Granular Matter最新文献

We modified the inelastic bouncing ball model (IBBM) to account for the role of collision time ({tau }_{c}) in defining the dynamics of vertically vibrated confined granular systems. Although ({tau }_{c}) was surmised to be consequential for dissipative systems, previous studies on the accuracy of IBBM did not formally incorporate ({tau }_{c}) as a dynamical variable of the model, focusing instead on other factors during flight, such as air friction. We utilized the discrete element method (DEM) to study the role of ({tau }_{c}) in the granular dynamics, and to cross-validate the efficacy of our reformulation of IBBM to account for the effect of collisions. When the ({tau }_{c}) value is greater than that of ({t}_{0}), which is the first instance that the container acceleration exceeds the gravitational acceleration (g), the time-of-flight decreases, and the location of the bifurcation point shifts in the bifurcation diagram (time-of-flight versus dimensionless acceleration). We model ({tau }_{c}) as representing the range of uncertainty in the occurrence of ({t}_{0}). Assuming a separation of timescale between the dynamics of the collision between the center-of-mass (CM) of the granular system and the container, and the time-of-flight of the CM itself, we propose a supporting but separate model for the dependence of ({tau }_{c}) on Γ. The time-of-flight duration is determined when ({tau }_{c}) is known in the modified IBBM that now produces bifurcation diagrams which are in closer agreement with the DEM simulation results.

Graphical Abstract

Producing dense and homogeneous powder layers with smooth free surface is challenging in additive manufacturing, as interparticle cohesion can strongly affect the powder packing structure and therefore influence the quality of the end product. We use the Discrete Element Method to simulate the spreading process of spherical powders and examine how cohesion influences the characteristics of the packing structure with a focus on the fluctuation of the local morphology. As cohesion increases, the overall packing density decreases, and the free surface roughness increases, which is calculated from digitized surface height distributions. Local structural fluctuations for both quantities are examined through the local packing anisotropy on the particle scale, obtained from Voronoï tessellation. The distributions of these particle-level metrics quantify the increasingly heterogeneous packing structure with clustering and changing surface morphology.

The mechanical properties of rockfill materials are not only influenced by microscopic factors such as particle morphology and gradation, but also closely related to different loading stress paths. It is of great significance to study the microscopic mechanical properties of rockfill materials under different stress paths for revealing the macroscopic mechanical properties as well as the microscopic deformation and failure mechanisms of rockfill materials. In this paper, based on the results of triaxial tests, a series of numerical triaxial simulation tests under different stress paths were carried out using the discrete element particle flow method, and the deformation, strength change rules, and fine structure evolution mechanism under three stress paths were explored. The results demonstrated that there were significant differences in the effects of stress paths on the stress–strain and strain-volume change characteristics of the rockfill materials. Stress paths exhibited little effect on the strength characteristics. The anisotropy of strong contact number and strong contact force was the microscopic source of macroscopic strength. The contact situation between the particles was the main microscopic factor affecting the macroscopic deformation. The intrinsic mechanism of macroscopic deformation properties could be revealed by the average coordination number and porosity. The stress path affected the growth rate of the number of bond failures and the total number of failures. The relationship between macroscopic mechanical properties and microstructural evolution under different stress paths was also discussed. The findings can provide meaningful insights into the deformation control and stability analysis of rockfill engineering.

Graphical Abstract

Railway ballast undergoes rearrangement, abrasion, and even breakage, when subjected to high-speed train loads. To reproduce the deformation and degradation behavior of ballast under realistic boundaries used in laboratory triaxial tests, bonded particle clusters and clumps sampled within flexible and rigid boundaries were established, using the discrete element method and finite difference method. The models were then calibrated and validated against a series of experimental results. It is found that boundary condition has a considerable effect on the contact force chains and coordination number. The flexible boundary induces more uniform stress distribution between particle contacts, and consequently higher strength, lower dilation, and impartial breakage. A unimodal frequency distribution of the coordination number is observed when using flexible boundary, while rigid boundary can result in multi-modal distribution in breakable specimens. The flexible boundary also induces more particle breakage with high fragmentation. The rigid boundary specimens exhibit a bimodal distribution of particle breakage along the specimen height after test, with fewer fragments existing in the middle part; however, a unimodal distribution of particle breakage is found in the flexible boundary ones, which agrees more with the laboratory observation.

Granular media is ubiquitous, playing a vital role in a diverse set of applications. The complex microstructure of granular media results from assorted particle shapes, morphologies, and packings, make it difficult to predict its macroscopic behavior. Under compression, these complex microstructures enable highly anisotropic and heterogenous behaviors, including creation of highly-loaded particles (i.e. force chains) supported by clusters of minimally-loaded particles. While many existing constitutive models relate state variables describing microscale behavior to continuum properties, these models do not generally consider the mesoscale interactions between the force chain network and minimally-loaded particles. Here, we develop a micromechanics model that connects micro-scale force chain mechanics to macro-scale mechanical behavior through explicit consideration of the interaction between force chains and minimally-loaded particles. We first examine the elastic behavior of a force chain using a spring model, explicitly considering the mesoscale interactions between the force-chains and surrounding regions. We then construct an equivalent inclusion problem to calculate macroscopic mechanical response as analytical functions of microscopic properties, with proper consideration of mesoscale interactions. We present our calibration and validation approaches, showing the model’s predictive abilities. Finally, we examine the effect of relevant microscopic quantities on macroscopic response, demonstrating the importance of these mesoscale interactions on bulk deviatoric behavior.

Particulate precipitation, deposition, and accumulation, including the formation of salt and mineral crystals, frequently occur in a wide range of subsurface applications involving multiphase flow through porous media. Consequently, there has been a considerable emphasis on researching and understanding these phenomena. However, modeling particle dynamics in flows through porous media with low Reynolds numbers has always been a challenging problem as it requires resolving fluid flow around the moving solid particles, the solid–solid contact mechanics, and the solid–fluid coupling. The discrete element method coupled with fluid solvers has been widely used to study particle-laden flow. Most fluid-solid numerical schemes involve solving the full or generalized Navier–Stokes equations, which often yields relatively accurate fluid-solid interactions at the cost of computation time and particle shape limitations. In this paper, we present a novel method to study mono-layered particle-laden flow by coupling the level set discrete element method (LS-DEM) with Hele-Shaw flow model. Utilizing the Hele-Shaw flow model allows us to simplify flow computation, while incorporating LS-DEM enables the simulation of arbitrarily shaped particles. Cases of mono-layered particle flow through a simplified micromodel geometry are studied and validated against published experimental results. Moreover, the effects of particle friction and shape on clogging statistics are investigated.

Graphical Abstract

The work aims to numerically investigate the quasi-static response of partially fluid-saturated concrete under two-dimensional uniaxial compression at the mesoscale. We investigated how the impact of free pore fluid content (water and gas) affected the quasi-static strength of concrete. The totally and partially fluid-saturated concrete behavior was simulated using an improved pore-scale hydro-mechanical model based on DEM/CFD. The fluid flow concept was based on a fluid flow network made up of channels in a continuous region between discrete elements. A two-phase laminar fluid flow was postulated in partially saturated porous concrete with very low porosity. Position and volumes of pores/cracks were considered to correctly track the liquid/gas content. In both dry and wet conditions, a series of numerical simulations were performed on bonded granular specimens of a simplified spherical mesostructure that mimicked concrete. The effects of fluid saturation and fluid viscosity on concrete strength and fracture, and fluid pore pressures were investigated. It was found that each of those effects significantly impacted the hydro-mechanical behavior of concrete. Due to the rising fluid pressure in pores during initial specimen compaction under compressive loading that promoted a cracking process, the compressive strength increased as fluid saturation and fluid viscosity decreased.

Graphical abstract

DEM-CFD results for fully saturated specimen: evolution of maximum pore water pressure against vertical normal strain during uniaxial compression (from zero up to peak stress for).