Granular Matter最新文献

This paper analyzed the influence of the inherent anisotropy of sand on active and passive arching by simulating the trapdoor emplying the discrete element method (DEM). The inherent anisotropy is reflected by the bedding plane angle α of particles. The granular material constitutive responses are captured on representative volume elements (RVEs). A new modeling method is employed to prepare particle specimens, aiming to obtain a more uniform soil model. The results indicate that the discrete element method can simulate the influence of the inherent anisotropy of granular material on the evolution of soil arching. An asymmetric arching evolution phenomena is observed in the α other than 0° or 90° cases, which leads to obvious asymmetric deformation and stress distribution in the soil. As the filling height increases, this phenomenon becomes more and more obvious. From a microscopic perspective, the reorientation of the contact normal fabric caused by particle rotation is the main reason for the differences in soil arching evolution with different α. This study provides a theoretical basis for predicting ground deformation failure caused by underground engineering activities and changes in surrounding environmental conditions.

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Gravitational mass movements represent a serious threat for populations and infrastructures. Their dynamics are influenced by topography, mechanical and rheological properties of the flowing material, and the potential erosion or entrainment of bed material. A longstanding challenge involves theorizing the complex influence of material rheology and entrainment on avalanche mobility to improve predictions of flow run-out and impact, crucial for hazard assessment. To enhance process understanding and improve snow avalanche physics-based models, we investigate the interplay between cohesion and entrainment on the mobility of cohesive granular flows over an erodible bed. We conducted 2D DEM simulations of an avalanche slope where cohesive granular materials release and flow over continuous erodible beds of bonded particles. A comprehensive sensitivity analysis focused on the influence of released mass, cohesion, and maximum erodible depth on avalanche mobility and entrainment rate. Our results indicate that inter-particle cohesion significantly influences flow mobility, with highly cohesive beds exhibiting limited entrainment rates and run-out distances. The study reveals that the propensity of particles to form new cohesive bonds during flow significantly affects mobility. Instances where bond formation is feasible (adhesive) show considerably lower mobility and entrained mass compared to scenarios where bonds cannot form during flow (non-adhesive). Finally, we propose a scaling law relating the ratio between actual and maximum entrainment rates to the ratio between bed cohesion and a pressure term accounting for dynamic and gravitational contributions. This study enhances our understanding of geophysical mass flow dynamics and highlights the crucial role of cohesion and entrainment in flow mobility.

Graphical Abstract

The soil fabric varies significantly depending on the deposition process that forms the grain skeleton. Each deposition method produces a specific type of soil fabric, which can be linked to a particular soil density. When represented as relative density, determined using limit densities from standard index tests, a wide range of relative densities can be observed for different sands produced by the same deposition method. The influence of this variation in relative density, resulting from a single deposition method, on the development of the excess pore water pressure (PWP) should be further investigated. A fast testing of the excess PWP accumulation in sandy soils during undrained cyclic shearing can be easily performed using the newly developed PWP Tester. In the PWP Tester, specimens are prepared through sedimentation in water, which yields a comparable fabric in different sands but significantly different relative densities. Despite these relative density differences, the rate of the excess PWP evolution during undrained shearing is remarkably similar among different sands. This indicates that relative density should not be regarded as a primary factor influencing the development of the excess PWP and that the soil fabric plays equal or even a greater role.

We investigate the effect of particle deformability on the flow behavior in a 2D silo. We use a novel Smoothed Particle Hydrodynamics-Discrete Element Method (SPH-DEM) approach that explicitly models the particles’ deformation. We identify a two-fold mechanism through which particle deformation influences silo flow: (i) the spatial arrangement of particles and (ii) the velocity distribution of particles at the outlet. Specifically, we observe—for orifices larger than five times the particle diameter—that the velocities at the outlet follow the same distribution for both hard and soft particles. Thus, we are able to collapse appropriately scaled velocity profiles at the outlet onto a single master curve. Also, we find that our velocity scaling should take the different spatial organization of soft and hard particles near the orifice into account. Finally, we explore the effect of particle deformation on the silo discharge rate. By introducing a deformability parameter (alpha ), we propose an extended version of the Beverloo equation that accounts for the influence of particle deformation on the flow rate. Interestingly, we find that this deformability parameter should be chosen as the ratio of the stress at the bottom of the container and the bulk modulus of the material.

As a kind of geophysical flow in high and cold region, rock-ice avalanches increase in volume and potential impact by eroding and entraining bed material during their movement, thereby posing significant risks to human lives and infrastructure located downstream. This granular process is regulated by the basal stress and its variations at the flow-bed interface. It is imperative to offer a comprehensive understanding of the basal stresses produced by granular flows in order to enhance hazard risk management strategies. In this study, we conducted a series of discrete element method (DEM) simulations of rock-ice avalanches under steady-state conditions to enhance our microscopic understanding of particle-bed interactions. The quantitative indices of basal stress fluctuation, specifically the maximum stress and the standard deviation of stress, as well as the microscopic indices of particle interaction, including the Savage number, granular temperature, and particle free space, are assessed through numerical simulation. The results indicate that as the Savage number increases, the mode of particle interaction with the bed shifts from continuous contact to random collisions, leading to significant fluctuations in basal stress. Furthermore, variations in stress fluctuation are correlated with granular temperature, indicating a dependence on random motion of particles. In conclusion, a microscopic mechanism underlying stress fluctuations is proposed based on particle dynamics. As the macroscopic flow intensifies, the available free space surrounding the particles increases, resulting in an elevated local velocity due to the random motion of the particles, which generate a greater impact force on the bed.

Graphical Abstract

This study investigates the evolution of downsized barchan dunes in a water tunnel experiment, emphasizing the influence of inflow velocity on their crescent-shaped attractor. Based on image processing methods, the kinematic and morphological features of barchan dune evolution are determined. The results reveal that the varying inflow velocity will affect the crescent-shaped attractor. Furthermore, a cubic relationship is established between flow velocity and the aspect ratio of barchan dunes. Additionally, there is a morphological threshold in the evolution of the crescent-shaped attractor to evaluate its stability. This study highlights the sensitivity of the crescent-shaped attractor to inflow velocity and confirms its steady evolutionary pattern, and it provides a new perspective on the formation of the barchan dune swarms in the field of wind-blown sand dynamics.

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The distribution of rock fragmentation after blasting is an important indicator for assessing the effectiveness of mine blasting. The quantitative characterization of blasting fragmentation is a challenging problem for the evaluation of blasting effects. The use of U-Net network technology to segment blasting images provides a new means for obtaining quantitative statistics from blasting fragmentation. Although the U-Net network is generally capable of segmenting images, there are issues in improving the accuracy and efficiency for ores. To solve these problems, this paper proposes a new network structure - ResOreNet. ResOreNet first integrates the Feature-Fusion module into the U-Net network to become a FU-Net network that enhances the model’s identification of target locations and morphological details, thereby improving the accuracy of ore image segmentation. More specifically, it incorporates the residual network into the FU-Net network, which effectively solves the phenomenon of blurring the boundary of the mineral rock image segmentation caused by the overfitting of the model, and the introduction of the residual network effectively mitigates the problem of the gradient vanishing of the loss function during the backpropagation, and also further improves the computational efficiency of the model, and provide a new technical means for obtaining evaluation indicators of blasting effects in mines.

A rheological model for loose granular media is developed to capture both solid-like and fluid-like responses during shearing. The proposed model is built by following the mathematical structure of an extended Kelvin–Voigt model, where an elastic spring and plastic slider act in parallel to a viscous damper. This arrangement requires the partition of the total stress into rate-independent and rate-dependent stress components. To model the solid-like behavior, a simple frictional plasticity model is adopted without modifications, thus contributing to the rate-independent stress. Instead, the fluid-like or rate-dependent stress is further decomposed into deviatoric and volumetric parts, by proposing a new formulation based on a combination of the (mu (I)) relation, originally developed under pressure-controlled shear, with a pressure-shear rate relation derived under volume-controlled shear. The proposed formulation allows the model to capture both the increase in the friction coefficient and the enhanced dilation at high shear rates. High-fidelity simulation data, obtained from discrete element method and multiscale modelling, are used to evaluate the performance of the proposed constitutive model. The model provides accurate results under both drained and undrained simple shear paths across a wide range of shear rates. Furthermore, it successfully reproduces at much lower computational cost the flowslide mobility computed through multiscale simulations, which is primarily regulated by the shear rate dependence of the material properties during the dynamic runout stage.

Mixing performance and heat transfer was investigated in dry granular flows in cylindrical geometry where heat is transferred from cylindrical walls to granular bed. The discrete element method (DEM) was used to simulate these flows and to investigate the effect of different parameters on mixing and heat transfer that include impeller speed, blade rake angle, number of blades and polydispersity. The effect of impeller rotation on heat transfer was also investigated. Mixing quantification was done by using the latest subdomain mixing index (SMI). Results of DEM simulations for these parameters were concluded for mono and poly-dispersed flows. Velocity and heat transfer profiles were drawn. Better mixing was observed in the case of four blades. Higher impeller speed also showed better mixing and heat transfer. In this study, the effect of polydispersity—an often-overlooked factor—is studied. In all cases it was observed that polydispersity had a negative effect on both mixing and heat transfer due to enhanced segregation and reduced thermal conduction. It is also the first-of-its-kind analysis of coupled impeller-geometry effects on particulate mixing and thermal transport in granular media.

Graphical Abstract

Vibratory finishing is widely used in aerospace, weapon industry, high-grade computer numerical control machine tools, rail transportation equipment, and other high-end equipment manufacturing industries. Processing parameters are critical to processing efficiency and processing effect. Island-less bowl vibratory finishing equipment is commonly used to process medium and large disk parts. Compared with traditional bowl vibratory finishing equipment, the lack of an island structure in the shape of the container and the low motor mounting position can lead to changes in the container motion and the flow field of the particulate medium. In this work, a bowl vibratory finishing mathematical model and numerical model were established, the velocity of the granular media at different locations in the container and the magnitude of the force was analyzed, the change rule of the container amplitude size, granular media motion characteristics and force behavior under different processing parameters was elucidated, the container amplitude test experiments and the force test experiments were carried out. The results show that during bowl vibratory finishing, the container performs periodic three-dimensional elliptical motion in space. The lack of island structure in island-less bowl vibratory finishing results in lower motion speed and frequency of the granular media in areas far away from the container wall. The granular media makes a spiral motion around the container’s central axis consisting of circular and tumbling motions in an extended period and performs irregular velocity-changing annular spiral motion in a short period. The phase difference will change the motion direction of the granular media. The mass of the upper eccentric block affects the intensity of the circular motion of the granular media, and the mass of the lower eccentric block affects the intensity of the tumbling motion of the granular media. Selecting a phase difference of 90°, increasing the motor rotational speed and the eccentric block’s mass can increase the granular media’s velocity and force. This study provides a reference for improving processing efficiency and changing granular media flow field motion.

Graphical Abstract