Granular Matter最新文献

We present an experimental investigation on the flow and clogging of bi-disperse mixtures of coarse and fine grains of different densities passing through small orifices. We vary the density ratio (coarse/fine) from 1.87 down to 0.79 by using amaranth seeds, glass and ceramic beads of similar size as the fine species in combination with 2.0 mm glass beads as the coarse grains. We analyzed the effect of the density ratio on the effective flow rate of the coarse species, the segregation during flow and the clogging for a range of orifice diameters. As in previous studies, the flow of the coarse grains is facilitated by the fine species, which prevents clogging. We show that the effective flow rate of the coarse species is virtually independent of the density ratio. These results suggest that in practical applications with the goal of clogging reduction, the density of the fine species used to ease the flow is not a relevant parameter and can be selected based on practical or economic constraints.

Graphic abstract

Schematic diagram of the flow of large grains through a small orifice when they are diluted in a mixture with fine grains

The fluidized bed bioreactor is an economical and efficient method for wastewater treatment. In the fluidized bed bioreactor, fluidized particles carrying microorganisms consume the organic pollutants in wastewater. The collision and friction between carrier particles in the fluidized bed can affect the efficiency of wastewater treatment. Therefore, understanding the hydrodynamics of fluidized bed bioreactors is crucial. In this study, the particle collision velocity depending on particle volume fraction and granular temperature, as well as considering the influence of particle surface roughness and elasticity through the critical Stokes number, a dynamic restitution coefficient model for wet rough particles is developed to provide a more accurate description of the collision behavior between wet rough particles. The model is incorporated into the kinetic theory of rough spheres to perform numerical simulations on the hydrodynamic characteristics of a three-dimensional liquid-solid tapered fluidized bed using the two-fluid model. The simulation results exhibit better agreement with experimental data by Wu et al. compared to prior studies. Furthermore, sensitivity analyses are conducted on drag force, virtual mass force, and lift force. It is observed that the Koch-Hill drag model predicts the bed expansion heights closest to the measured results. Additionally, the impacts of static bed height and particle density on the fluidized bed hydrodynamics are investigated. Simulation results indicate that an increase in static bed height initially leads to an increase and then a decrease in particle collision velocity. Within the current study scope, particle collision velocity exhibits a monotonic increase with increasing particle density.

Granular materials belong to the class of complex materials within which rich properties can emerge on large scales despite a simple physics operating on the microscopic scale. Most notable is the dissipative behaviour of such materials mainly through non-linear frictional interactions between the grains which go out of equilibrium. A whole variety of intriguing features thus emerges in the form of bifurcation modes in either patterning or un-jamming. This complexity of granular materials is mainly due to the geometrical disorder that exists in the granular structure. Diverse configurations of grain collections confer to the assembly the capacity to deform and adapt itself against different loading conditions. Whereas the incidence of frictional properties in the macroscopic plastic behavior has been well described for long, the role of topological reorganizations that occur remains much more elusive. This paper attempts to shed a new light on this issue by developing ideas following the configurational entropy concept within a proper statistical framework. As such, it is shown that contact opening and closing mechanisms can give rise to a so-called configurational dissipation which can explain the irreversible topological evolutions that granular materials undergo in the absence of frictional interactions.

Graphical Abstract

The response of dry sand to rapid penetration by a rigid projectile is investigated through a series of high-speed penetration experiments. A ballistic range is used to vertically launch cylindrical projectiles and a scaled version of a 155 mm M107 projectile at impact velocities of approximately 200 m/s into sand targets. A photon Doppler velocimeter is used to track projectiles from impact to rest in the soil target. Data collected from the experiments include the evolution of the cavity crown along with displacement, velocity, and acceleration time history. Analysis of the results reveal that the soil bulk density has a major role in penetration resistance at high relative densities. The role of bulk density diminishes at lower relative densities. Furthermore, the shape of the projectile nose has limited influence on the penetration response, due to the formation of a kernel of crushed sand at high velocities. The crushed sand kernel, known as the false nose, has a curved surface, and it can be approximated as a cone with a 60° apex angle. Only projectiles with a nose sharper than this value affect penetration resistance, while blunter noses effectively behave as 60° cones due to the formation of the false nose. A phenomenological equation of penetration resistance comprising inertial and frictional bearing resistance is used to describe the penetration response and predict the depth of burial (DoB) of the projectile in the soil target with reasonable accuracy.

Graphical Abstract

Laboratory made granular–granular impact craters have been used as model analogues of planetary impact craters. These kind of craters have been observed and studied using profilometry techniques that allow to retrieve important morphologic features from the impacted surface. In this work, we propose to use a Time-of-Flight camera (Microsoft Kinect One) for the acquisition of depth data. We show comparisons between the typically used technique and the analysis derived from the Time-of-Flight data. We also release craterslab, a Python library developed to automate most of the tasks from the process of studying impact craters produced by granular projectiles hitting on the surface of granular targets. The library is able to acquire, identify, and measure morphological features of impacted surfaces through the reconstruction of 3D topographic maps. Our results show that using a Time-of-Flight camera and automating the data processing with a software library for the systematic study of impact craters can produce very accurate results while reducing the time spent on different stages of the process.

Graphical abstract

Three-dimensional representation of the lunar crater Werner by craterslab as a tool for morphometric analysis of natural and laboratory impact craters.

The laser-beam powder bed fusion process for metals, commonly abbreviated as PBF-LB/M, is a widely used process for the additive manufacturing of parts. Numerical simulations are useful to identify optimal process parameters for different materials and to obtain detailed insights into process dynamics. The present work uses a single-phase incompressible Smoothed Particle Hydrodynamics (SPH) scheme to model PBF-LB/M which was found to reduce the required computational time and significantly stabilize the partially violent flow in the melt pool in comparison to a weakly compressible SPH approach. The laser-material interaction is realistically modelled by means of a ray tracing method. An approach to model the effective thermal coductivity of the powder bed is proposed. Excellent agreement between the simulation results and experimental X-ray analyses of the transition from conduction melting mode to keyhole mode including geometric properties of the vapor depression zone was found. These results prove the usability of SPH as a high precision simulation tool for PBF-LB/M.

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The investigation of rockfill materials poses challenges due to their large particle size, associated high cost, and long laboratory testing duration. As a result, empirical correlations based on historical experimental studies are commonly used to design and analyse rockfill structures. However, the extensive use of rockfill in a wide range of applications and limited understanding of its mechanical behaviour emphasize the need for further research. These make it necessary to develop a robust technique capable of capturing key parameters such as particle shape and breakage, allowing for the simulation and study of large-scale assemblies with realistic boundary conditions. Given that the behaviour of rockfill is highly scale-dependent, primarily due to particle breakage, the simplified laboratory tests on the scaled-down assemblies can be misleading. Particle breakage is a fundamental phenomenon in the mechanical behaviour of rockfill and significantly affects shear strength, deformability, and porosity under different stress levels. The particle breakage is influenced by factors such as the rockfill’s maximum particle size, mineralogy, particle shape, gradation, and confining stresses. This study adopts a computationally efficient breakage method called the Modified Particle Replacement Method (MPRM) based on the Discrete Element Method. A Tile-Based Flexible Membrane (TBFM) for triaxial test modelling has been developed by employing segmental rectangular walls to create a deformable membrane. The effects of critical parameters, including particle shape, confining stress, membrane resolution, degree of flexibility, and the characteristic strength of the particles, are examined. The findings of the combined MPRM-TBFM approach demonstrate the significant influence of membrane flexibility on volumetric-related behaviour.

Graphical Abstract

Laser Powder Bed Fusion (L-PBF) has immense potential for the production of complex, lightweight, and high-performance components. The traditional optimization of process parameters is costly and time-intensive, due to reliance on experimental approaches. Current numerical analyses often model single-line scans, while it is necessary to model multiple fully scanned layers to optimize for bulk material quality. Here, we introduce a novel approach utilizing discrete element simulations with a ray tracing-modeled laser heat source. Our approach significantly reduces the cost and time consumption compared to conventional optimization methods. GPU acceleration enables efficient simulation of multiple layers, resulting in parameters optimized for bulk material. In a case study, parameters were optimized for AlSi10Mg in just 5 days, a process that would have taken over 8 months without GPU acceleration. Experimental validation affirms the quality of the optimized process parameters, achieving an optical density of 99.91%.

Graphical Abstract

Optimization using the accelerated simulation yielded an optimized parameter set within 5 days. This resulted in apart with an optical density of 99.91%.

We consider a dense aggregate of elastic, frictional particles isotropically compressed and next uniaxial strained at constant pressure. We show how failure can be predicted if fluctuations in the kinematics of contacting particles are introduced. We focus on the second order work and the possibility that at some stressed states it becomes negative under proper perturbations. Our analysis involves both a theoretical model and numerical simulations based upon the distinct element method (DEM). The theoretical model deals with contacting particles with incremental relative displacements that deviate from the average deformation in order to ensure their equilibrium. Because of this, the macroscopic stiffness tensor of the aggregate, that relates increments in stress with increments in strain, does not have the major symmetry. Consequently, in the hardening regime, we predict stressed states in which the second order work vanishes. The model seems transparent, and it makes clear and illustrative the role played by the fluctuations introduced in the kinematics of contacting particles in relation to the vanishing of second order work in an aggregate of compressed particles. The comparison with numerical simulations data supports the model.

Graphical Abstract

Statistical representation of the aggregate: conditional average.

We introduce a novel numerical method for the simulation of soft granular materials, in which the particles can undergo large strains under load without rupture. The proposed approach combines an explicit total Lagrangian formulation of Material Point Method (TLMPM) with the Contact Dynamics (CD) method. The TLMPM resolves particle bulk deformations whereas the CD treats contact interactions between soft particles. The efficiency and accuracy of this approach are illustrated by analyzing diametral compression of a soft circular particle and the compaction of an assembly of soft particles up to very high levels of packing fraction. We show that although the assembly undergoes a jamming transition, the particles continue to rearrange as they get increasingly distorted under load. Interestingly, as the packing fraction increases, a transition occurs to a regime fully governed by particle shape change. The evolution of the global stress as well as the connectivity of the particles as a function of the packing fraction are discussed and a predictive model relating stress to packing fraction beyond jamming transition is proposed.

Graphical Abstract