Granular Matter最新文献

Soil-rock mixture (S-RM) is widely distributed in some accumulation slopes and commonly used as a backfill material in the field of geotechnical engineering. The mechanical properties of S-RM play a pivotal role in ensuring the stability of geotechnical engineering projects. The discrete element method (DEM), which can construct S-RM’s microstructure model, is an effective tool for studying its mechanical properties. Currently, the most realistic and precise approach for constructing a three-dimensional (3D) microstructure model of S-RM is digital image processing (DIP) technology using computed tomography (CT) scanning device or 3D laser scanning device. However, these devices are very expensive. This study aims to develop an economical and accurate DEM for constructing the 3D microstructure of S-RM using DIP technology with a conventional digital camera. Firstly, a digital camera was used to capture three sets of 2D images on real rock blocks around four circles at different angles. DIP technology was then applied to process the 2D images and construct the refined 3D rock block grid models. Subsequently, the geometric parameters of the grid models were compared with those of the corresponding real rock blocks to validate the accuracy and applicability of this method. The microstructure model of S-RM in the large-scale direct shear test was then established and verified for DEM simulations. Finally, the mechanical properties of S-RM were analyzed based on the evolution of the shear band, the rotation of rock blocks, and the change of contact force chain.

Graphic abstract

This work introduces a polydisperse aerosol testing system for respirable dust virtual impactor evaluation for the first time, and characterizes its performance. The aerosol in the dust chamber diluted and mixed completely, its diameter distribution follows a lognormal distribution. Although the aerosol concentration increased from top to bottom in the chamber, the particle distribution in the same measurement plane was relatively uniform. The sample flow rate of the test system is stable, and the change of temperature on the flow rate is negligible. The most common virtual impactor with the operating point flow rate of 4 L/min was selected as the subject, and its separation performance, flow variation and load characteristics were evaluated by this test system. The results show that the cutoff diameter deviation is − 2.8% and the maximum deviation from the BMRC curve is − 4.85%. The deviation in the cut-off diameter was more likely to be caused by the minor flow rate than that of a major flow rate. With an increase in the loading time and dust concentration, the virtual impactor was overloaded, which required maintenance according to its actual usage.

Graphical Abstract

Rock fragment size is a key variable in several mining stages such as underground mine design, equipment selection, and mineral processing. In Block Caving, rock fragment sizes are affected by fragmentation during gravity flow in the ore column while ore is being extracted from drawpoints. Additionally, smaller fragments can percolate between large fragments during gravity flow. These two phenomena — rock fragmentation and particle percolation — are not easy to simulate at a large scale in Block Caving. In this paper, a fragmentation model in a cellular automata gravity flow simulator is proposed to model rock fragmentation during flow at large scales. The fragmentation model uses the rock strength, vertical stresses, and travel distance as inputs to estimate the rock breakage and was calibrated with experimental and mine data. The mine scale results show an error of 9% and 7% of the fragmentation in the zones evaluated. This error rate is considered low due to the variability of the phenomena involved. Then, integrating a fragmentation model into a gravity flow simulator can more realistically represent ore fragmentation in caving-mine to generate flow simulations.

The repose of granular materials is investigated via two different Discrete Element Method (DEM) implementations in comparison with an experimental reference from a recently proposed benchmark setup. On a methodological standpoint, a rigorous measurement method of the angle of repose (AOR) is first proposed for plane-strain and axisymmetric conditions as encountered in the reference experiments.Additionally, two systematic procedures are designed in order to also determine the void ratio of the heap, as a fundamental property of granular matter possibly influencing the AOR. A physical discussion is then developed on the role of particle shape, considering the non-spherical nature of reference particles with a convexity value of (C = 0.954). Adoping non-convex multi-spheres aggregates (i.e. clumps), the first DEM modelling approach successfully predicts the AOR within a 8% tolerance. After a convex simplification that neglects local concavities, another approach based on potential particles underestimates to a greater extent the AOR, bringing it down from (35.95 pm 0.88^{circ }) to (31.26 pm 0.95^{circ }). For the loading setup(s) at hand, the AOR is eventually shown to bear no constitutive nature. It is for instance independent of initial void ratio but is still different than the critical friction angle. The latter may actually serve as a lower bound for the process-dependent AOR. These conclusions are drawn from a statistical analysis of a large set of results, accounting for the random nature of the microscopic arrangement in the studied process.

Particle morphology at different length scales is important in understanding the mechanical behaviour of granular materials. In this sense, it is crucial to accurately describe and measure the size and shape of the grains using suitable definitions of morphological descriptors. Most of the research up until this point has analyzed particle shape in a two-dimensional framework, and sieving has typically been used to determine size. This paper describes the use of x-ray micro-computed tomography (µCT) which enables the visualization and quantification of three-dimensional particle morphology. Spherical harmonic analysis was used to reconstruct the three-dimensional (3D) realistic surface of the granular particles. 3D morphological descriptors were then introduced and computed to obtain the overall form, local features, and surface textures of the particle morphology based on the spherical harmonic reconstructed surface. To describe the fractal nature of the surfaces of natural sand particle morphology, the 3D fractal dimension was quantified using spherical harmonic-based fractal analysis. Complete volume-based distributions of particle morphological descriptors were presented and compared for four different sand samples with different grain size and shape characteristics. According to the statistical analysis, there is a clear correlation between the shape parameters at various characteristic scales, indicating that they are not independent measures. The correlation between any two parameters was observed to rely on the distance between the characteristic scales of the morphological parameters.

Graphic abstract

The apparent friction coefficient, namely Heim’s ratio, is a prevalent dimensionless parameter to represent landslides’ mobility. Many empirical and theoretical models have been developed on the Heim’s ratio and landslides’ volume. However, studies on the ratio and their slope angles are lacking. Here, we performed a series of laboratory landslides at different slope angles to explore their mobility and motion characteristics. Our results show that the runout of the laboratory landslides decreases linearly with an increase in slope angles. A theoretical relationship between the apparent friction coefficient and slope angle is proposed, based on a hypothesis that the ratio of energy dissipation occurring when an object collides with a plane is power law to its impact angle. The relationship shows well approximation to our experimental data, and data from Crosta (IOP Conf. Ser. Earth Environ. Sci. 26:012004, 2015) and natural landslides. We also obtain that the coefficients of apparent friction and effective friction are almost identical at low slope angles. The dimensionless length and area of the laboratory landslides increase first and then decrease during their whole motion. The maximum area of them during their motion decreases with an increase in slope angles. The study will support studies on the morphological variation during the whole motion and mobility of landslides.

This paper studies the effects of angularity and coarse particle content on the shear behaviour of granular mixtures via the discrete element method. The contributions of different contact types to the shear strength are quantified, which can be used to classify the structure of granular mixtures. After that, a microscopic analysis of the effect of angularity on the thresholds of the granular mixtures is presented. Finally, a method that can predict the critical friction angle of granular mixtures at arbitrary coarse particle contents is proposed. The validity of the prediction method is verified by comparisons with the experimental and numerical data in other studies.

Graphical Abstract

The flow behavior of particles is simulated with an Eulerian-Eulerian two-fluid model based on kinetic theory of granular flow (KTGF) in a jet bubbling bed. A second-order moment (SOM) model is applied to explore the anisotropy flow behavior of particles through kinetic interaction of particle collisions. The particle frictional stresses are calculated using equations proposed by Johnson and Jackson (J Fluid Mech 176:67–93, 1987) and Schaeffer (J Differ Eq 66:9–50, 1987). The predictions of the equivalent bubble diameter and porosity are in good agreement with experimental data by Kuipers et al. Simulated comparisons between the KTGF model and the SOM model show that the SOM model is superior to the KTGF model in capturing the inhomogeneity and anisotropy of the flow field. The simulated results demonstrate that the axial second-order moment component is significantly larger than the radial second-order moment component, and they exhibit obvious anisotropy. Finally, the impacts of jet velocity, particle diameter, and restitution coefficient on the second-order moments are analyzed, respectively. It is found that the enhancement of jet velocity and particle diameter intensifies the anisotropy of flow structure, and a higher restitution coefficient weakens the anisotropy due to the reduction of energy dissipation.

Graphical abstract

We explore computationally efficient techniques to improve the XRCT image processing of low resolution and very noisy images for use in reconstruction of the fabric of densely packed, natural sand deposits. To this end we evaluate an image preprocessing workflow that incorporates image denoising, single image super resolution, image segmentation and level-set (LS) reconstruction. We show that, although computationally intensive, the Non-Local Mean (NLM) filter improves the quality of XRCT images of granular material by increasing the signal-to-noise ratio without impairing visible structures in the images, and outperforms more traditional local filters. We then explore an image super-resolution technique based on sparse signal representation and show that it performs well with noisy data and improves the subsequent stage of binarization. The image binarization is performed using a Hidden Markov Random Fields (HMRF) with Weighted Expectation Maximization (WEM) algorithm which takes the spatial information into account and performs well on high resolution images, however it still struggles with low quality images. We then use the level set method to define the grain geometry and show that the Distance Regularized LS Evolution (DRLSE) is an efficient approach for data sets with large numbers of grains. Finally, we introduce a penalty term into the evolution of the LS function, to address the issue of adhesion of much finer particles, such as clay, on the surface of the reconstructed avatars, while maintaining the main morphological details of the grains.