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Despite their widespread adoption, particle-scale simulation methods, such as the Discrete Element Method (DEM), for electrically charged particles in several natural processes and industrial transformations do not include realistic polarization effects. At close distances, these can dominate the particle motion and are impossible to predict by the commonly adopted Coulomb point-charge approximation. Sophisticated mathematical tools can account for uneven charge distributions, predicting an attractive force between a charged particle and a neutral particle or possible attraction between two like-charged particles. Such approaches are accurate but too complex for implementation in DEM simulations of many interacting particles. We propose a novel, simpler yet realistically accurate effective dipole model. By attributing a net charge and an induced effective dipole to each sphere, the interaction force between charged polarizable particles is computed in closed form. A comparison of a rigorous solution and the proposed dipole approach for two spherical particles reveals significant improvement over the commonly employed Coulomb law. The effects of particle size ratio and charge ratio on the interaction force are discussed. Then, the dynamic DEM simulation of a shaker filled with a binary mixture of differently sized particles that are all positively charged is shown to predict the counterintuitive formation of fine-on-coarse aggregates. [ABSTRACT FROM AUTHOR]

The discrete element method (DEM) is used to simulate the behavior of a model sand under cyclic stress. Two approaches are employed in the contact model to account for the effect of anisotropic particle shape: (1) spheres with a rolling resistance moment and (2) clumps of spheres. Model parameters are calibrated using experimental results from drained monotonic triaxial tests on NE34 sand. Then, a series of cyclic triaxial tests is done on a homogeneous elementary volume sample with varying density index ( I D ) and cyclic stress ratio (CSR). Both macroscopic and micromechanical characteristics of the material are examined under cyclic loads. In particular, the evolution of Young’s modulus (E) and the damping ratio (D) with strain amplitude are evaluated at varying I D and compared with values from the literature. An analysis of the coordination number (Z), orientation of strong and weak contact forces, friction mobilization, sliding contacts and fabric evolution links the observed macroscopic behavior of energy dissipation to the phenomenon of frictional sliding at the grain scale. [ABSTRACT FROM AUTHOR]

Pile-supported embankments are typically composed of soil-rock mixtures. within these structures, while the soil arching effect is crucial for effective load transfer, it remains incompletely understood, particularly when the impact of various loading conditions needs to be considered. This study investigates this problem using a 1 g physical experimental modeling approach. Subsequently, a DEM model for a full-scale pile-supported embankment of high-speed railways, accounting for multiple pile interactions, is established with proper model calibration. Numerical simulations are conducted to explore the load transfer mechanism and soil arching processes under self-weight, embankment preloading, and train-induced dynamics influences. The findings indicate that under self-weight, fully developed soil arching structures can be achieved with a sufficiently high embankment height, although they can diminish as the soil-pile relative displacement increases. However, during embankment preloading processes, represented by static loading, pressure can be transferred from pile caps to subsoil regions, potentially compromising the integrity of soil arching structures. Train-induced dynamics effects are modeled as cyclic loading inputs, revealing that an increase in loading frequency leads to weakened dynamic pressure fluctuation for both pile caps and subsoil regions, with a limited impact on the valley values of the pressures. Additionally, a higher loading frequency corresponds to smaller accumulated loading plate settlements. [ABSTRACT FROM AUTHOR]

This article presents a novel approach to calibrating the digital twin of a laboratory mill used for copper ore milling. By integrating computer vision techniques for real-time data extraction and employing DualSPHysics simulations for various milling scenarios, including balls only, balls with ore, and balls with slurry, we achieve a high degree of accuracy in matching the digital twin's behavior with actual mill operations. The calibration process is detailed for mills with three different diameters, highlighting the adjustments in simulation parameters necessary to account for the absence of ore. Understanding the dynamics between the suspension within the mill and the operation of the grinders is crucial for the future improvement of the grinding process. This knowledge paves the way for optimizing the process, not only in terms of the quality of the end product but primarily in terms of energy efficiency. A profound understanding of these interactions will enable engineers and technologists to design mills and grinding processes in a way that maximizes efficiency while minimizing energy consumption. [ABSTRACT FROM AUTHOR]

Although particle corner breakage has been proved to be the primary mode of particle breakage for coral sand, current studies of pile penetration have continued to present the particle fracture breakage mode, causing the mechanism of pile behaviours in coral sand to remain unclear. This study investigates the particle corner-breakage effect on pile penetration in coral sand at both the macro- and microscales via indoor pile penetration model testing and three-dimensional discrete element method (DEM) simulations. According to the study findings, DEM simulations revealed that particle corner breakage has a more obvious soil contraction effect than fracture breakage. Thus, this study is the first to explain the particular turn of the pile skin friction in coral sand by the dual effects of breakable corners. Next, relationships between the particle breakage mode and the controversial breakage zone around pile tips have been accomplished. Moreover, the decrease in effective contacts and the change in soil skeleton have been proved to be essential factors behind the narrower penetration-affected width in breakable corner grains. The study suggests that neglecting the particle corner-breakage effect can lead to a hidden danger affecting engineering safety in angular granular soil. [ABSTRACT FROM AUTHOR]

Scalping techniques are commonly employed to handle oversized particles in gravelly soils for performing laboratory tests, which will cause significant gravel content (GC) effect on the predicted mechanical behaviors of prototype soils. To solve this obstacle, this study tries to establish the scaling law for correcting the GC effect, which controls the same equivalent skeleton void ratio between the prototype and model soils. The proposed scaling law characterizes the mechanical parameters using the newly introduced indexes including the degree of heterogeneity and the scaling factor for parameter A of the Harding equation. Then a series of DEM simulations of element tests were performed on sand-dominant gravelly soils to parameterize and check the applicability of the proposed scaling law. The simulation results show that both the converted shear responses and shear modulus reduction curves through the proposed scaling law are highly consistent with that of the prototype soils. This suggests that in dealing with deformation problems from small to medium strains, Rocha's assumption is highly applicable for correcting the GC effect. When the adopted scalping techniques control the change in GC (Δ GC) below 20 %, the predicted liquefaction resistance will be very close to that of the prototype soils regardless of the earthquake magnitudes. However, when Δ GC is larger than 40 %, the prediction error in liquefaction resistance will be larger. Based on these simulation results, it is recommended that when selecting scalping techniques for laboratory tests on gravelly soils containing oversized particles, it is preferred that Δ GC be kept below 20 %. [ABSTRACT FROM AUTHOR]

In practically every industry, mixing is a fundamental process, yet its 3D analysis is scarce in the literature. High-resolution computed tomography (micro-CT) is the perfect X-ray imaging tool to investigate the mixing of granular materials. Other than qualitative analysis, 3D micro-CT images provide an opportunity for quantitative analysis, which is of utmost importance, in terms of efficiency (time and budget) and environmental impact of the mixing process. In this work, lacunarity is proposed as a measure of mixing. By the lacunarity calculation on the repeated micro-CT measurements, a temporal description of the mixing can be given in three dimensions. As opposed to traditional mixing indices, the lacunarity curve provides additional information regarding the spatial distribution of the grains. Discrete element method simulations were also performed and showed similar results to the experiments. [ABSTRACT FROM AUTHOR]

Powder deaggregation in Dry Powder Inhalers (DPI) with carrier-based formulations is a key process for the effectiveness of drug administration. Carrier-wall collisions are one of the recognised mechanisms responsible for active pharmaceutical ingredient (API) aerosolisation, and DPI geometries are designed to maximise their efficacy. The detachment of fine and cohesive API particles is investigated at a fundamental level by simulating with DEM the normal collision of a carrier sphere with an API particle attached. The impact velocity at which detachment occurs (escape velocity) is determined as a function of key parameters, such as cohesiveness, coefficient of restitution, static and rolling friction. An analytical model for the escape velocity is then derived, examining the role of the initial position of the particle, cohesion model and particle size. Finally, the results are framed in the context of DPI inhalers, comparing the results obtained with impact velocities typically recorded in commercial devices. [ABSTRACT FROM AUTHOR]

We characterize pellet and sinter particles used in the blast furnace in terms of DEM simulation parameters. By designing an innovative rotating tumbler setup whose length can be adjusted, we measure the dynamic angle of repose at six different rotation speeds for two different tumbler lengths. DEM simulations are performed in identical geometry by identifying the key simulation parameters which significantly affect the simulation results. Most of these parameters are directly measured using laboratory experiments. Using a novel image analysis based approach, we measure rolling friction for nearly spherical pellet particles and highly irregular shape sinter particles. Using these measured values of the rolling friction in DEM simulations, we obtain a very good agreement between experimental and simulation data without any calibration. The methodology presented here can be used for estimating rolling friction of non-spherical particles for usage in DEM simulations of bulk solids along with other DEM parameters. Unlabelled Image • Characterization of sinter and pellet particles used in the blast furnace • Dynamic angle of repose measurement in rotating tumbler of adjustable length • Present a methodology for mimicking experiments quantitatively in DEM simulations • Estimation of rolling friction of non-spherical particles using particle images • More efficient approach for bulk calibration of DEM parameters for industrial feed [ABSTRACT FROM AUTHOR]

Many reptiles, known as 'sand swimmers', adapt to their specific environments by vibrating or rotating their body. To understand these type of interactions of active objects with granular media, we study a simplified model of a self-excited spherical object (SO) immersed in the granular bed, using three-dimensional discrete element method (DEM) simulations. Modelling the vibration by an oscillatory motion, we simulate the longitudinal locomotion of the SO in three modes: transverse vibration, rotation around different axes, and a combination of both. We find that the mode of oscillation in y direction coupled with rotation around x-axis is optimal in the sense that the SO rises fastest, with periodic oscillations, in the z direction while remaining stable at the initial x position. We analyze the physical mechanisms governing the meandering up or down and show that the large oscillations are caused by an asynchronous changes between the directions of oscillation and rotation. We also observed that the SO's rising rate is sensitive to three parameters: the oscillation amplitude, the oscillation frequency, f, and the rotation angular velocity, Ω. We report the following results. 1. When the frequencies of the rotation and transverse motion are synchronised, SO rises when Ω < 0 and sinks when Ω > 0 ; the average rising/sinking rate is proportional to | Ω | . 2. The rising rate increases linearly with the oscillation amplitude. 3. There exists a critical oscillation frequency, above and below which the rising mechanisms are different. Our study reveals the range of parameters that idealized 'swimmers' need to use to optimize performance in granular environments. [ABSTRACT FROM AUTHOR]

Bentonite pellet mixtures are candidate material for the sealing of galleries in radioactive waste disposals. The hydromechanical behaviour of assemblies of bentonite pellets is investigated upon partial hydration through (1) suction-controlled swelling pressure tests in the laboratory and (2) discrete element method simulations. The combination of these experimental and numerical approaches highlights that, before the mixture homogenization, the swelling pressure develops in two phases. The first phase is characterized by an increase in contact forces between pellets. The second phase is characterized by plasticity at contacts between pellets and is controlled by the progressive decrease in pellet strength and stiffness upon hydration. In addition, numerical results highlight that the swelling pressure measured in the laboratory can be influenced by the sample preparation, the cell size, and the diameter of the pressure sensor. [ABSTRACT FROM AUTHOR]

The inorganic fillers in dental resin composites can enhance their mechanical properties and reduce polymerization shrinkage. When the usage amount of inorganic fillers is closed to maximum filler loading (MFL), the composites will usually achieve optimal performances. This study aims to develop a method that can predict the MFL of dental resin composites for the optimization of filler formulations. A method based on discrete element method (DEM) simulations and experiments was firstly developed to predict the MFL of spherical silica particles for single-level and multi-level filling. The results indicate that the presence of modifier can increase the MFL, and the MFL increment can be exponentially changed with the content of the modifier. Compared with the single-level filling, the addition of secondary fillers is beneficial to increase the MFL, and the increment can be affected by the particle size and size ratio. The prediction results show a good agreement with the experiment results. The accuracy of prediction results indicates a great potential of DEM simulations as a numerical experimental method in studying the MFL, and provides an effective method for the optimization of filler formulations. [ABSTRACT FROM AUTHOR]

This work focuses on the behaviour of granular materials under unsteady, simple shear conditions and, in particular, on from solid- to fluid-like phase transition. The authors introduce a theoretical model, based on continuum mechanics, able of predicting the mechanical behaviour of granular media under both quasi-static and dynamic conditions. The model assumes a parallel scheme where confining and shear stresses are computed as the sum of two contributions: the quasi-static and the collisional one. The quasi-static contribution is obtained by employing an elastic–plastic model including the critical state concept, while the collisional one is derived from the kinetic theory of granular gases. In order to test the model under unsteady conditions, DEM numerical simulations of time evolving homogeneous shear flows have been performed by considering an assembly of frictional, deformable spheres, under constant volume conditions. Simulations have been performed by systematically changing both void ratio and shear rate. The comparison between theoretical model predictions and DEM results is done in terms of time evolution of stresses and granular temperature. Suitable initial conditions are imposed to reproduce both solid to fluid (liquefaction) and fluid to solid (solidification) phase transitions. [ABSTRACT FROM AUTHOR]

Prior laboratory and in situ investigations show that monotonic preshearing can have a significant effect on the cyclic response of granular materials. The underlying mechanics that are responsible for these changes in behavior in response to preshearing are not well characterized, however. Herein, we use the discrete-element method (DEM) to simulate undrained monotonic and cyclic simple shear tests with the constant volume method. Through published comparisons to laboratory data, DEM simulations have been shown to reasonably predict the cyclic response of granular materials, making them an appropriate tool for studying the effects of initial static shear stress on liquefaction initiation. Mechanical coordination number and the normal force-weighted fabric tensor are used to describe the evolution of fabric after monotonic preshearing and during cyclic loading. We find that higher initial static shear stress induces smaller cyclic strength in stress-controlled cyclic simple shear tests, but that there is no such effect in strain-controlled cyclic simple shear tests in which the normal force-weighted anisotropy is removed in the first three loading cycles. In addition, the stability of specimens is found to be closely related to pore-pressure increases. Finally, we show that the effect of initial static shear stress on liquefaction resistance is a combination of reduced stability before cyclic loading and accumulated shear strain during cyclic loading. [ABSTRACT FROM AUTHOR]

All‐solid‐state batteries constitute a very promising energy storage device. Two very important properties of these battery cells are the ionic and the electrical conductivity, which describe the ion and the electron transport through the electrodes, respectively. In this work, a numerical method is presented to model the electrical conductivity, considering the outcome of discrete‐element method simulations and the intrinsic conductivities of both the active material particles and the conductive additive particles. The results are calibrated and validated with the help of experimental data of real manufactured electrodes. The tortuosity, which strongly influences the ionic conductivity, is also presented for the analyzed electrodes, taking their microstructure into account. [ABSTRACT FROM AUTHOR]

Recent advances in shear-thickening suspension rheology suggest a relation between (wet) suspension flow below jamming and (dry) granular physics. To probe this connection, we simulated the contact force networks in suspensions of non-Brownian spheres using the discrete element method, varying the particle friction coefficient and volume fraction. We find that force networks in these suspensions show quantitative similarities to those in jammed dry grains. As suspensions approach the jamming point, the extrapolated volume fraction and coordination number at jamming are similar to critical values obtained for isotropically compressed spheres. Similarly, the shape of the distribution of contact forces in flowing suspensions is remarkably similar to that found in granular packings, suggesting potential refinements for analytical mean field models for the rheology of shear thickening suspensions. [ABSTRACT FROM AUTHOR]

Segregation of granular homogeneous mixtures is a common problem faced by many industries. In recent years, periodic flow inversions have shown to mitigate both size and density segregation. However, they are not fully explored. In this paper, we investigate the effect of bend angle (θ) in the chute flow using three-dimensional Discrete Element Method (DEM) simulations. We observed a significant difference in size segregation when changing the bend angle (θ) in terms of qualitative behavior and extent of segregation. For moderate and dense regimes, minimum segregation is observed for bend angle range of 120°–150°. Based on our results, we propose to use chutes with periodic 150° bends instead of vertical chutes, to reduce segregation and thus improve mixing. Image 1 • Investigated the effect of bend angle on size segregation in periodic flow inversion using DEM simulations. • A drop in segregation intensity is observed with increasing bend angle of the chute. • Reversal in Segregation flux direction is observed for moderate regime. • For moderate and dense regimes, minimum segregation is observed for bend angle range of 120°–150°. [ABSTRACT FROM AUTHOR]

Summary: Three‐dimensional Discrete Element Method simulations have been performed to study the deformation of cross‐anisotropic granular materials under principal stress rotation (PSR), for rotation planes oriented at different angles θ with respect to the bedding plane. The simulations have been conducted with a novel technique for applying specified stresses at three‐dimensional boundaries. The results are qualitatively in agreement with experimental results from literature. Cumulative volume contraction is always observed under continuous PSR and increases with increasing θ. The dilatancy rate decreases with increasing number of PSR cycles, tending to zero. The noncoaxiality angle between the strain increment and the stress in the PSR plane increases with increasing number of cycles, reaching the same asymptotic value for samples of various densities and for various θ. Periodic oscillations of the dilatancy rate and noncoaxiality angle within each PSR cycle are observed with an increasing oscillation magnitude with increasing θ, due to the larger fabric anisotropy within the PSR plane. When θ = 30 or 60°, significant noncoaxial strain accumulation occurs in the plane perpendicular to the PSR plane due to the oblique angle between the PSR plane and the bedding plane, echoing the major principal fabric direction's being neither parallel nor perpendicular to the PSR plane. The macroscopic behavior of the samples is related to the microscopic parameters including coordination number and fabric anisotropy. With increasing number of cycles, the difference between normalized stress/strain/fabric increment tensors tends to become constant, with only a small lag between each pair, irrespective of θ. [ABSTRACT FROM AUTHOR]

Featured Application: Nanoparticle suspensions are used in a variety of applications involving the transport of reagents and products to and from structured material surfaces. The more efficient alignment of particle layers adjacent to, and in contact with, a reactive surface, such as found in a fuel cell, bio-medical and electrochemical device, is often reliant upon the effective control of the dynamics of particle assembly in narrow conduits. One such control is possible by spectral thermal pulsing to generate a controlled Brownian motion of particles. This is demonstrated by numerical simulation here to be highly effective when particles are conveyed in viscous fluids close to a neutrally buoyant condition. Inter-particle and particle-wall connectivity in suspension flow has profound effects on thermal and electrical conductivity. The spectral impulse generation and the imparting of kinetic energy on the particles is shown through a mathematical analysis to be effective as a means of achieving an approximate equivalent of a Langevin thermostat. However, with dilute suspensions, the quadratic form of the thermal pulse spectra is modified with a damping coefficient to achieve the desired Langevin value. With the dense suspension system, the relaxation time is calculated from the non-linear differential equation, and the fluid properties were supported by the viscosity coefficient. A "smoothed" pulse is used for each time-step of the flow simulation to take care of the near-neighbor interactions of the adjacent particles. An approximate optimal thermostat is achieved when the number of extra pulses introduced within each time step is found to be nearly equal to the co-ordination number of each particle within the assembly. Furthermore, the ratio of the particle kinetic energy and the thermal energy imparted is found to be never quite equal to unity, as they both depend upon the finite values of the pulse duration and the relaxation time. [ABSTRACT FROM AUTHOR]

• High-accuracy separation process for swollen cables by mechanical agitation. • The effect of mechanical factors on the separation rate of PVC and Cu. • Force analysis and calculation on the cables in treatment by CFD-DEM simulation. • High correlation between simulated forces and experimental separation rate. • Separation mechanism under various conditions for optimal industrial design. In this study, a high-accuracy separation process is proposed for recycling pure polyvinyl chloride (PVC) and Cu from the thin electric cables of electrical, electronic, and automotive wastes by PVC swelling and mechanical agitation in hydrophobic organic solvent mixed with water. The high stirring speed and low blade height combined with proper blade type and reactor tank shape ensure a separation rate of over 98%. By conducting computational fluid dynamic and discrete element model simulations, quantitative force, fluid velocity, and data visualization analyses were performed. The obtained separation rate exhibited strong positive correlations with the resultant, drag, and centripetal forces at various stirring speeds and blade heights. Using the experimental and simulation data, a plausible separation mechanism was suggested. It was found that Cu pieces could slip out from swollen PVC covers under the action of external forces, while the stirring speed should be high enough to apply sufficient external forces to cables via either blade-to-cable collisions or fluid drag. Furthermore, the vertical motion of cables induced by the low blade height was essential because the rotation in the bottom reactor part inhibited the slipping of Cu pieces. [ABSTRACT FROM AUTHOR]

Acerola is renowned as a potent natural source of vitamin C. However, its high perishability presents challenges for harnessing its health benefits. In this study, we explored an alternative method, the Rotary Dryer with Inert Bed (RDIB), to transform acerola pulp into a powdered form. Through both experimental and numerical investigations, we quantified how various operational variables affect RDIB's performance in acerola pulp dehydration. In numerical simulations using the Discrete Element Method (DEM), we analyzed the number and forces of particle collisions. We combined DEM results into a single response using a desirability function, pinpointing the optimal configuration: a drum with three continuous flights, each 25 mm long. Subsequently, experimental tests with this optimal setup considered temperature (T), inert fraction (FI), and maltodextrin concentration (M) effects on drying yield and bioactive compound levels. Multi-response optimization aimed to maximize both yield and bioactive compounds, yielding the optimal conditions: M = 6.25%, FI = 46.19%, and T = 77.2 °C. Thus, the results of this study show that RDIB efficiently dehydrates acerola pulp and has promising potential for industrial applications, facilitating the production of high-quality fruit powder. [Display omitted] • A nonconventional rotary drum for fruit dehydration was investigated. • Through both experimental and DEM simulations the best configuration was found. • The rotary dryer with inert bed was efficient to transform acerola pulp into a powdered form. • An optimization study led to the best conditions for process yield and product quality. [ABSTRACT FROM AUTHOR]

Many reptiles, known as ‘sand swimmers’, adapt to their specific environments by vibrating or rotating their body. To understand these type of interactions of active objects with granular media, we study a simplified model of a self-excited spherical object (SO) immersed in the granular bed, using three-dimensional discrete element method (DEM) simulations. Modelling the vibration by an oscillatory motion, we simulate the longitudinal locomotion of the SO in three modes: transverse vibration, rotation around different axes, and a combination of both. We find that the mode of oscillation in y direction coupled with rotation around x-axis is optimal in the sense that the SO rises fastest, with periodic oscillations, in the z direction while remaining stable at the initial x position. We analyze the physical mechanisms governing the meandering up or down and show that the large oscillations are caused by an asynchronous changes between the directions of oscillation and rotation. We also observed that the SO’s rising rate is sensitive to three parameters: the oscillation amplitude, the oscillation frequency, f, and the rotation angular velocity, Ω. We report the following results. 1. When the frequencies of the rotation and transverse motion are synchronised, SO rises when Ω<0 and sinks when Ω>0; the average rising/sinking rate is proportional to |Ω|. 2. The rising rate increases linearly with the oscillation amplitude. 3. There exists a critical oscillation frequency, above and below which the rising mechanisms are different. Our study reveals the range of parameters that idealized ‘swimmers’ need to use to optimize performance in granular environments.

In gravity-driven flows of different-sized (same density) particles, it is well known that larger particles tend to segregate upward (toward the free surface), and the smaller particles downward in the direction of gravity. Alternatively, when the particles are of the same size but different density, lighter particles tend to segregate upward and heavier particles, downward. When particles differ in both size and density, true of most mixtures of interest in industry and nature, the details are complicated and no rule based on gravity alone has captured the segregation behaviours. Gradients of granular temperature and kinetic stress (i.e., energy and stress associated with velocity fluctuations) offer alternative segregation driving forces, but have, until recently, been discounted as these dynamics are relatively small in dense flows. Recently, gradients in kinetic stress have been shown to play a significant role in segregating densely sheared particle mixtures, even where the kinetic stress is a relatively small percentage of the total stress. We review recent modelling advances accounting for this effect and validation in computational experiments. We show how this framework may be useful in capturing the complicated segregation phenomenology that emerges for dense sheared flows of particles different in both size and density.

Abstract The influence of transverse mechanical vibrations on dense flows of granular material down inclines with a rough bed is analyzed using 3D DEM simulations and compared with experimental results. The vibrations make appear two distinct behaviors: a gravity-driven regime and a vibration-driven regime. In the gravity-driven regime, our results are consistent with previous studies from the literature. The vibrations induce particles velocity fluctuations in the vibration-driven regime, which generate a granular temperature profile along the depth of the packing related to the velocity profile. The velocity profile is consistent with a creeping flow which makes appear a critical length scale related to a nonlocal rheology. Our results suggest that this length scale emerges from the characteristic damping length of transverse vibration waves which propagate in the bulk of the granular material. These results could be of interest for the optimization of powders conveying. Graphical ABSTRACT Unlabelled Image Highlights • Vibrations allow the transport of granular materials to be tuned. • Vibrations allow possible flows below the angle of avalanche. • Vibrations induce a nonlocal rheology related to a characteristic length. • This characteristic length depends on the acceleration of the vibrations. [ABSTRACT FROM AUTHOR]

When dealing with unsaturated wet granular materials, a fundamental question is: What is the effect of capillary cohesion on the bulk flow and yield behavior? We investigate the dense-flow rheology of unsaturated granular materials through experiments and discrete element simulations of homogeneous, simple annular shear flows of frictional, cohesive, spherical particles. Dense shear flows of dry, cohesionless granular materials exhibit three regimes: Quasistatic, inertial, and intermediate [B. Andreotti et al., Contemp. Phys. 55, 151–152 (2013)]. Herewith, we show that the quasistatic and the intermediate regimes persist for unsaturated materials and that the rheology is essentially described by two dimensionless numbers: The reduced pressure P* comparing the cohesive to confining forces and the inertial number I, for a wide range of liquid content. This is consistent with recent numerical simulations [S. Khamseh et al., Phys. Rev. E 92, 022201 (2015)]. Finally, we measure the effective friction coefficient and the solid fraction variation throughout the wet bed. From this, we show that, in the quasistatic regime, the Mohr–Coulomb yield criterion is a good approximation for large enough P*. The experimental results agree quantitatively with the numerical simulation ones, provided the intergranular friction coefficient μ is set to its physical value identified from dry material rheology [M. Badetti et al., Eur. Phys. J. E 41, 68 (2018)]. To directly and quantitatively determine what happens inside the sheared granular bed, x-ray tomography measurements are carried out in a custom-made setup that enables imaging of a wet granular material after different shear histories. For the explored range of liquid content, samples remain homogeneous but exhibit some complex microscopic morphologies far from simple capillary bridges. From the x-ray microtomographic images, we can clearly distinguish liquid capillary bridges and liquid clusters by their morphologies. We see that the total number of capillary bridges decreases when one increases the liquid content and interestingly increases, at the expense of other morphologies, when we increase the shear strain. This explains the concordance between the experimental and numerical measurements since the numerical model is restricted to the pendular state, for which the liquid phase is completely discontinuous and no coalescence occurs between liquid bridges. [ABSTRACT FROM AUTHOR]

Based on the Brazilian test results of 23 kinds of transversely isotropic rocks, five trends are obtained for the variation of normalized failure strength (NFS) as a function of the weak plane-loading angles. For each angle, three kinds of fracture patterns are obtained. Furthermore, a new numerical approach based on the particle discrete element method is put forward to systematically investigate the influence of the micro-structure of rock matrix and strength of weak plane on NFS and fracture patterns. The results reveal that the trend of NFS and fracture patterns are slightly influenced by coordination number of rock particles and tensile strength of weak plane, but greatly influenced by percentage of pre-existing cracks and shear strength of weak plane. Micro-parameters of the numerical approach are calibrated to reproduce behaviours of transversely isotropic rocks with different trends, and the simulation results are well matched with experimental results in terms of NFS and fracture patterns. Finally, the numerical approach is applied to study the failure process of layered surrounding rock after tunnel excavation. The simulation results also agree well with observation results of engineering projects. [ABSTRACT FROM AUTHOR]

Porous materials, especially ceramics, are used in an ever-expanding range of functional applications. In most cases there are minimum mechanical requirements which limit the porosity level and thus the functional performance provided by the pore surface or volume. In order to design porous materials with the best compromise between functional and mechanical performance, a sound understanding of microstructure-mechanical properties relationships is required. In the current work, discrete simulations are used to assessed the Young's modulus and fracture toughness of various realistic porous microstructures obtained via partial sintering of powders. Scaling laws relating these quantities to microstructural parameters are derived and it is demonstrated that the proportionality between Young's modulus and fracture toughness, often claimed for partially sintered materials, is actually an approximation of a more general relationship. The proposed scaling laws suggest new strategies to build microstructurally tougher and strain tolerant porous materials. It is shown that strain tolerant microstructures can be designed by introducing controlled heterogeneity and hierarchy. Finally, the proposed scaling relationship between Young's modulus and fracture toughness is simplified to give it a practical use and verified for a wide range of porous microstructures, including hierarchical ones. [ABSTRACT FROM AUTHOR]

Inter-particle and particle-wall connectivity in suspension flow has profound effects on thermal and electrical conductivity. The spectral impulse generation and the imparting of kinetic energy on the particles is shown through a mathematical analysis to be effective as a means of achieving an approximate equivalent of a Langevin thermostat. However, with dilute suspensions, the quadratic form of the thermal pulse spectra is modified with a damping coefficient to achieve the desired Langevin value. With the dense suspension system, the relaxation time is calculated from the non-linear differential equation, and the fluid properties were supported by the viscosity coefficient. A “smoothed” pulse is used for each time-step of the flow simulation to take care of the near-neighbor interactions of the adjacent particles. An approximate optimal thermostat is achieved when the number of extra pulses introduced within each time step is found to be nearly equal to the co-ordination number of each particle within the assembly. Furthermore, the ratio of the particle kinetic energy and the thermal energy imparted is found to be never quite equal to unity, as they both depend upon the finite values of the pulse duration and the relaxation time.

Structural defects such as joints or faults are inherent to almost any rock mass. In many situations those defects have a major impact on slope stability as they can control the possible failure mechanisms. Having a good estimate of their strength then becomes crucial. The roughness of a structure is a major contributor to its strength through two different aspects, i.e. the morphology of the surface (or the shape) and the strength of the asperities (related to the strength of the rock). In the current state of practice, roughness is assessed through idealized descriptions (Patton strength criterion) or through empirical parameters (Barton JRC). In both cases, the multi-dimensionality of the roughness is ignored. In this study, we propose to take advantage of the latest developments in numerical techniques. With 3D photogrammetry and/or laser mapping, practitioners have access to the real morphology of an exposed structure. The derived triangulated surface was introduced into the DEM (discrete element method) code PFC3D to create a synthetic rock joint. The interaction between particles on either side of the discontinuity was described by a smooth-joint model (SJM), hence suppressing the artificial roughness introduced by the particle discretization. Shear tests were then performed on the synthetic rock joint. A good correspondence between strengths predicted by the model and strengths derived from well-established techniques was obtained for the first time. Amongst the benefits of the methodology is the possibility offered by the model to be used in a quantitative way for shear strength estimates, to reproduce the progressive degradation of the asperities upon shearing and to analyze structures of different scales without introducing any empirical relation.

In pharmaceutical industry the coating of granulates or tablets is an essential production step, which is often realised in a rotating drum moving the tablet bed and a number of spray nozzles installed above this bed. The nozzles atomise the coating liquid through a high-velocity annular air-stream and apply steering air nozzles for producing an oval-shaped spray. The operational conditions determine the produced droplet size spectrum. The collision of the droplets with the solid particles or tablets in the drum and the resulting coating and film thickness strongly influences the product properties of granulates or tablets. In order to be able to predict the coating result, without running time-consuming lab-scale experiments, it is intended to numerically simulate the entire tablet coating process by combining Computational Fluid Dynamics (CFD), the Discrete Element Method (DEM) and the Lagrangian Parcel Concept (LPC) for simulating the spray propagation originating at the nozzle. For doing so, knowledge on the spray droplet properties produced by the two-fluid spray nozzles is indispensable. Therefore, it is necessary to characterise as a first step the local droplet size and velocity distributions in different spray cross-sections, which is the main focus of the present contribution. Such measurements are efficiently done by applying Phase Doppler Anemometry (PDA). It was found that the velocity of the spray emitted from the two-fluid nozzle exceeds almost 200 m/s and the number mean droplet diameter was well below 20 μm. In the initial spray region droplet break-up was observed whereas downstream of 50 mm a slight growth of the mean droplet size indicated further coalescence. Due to the applied pattern air, the spray flux distribution at the normally located tablet bed (i.e., 100–150 mm downstream the nozzle) has an oval shape and the mean droplet velocity still exceeds almost 50 m/s with a mean droplet diameter of about 10 μm. The cross-sectional measurements are used to provide proper inlet conditions for the spray simulations, while the other cross-sections are used for validating the computed spray expansion and dispersion. For demonstrating the effect of a real measured spray flux distribution on the tablet coating quality, additionally DEM simulations were conducted. Here, the measured spray flux data were mapped onto the temporarily evolving and moving tablet surface simulated for a lab-scale test drum by DEM. The uniformity of tablet coating (inter-tablet variability) was judged by calculating the Coefficient Of Variation (COV) for the considered different tablet shapes and operational conditions. [Display omitted] • Numerical simulation and modelling of tablet coating by sprays in a rotating drum • Computational Fluid Dynamics (CFD), Discrete Element Method (DEM) and Lagrangian Parcel Concept (LPC) • Highly resolved spray measurements by PDA (phase-Doppler anemometer) for providing inlet conditions • Atomisation and steering air flow yield a very complex spray pattern with very high velocities • Consideration of fully resolved spray pattern of coating liquid from measurements in DEM simulations [ABSTRACT FROM AUTHOR]

In gravity-driven flows of different-sized (same density) particles, it is well known that larger particles tend to segregate upward (toward the free surface), and the smaller particles downward in the direction of gravity. Alternatively, when the particles are of the same size but different density, lighter particles tend to segregate upward and heavier particles, downward. When particles differ in both size and density, true of most mixtures of interest in industry and nature, the details are complicated and no rule based on gravity alone has captured the segregation behaviours. Gradients of granular temperature and kinetic stress (i.e., energy and stress associated with velocity fluctuations) offer alternative segregation driving forces, but have, until recently, been discounted as these dynamics are relatively small in dense flows. Recently, gradients in kinetic stress have been shown to play a significant role in segregating densely sheared particle mixtures, even where the kinetic stress is a relatively small percentage of the total stress. We review recent modelling advances accounting for this effect and validation in computational experiments. We show how this framework may be useful in capturing the complicated segregation phenomenology that emerges for dense sheared flows of particles different in both size and density. [ABSTRACT FROM AUTHOR]

We study numerically the propagation of an acoustic pulse through a loaded granular material under the hypothesis that the conventional modeling of solid friction used in soft sphere discrete element modeling remains valid at acoustic time scales. As the pulse crosses the material, it temporarily suppresses sliding contacts, making it difficult to prepare states that correspond to experimental conditions. The pulse speed is strongly affected by the loading in a very anisotropic way, varying by as much as a factor of two depending on the propagation direction. We separate the contribution of the contact network from that of sliding contacts, and show that sliding contacts can reduce the propagation speed as much as changes in coordination number. Sliding contacts have a characteristic acoustic signature: pulse speed depends on sign (compression or rarefaction), even at very small amplitudes. [ABSTRACT FROM AUTHOR]

One of the most important operations in the food, chemical, and pharmaceutical industries is powder mixing. In the pharmaceutical industry this operation is currently performed mainly in batch mode. However, the Food and Drug Administration (FDA), using the Process Analytical Technology (PAT) initiative, has been working on the promotion of the application of continuous processes in the pharmaceutical industry. The main goal of this study was to understand the powder phenomena inside the mixer and monitor mixing uniformity using experiments and simulations by Discrete Elements Methods (DEM). The experimental results showed highest relative standard deviation (RSD) using the lowest active pharmaceutical ingredient (API) concentration (2.5%), and after using image analysis part of this effect was attributed to the position of the feed inlet. The two experiments with the highest RSD (50 and 70 RPM) were replicated using a new feeding position (25°). The RSD values for the new feeding position demonstrated an improvement in mixing uniformity. The new feed position was studied in more detail using DEM by performing a simulations set that included simulations at two mixer speeds (50 and 70 RPM), three concentrations (2.5%, 10.5%, and 50%), and two different material properties (with and without cohesion). Values of hold-up, velocity profile, mean residence time (MRT), and mixing uniformity were compared for both feed positions. Also the blend uniformities inside the tumble, at each tumble exit, and at the final exit of the system were compared. The results show a positive effect of the angle in the uniformity inside the mixer, in the exit points, and at the exit of the system. Additionally, a relationship between RSD and concentration was found. [ABSTRACT FROM AUTHOR]

This work presents an experimental and numerical investigation of the shear flow of a cohesive, caking powder. Utilizing potassium chloride (KCl) as a model material, the bulk's flow behavior with respect to storage time is measured with a Schulze ring shear tester. Our results suggest that KCl cakes on a characteristic timescale $$t_\mathrm{c}$$ , which is independent of the normal load. Based on a detailed product characterization, a generalized elastoplastic contact model for discrete element simulations combined with an approach of successive calibration is proposed. Mircoscopic parameters, which are not measured directly, are used as effective ones to fit the flow behavior of an idealized ensemble of particles to the experimental findings. Within this process, we investigate the influence of these microscopic parameters on macroscopic quantities as well as the bulk's structure. Successful calibration confirms that the rheology of complex bulk materials, such as KCl, can be predicted efficiently with simplified numerical simulations. [ABSTRACT FROM AUTHOR]

We investigate the mechanical properties of inclined frictional granular layers prepared with different protocols by means of DEM numerical simulations. We perform an orthotropic elastic analysis of the stress response to a localized overload at the layer surface for several substrate tilt angles. The distance to the unjamming transition is controlled by the tilt angle $$\alpha $$ with respect to the critical angle $$\alpha _c$$ . We find that the shear modulus of the system decreases with $$\alpha $$ , but tends to a finite value as $$\alpha \rightarrow \alpha _c$$ . We also study the behaviour of various microscopic quantities with $$\alpha $$ , and show in particular the evolution of the contact orientation with respect to the orthotropic axes and that of the distribution of the friction mobilisation at contact. [ABSTRACT FROM AUTHOR]

Microbially induced calcite precipitation (MICP) is a recent technique used to reinforce the soil. In the present work, a 3D Discrete Element Method (DEM) model for biocemented sand is set up. It takes into account real contact properties, such as the cohesive contact surface area distribution and the percentage of cemented bonds, which have been computed from 3D synchrotron X-ray images at high resolution of biocemented sand with different amounts of calcite, ranging from 0 to 14.9% in volume (i.e. 19.6% in mass). Triaxial tests under different confining pressures are simulated. The macroscopic response of numerical and physical specimens are compared. Overall, these simulations show that the above contact properties play an important role on the macroscopic response and the proposed numerical model allows to describe with a good accuracy the macroscopic responses of lightly cemented samples (below 4% in volume), whereas it requires future developments to well represent the behavior of medium and highly cemented samples. [ABSTRACT FROM AUTHOR]

Writing the stress as a sum over contact forces allows one to partition stress increments according to their microscopic, grain-level causes. We present two ways of doing this: the first way differentiates between normal and tangential contact forces, while the second ascribes different parts of a change in stress to different physical processes such as contact network anisotropy, non-affine motions and sliding contacts. These two partitioning methods can be combined. We then use them to analyze simulations of failure in biaxial tests, leading to several results. We show that the granular material becomes effectively frictionless well before failure. In addition, by partitioning the second order work, one can attribute a part of the destabilization to each of the various physical processes mentioned above. [ABSTRACT FROM AUTHOR]

Abstract: Three dimensional (3D) DEM (discrete element method) simulations of drained triaxial compression and plane strain tests are presented for both dense and loose assemblies of polydisperse spheres using a periodic cell. In the work reported, drained tests were modelled by deforming the samples under constant mean stress conditions. The drained behaviour is shown to be qualitatively similar to published physical experimental results. The Bishop''s formula for the estimation of the intermediate principal stress is evaluated. The existence of critical density is shown to be independent of initial packing densities and strain conditions. Different failure criteria have been compared based on the DEM simulation results, and the Lade criterion is found to be the most appropriate one. A new microscopic fabric parameter is introduced to give insight to structural anisotropy under general 3D fabric conditions. It is found that two parameters characterize the evolution of the stress and fabric respectively independent of strain conditions. [Copyright &y& Elsevier]