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[Display omitted] • Discovered different properties among various stationary particle bed configurations. • Revealed significant reduction in thermal conductivities when particles were flowing. • Measured near-wall thermal resistance of three types of moving Carbo particles. • Used flowing particle thermophysical properties to model heat transfer coefficients in MPBE. • Modeled HTCs in MPBEs with various particle sizes, flow velocities, and temperatures. Accurate determination of heat transfer coefficients for flowing packed particle beds is essential to the design of particle heat exchangers and other thermal and thermochemical equipment. While such dense granular flows mostly fall into the well-known plug-flow regime, the discrete nature of granular materials alters the thermal transport processes in both the near-wall and bulk regions of flowing particle beds from their stationary counterparts. As a result, heat transfer correlations based on the stationary particle bed thermal conductivity could be inadequate for flowing particles in a heat exchanger. Most earlier works have achieved a reasonable agreement with experiments by treating granular heat transfer media as a plug-flow continuum with a near-wall thermal resistance in series. However, the thermal conductivity values of the continuum were often obtained from measurements on stationary beds owing to the difficulty of flowing bed measurements. In this work, it was found that the properties of a stationary bed are highly sensitive to the method of particle packing and there is a decrease in the particle bed thermal conductivity and increase in the near-wall thermal resistance, measured as an effective air gap thickness, on the onset of particle flow. These variations in thermal conductivity of stationary and flowing particle beds can lead to errors in heat transfer coefficient calculations. Therefore, the heat transfer coefficients for granular flows were calculated using experimentally determined flowing particle bed thermal conductivity and near-wall air gap for ceramic particles − CARBO CP 40/100 (mean diameter = 275 µm), HSP 40/70 (404 µm) and HSP 16/30 (956 µm); at velocities of 5–15 mm·s−1; and temperatures of 300–650 °C. The thermal conductivity and air gap values for CP 40/100 and HSP 40/70 were further used to calculate heat transfer coefficients across different particle bed temperatures and velocities for different parallel-plate heat exchanger dimensions. These calculations, which show good agreement with measured HTC values reported in literature, can be used as a guide for heat exchanger designs. [ABSTRACT FROM AUTHOR]

Rotary drums are widely used in industry, however, determining granular dynamics within them is still a complex task, thus remaining a focal point of research over the years. In this context, experimental techniques require numerical support to obtain data that is not easily acquired experimentally. One well-established numerical technique for studying particulate systems is the Lagrangian approach embed with Discrete Element Method (DEM). Although, DEM has limitations related to the definition of input parameters. It is crucial that DEM input parameters be measured and/or calibrated to reliably represent the granular behavior. These parameters are related to particle properties such as density, Young's modulus, Poisson's ratio, among others, and particle interactions, such as coefficients of restitution, static friction, and rolling friction. This study explores the experimental determination of interaction parameters preceding the acquisition of DEM input parameters in a rotary drum. Statistical analysis highlights the importance of each parameter and their combined interactions with the static angle of repose. The granular materials used were: soybeans, glass beads, polyacetal, 6 mm steel, and 4 mm steel. The coefficient of restitution was determined via free-fall methodology using equipment that verifies particle trajectory, static friction coefficient through equilibrium between weight and resistance forces, rolling friction coefficient using a launching device on surfaces of different roughness, and static angle of repose in a rectangular box with sections where particles rested. After removing the partition, angles were measured at both top and bottom sections. Regression techniques using a neural network created in Python were employed for both measurements. Experimental investigations and numerical simulations using DEM indicated that the coefficient of restitution depends on the thickness and type of material of the flat surfaces, showing an asymptotic behavior with increasing surface thickness. The static friction coefficient increased on rougher surfaces and in particles with greater imperfections. Rolling friction coefficient also increased on rough surfaces. Analysis of the angle of repose revealed that granular properties influence the response, such as static and rolling friction coefficients, whereas the coefficient of restitution had no significant influence. Neural network correlation coefficients exceeded 0.95, indicating a strong association between the variables. Our findings revealed that mathematical models can be used in conjunction with static repose angle experiments to calibrate DEM parameters for more robust and complex simulations. This research also quantifies and provides insights into the interactions between DEM parameters in direct measurement experiments, promoting more efficient calibration for future research. [Display omitted] • Restitution coefficient varies with surface thickness and material. • Increased surface roughness and imperfections raise static friction coefficients. • Rougher surfaces and smaller particles lead to higher rolling friction coefficients. • Granular properties, diameter, and density affect the static angle of repose. • Artificial Neural Networks have been employed for DEM parameters determination. [ABSTRACT FROM AUTHOR]

Understanding motion-induced terrain deformation is essential for improving predictive models in geotechnical engineering, infrastructure development, and robotics. The existing analysis assumes a uniform and unchanged environment and lacks a description of the terrain deformation induced by motion, which is particularly significant when the grains are loosely packed. To address this gap, we performed numerical investigations to mathematically model the changes in the surrounding environment and their corresponding effects on resistance. Specifically, we examined the response of granular materials and the energy variation of the particles during motion. Increased energy in the direction of motion was categorized as influences above or below the medium surface. Aligned with the influencing factors, two variables were considered to quantify the impact on the drag force. Building upon the energy analysis and the framework of Resistive Force Theory, we proposed a drag force model for buried motion in loose granular terrain. Compared with the experimental data, the average predictive error decreased from 17.8% to less than 2.7%. Our study provides a feasible approach for incorporating the effects of terrain deformation into a drag force model, thereby enhancing the accuracy and contributing to the development of granular locomotion. • The impact of motion-induced terrain deformation on the surface layer is modeled. • Granular pile formation leads to increased depth of buried objects. • The flow of particles raises local packing density. • A drag force model integrated with environmental changes is established and verified. [ABSTRACT FROM AUTHOR]

This study explores how a continuous drill of T-shaped rotor interacts with granular media and illustrates an universal torque bifurcation mechanism. Experiments shew that the drilling torque linear increases with depth initially, followed by bifurcation. The bifurcation types are intricately linked to the motion parameters of the rotor, while the depth of bifurcation and saturated torque are directly influenced by rotor geometry. DEM simulations link torque bifurcation to a low-stress fluidized region around the rotor, while the region is enclosed by a fluid–solid interface and influenced by the motion parameters. Moreover, force chain analysis demonstrates the irreversible state change of granular media disturbed by the rotor. Leveraging these insights, we propose two scaling laws quantifying the bifurcation point depth and the saturated torque, then perform qualitative analysis to explain the experimental phenomena. These findings may hold significant promise for practical engineering applications. [Display omitted] • Rotor geometry affects torque magnitude and bifurcation depth. • Rotor motion determines the appearance of saturated torque. • Bifurcation hinges on the expansion of a low-stress fluidized zone. • Rotor induces irreversible impact on granular media state. • Scaling analysis can compute bifurcation depth and saturation condition. [ABSTRACT FROM AUTHOR]

The Cosserat continuum, which contains a length scale factor l reported to link with the material's grain size, is seemingly reasonable to describe the size effect of granular media. However, the physical meaning of l is still unclear, and its potential in describing the size effect is seldom studied. In this work, the performance of the conventional Cosserat model in capturing the size effect of granular media is firstly assessed. A new multiscale Cosserat constitutive model with an additional higher-order internal length scale vector D q is then proposed based on the conventional Cosserat theory. The advantage of the proposed multiscale Cosserat model in size effect simulation for granular media is discussed through numerical simulations of centrifuge slope experiments and triaxial compression tests. The analysis results denote that the multiscale Cosserat continua are more sensitive to size effect than the conventional Cosserat continua, and the length scale vector D q is like a symmetry-breaking factor that can trigger asymmetric deformation in structures; the influences of length scale factor l and D q are seemingly opposite on stress concentration. This subtly improved constitutive model has the potential to simulate the size effect of a wider variety of materials. • A multiscale Cosserat model that contains higher order lengths was developed. • The new model can phenomenologically simulate the failure mode of the soil. • The new model can simulate the size effect caused by the structure size changes. • The new model is more sensitive to size effect than the conventional one. • The length scales of the new model can perform as symmetry-breaking factors. [ABSTRACT FROM AUTHOR]

Granular heat transfer is encountered in many industries, including food production, solar energy harvesting and nuclear engineering. The present study investigates influence of vibration characteristics on granular heat transfer in a vibrationally fluidized bed of particles using a thermal discrete element method. The bed is heated by transferring a constant heat flux from its bottom wall. Vibrations generally had two influences: first, particles reached a higher average temperature, and second, they attained more uniform temperature distribution. The particles average temperature generally increased by increasing vibration amplitude and frequency. However, the best temperature uniformity occurred in an optimal set of amplitudes and frequencies. Contrary to claims in previous studies, the effective thermal conductivity (ETC) of the vibrated bed was not the only determining parameter for quality of heat transfer. Other parameters such as flow patterns, convective and diffusive motions must be also analyzed to evaluate the heating process in vibrating beds. Evolution of temperature profile of particles after 10 min of heating within (a) a fixed bed, and beds vibrated with frequency of 20 Hz and with amplitudes of (b) 1 mm, (c) 3 mm, and (d) 6 mm. [Display omitted] • Vibration characteristics effects on heat transfer in granular beds were studied. • Vibrations enhance the average temperature and temperature uniformity of particles. • Particles average temperature increase by increasing amplitude and frequency. • Maximum temperature uniformity occurred in an optimal amplitude and frequency set. • Generation of heating cycles plays a significant role in granular heat transfer. [ABSTRACT FROM AUTHOR]

The reliable modelling of irregular grain shapes and generating realistic particle packs still remain urgent problems limiting the application of the particle-based numerical techniques in many industries dealing with granular materials. An effective computational approach for generating irregular three-dimensional (3D) grains by dynamic agglomeration of a large number of movable and interacting particles is proposed. The constructed grains are then randomly packed within an arbitrary-shaped container. The molecular dynamics-based concept is used for producing the grains with realistically complex configurations and adjustable shape morphology. The identical molecular dynamics technique coupled with the full six-degree-of-freedom rigid-body dynamics of the agglomerates is applied for grain packing. A database of 3D models of unduplicated irregular particles has been created. Several examples are considered to demonstrate the computational efficiency and controllability of the presented algorithms. The numerically reconstructed packings qualitatively and quantitatively satisfy the desirable properties of the real one in terms of the morphological features, void ratio, size distribution, and generally accepted shape characteristics of the grains (such as aspect ratio, sphericity, roundness, and roughness). Not only statistical, but also physical correspondence is established through the numerical reproduction of the angle of repose tests. An experimental verification of the model is performed. [Display omitted] • An approach for generating and optimal packing of irregular 3D grains is presented. • The method uses molecular dynamics with intergranular and grain-wall interactions. • A database of 3D grains with unduplicated complex configurations has been created. • A verified versatile computational model and cost-effective algorithms are proposed. • The model reproduces the morphological features of real grains and grain assemblies. [ABSTRACT FROM AUTHOR]

Granular media, a macroscopic amorphous matter composed of mesoscopic discrete particles, exhibits different behaviour characteristics (e.g., solid-, liquid-, and gas-like phases) depending on the volume fraction and loading conditions. Therefore, it poses a significant challenge to three-phase modelling and analysis. In this study, a description of multiple phases in granular media is realised by establishing the transition criteria and parameter conversion relations between different phases, using the already established theories of the elastic-plastic constitutive relation in solid mechanics, the viscoplastic constitutive relation in rheology, and the kinetic theory of granular flow. The new model realises the coexistence of different phases and the conversions therebetween. Smoothed discrete particle hydrodynamics are introduced to discretise the new theoretical model, and the correspondence between the smoothed and actual particles for different phases and discrete formulas under different models are described in detail. Two typical cases are used for numerical tests: the collapse of one side of a three-dimensional granular column and the collapse and flow of a granular column on an inclined surface. The calculated morphology of granular flow, distributions of velocity vectors, scaling law of spreading range under different height-width ratios, and the transient characteristics of particle spreading all accord well with the experimental results. Furthermore, quasi-static, slow, fast, and other phases of particle motions are well captured. Thus, we verify the feasibility of the new model and method for the multiple-phase simulation of granular media. [Display omitted] • A constitutive model for solid, liquid, and gas phases of granular media established. • Transition and conversion strategies of model and method between different phases. • Smoothed discrete particle hydrodynamics first used to simulate the three phases. • Quasi-static, slow, fast, and other phases of particle motions all well captured. • Quasi-static, slow, fast, and other phases of particle motions all well capture. [ABSTRACT FROM AUTHOR]

Understanding of the rheological behaviour of fully three-dimensional and non-uniform particle flow is of great interest. We analyse the resistance exhibited by a granular bed as a rotating impeller is penetrated into it, from which the rheological characteristics of the bed are deduced. For this end, the transient rheological response of both spherical and rodlike particles is simulated by Discrete Element Method. Transition from quasi-static to intermediate flow regime of rodlike particles is found to occur at a much larger shear strain rate than that of spherical particles. The relationship between the bulk friction coefficient and the inertial number is not monotonic. The viscosity of particle flow is inferred from the blade torque and is related to the inertial number and granular temperature, for which a power law is obtained covering both quasi-static and intermediate flow regimes. It can be used for obtaining flow field in complex geometry and dynamics using continuum mechanics by Computational Fluid Dynamics. [Display omitted] • Rheological characteristics of a dense granular system is investigated. • An impeller with twisted blades is penetrated into a particle bed. • Different flow regimes may prevail during the penetration. • Viscosity of granular flow is related to inertial number and granular temperature. • A power law model of viscosity is proposed that is obtainable from impeller torque. [ABSTRACT FROM AUTHOR]

Many materials, such as sand, rice, wheat, or other kinds of seeds, consist of numerous individual grains that determine the visual appearance of these materials. When generating images of these mixtures, the primary challenge is to simulate the interaction of light with each individual grain. While subsurface scattering effects are crucial for producing realistic images, the computation of light transport using standard path tracing methods for each grain can be prohibitively expensive. Although there have been several methods developed to address this issue, they all assume that bounding spheres of individual grains do not intersect. This restriction limits the application of these methods to almost spherical grains. Nonetheless, various grains, such as seeds and rice, are non-spherical, making this assumption lead to impractical stackings in situations involving coarse-grained materials. We address this issue by presenting a subsurface scattering model that utilizes a neural network and is trained using an optimal transport framework. Our model surpasses path tracing approaches conclusively, allowing for efficient rendering of granular mixtures that were previously unfeasible. Additionally, this method can be utilized in large-scale procedural generated scenes based on sphere packings and obtains similar results as previous methods in these cases. [Display omitted] • Mixtures of non-spherical grains can be rendered efficiently. • Neural network-based subsurface scattering models speed up rendering times. • Optimal transport can be used for stable training of subsurface scattering models. • State-of-the-art grain rendering can be extended by neural network-based approaches. [ABSTRACT FROM AUTHOR]

Interparticle Van der Waals force contributes to the overshoot in the bed pressure drop at the minimum fluidization velocity during the transition from static to fluidized bed conditions, which is a well-known phenomenon in the fluidization of fine particles. In this study, two adhesive particle pressure closures considering the effect of interparticle Van der Waals force are used in two-fluid model simulations with the intention to generate the pressure overshoot. The first adhesive pressure model developed within the context of the kinetic theory of rapid granular flows failed to produce the overshoot due to the dominance of multiple and long duration contacts in the fixed-bed flow. Another closure based on the coordination number was then proposed to represent long-lasting interparticle contacts, which gave an adhesive contribution much larger than the one of the kinetic theory model and was able to create the pressure drop overshoot. [Display omitted] • Two adhesion pressure closures are given for the Van der Waals force effect in TFM. • The kinetic theory fails to predict a pressure overshoot in a fluidized bed. • The coordination number model is successful in generating the pressure overshoot. • Interparticle Van der Waals force contributes to the pressure overshoot phenomenon. [ABSTRACT FROM AUTHOR]

[Display omitted] • The study covers density-difference-driven segregation of dense granular flow. • Experiments are done in an annular shear cell developing non-uniform shear rates. • A continuum model for describing density-driven segregation is proposed. • The model well predicts density-driven segregation of dense granular flow. • The shear localization features strongly influence segregation by density. Granular materials segregate spontaneously due to differences in particle size, shape, density and flow behaviour. In this paper we experimentally investigate density-difference-driven segregation for a range of density ratios and a range of heavy particle concentrations. The experiments are conducted in an annular shear cell with rotating bumpy bottom that yields an exponential shear profile. The cell is initially filled with a layer of light particles and an upper layer of heavier grains and, on top, a load provides confinement. The segregation process is filmed through the transparent side-wall with a camera, and the evolution of particle concentration in space and time is evaluated by means of post-processing image analysis. We also propose a continuum-approach to model density-driven segregation. We use a segregation-diffusion transport equation, constitutive relations for effective viscosity and friction coefficient, and a segregation velocity analogous to the Stokes' law. The model, which is validated by comparison with experimental findings, can successfully predict density-driven segregation at different density ratios and volumetric fraction. [ABSTRACT FROM AUTHOR]

This paper presents a computational study on the interactive evolution of particle shape and particle breakage for granular material under confined compression. A novel multiscale approach combining peridynamics and non-smooth contact dynamics is employed. Breakage of more than 12,700 particles with different size, shape, and loading condition are simulated with results examined statistically. It is found that, with the progress of particle breakage, the shape of fragments tends to achieve a steady state distribution. Particle shape has profound influence on the strength and crushing pattern of individual particles and such influence is best reflected by sphericity. The average particle strength appears linearly correlated with particle shape factors and sphere-like particles tend to possess higher strength and break in a major-splitting mode. The findings help to decode the intricate crushing behavior in granular media and shed lights on future development of physically informed, simplified models for predicting particle breakage. Unlabelled Image • Grain crushing modeled by a hybrid peridynamics and contact dynamics approach. • Simulations validated by macroscopic data of sand under compression. • Two-way influence between particle crushing and particle shape revealed. [ABSTRACT FROM AUTHOR]

• We found the thermoelectric field distribution of spheroid inside a matrix material • The Maxwell–Garnett approach was adopted for granular thermoelectric materials • The model was illustrated for MgAg 0.97 Sb 0.99 inclusions placed in a matrix of Bi 2 Te 3 • Impact of the spheroid geometry on effective properties of a composite is analyzed The paper studies the problem of an extended thermoelectric spheroid placed in a thermoelectric matrix. The external electric field is assumed to be uniform. Despite the nonlinear nature of the equations, the solution of the problem can be found as a sum of Legendre functions and polynomials. The solution obtained is used to find the effective values of electrical and thermal conductivities, as well as the thermopower for a composite medium consisting of a set of randomly located spheroids. As an example of the model application, we consider a composite consisting of spheroidal inclusions MgAg 0.97 Sb 0.99 placed in a matrix of Bi 2 Te 3. [ABSTRACT FROM AUTHOR]

This paper presents a numerical investigation of a disk-shaped rotor rotating inside a granular medium using the discrete element method, and the torque, quantitatively characterizing the resultant of drag forces on the rotor from surrounding particles, is mainly explored. The existence of a dumbbell-shaped region induced by the rotating rotor may essentially give rise to the steady state independent of initial loading conditions, in which the pressure difference between the upper and lower surfaces of the disk disappears and the torque always tends to a stationary value. Furthermore, the rotation of the rotor induces an anisotropic structure with the formation of arches that screen the vertical pressure, leading to the occurrence of the low-pressure region above the rotor. Finally, a scaling law of the torque with the depth is proposed, in which the rotation speed of the rotor as an affecting factor is also taken into account. Unlabelled Image • The drag force experienced by a disk-shaped rotor is studied based on DEM. • The steady state is independent of the initial loading conditions. • The rotation of the rotor induces a dumbbell-shaped activated region. • The formation of arch structure screens the vertical pressure. • The scaling law of the torque with the depth and the rotation speed is proposed. [ABSTRACT FROM AUTHOR]

The second part of this bipartite series on the micromorphic contribution of particle-based microstructures to continuum-mechanical theories is mainly concerned with the relation between particle and continuum kinematics. Extending the governing relations and the stress homogenisation of Part I by the introduction of a tensor-valued micromorphic angular momentum balance and a micromorphic balance of mechanical work yields new insights in the impact of dyadic stress moments in micromorphicity. Furthermore, it contributes to identify work-conjugated pairs of standard and extended micromorphic continuum contributions based on an investigation of ensembles of deformable particles taken as a Representative Elementary Volume (REV) both at the mesoscopic and the macroscopic scale. By use of geometrically linear kinematics, it is furthermore shown how micromorphic deformations subdivide in micropolar and microstrain contributions. Deriving the specific volume-averaging formalism for deformation measures of REV with a finite number of particles additionally allows for a direct evaluation of the individual contributions based on Discrete-Element (DE) simulations, thus yielding an excellent possibility to include particle mechanics in continuum-mechanical approaches with complex microstructures. [ABSTRACT FROM AUTHOR]

We propose a new way to estimating interparticle contact forces in granular materials, based on the combination of experimental measurements and numerical techniques that take the contact laws respectively from Molecular Dynamics and Non-Smooth Contact Dynamics discrete element methods. Tests are performed in quasi-static conditions on a two-dimensional granular assembly; 80 MPixel pictures of the assembly are shot throughout each test. Image processing and Digital Image Correlation are used in order to get information on the geometry of the assembly (particles and contact points position) and the rigid-body motion of particles (displacements and rotation); based on this information, numerical methods can be applied to assess contact forces. 2D DEM simulations are also performed and the same information is extracted and used for contact force estimation, so that the numerical methods can be validated. The reproducibility of the original set of contact forces is shown to be dependent on the displacement history for the first method, based on contact elasticity, and on the degree of force indeterminacy for the Contact Dynamics-based one. The validation phase also includes a perturbation analysis to predict the influence of measurement error (bad contact detection) when applying the Contact Dynamics-based method, which shows the robustness of this method. After this validation phase, the methods are applied to experimental data. Measurement error on kinematic measurements turns out to be quite significant for the first approach, while it does not affect the second, for which the accuracy of the force estimation can be assumed to be mainly dependent on the degree of force indeterminacy of the system. [ABSTRACT FROM AUTHOR]

We propose a model for the study of rock filled gabions, based on the theory of continua with microstructure, where the interlocking of stones through contacts figures explicitly. In this model, each stone is partly locked by its first neighbours, appearing as a continuous deformable box. While stones are material items, as such endowed with inertia, boxes are configurational features, as e.g., crack tips or phase boundaries. Two kind of internal actions are then included in the model: (1) material actions that represent the internal forces and moments arising in the boxing process and (2) configurational actions that possibly drive boxes to change shape or make contacts within them be updated. Following the theory of continua with microstructures, each material and configurational element of the continuum is endowed with geometrical properties, working with respect to the above mentioned internal actions. In order to reduce the complexity of this geometry (and of the ensuing mathematical problem), preserving nonetheless a sufficiently accurate description of the boxing process, we will consider stones and boxes having a one-to-one permanent connection, represented by a symmetric traceless tensor, akin to a fabric tensor. The set of equilibrium equations that define the mechanical problem is then divided into two groups: (1) balance of material momentum and moment of momentum, expressed in the usual vectorial space of forces and (2) balance of configurational dynamical entities, expressed in the space of second order tensors. The dissipative and non-dissipative parts of the stresses are then related to a free energy potential and to a dissipation potential, within the framework of rate independent processes. [ABSTRACT FROM AUTHOR]

This paper is a study of the dependence of the volume of voids in a granular material on the particle size distribution. It has previously been proposed that the volume of voids is proportional to the volume of the smallest particles. In a particle size distribution which is progressively becoming wider (e.g. as occurs due to crushing during the compression of sand), the smallest size of particle decreases, yet there are only ever a few of these particles out of many thousands or millions. This paper attempts to identify which particles govern the overall density of a granular material, and a new definition of the 'smallest particles' is proposed. These particles are shown to govern the void space in a range of simulations of spherical and non-spherical crushable particles. The theory also applies to idealised Apollonian sphere packings. [ABSTRACT FROM AUTHOR]

The present article concerns a homogenisation strategy that links particle-based interaction forces to stress quantities of kinematically extended microcontinuum theories. The homogenisation procedure is based on the investigation of the balance relations for single, homogeneously deformable particles and for ensembles of particles forming a Representative Elementary Volume, thus exhibiting the relation of particle-based forces and stresses with characteristic micromorphic stress states. The formalism includes the established homogenisation procedure for rigid particle ensembles linking particle-interaction forces to characteristic micropolar stress states as a special case of microcontinua. In order to demonstrate the applicability of the proposed homogenisation technique, a localisation problem of granular media is investigated by use of the Discrete-Element Method and a subsequent homogenisation of the microstructural information. A comparison between the arising micromorphic and micropolar cases is presented. The former is a result of microstructures that include deformable components, while the latter is already resulting from rigid-particle microstructures. In both cases, the averaged extended stress states are predominant in the region of localised zones. [ABSTRACT FROM AUTHOR]

The arching effect significantly affects the mechanical properties such as deformation and loading transmission of the granular media. To visually observe the arching process and force chain distribution, with two steel columns set underneath in the device to cause localized deformation, two-dimensional loading model experiments of photoelastic particles were conducted. By exploring the arching mechanism, the following main conclusions were drawn: (1) Two kinds of arches were found; one is called "stress arch", which can block the loading transmission path and shield the passing stress, while the other is an intrinsic arch generated under appropriate compressive state that can cause mesoscopic inhomogeneity of deformation. (2) Both the geometric parameters and the stress state affect the formation of the stress arch; for instance, the accumulating space S of the steel columns will increase the influence range of the stress arch until its disintegration happens, which is consistent with the trap-door experiments. Unlabelled Image • Arching effect of photoelastic particles formed over two steel columns was observed. • Stress arch and intrinsic arch were found in the experiments. • Arches in media can block the passing load or cause inhomogeneous deformation. • Geometric parameters and stress state can affect the formation of the stress arch. • Morphological changes of stress arches include triangle, trapezoid, and arc shapes. [ABSTRACT FROM AUTHOR]

The strength of wet granular ice layers is an important quantity for modelling the behaviour of ice accretions encountered in aircraft engine icing since it determines the dynamics of the collision of ice crystals, their penetration into the layer, breakup and the characteristics of the ejected material. In the present study, impact tests of 3.18 mm nylon spheres are conducted on wet granular ice layers for a dynamic strength characterization. An experimental setup and methodology are designed to measure the crater depth for impact velocities ranging from 5 ms − 1 to 90 ms − 1. Experiments are conducted using both ice layers generated by ice crystal accretion on a solid substrate inside an icing wind tunnel and artificial ice layers prepared in a laboratory environment. By fitting the Poncelet penetration equation to the experimental data, two dimensionless parameters are derived that characterize the strength of the investigated ice layers in relation to the layer inertial resistance and apparent yield strength. A key finding is that strong connections between individual ice particles exist for ice layer generated in the wind tunnel, originating from re-freezing of liquid water near the particle contacts. Only by re-enforcing the ice particle connections in ice layers prepared in the laboratory, comparable strength values are obtained. The presented methodology enables measurement of ice layer strength and comparison at different facilities as well as conducting advanced experiments with realistic ice layers in a laboratory environment. • Ice layer impact experiments conducted in icing wind tunnel and icing laboratory • Deeper impact craters in ice layers generated at positive wet-bulb temperatures • Connection strength between ice crystals significantly affects ice layer strength • Representative granular ice layers can be replicated in a laboratory environment [ABSTRACT FROM AUTHOR]

Producing of granular media with irregular shapes is one of the significant challenges in different fields related to granular particles. Representing inaccurate and simplified particles can result in wrong predictions. In this paper, a new statistical and optimization-based algorithm is presented by which 3D particles with complex shapes are generated on a random container. Stochastic conditional sampling along with an optimization method are combined such that particle with realistically and controllable shapes are generated. Depending on the initial shape of the container, a pack of spheres is generated. Then, an iterative optimization-based conditional sampling is proposed by which other complex shape descriptors are considered and can help to avoid artifacts and unrealistic shapes. The proposed method is applied to two examples wherein the particles reproduction and controllability of algorithm is demonstrated. The performance of the algorithm, in terms of shape reproduction, is also quantified using various utilized statistical measures. Unlabelled Image • An automatic algorithm for producing irregular particles is presented. • The proposed method is based on the input morphological statistics. • An optimization algorithm is developed to include the statistics. • The results are compared with the initial data. [ABSTRACT FROM AUTHOR]

The particles velocity vectors; (a) creation, (b) fully-developed pattern, (c) movement, and (d) destruction of a convection roll. • Influence of vibration amplitude and frequency on mixing in fluidized beds. • As the vibration strength increased, the convection rolls became fully developed. • The generation and movement of convection rolls played a fundamental role in mixing. • Mixing of particles was generally improved by increasing the vibration amplitude. • An optimal frequency was found in each amplitude, by which the best mixing occurred. The uniform mixing is necessary in many processes such as those used in pharmaceutical, chemical and petrochemical industries. The present work studies the influence of vibration characteristics on mixing of binary mixtures of spheres produced in a vibrationally fluidized bed by using discrete element method. Solid-like flow pattern was observed in low frequencies and amplitudes, while as the vibration got stronger, particles behaved fluid-like, and convection rolls were generated. The generation and movement of convection rolls played a fundamental role in mixing within vibrated beds. It was realized that mixing was generally improved by increasing the vibration amplitude. In contrast, an optimal frequency was found in each amplitude, by which the best mixing occurred. The particles average velocity always increased by amplitude at a fixed frequency, while the mixing was not necessarily improved. It was found that the results were in qualitative agreement with some previous experimental and numerical studies. [ABSTRACT FROM AUTHOR]

Structural characteristics are considered to be the dominant factors in determining the effective properties of granular media, particularly in the scope of transport phenomena. Enhancing heat management requires an improved fundamental understanding of thermal transport in granular media. In this study, the effects of packing structure on heat transfer in granular media are evaluated at macro- and grain-scales. At the grain-scale, a gas-solid coupling heat transfer model is adapted into a discrete-element-method to simulate this transport phenomenon. The numerical framework is validated by experimental data obtained using a plane source technique, and the Smoluchowski effect of the gas phase is found to be captured by this extension. By considering packings of spherical SiO 2 grains with an interstitial helium phase, vibration induced ordering in granular media is studied, using the simulation method developed here, to investigate how disorder-to-order transitions of packing structure enhance effective thermal conductivity. Grain-scale thermal transport is shown to be influenced by the local neighbourhood configuration of individual grains. The formation of an ordered packing structure enhances both global and local thermal transport. This study provides a structure-based approach to explain transport phenomena, which can be applied in properties modification for granular media. • A framework combining finite and discrete element simulations is proposed to study heat conduction in granular media. • Finite element analysis is used to improve the conventional analytical heat transfer model with an empirical correlation. • The modified heat transfer model is incorporated into the discrete element simulation. • Effects of structural transitions on effective thermal conductivity are evaluated by grain-scale structure index. [ABSTRACT FROM AUTHOR]

Suffusion occurs when fines are plucked off by seepage forces and transported throughout the pores of the matrix constituted by coarser soil particles. Natural or human made soils are seldom homogeneous, which makes suffusion more complex. Suffusion is usually combined with the self-filtration of the fine particles and the transport of these fines may cause the soil structure to become looser. At the same time a clogging may occur which could reduce the permeability leading to an increase of excess pore pressure. The combination of these two phenomena will result in strength degradation. Currently, most suffusion analyses are performed without taking into account the soil's spatial variability. In this paper, a four-constituent continuum finite difference model for suffusion has been extended through the self-filtration process. A random field theory was at this point introduced into the finite difference code to investigate soil suffusion with a randomly distributed initial porosity and fines content. A probabilistic study using the Monte Carlo method was conducted to analyze the effect of the variance, the spatial correlation length, and the cross correlation of the randomly distributed initial porosity and fines content on the eroded mass and on the evolution of the hydraulic conductivity under 1D and 2D conditions. Based on all the simulations, it was possible to quantify the probability of particle blockage during erosion. [ABSTRACT FROM AUTHOR]

In this work we present a new experimental technique alternative to Acoustic Emissions, which makes use of sound signal sensing, to complement the assessment of the mechanical behaviour of sand grains. A post-processing protocol is also proposed. By using this technique in combination with microscopy during in-situ compression tests, it was possible to capture complementary features in the complex mechanical behaviour of granular materials. In addition to that, dominant acoustic frequencies in the sound emitted by sand grains during failure were captured. The method has been applied to study the effects of chemical composition, morphology and grain sizes on the failure mechanics of three types of single sand grains. The advantages of this approach over other traditional techniques to capture failure events during cracking of granular materials have also been clearly demonstrated. • This work presents a novel technique, which makes use of sound to assess the mechanical behaviour of sand grains. • A detailed post-processing protocol in the time and frequency domain is proposed. • The technique is applied to investigate effects of composition, morphology and grain size upon the failure of sand grains. • The method is calibrated using microscopy images captured during in-situ compression tests. • Calibration proved that the method captures complementary features in the behaviour of granular materials. • The advantages and limitations of this approach over other traditional techniques are discussed. [ABSTRACT FROM AUTHOR]

Abstract Impact-driven compaction is a proposed mechanism for the lithification of primordial bimodal granular mixtures from which many meteorites derive. We present a numerical-experimental mesoscale study that investigates the fundamental processes in shock compaction of this heterogeneous matter, using analog materials. Experiments were performed at the European Synchrotron Radiation Facility generating real-time, in-situ , X-ray radiographs of the shock's passage in representative granular systems. Mesoscale simulations were performed using a shock physics code and set-ups that were geometrically identical to the experiments. We considered two scenarios: pure matrix, and matrix with a single chondrule. Good agreement was found between experiments and models in terms of shock position and post-shock compaction in the pure powder setup. When considering a single grain embedded in matrix we observed a spatial porosity anisotropy in its vicinity; the compaction was greater in the region immediately shockward of the grain, and less in its lee. We introduced the porosity vector, C , which points in the direction of lowest compaction across a chondrule. This direction-dependent observation may present a new way to decode the magnitude, and direction, of a single shock wave experienced by a meteorite in the past [ABSTRACT FROM AUTHOR]

Abstract The paper aims to clarify the stress tensor definition and its symmetry property that applies to granular media, and conducts 3D Discrete Element Method (DEM) inspection of the stress tensor definitions provided in the literature. Various stress tensor formulas under static and dynamic conditions are summarized, compared and numerically inspected through different types of simulation, such as gravitational deposition, isotropic/oedometer compression and high-strain-rate (HSR) oedometer impact. The stress tensor symmetry is particularly discussed from the perspective of applying classical continuum mechanics to granular media. It is proved analytically and numerically that the stress tensor should be calculated by Bagi's formula, not Weber's formula or Drescher's formula, for a particle assembly or representative volume element (RVE) in static equilibrium. We propose to modify the De Saxcé and Nicot formulas by incorporating the boundary-radius-gap term such that they are consistent with Bagi's formula, which is particularly well-suited for studying granular phenomena that transition between static, quasi-static and dynamic conditions. It is shown from the perspective of stress tensor calculation that the number of particles in the RVE does not need to be large. Symmetry of averaged stress tensor can be accurately satisfied in static equilibrium of the granular DEM RVE, however it cannot in quasi-static or dynamic states due to imbalance of angular momentum of the granular DEM RVE (in comparison to the balance of angular momentum which is always satisfied in classical continuum mechanics). When the stress tensor definition is extended to "discontinuous" state with regard to discrete granular DEM RVEs, the calculated values need to be treated with caution. [ABSTRACT FROM AUTHOR]

Highlights • Granular flows over a static pile have been investigated in an inclined channel. • The velocity profiles of the avalanches follow a pure parabolic decrease. • The velocity profiles can be predicted with the flow rate. • A shear-thinning viscosity model accounting for the velocity profile is deduced. Abstract Granular flows over a static pile have been investigated in an inclined channel. Instead of dividing the granular motions into fast flow with linear velocity profile and creeping motion with exponential velocity profile, we set the boundary of the flowing layer as the locus of points where the particle velocity decreases down to 1% of the surface velocity, and unify the granular motions with a single velocity profile making the application more convenient. The velocity profile of the granular flows follows a parabolic form, and can be predicted once the surface velocity or the flow rate is determined. Moreover, we propose a viscosity model to account for the parabolic velocity profile in the flowing layer, and validate this model by employing FT4 Powder Rheometer. The apparent viscosity is approximated as the sum of frictional and collisional-translational contributions. In analogy with the turbulent mixing-length theory, the momentum transfer is enhanced by the fluctuation, and the apparent viscosity in the granular flows depends both on the flowing-layer thickness and the local shear rate. Graphical abstract Image, graphical abstract [ABSTRACT FROM AUTHOR]

Highlights • Non-spherical polydisperse particles are measured using a caliper. • A granular packed-bed is generated using DEM code. • The flow inside of the bed is computed using CFD. • Numerical predictions are only 16.0% from experimental data. • The numerical workflow can explore parameter ranges unaccessible to the experiment. Abstract This work presents a numerical workflow to generate a virtual packed bed made of non-spherical polydisperse particles, and subsequently predict its permeability. Wood chips were taken as an illustration. First, chips are sized before being recreated numerically. Then, using LMGC90, a DEM code, a packed bed made of those chips was generated. Once bed internal had been sampled, CFD tools belonging to the OpenFOAM library were used to mesh the geometry (snappyHexMesh) and compute fluid motion (simpleFoam). Finally, using numerical results, the bed permeability was computed in both Stokes and inertial regimes - turbulence being described by Launder-Reece-Rodi model. In parallel, experimental measurements of the permeability of a packed bed, made of the exact same wood chips, was carried out. These experiments were used as a reference to challenge numerical results. The permeability value delivered by the workflow is 16.0% higher than the experimental value. This value has to be compared with Kozeny-Carman equation estimation which overestimates bed permeability by 115%. Going one step further, this framework was successfully used to compute inertial effects constant of the Forshheimer equation for our packed bed. Throughout this article, a special care has been taken in explaining and evaluating the impact of all the key parameters, namely, number of particles that have to be sized, mesh refinement level, numerical domain dimensions. This workflow opens the door to numerical estimation of bed tortuosity, dispersion coefficients, volumetric heat exchange coefficients, and much more, using the particle size distribution as unique input data. [ABSTRACT FROM AUTHOR]

• The desaturation process of granular media was explored through CT scanning and MCMP-LBM simulation. • The water retention capacity of granular media is inversely proportional to both the porosity and contact angle. • The wetted ratio of the solid surface shows a negative correlation with the contact angle and a positive correlation with the porosity. • Under low saturation conditions, LBM simulation has a distinct advantage over CT scanning. Most soils in nature are in unsaturated states. The key to correctly understanding its hydraulic characteristics lies in mastering the correlation between its microstructure and macro water retention characteristics. Based on an independently developed miniaturized in-situ unsaturated soil–water characteristic instrument, three-phase topological morphology and micrographic information (pore throat, liquid cluster, interface area) of unsaturated granular media were explored, supplemented by CT scanning under a high resolution (5.6 μm/pixel). In addition, three-dimensional reconstruction of CT images was performed for simulating two-phase flow in porous granular media using the lattice Boltzmann method. The experimental results indicate that the water retention capacity, air-entry value, and residual saturation of unsaturated granular specimens are negatively correlated with the porosity of specimens. As the saturation decreases, the specific air–water interfacial area first increases and then decreases, with the peak value appearing around 20% of saturation. Based on LBM simulation, the water retention capacity of the specimen is negatively correlated with the contact angle, while the liquid–solid characteristic interface area and relative permeability are positively correlated. The experimental and simulation results demonstrate good consistency with each other. [ABSTRACT FROM AUTHOR]

In recent years, the multi-particle finite element method (MPFEM) has demonstrated its suitability for the micromechanical modelling of granular systems made up of highly deformable particles. This method combines the advantages of the finite element method to compute particles' deformations and of the discrete element method to simulate particles' interactions. Being computationally expensive, its main drawback is the limitation of the number of particles which can be included in a simulation. However, using a multi-scale approach, it is possible to deduce mesoscopic material properties from the simulation of a relatively small number of particles within an elementary volume, through appropriate averaging techniques. Such simulations are significantly influenced by the boundary conditions (BCs) applied. Periodic BCs are commonly admitted to be the most accurate. However, they are technically rather difficult to implement in the MPFEM because of the multiple contacts which need to be handled in the model. As a result, simplified BCs involving for example, rigid walls, remain used. This study analyses the effect of different types of BCs on the simulated mechanical response of a numerical granular sample, under quasi-static loading. Two different techniques for implementing full or simplified periodic BCs, introduced by Schmidt et al. [1] and Loidolt et al. [2] , respectively, were compared with simpler, non-periodic BCs. In the comparison, the same initial microstructure was used; it consisted of an assembly of 50 spherical particles with a periodic topology. The results suggest that considering the additional computational cost induced by the implementation of multi-point constraints, the advantage brought by periodic BCs is relatively limited. The study also highlights the differences between BC types and provides interpretations relative to the particularities of the numerical method used. [ABSTRACT FROM AUTHOR]

Blocky rock and stone structures (such as jointed rocks mass, masonry structures and railway ballast) are often submitted to dynamic effects. Their mechanical behaviour can be simulated with the discrete element method (DEM) using polyhedral elements. DEM models differ in how they compute the contact forces. Several studies have compared different contact constitutive laws, but little information is available in the literature on which contact treatment method (i.e. number of contacts between two elements, contact detection, contact orientation, area and overlap computation) to choose. The inclined slope, the perpendicular impact and the rocking block problems were simulated and applied as benchmark tests in the present study to compare three most popular polyhedral DEM contact treatment models for rigid elements, namely the Eliáš (available in Yade), the Gilbert-Johnson-Keerthi (GJK, widely implemented, here tested with PFC3D) and the Sub-Contact model (available in 3DEC), to see which ones could be precisely calibrated to analytic and experimental solutions. Sensitivity studies were also carried out on the effect of subdivision density of element surfaces and on the admissible timestep length. Experiences showed that while there are serious doubts about the applicability of the Eliáš volumetric model as it is sensitive to both variations in geometry and the results vary randomly with the timestep length, the GJK model is suitable to simulate granular materials because of its computational efficiency and robustness, and the Sub-Contact method is the proper choice to model structures where the contact surfaces are relatively large, as it operates multiple contacts between two elements and gives information about the force distribution on the surface of the elements. [Display omitted] [ABSTRACT FROM AUTHOR]

Particle shape has a significant effect on the shear strength and shear dilatancy properties of granular materials. This study presents a granular thermodynamic theory investigate the effects of particle shape and distribution on the mechanical properties and behavior of granular materials. The theory model outcomes closely resemble experimental results, effectively capturing the impact of particle shape on the mechanical properties and shear dilation of granular materials. The paper analyzes the relationship between a granular system's plastic behavior and granular temperature and explores the influence of irregularly shaped granular on the system's strength and kinetic energy activity. The granular temperature behavior during external loading is studied, showing that granular temperatures increase to a maximum value before gradually decreasing to a more stable state. Simulation results based on the proposed model also indicate obvious effects of nonuniform particle-shape distribution on the mechanical properties of granular materials, providing a comprehensive insight into the underlying mechanisms of the particle-shape effects from the perspective of thermodynamics. [ABSTRACT FROM AUTHOR]

• Green roof designs rely on hydrological behaviour of granular substrates. • Substrate hydraulic properties are closely related to their Grain Size Distribution. • Fractal approximations in size distribution models are physically unrealistic. • Pore and Grain size distributions are directly related to each other. • More realistic Multifractal-based models are proposed here. Hydrological behaviour of granular substrates is of critical interest in Nature-based solutions (NBS) like green roofs. To simulate this behaviour in a physically realistic manner it is indispensable to model the substrate's hydraulic conductivity (HC) as it determines infiltration rate at various degrees of saturation. Since HC is directly dependent on water content retained by the substrate, it is necessary to physically model this water retention (WR) behaviour too. Capillary water is stored or retained in pore spaces and this water content that can be retained by a substrate under different suction pressures is therefore dependent upon its pore size distribution (PSD). Since pores in any granular media are spaces where grains are absent, their size distribution too is intrinsically related to the substrate's grain size distribution (GSD) which provides the probability of finding grains smaller than some threshold diameter d g , t . Although earlier studies have attempted to model PSD, WR and HC, they frequently use simplifying mono-fractal approximations, whereas this study proposes a more generalized multifractal-based approach. Furthermore, while it is quite usual to incorporate pore tortuosity through some indirect parameter l in the HC model, a related ink-bottle effect which even though capable of affecting WR behaviour is commonly ignored. Therefore, this paper attempts to address the aforementioned research gaps in modelling GSD, PSD, WR and HC by i) investigating the somewhat overlooked question of similarity in multifractal behaviour between grain size fields and substrate density fields, and consequently suggesting an improved method for estimating universal multifractal (UM) parameters of grain size fields in a more reliable manner from just conventional GSD measurements in order to be directly used in the multifractal GSD model, ii) proposing a new UM-based PSD model, and subsequently using it to obtain a new UM-based WR model with a parameter to directly represent ink-bottle effect - a consequence of the substrate's pore configuration or arrangement, iii) using this UM-based WR model to suggest a new UM-based HC model without the necessity for a separate pore tortuosity parameter. Finally, the proposed models have been validated by using experimental measurements from 4 different commercially used green roof substrates. [ABSTRACT FROM AUTHOR]

A simple method to estimate percolation thresholds p c in lattice models is presented. The technique is based on the calculation of the probabilities R L X (p) for finding a percolating cluster of type X [ X could be horizontal (H), vertical (V), average (A), intersection (I), or union (U)] on a finite lattice of side length L at concentration p of occupied sites. The functions R L X (p) are obtained by numerical simulations. Then, (1) effective percolation thresholds p c X , r (L) are estimated from the condition R L X (p) = r (where r is a parameter ranging between 0 and 1); and (2) the percolation threshold in the thermodynamic limit is determined by extrapolating the effective percolation thresholds to infinite L. The methodology presented herein contains and extends the classical scheme developed by Yonezawa, Sakamoto and Hori, offering a more complete and versatile theoretical/simulation framework to calculate percolation thresholds. The approach is validated by successfully comparing with well-known results obtained for the problem of random site percolation on square lattices. Taking advantage of the behavior of the curves of p c X , r (L) for different values of the parameter r , more general scaling relations are proposed for the effective percolation thresholds. Finally, the technique is applied to model experimental data of clogging transitions in a two-dimensional silo with a vibrated base. • A new procedure to calculate percolation thresholds in lattice systems is presented. • Multiple effective thresholds are obtained from the percolation probability functions. • The method is applied to the problem of random site percolation on square lattices. • The obtained results are in good agreement with previous calculations in literature. • The clogging transition in a 2D silo is discussed in terms of percolation theory. [ABSTRACT FROM AUTHOR]

Abstract The rheological properties and dynamic behaviors of granular media under oblique impact are very sensitive to the variation of packing fraction. In this work, granular beds with different packing fraction have been obtained by changing the mixture ratio and diameter ratio of two components. Then, the influence of packing fraction on the dynamic characteristics of the two-dimensional granular beds subjected to oblique impact has been investigated using the discrete element method. Simulation results exhibit that there exists a critical packing fraction for the reversal of the penetration depth-packing fraction dependence. The linear relationship of horizontal penetration depth and impact velocity is still remained, but vertical penetration depth and impact velocity is held at the expense of correlation coefficient. However, if the packing fraction exceeds the critical domain of packing fraction, the resistance force cannot be decomposed into two independent functions proportional to the square of velocity and depth any more for impact angles larger than the critical angle, vice versa. Meanwhile, the dependence of stopping time on the impact velocity exhibits a criticality characteristic when packing fraction is in the critical range, that is, for an impact angle lower than critical angle, the stopping time generally increases linearly with impact velocity, while it decreases in an exponential manner for a larger impact angle. Based on the simulation results, the scaling relations between penetration depth, stopping time and packing fraction, impact angle have been proposed. Meanwhile, a phenomenological model describing the resistance force has been established considering the packing fraction Graphical abstract Unlabelled Image Highlights • A model describing the resistance force is presented considering the packing fraction. • The scaling laws between penetration depth, stopping time and packing fraction, impact angle have been proposed. • There is a critical packing fraction ϕ c for the reversal of the penetration depth-packing fraction dependence. [ABSTRACT FROM AUTHOR]

The effect of grain shape and fracture on the interaction of high velocity granular slugs with rigid stationary targets is analysed for targets in normal and inclined orientations. The granular slugs comprise spherical, rod-shaped or cubic grains and are constructed by connecting together spherical sub-particles with either rigid or beam connectors. The case when grain fracture is suppressed (rigid connectors between sub-particles) is first analysed. With increasing grain aspect ratio, the grains tend to slide rather than roll on the target surface and this increases frictional interactions with the target surface. However, these enhanced frictional forces do not affect the momentum transmitted into normally oriented targets due to the symmetry of the problem. By contrast, the break in the symmetry for inclined targets results in the transmitted momentum increasing with grain aspect ratio. Fracture of the grains (as modelled by the fracture of the beam connectors between sub-particles) is shown to affect the momentum transmitted into the inclined targets. This is a consequence of fracture resulting in a change in grain shape. In this case the simulations show that the transmitted momentum is a function of the initial grain shape, the fracture properties of the grains and the impact velocity. In fact, grain fracture results in an enhanced transmitted momentum for initially cubic grains but fracture of grains with a high initial aspect ratio results in a reduction in transmitted momentum as these grains fragment into more spherically shaped grains. [ABSTRACT FROM AUTHOR]

Quantitative image analysis often depends on accurate classification of pixels through a segmentation process. However, imaging artifacts such as the partial volume effect and sensor noise complicate the classification process. These effects increase the pixel intensity variance of each constituent class, causing intensity values of one class to overlap with another. This increased variance makes threshold based segmentation methods insufficient due to ambiguous overlap regions in the pixel intensity distributions. The class ambiguity becomes even more complex for systems with more than two constituents, such as unsaturated moist granular media. In this paper, we propose an image processing workflow that improves segmentation accuracy for multiphase systems. First, the ambiguous transition regions between classes are identified and removed, which allows for global thresholding of single-class regions. Then the transition regions are classified using a distance function, and finally both segmentations are combined into one classified image. This workflow includes three methodologies for identifying transition pixels and we demonstrate on a variety of synthetic images that these approaches are able to accurately separate the ambiguous transition pixels from the single-class regions. One of these methods does not require any parameter tuning and is entirely unguided, whereas the other two require manual threshold specifications and are thus user-guided. For situations with typical amounts of image noise, misclassification errors and area differences calculated between each class of the synthetic images and the resultant segmented images range from 0.69 to 1.48% and 0.01 to 0.74%, respectively, showing the segmentation accuracy of this approach. We demonstrate that we are able to accurately segment x-ray microtomography images of moist granular media using these computationally efficient methodologies. [ABSTRACT FROM AUTHOR]

A new conditional modeling along with a level-set method that can produce high quality and large packs of irregular particles is proposed. First, an initial pack of particles is generated statistically using the particles provided through an X-ray image. Then, the mismatch locations, wherein an error map identifies the particles manifest unrealistic shapes, are detected. Next, a level-set algorithm is applied to refine the defected particles using the statistical distributions extracted from the actual particles. The generated packs are statistically and physically (i.e. flow modeling) compared with the original particles and they all represent an excellent agreement. [ABSTRACT FROM AUTHOR]

The deformation of square honeycomb core, stainless steel sandwich panels by the impact of explosively accelerated granular matter has been investigated and compared to results from a previous study using equivalent (same material and mass per unit area) solid plates subjected to similar impulsive loadings. Spherical explosive charges surrounded by 25–150 kg mass annular shells of water-saturated granular media (either fused silica or zirconia particles) were suspended above the center of the edge clamped test panels. The radially expanding granular particle front velocities were measured from high-speed video images, and revealed that the granular matter had been accelerated to velocities of 500–1200 m/s after detonation. A Kolsky bar was used to measure the time-dependent pressure and impulse at a position equivalent to the panel center, while the permanent deflections of the sandwich panels were determined by profilometry after the experiments. Even though fracture of electron beam welds used to attach the back face sheet to the sandwich panel core occurred in all the tests, the permanent deflections of the sandwich panel back faces were significantly less than those of equivalent solid plates, and were accompanied by minimal core compression. Discrete particle simulations of the granular matter acceleration and impact loading of the sandwich panels indicated that their superior deflection benefit arose from their high bending resistance rather than particle-structure interactions. This benefit was offset when the rear face of the sandwich was kept the same distance from the impulsive source as that of the solid plate since the impact face of the sandwich panel was closer to the impulsive source, subjecting it to a higher impulse than the solid plate. However, a substantial deflection reduction was still achieved by use of a strong core sandwich design. [ABSTRACT FROM AUTHOR]

The packing density and initial modulus of binary mixtures were studied using the discrete element method (DEM). In addition, the behavior of particle contacts (microscopic behavior) was examined. The binary mixture contains two similar ellipsoids of different sizes. The particle size ratio ( r ), the ratio of particle sizes between a small particle and a large particle is either 0.1 or 0.2. The particle shape of these binary mixtures is described by the ratio between the major length and the minor length of an ellipsoid (AR = 1.2, 1.5, or 1.7). Very dense samples of different AR and fines contents were generated. In general, the study shows that r affects the initial modulus but not the relationship between void ratio and fines content. The behaviors of samples (AR = 1.5 and 1.7) are similar but different from those of AR = 1.2. The DEM results were fitted by three empirical models for binary mixture. These three models described the relationship between void ratio and fines contents very well. Degree of fitness is related to the number of parameters in the model. Although packing density is not affected by sample shape (cube versus cuboid), initial modulus is affected by sample shape. Microscopic investigation on particle contacts indicated that AR affects the number of contacts but not the distributions of contacts among large-to-large, small-to-small, and large-to-small particles. Greater initial modulus was found for samples of r  = 0.1. When less than 20% of small particles were introduced to the large particles, initial modulus decreases significantly from the initial modulus of monodisperse samples. [ABSTRACT FROM AUTHOR]

In most particulate classification systems, feed rates in excess of 80% of the designed capacity leads to inefficiency and conversely feed rates below this value significantly diminishes the operational efficiencies. It therefore implies that maximum efficiency is only attainable at the expense of low capacity, and vice versa . This problem is caused by transience in granular flow due to start-ups and fluctuating feed-rates, in addition to fluctuations in feed material properties. If these variations are not checked, they cause instabilities, resulting in chaotic saddles responsible for in-process systemic error generation. These errors produce intermittent disruptions in production process and control. We have applied perturbation theory to study the effects of infinitesimal changes on the material balance analysis of the unit operation. The problem was identified as one of the highly multi-stable dynamic systems, characterized by ‘predator-prey’ phenomenon in dynamical systems theory. The study allowed formulation of optimal state equations, whose numerical solutions resulted in establishment of optimal operating conditions required to sustain stability, and consistently high tonnages and efficiency up to 99% simultaneously. The study also led to development of an optimization algorithm, which upon validation with experimental data showed a close relationship, with a minimal absolute error of 0.8 and a relative error of 6%. Finally, a representative case study was conducted on screen dimensioning, based on the determined parameters. Successful evolution of this methodology may be applied for up-scaling of real systems in future. [ABSTRACT FROM AUTHOR]

• High velocity penetration of a projectile into a granular medium is studied. • The false nose formed during penetration of flat and shallow noses is investigated. • The false nose shape of the target soil material is equal to the optimal shape. • Flat and hemispherical nose projectiles and spherical projectiles are considered. • The approach is compared with many test results and very good agreement is obtained. The present article deals with a particularly interesting phenomenon of a false nose that is formed during the high velocity penetration of a projectile into a granular soil like sand. The false nose is composed of compacted, fractured, and comminuted sand particles. It is formed as a cap in front of flat nose projectile and accompanies the projectile during its penetration. Only limited sparse experimental evidence exists on the false nose shape that is mostly observed after the penetration event. So far, no theoretical study was carried out to assess the false nose shape and its effect on penetration. The present paper conjectures that the shape of the false nose is equal to the optimal shape formed from the target soil material that yields the minimal resistance during its penetration in the soil. This nose together with the original flat nose projectile behind, yield a stable deep penetration that is comparable to projectiles with shaped noses. The present study extends the approach of a full false nose in front of a flat nose projectile, to a partially added false nose in front of a hemispherical, spherical, or other short noses. Analysis of experimental images of false noses verifies the above conjecture and prove that the shape of the false nose is very close to the optimal shape of a projectile nose that is made of compacted soil. These findings enable implementation of the well-proven DISCS approach (which was originally developed for penetration of projectiles with slender noses into various compressible media) and extend it for flat, spherical and hemispherical projectiles penetration into a granular soil. The efficiency of the proposed approach is demonstrated through numerous examples for the above types of projectiles, at various impact velocities, showing very good agreement with experimental data. [ABSTRACT FROM AUTHOR]

Membrane fouling is considered the major limitation of membrane bioreactors (MBRs). This paper provides an overview on fouling mitigation in MBRs using granular materials. Adsorbents addition extends filtration period, improves critical flux as well as sludge properties (increased flocs size, reduced soluble EPS, improved dewaterability). However, determination of optimal dosages of adsorbents is needed to balance cost savings from fouling mitigation versus cost of adsorbents and sludge handling. The abrasion from granular media reduces cake layer formation, extends membrane filtration period, increases flux (∼20–30%), and reduces aeration intensity by 50%. Finding appropriate aeration intensity and optimum dose for different media is critical for full-scale application. Granular sludge substantially reduces fouling in MBRs; but, optimal operational conditions for long-term granule stability are required. Quorum quenching (QQ) mitigates biofouling (energy savings ∼27–40%). Cost savings from QQ need assessment against the production and application of QQ approaches. [ABSTRACT FROM AUTHOR]

The article shows a method aimed at taking into account the irregularity in the distribution of stresses along wooden and reinforced concrete ties transferred to the ballast body. Based on the comparison of the theoretical values of stresses and the values of stresses actually measured in an experimental way, a conclusion has been made as to the viability of the present method implemented on the base of the mechanics of granular media. [ABSTRACT FROM AUTHOR]

Based on the configuration of hexagonal packing of identical particles, the radial and the circumferential heterogeneous degrees for granular materials are defined. The relationship between original heterogeneous degrees and strain distribution are concerned. The consistency of strain distribution and heterogeneity is verified. Both configurations and force chains in specimens are analyzed to explore different deformation modes. In the loading process, the first peak stress is defined; the linear relationship between average heterogeneous degrees and first peak stress is clarified. According to the tensor theory, some quadratic invariants of the couple stress tensor are investigated to study the scope of the couple stress effect. Analyses of the volume average of the invariants are carried out to determine the effective range of the couple stress, namely the characterization size of the couple stress. [ABSTRACT FROM AUTHOR]