Granular Matter最新文献

Water can strongly affect the mechanical behavior of granular materials. In this study, numerical simulation is conducted to investigate the effect of water content described by saturation on the collapse of granular columns. A coupled CFD-DEM model is adopted for the wet granular materials from pendular state to capillary state (saturation from 30 to100%), and the discrete element-liquid bridge model is adopted for the wet granular materials of pendular state (saturation less than 30%). The influence of saturation, particle radius, and friction on the shape of the deposit is studied, and the final deposit boundary is fitted by a bilinear model. In addition, the fluidity is studied by the motion of mass center of granular materials. Numerical examples show that within the saturation range from 0.1 to 0.5% and from 30 to 100%, the water has an obvious effect on final deposit shape. Within the saturation range of 0.5–30%, the water has little effect on the final deposit shape. For the saturation range of 0.1–0.5%, the fluidity decreases with the increase of saturation, and when the saturation is more than 30%, the fluidity increases with the increase of saturation. The study revealed the influence of interstitial water on the fluidity of granular materials, which is significant for the researches of geological engineering problems, such as landslides.

Graphical abstract

A model was proposed to predict the permeability of granular soil with various gradations. The pore size distribution for different particle sizes was determined by considering different particle combinations and occurrence probabilities, which was then used in the fractal and capillary model for predicting soil permeability. The proposed model was verified by experiments and exhibited higher accuracy than other models. Upon verifying by over 60 tests, the mean absolute percent error using this proposed model was 18% for the permeability predictions of spherical granular soils. The pore size distribution predicted by the model was verified by computed tomography to capture the pore characteristics of different soil gradations. The proposed model only requires three parameters (soil particle size distribution, void ratio, and relative density) to predict soil permeability, and no empirical or calibrated parameters are needed. A parametric analysis showed that the gradation significantly affected soil permeability. Even when porosity and the characteristic particle size, d₅₀, are the same, as the particle size distribution narrows and the gradation range decreases, the permeability coefficient can increase by 80%. The permeability increased with the void ratio and decreased as the relative density increased. The proposed model provides a practical approach for predicting the permeability of granular soils and considers the effect of the particle size distribution.

Graphic abstract

In this paper we show DEM simulations of static and cyclic large triaxial tests on a sample of railway ballast. The sample is reconstructed from X-Ray tomography images of an untested laboratory sample, recovered by impregnation with an epoxy resin. Measurements of both shape and fabric are carried out; the sample shows a high anisotropy of particle orientations due to the preparation procedure and a high shape heterogeneity. A DEM model is then generated using clumps to model single particles, preserving the shape of each particle and the fabric of the sample. Results of static and cyclic simulations are shown and compared with previous simulations on numerically generated samples, showing the importance of an accurate representation of the whole range of particle shapes, as well as confirming the effect of particle anisotropy on the mechanical response.

Graphical Abstract

The concept of force chains transmitting stress through granular materials is well established; however identification of individual force chains and the associated quantitative analysis is non-trivial. This paper proposes two algorithms to (1) find the network of percolating contacts that control the response of loaded granular media, and (2) decompose this network into the individual force chains that comprise it. The new framework is demonstrated considering data from discrete element method simulations of a ribbed interface moving against a granular sample. The subset of contacts in the material that transfers load across the sample, namely the percolating contact network ((G_{perc})), is found using the maximum flow algorithm. The resulting network is fully-connected and its maximum flow value corresponds to the force percolating the system in the direction normal to the ribbed wall. (G_{perc}) re-orientates in response to the ribbed interface movement and transmits 85–(95%) of the stress, with only 40–(65%) of the contacts in the sample. Then, (G_{perc}) is split into individual force chains using a novel implementation of the widest path problem. Results show that denser materials with increased force-chain centrality promote a higher density of force chains, which results in a higher macro-scale strength during interface shearing. The contribution of force chains in the network is revealed to be highly centralized, composed by a small set of strong and long-lived force chains, plus a large set of weak and short-lived force chains.

Rockfill is a common irregular granular material used in most dam construction projects. The purpose of this paper is to investigate the distribution and size-dependent properties of the mechanical parameters describing the elastic properties and crushing strength of rockfill particles. These statistics can be used as a reference to calibrate the input parameters of numerical models when studying the macroscopic behavior of rockfill with particle breakage using the discrete element method. A series of limestone particles ranging in diameter from 20 to 240 mm were measured in this study using a single particle compression test. The elastic modulus, elastic contact stiffness, tensile stress and fracture force were then determined by characterizing each experimental force–displacement curve. Classical statistical methods were used. It has been shown that Weibull, lognormal and logistic functions can all represent the distributional features of the elastic modulus, tensile stress and fracture force, with the lognormal function being the optimal type here. As the grain size increases, the elastic modulus and tensile stress decrease, while the fracture force rises. Empirical models of power functions effectively reproduced these size-dependent laws. Meanwhile, the relationship between these parameters was also established. Finally, the lognormal function was adopted to express the randomness of the maximum elastic contact stiffness. Some suggestions were made after discussing the positive association between the maximum elastic contact stiffness and grain size. Moreover, the evaluation of the loading strain rates of individual particles tested shows that the present conclusions are applicable to quasi-static case.

Graphical abstract

The contact overlap algorithms and contact models of both polyhedron-polyhedron and polyhedron-boundary contact in discrete element method (DEM) has been proposed. The overlap volume between contacting polyhedrons is explicitly calculated based on geometric dualization theory. The polyhedron-boundary contact is transferred to the contact between polyhedron and triangles. An improved parallelizing by candidate contact pair algorithm is provided to accelerate the accurate contact overlap algorithms. Furthermore, a DEM framework based on graphics processing unit, named as CoSim-DEM, has been developed to realize the high-performance simulation. The algorithms are validated using hopper flow experiments with 3D printed particles, and DEM parameters are calibrated by the experimental tests. Two benchmarks are used as the case extension of the algorithms: one is the interaction between a retaining rigid wall and granular material for quasi-static analysis; the other is the conveying motion of granular material in a screw conveyor for dynamic analysis. Finally, the computational efficiency of the developed algorithms is analyzed. All results indicate that the developed CoSim-DEM can be better used in the simulation of granular materials with polyhedral particles.

The unique behavior of a bed of monodispersed dry granular matter confined in a rotating vertical cylinder has been investigated using DEM. At a constant rotational speed, the concave free surface shape exhibits an apparent similarity with the hydrodynamic counterpart of the same problem; but there are some unique differences arising from the particulate nature of the medium. In the rotating granular bed, the free surface profile is independent of the density and the size of the particles, but the local gradient of the free surface is not independent of the rotational speed. The results of a simulated experiment exhibit excellent matching with the free surface profiles obtained computationally and thus establish the efficacy of DEM in simulating the rigid body rotation of a particulate medium. However, the simulation results strongly suggest that though globally the bed behavior resembles solid body rotation, locally, the motion of the particles and their distribution is influenced by the discrete nature of the medium.

Graphical abstract

Using iron ore tailings (IOTs) as the main aggregate for concrete will not only save crushed stone mining but will also reduce the environmental impact of IOTs. A discrete element model of concrete with realistic IOTs shape was developed using particle flow coding 3D technique. The accuracy of the numerical model was verified with the laboratory uniaxial compressive test results, and the damage of concrete with IOTs of 40% under different working conditions, including stress amplitude or maximum stress in a cycle and the number of cyclic loadings, was also observed in detail. Combining the quantification of damage particles estimation, the macroscopic and microscopic damage mechanism of concrete under cyclic loading was revealed from the perspectives of both IOTs content and particle size. The results show that the maximum stress of cyclic loading is a more important factor than the stress amplitude to control the number of fractures generated. Although the increase of IOTs content can improve the compressive stress of concrete, the reduction of IOTs particle size can curb fracture formation.

Graphical abstract

This study has considered the behaviour of granular materials subjected to drained cyclic loading under constant mean effective stress. Using the discrete element method, cubical, isotropically compressed samples were subjected to 50 loading cycles at different values of mean stress ((p' =) 100, 200, 300 kPa) and different loading amplitudes ((zeta =) 5%, 10% and 20% of (p')). At low cycle numbers, the deformation mechanism is controlled by contractive volumetric strains, before transitioning to the ratcheting regime, characterised by the persistent accumulation of plastic strains. An energy/work analysis showed that the volumetric work per cycle decreased as hysteresis loops tighten. During ratcheting, most boundary work was dissipated by contact sliding. The mechanical response was controlled by (zeta), with little to no influence of (p'). For (zeta = 5%), deformations were confined to the elastic range, with no increase in secant stiffness (G_{sec}) or shear strength after cyclic loading. For (zeta = 10%), (G_{sec}) and the shear strength increased after cyclic loading, although no significant expansion of the yield surfaces was observed. The largest loading amplitude ((zeta = 20%)) induced yielding at low cycles, leading to significant changes in the fabric, volume and yield surfaces of the samples, and a significant increase of shear strength and (G_{sec}). At the micro-scale, graph theory was used to quantify the evolution of the contact network. After (sim 20) loading cycles, the network reached a steady-state of constant but persistent topology changes in the material, with most of the topology retained between loading cycles.

Jojoba granular matter has received exceptional attention since it contains a unique waxy oil. The Jojoba plant is a promising crop for arid and marginal areas with probable value in combatting desertification and soil degradation in dry regions. This study was carried out to estimate some selected physical, mechanical, and aerodynamic properties of Jojoba grains and provide some essential parameters for modeling their granular flow through computational methods. The considered properties were geometrical dimensions, unit volume, unit mass, thousand-grain weight, projected and surface areas, sphericity, bulk and true densities, porosity, static and dynamic angles of repose, static coefficient of friction, terminal velocity, and drag coefficient. At a moisture content of 6.5%, the Jojoba grain properties significantly reveal strong correlations to the unit mass and length at (ple 0.01), where simple linear equations were developed. From the linear dimensions, the grain shape is considered oval or elongated, with an estimated sphericity of 70%. The results showed that the physical characteristics, such as bulk and true density, were 688 and 831 kg m⁻³, and the average values of unit mass and thousand-grain weight were 0.94 g and 1088.9 g, respectively. The aerodynamic properties of the average terminal velocity and drag coefficient were 18.7 ms⁻¹ and 0. 3, respectively. The rubber surface offered the maximum static coefficient, followed by galvanized steel and Plexiglas in descending order. The mean values of the static and dynamic angles of repose were 40 and 24°, respectively. The obtained data could be necessary for different optimal designs, computational modeling, and development of industrial processes, such as separation, handling, storage, squeezing, sorting, cleaning, harvesting, and post-harvesting.