simulations are based on Immersed Boundary Methods (IBMs), for which modifications of the coarse-graining method are necessary due to certain peculiarities of IBMs, such as the replacement of the particles’ interiors by pseudo-fluid. We therefore derive mathematically exact adaptations of the coarse-graining method for two distinct common IBM versions, implement one version to obtain coarse-grained fields from sediment transport simulations based on this version, and validate the implementation.