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We demonstrate that the Delaunay-based strain definition proposed by Bagi (Mech Mater 22:165–177, 1996) for granular media can be straightforwardly translated into a particle-based numerical method for continua. This method has a number of attractive features, including linear completeness and satisfaction of the patch test, exact conservation of linear and angular momenta in the absence of external forces and torques, and anti-symmetry of the gradient vectors for any two points not both on the boundary of the computational domain. The formulation in effect relies on nodal (particle) interpolation of the deformation gradient and is therefore inherently unstable. Drawing on the analogy with granular media, a pairwise interaction between particles is included to alleviate this issue. The underlying idea is to define a local, non-affine deformation of each bond or contact, and to introduce pairwise forces via a stored-energy functional expressed in terms of the corresponding local displacements. In this manner, a generalisation of the Ganzenmüller (Comput Methods Appl Mech Eng 286:87–106, 2015) hourglass stabilisation procedure to non-central forces is obtained. The performance of the method is demonstrated in a range of problems. This work can be considered a first step towards the development of a macroscopically consistent discrete method for granular materials.

Direct reduction ironmaking has gained popularity as a low carbon alternative to the typical blast furnace ironmaking route. A popular method of producing direct reduced iron is through the reduction of iron ore pellets in a reduction shaft furnace. Critical to this process is the use of a reliable continuous pellet feed system to provide a steady flow of pellets to the furnace. Therefore, any disruption in pellet flow can have a significant negative impact on the production rate of iron. An iron ore pellet feed system for a direct reduction ironmaking furnace is jamming during winter operation. The pellets are jamming in a hopper at the top of the feed system above the furnace, and a hot gas, that seals off the furnace flue gas, flows counter to the pellets. A computational model of the feed system is built utilizing the discrete element method and computational fluid dynamics, using Siemen’s commercial multiphysics software Star-CCM+, to study the conditions that cause the jam to occur. The study is divided into six parts: pellet bulk flow calibration, computational cost reduction, modeling of the baseline operation, modeling the effect of moisture, development of a thermal model, and investigation of the minimal amount of icy and wet material to jam the system. The findings show that the location of jamming during operation matches the area in the simulation where it is most likely to occur, and that moisture alone is unlikely to result in jamming. Results indicate that the system will jam when charged with a minimum of 15% icy pellets, and when charged with 10% icy together with 5% wet pellets. Experimental work is recommended to validate the findings and to calibrate the simulations accordingly.

In a recent article (Frenning in Comp Part Mech 24:1–4, 2021), we demonstrated that a Delaunay-based strain estimate could be used as a starting point for the development of a particle-based method for continua. In this article, we argue that the Voronoi diagram, dual to the previously used Delaunay tetrahedralization, provides a more natural description of the underlying particulate system. For this reason, a Voronoi-based estimate of the deformation gradient is derived and used to the same effect. Although the gradient vectors cease to be antisymmetric, sums over nearest neighbors vanish, which results in a formulation that not only is linearly complete but also satisfies the patch test irrespective of initial particle placement. Pairwise forces, inferred from the local (nonaffine) deformation of each bond or contact, impart a physical stabilization. Forces are obtained from a discrete Lagrangian, thus ensuring that linear and angular momenta are conserved in the absence of external forces and torques. Methods to enforce different types of boundary conditions are described; these are exact for linear displacements, for constant stresses and for free surfaces. The performance of the method is assessed in a number of numerical tests.

Controllable continuous feeding of biomass feedstock in a biorefinery is critical to upscaling current ethanol conversion techniques to a commercial scale. Mechanical pretreatment of biomass feedstock performed using a compression feed screw (CFS) improves the ethanol yield but is subject to flowability issues, especially the plugging of biomass. The mechanical behavior, and hence, the flowability of biomass feedstock, is strongly affected by several factors, including preparation method, moisture content, physical composition, and particle size distribution. In addition, the current design of CFS is guided by limited experimentation and even fewer theoretical correlations. This thesis aims at developing computational methods to model the flow of densified feedstock in a CFS and experimental techniques to characterize the mechanical properties required for the model. We adopted a modified Drucker-Prager Cap constitutive (mDPC) law for milled corn stover (a widely used feedstock for bioethanol production) to model the material’s rate-independent bulk behavior in a CFS. The mDPC elastoplastic law captures the frictional shear and permanent volumetric changes in corn stover using a continuous porosity-dependent yield surface. The parameters of the mDPC model are calibrated using a unified set of single-ended die compaction and multiple shear failure tests. In addition, we quantified the changes in the mDPC parameters with moisture content up to the water-holding capacity of corn stover particles. A Coupled Eulerian-Lagrangian Finite Element Method model developed for the CFS geometry predicts the deformation of the material using the calibrated mDPC parameters. We model the interaction between the material and the CFS surface using a Coulomb wall friction coefficient calibrated using the Janssen-Walker method for a punch and die system. A laboratory-scale compression feed screw is designed and fabricated to characterize the flow of dense granular materials in collaboration with undergraduate students in the School of Mechanical Engineering. FEM model predictions of feeding torque and mass flow rate are validated against the laboratory-scale feeder for microcrystalline cellulose Avicel PH-200 and milled corn stover. The model predictions agree with the experiments for Avicel PH-200 but have a higher error in the case of corn stover. Some physical effects, such as shear hardening and particle erosion observed in milled corn stover, are not captured using the current implementation of the mDPC model, which explains the different model accuracies for both materials. The continuum model is used to uncover material density distribution, torque, and pressure inside the CFS, otherwise challenging through experiments. The FEM model showed a significantly higher sensitivity of the feeder performance to two material properties, namely the hydrostatic yield stress and the wall friction coefficient. The characterized variation of material properties with moisture content and the effect of each material property on the feeder performance provide strategies to engineer the feedstock for better flowability. Further, the continuum model offers a method to study design changes before manufacturing the equipment. Finally, we propose the possibility of a reduced-order analytical model based on the critical material properties and the material deformation mechanism demonstrated by the FEM model.

The current state of preservation of cantilever retaining walls has attracted considerable attention in the last decade in Switzerland, as destructive tests have detected strongly localized corrosion of the main reinforcement in many walls built in the 1970s. This has been identified as a potential threat that could lead to an unpredictable brittle collapse of the wall, which may cause severe damage to high-traffic roads and even victims. Typically, retaining walls are designed to withstand active earth pressure. This condition implicitly presupposes certain soil deformations, which require the wall to have a sufficient rotation capacity. However, corrosion damage can greatly reduce structural rotation capacity. Therefore, quantifying the earth pressure acting on corrosion-damaged cantilever retaining walls is essential to assess their safety reliably. This thesis studies the evolution of the earth pressure as a function of corrosion-driven wall displacement. Analytical, numerical, and experimental analyses are performed to quantify the history of earth pressure, from the construction of the wall to the moment of possible corrosion-induced collapse. The obtained results are generally valid for any problem involving the same failure mode as that resulting from a corrosion of the main reinforcement. The relevant failure mode is identified as a rigid-body rotation around its toe. The limit load is determined using a static and a kinematic solution based on the limit analysis theorems and compared to conventional design methods. This failure mode is further analyzed in scaled experiments, where different initial conditions and soil parameters are investigated. Loose, contractive soil requires much larger rotations to reach the residual state than dense soil. In addition, the unloading process is influenced by the initial stress state in the backfill. In uncompacted soil, the initial earth pressure is bilinearly distributed, whereas higher stresses are measured close to the wall surface in statically compacted samples. Slightly larger wall rotations are required to reach the active state in compacted backfills. By imposing the rotation of a single wall section, it is shown how an inhomogeneous distribution of the corrosion degree over the wall length can lead to a decreased limit load on the failing wall section due to the stress redistribution occurring in the backfill. Consequently, neighboring sections must withstand increased loads. A numerical framework for quantifying the earth pressure on cantilever retaining walls is developed based on experimental observations and widely known constitutive laws to guarantee applicability in practice. The framework is generally applicable and provides reliable results as it is validated using experimental data. The material behavior is calibrated through virtual element tests performed using the Level Set Discrete Element Method. In plane strain tests, the mobilized soil strength is higher than in triaxial tests, which confirms the experimental observations. Furthermore, the Level Set Discrete Element Method is used to analyze the earth pressure coefficient at rest, showing a correlation between the coefficient and the peak friction angle, which does not imply causation. Finally, the developed numerical models are applied to some case studies. Taking into account a more accurate structural model, it is apparent that actions and reactions can be decoupled to assess the safety of walls, as the earth pressure is not significantly influenced by the precise modeling of the elastoplastic wall behavior. Furthermore, the effects of cyclic atmospheric temperature changes are simulated and discussed, considering the implications for wall monitoring.

Archive upload of https://doi.org/10.1029/2021JF006460 -- Many subglacial environments consist of a fine-grained, deformable sediment bed, known as till, hosting an active hydrological system that routes meltwater. Observations show that the till undergoes substantial shear deformation as a result of the motion of the overlying ice. The deformation of the till, coupled with the dynamics of the hydrological system, is further affected by the substantial strain rate variability in subglacial conditions resulting from spatial heterogeneity at the bed. However, it is not clear if the relatively low magnitudes of strain rates affect the bed structure or its hydrology. We study how laterally varying shear along the ice-bed interface alters sediment porosity and affects the flux of meltwater through the pore spaces. We use a discrete element model consisting of a collection of spherical, elasto-frictional grains with water-saturated pore spaces to simulate the deformation of the granular bed. Our results show that a deforming granular layer exhibits substantial spatial variability in porosity in the pseudo-static shear regime, where shear strain rates are relatively low. In particular, laterally varying shear at the shearing interface creates a narrow zone of elevated porosity which has increased susceptibility to plastic failure. Despite the changes in porosity, our analysis suggests that the pore pressure equilibrates near-instantaneously relative to the deformation at critical state, inhibiting potential strain rate dependence of the deformation caused by bed hardening or weakening resulting from pore pressure changes. We relate shear variation to porosity evolution and drainage element formation in actively deforming subglacial tills., Funding: National Science Foundation through the Office of Polar Programs: PLR-1341499 and PLR-1744758 U.S. Army Research Laboratory: W911NF-12-R0012-04 National Science Foundation Major Research Instrumentation Program: ACI-1429830 Hewlett Family Stanford Graduate Fellowship

Particle morphology is of pivotal importance in granular materials at different length scales. This thesis begins with quantification and reconstruction of real particle shapes, then studies various kinds of granular material behaviour influenced by morphology features. The project background and related previous work are introduced in Chapters 1 and 2, respectively. Chapter 3 is firstly concentrated on reconstructing particle shapes using Spherical Harmonics (SH). DEM clumps are also used to approximate realistic particle shapes to simulate sand column collapse. The efficiency of SH analysis between different coordinate systems in FEM mesh qualities and reconstruction are also compared. Chapter 4 is on contact behaviour of rough spheres with different morphology features. Via FEM simulation, benchmarked with Hertzian solution, a semi-analytical model is proposed. In Chapter 5, to highlight the influences of contact curvature on single particle crushing behaviour, a rotational point loading in FDEM, benchmarked with an in-situ XCT experiment, is proposed. In Chapter 6, experiments of failure modes of cemented sands under different loading paths and the quantification of fracture fabric are conducted. FDEM simulations, benchmarked with a combined in-situ experiment with XCT and diffraction, of meso-scale concrete are also conducted. Effects of realistic aggregate morphology on overall concrete are investigated. In Chapter 7, permeability of uniformly graded 3D printed grains, is experimentally studied. Modified Kozney-Carman equation is also proposed. Chapter 8 concludes this work by summarising the findings and implications and provides an outlook on future works. This dissertation presents a new comprehensive understanding of granular behaviour influenced by its morphology features. Via the proposed framework combining both experimental and numerical information, it is helpful to design and optimise of various granular materials with specific morphology features.

Glacier surges are quasi-periodic episodes of rapid ice flow that arise from increases in slip rate at the ice–bed interface. The mechanisms that trigger and sustain surges are not well understood. Here, we develop a new model of incipient surge motion for glaciers underlain by sediments to explore how surges may arise from slip instabilities within a thin layer of saturated, deforming subglacial till. Our model represents the evolution of internal friction, porosity and pore water pressure within the till as functions of the rate and history of shear deformation, and couples the till mechanics to a simple ice-flow model. Changes in pore water pressure govern incipient surge motion, with less permeable till facilitating surging because dilation-driven reductions in pore water pressure slow the rate at which till tends towards a new steady state, thereby allowing time for the glacier to thin dynamically. The reduction of overburden (and thus effective) pressure at the bed caused by dynamic thinning of the glacier sustains surge acceleration in our model. The need for changes in both the hydromechanical properties of the till and the thickness of the glacier creates restrictive conditions for surge motion that are consistent with the rarity of surge-type glaciers and their geographical clustering.

The complexity and multiscale nature of material microstructures introduces significant intricacies to many mechanics problems for which we do not have a full theoretical understanding. Under loading, these microstructures can introduce significant nonlinearities that cannot be described sufficiently by current theories and models. This leads us to consider experiments we could perform to improve our understanding of such effects. This thesis describes the design of experiments exploring two aspects of material microstructure effects: (i) crack propagation and renucleation in soft brittle polymers and (ii) interparticle forces in granular materials. First, experimental and analysis methods are developed to study fracture mechanics in soft brittle polymers with the goal of developing a more detailed understanding of the effects of microstructural heterogeneities on crack propagation and renucleation in three-dimensions. To better understand these processes, experiments on crack propagation in thin soft polymers using confocal microscopy images are conducted. Traditional metrics associated with crack propagation including stress intensity factor (SIF, K) and energy release rate (ERR, G) are calculated by direct measurement of the crack tip opening displacement (CTOD, δt) on the sub-millimeter scale. Errors in these calculations are comparable to those reported in the literature for more traditional fracture experiment geometries. Fluorescent speckle images are captured using confocal microscopy imaging, a fast and low cost 3D optical imaging technique, to study crack geometry during propagation. Images of renucleation events are also captured allowing investigation of factors contributing to slow crack roughening observed by earlier researchers. The goal of this study is to provide an experimental method to enhance understanding of crack interactions with microstructural heterogeneities and of renucleation events, which can significantly improve our ability to design material toughness. To begin to understand the effects of engineered microstructural heterogeneities such as inclusions in materials, we must be able to produce such engineered systems and understand the interparticle interactions. To this end, a method to manufacture volumetrically speckled spheres in-house with controlled diameters was developed. Additionally, an experimental method combining confocal microscopy with digital volume correlation (DVC) was also used to study interparticle force transmission in 3D. Analysis of an in-plane 2D projection of volumetric surface data shows that three-dimensional effects play a significant role in the deformation of granular assemblies. Study of a single grain in 3D demonstrates progress in experimental capabilities and highlights the need for more studies to validate existing numerical models and theories for granular matter. Analysis of particle scale deformations and strains with the Granular Element Method (GEM) allows us to determine interparticle forces and understand the development and evolution of force chains in a granular assembly under a wide variety of loading conditions. These experiments can also lead to development of new understanding of the effects of inclusions on material properties, processes, and damage evolution.

Dry granular flows are common and important in the environment and industry, and yet their behaviour is very poorly understood. The dynamics of individual grains are governed by the simple and well-known laws of Newtonian mechanics, but how do these 'microscopic' particle-level laws translate into the 'macroscopic' collective motion of thousands or millions of grains, which flow like a liquid? Various rheological models of granular flows have been developed to facilitate a continuum approach, but hitherto they have only been applied to flows in very simple geometries such as parallel shear flow. In these applications, the flows are assumed to be quasi-steady and to vary only over very long distances in the streamwise direction. This approximation, related to the 'shallow water' model of hydraulics, greatly simplifies the equations of motion. However, the assumption is inappropriate for modelling flows interacting with basal features that vary over lengthscales comparable to the depth of the current, or for flows with abrupt time-dependence that cannot be assumed to be quasi-steady. We refer to these spatial and temporal inhomogeneities collectively as topography. In this thesis, we apply a common rheological model to problems involving various types of spatial or temporal topography. One problem that we shall particularly study concerns a flow down a chute that experiences a sudden increase in basal roughness, either spatially or in time. This change induces an evolution of the depthwise velocity profile that begins near the base but eventually spreads throughout the current. We introduce an adaptation of the $\mu(I)$ rheology and find the velocity profile that this rheological model predicts, using a technique similar to the Blasius boundary layer theory for Newtonian fluids flowing past an aerofoil. We validate the predictions of the rheological model by comparing them against the results of discrete particle model (DPM) simulations. We review existing techniques for DPM, and present a number of novel ways of employing these techniques. These methods allow us to reduce the computational cost of simulations while maintaining their realism. The internal profile of a granular flow, and its response to a change in basal conditions, are difficult to observe in experiments or in real life, since grains are opaque. However, the models studied here can help to make predictions about the depth and speed of the flows, or conversely to make inferences about the nature of the base, given measurements on the surface of the flow., EPSRC Studentship

The relative equilibria for the spherical, finite density three-body problem are identified. Specifically, there are 28 distinct relative equilibria in this problem which include the classical five relative equilibria for the point-mass three-body problem. None of the identified relative equilibria exist or are stable over all values of angular momentum. The stability and bifurcation pathways of these relative equilibria are mapped out as the angular momentum of the system is increased. This is done under the assumption that they have equal and constant densities and that the entire system rotates about its maximum moment of inertia. The transition to finite density greatly increases the number of relative equilibria in the three-body problem and ensures that minimum energy configurations exist for all values of angular momentum.

A method for modeling cubically anisotropic elasticity within the discrete element method is presented. The discrete element method (DEM) is an approach originally intended for modeling granular materials (sand, soil, and powders); however, recent developments have usefully extended it to model stochastic mechanical processes in monolithic solids which, to date, have been assumed to be elastically isotropic. The method presented here for efficiently capturing cubic elasticity in DEM is an important prerequisite for further extending DEM to capture the influence of elastic anisotropy on the mechanical response of polycrystals, composites, etc. The system demonstrated here uses a directionally assigned stiffness in the bonds between adjacent elements and includes separate schemes for achieving anisotropy with Zener ratios greater and smaller than one. The model framework is presented along with an analysis of the accessible space of elastic properties that can be modeled and an artificial neural network interpolation scheme for mapping input parameters to model elastic behavior.

Granular collisions are characterized by a threshold velocity, separating the low-velocity regime of grain sticking from the high-velocity regime of grain bouncing: the bouncing velocity, v b . This parameter is particularly important for nanograins and has applications for instance in astrophysics where it enters the description of collisional dust aggregation. Analytic estimates are based on the macroscopic Johnson-Kendall-Roberts (JKR) theory, which predicts the dependence of v b on the radius, elastic stiffness, and surface adhesion of grains. Here, we perform atomistic simulations with model potentials that allow us to test these dependencies for nanograin collisions. Our results not only show that JKR describes the dependence on materials parameters qualitatively well, but also point at considerable quantitative deviations. These are the most pronounced for small adhesion, where elastic stiffness does not influence the value of the bouncing velocity.

The available studies in the literature which dealt with the scale effects of strip footings on different sand packing systematically still remain scarce. In this research, the variation of ultimate bearing capacity and deformation pattern of soil beneath strip footings of different widths under plane-strain condition on the surface of loose, medium-dense and dense sand have been systematically studied using experimental and noninvasive methods for measuring microscopic deformations. The presented analyses are based on model scale compression test analysed using Particle Image Velocimetry (PIV) technique. Upper bound analysis of the current study shows that the maximum vertical displacement of the sand under the ultimate load increases for an increase in the width of footing, but at a decreasing rate with relative density of sand, whereas the relative vertical displacement in the sand decreases for an increase in the width of the footing. A well agreement is observed between experimental results for different footing widths and relative densities. The experimental analyses have shown that there exists pronounced scale effect for strip surface footing. The bearing capacity factors Nγ rapidly decrease up to footing widths B=0.25 m, 0.35 m, and 0.65 m for loose, medium-dense and dense sand respectively, after that there is no significant decrease in Nγ. The deformation modes of the soil as well as the ultimate bearing capacity values have been affected by the footing widths. The obtained results could be used to improve settlement calculation of the foundation interacting with granular soil., {"references":["H. M. Jaeger, S. R Nagel, and R. P. Behringer, \"Granular solids, liquids, and gases,\" Rev. Mod. Phys., vol. 68, no.4, pp. 1259-1273, Oct.1996.","S. J. Antony, \"Link between single-particle properties and macroscopic properties in particulate assemblies: role of structures within structures,\" Phil. Trans. R. Soc. 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Semi-solid deformation mechanisms are important in a range of manufacturing and natural phenomena, which range from squeeze casting to magma flows. Using fast synchrotron X-ray tomography and a bespoke precision thermomechanical rig, we performed a four-dimensional (3-D plus time) quantitative investigation of the granular behaviour of equiaxed dendritic three-phase materials. This methodology produced new insights into the formation of damage during the isothermal semi-solid compression (� 30% liquid fraction) of an Al–15 wt.%Cu alloy at both a macroscopic and microscopic level. Grain rearrangements, such as translation and rotation, were observed and lead to local dilatancy. The resulting flow of Cu-rich intergranular liquid into the dilated interstices gave rise to a local increase in liquid fraction, followed by rapid void growth above a critical axial strain of � 6.4%. The local normal and shear strain distributions were quantified using digital volume correlation, identifying dilatant shear bands. At a microstructural level, the individual grains were also seen to undergo intragranular deformation, leading to bending and fragmentation of dendrites as grains interlock. 2014 Acta Materialia Inc. Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/ licenses/by/3.0/).

International audience; The present paper is an exploratory investigation to overcome difficulties encountered in the simulation of dense granulates when a NSCD approach is used. Some simple examples permit identification of impulse indetermination as the main cause of the failure or the slowdown of the numerical algorithm. We propose to deal directly with the space of impulses with null resulting momentum. A prediction-correction scheme is detailed and enriched to restrict at best the interpenetration. Finally sophisticated tools are developed to preserve the sparsity of the matrices and consequently to ensure an efficient implementation of an adapted nonsmooth solver.

Non convex granular media are involved in many industrial processes as, e.g., particle calcination/drying in rotating drums or solid catalyst particles in chemical reactors. In the case of optimizing the shape of catalysts, the experimental discrimination of new shapes based on packing density and pressure drop proved to be difficult due to the limited control of size distribution and loading procedure. There is therefore a strong interest in developing numerical tools to predict the dynamics of granular media made of particles of arbitrary shape and to simulate the flow of a fluid (either liquid or gas) around these particles. Non-convex particles are even more challenging than convex particles due to the potential multiplicity of contact points between two solid bodies. In this work, we implement new numerical strategies in our home made high-fidelity parallel numerical tools: Grains3D for granular dynamics of solid particles and PeliGRIFF for reactive fluid/solid flows. The first part of this work consists in extending the modelling capabilities of Grains3D from convex to non-convex particles based on the decomposition of a non-convex shape into a set of convex particles. We validate our numerical model with existing analytical solutions and experimental data on a rotating drum filled with 2D cross particle shapes. We also use Grains3D to study the loading of semi-periodic small size reactors with trilobic and quadralobic particles. The second part of this work consists in extending the modelling capabilities of PeliGRIFF to handle poly-lobed (and hence non-convex) particles. Our Particle Resolved Simulation (PRS) method is based on a Distributed Lagrange Multiplier / Fictitious Domain (DLM/FD) formulation combined with a Finite Volume / Staggered Grid (FV/SG) discretization scheme. Due to the lack of analytical solutions and experimental data, we assess the accuracy of our PRS method by examining the space convergence of the computed solution in assorted flow configurations such as the flow through a periodic array of poly-lobed particles and the flow in a small size packed bed reactor. Our simulation results are overall consistent with previous experimental work.; Cette thèse porte sur l'étude numérique des écoulements fluide-particules rencontrés dans l'industrie. Ces travaux se situent dans le cadre de la compréhension des phénomènes qui se déroulent dans des tambours tournants et réacteurs à lit fixe en présence de particules de forme non convexe. En effet, la forme des particules influence de manière importante la dynamique de ces milieux. A cet effet, nous nous sommes servis de la plateforme numérique parallèle Grans3D pour la dynamique des milieux granulaires et PeliGRIFF pour les écoulements multiphasiques. Dans la première partie de cette thèse, nous avons développé une nouvelle stratégie numérique qui permet de prendre en compte des particules de forme arbitrairement non convexe dans le solveur Grains3D. Elle consiste à décomposer une forme non convexe en plusieurs formes convexes quelconques. Nous avons nommé cette méthode « glued-convex ». Le modèle a été validé avec succès sur des résultats théoriques et expérimentaux de tambours tournants en présence de particules en forme de croix. Nous avons aussi utilisé le modèle pour simuler le chargement de réacteurs à lits fixes puis des lois de corrélation sur les taux de vide ont été déduites de nos résultats numériques. Dans ces travaux, nous avons aussi testé les performances parallèles de nos outils sur des simulations numériques à grande échelle de divers systèmes de particules convexes. La deuxième partie de cette thèse a été consacrée à l'extension du solveur PeliGRIFF à pouvoir prendre en compte la présence de particules multilobées (non convexes) dans des écoulements monophasiques. Une approche du type Simulation Numérique Directe, basée sur les Multiplicateurs de Lagrange Distribués / Domaine Fictif (DLM/FD), a alors été adoptée pour résoudre l'écoulement autour des particules. Une série d'études de convergence spatiale a été faite basée sur diverses configurations et divers régimes. Enfin, ces outils ont été utilisés pour simuler des écoulements au travers de lits fixes de particules de forme multi-lobée dans le but d'étudier l'influence de la forme des particules sur l'hydrodynamique dans ces lits. Les résultats ont montré une consistance avec les résultats expérimentaux disponibles dans la littérature.

Using a granular-mechanics code, we study the impact of a highly porous granular body on a hard wall. The projectile consists of monodisperse adhesive micrometer-sized silica grains. For the impact velocities studied, v

Shear deformation of granular media leads to continual restructuring of particle contact network and mechanical interactions. These changes to the mechanical state include jamming of grains, collisions, and frictional slip of particles—all of which present abrupt perturbations of internal forces and release of strain energy. Such energy release events typically result in the generation of elastic waves in the kHz frequency range, termed acoustic emissions (AE). The close association between grain-scale mechanics and AE generation motivated the use of AE as surrogate observations to assess the mechanical state of complex materials and granular flows. The study characterizes AE generation mechanisms stemming from grain-scale mechanical interactions. Basic mechanisms are considered, including frictional slip between particles, and mechanical excitation of particle configurations during force network restructuring events. The intrinsic frequencies and energy content of generated AEs bear the signature of source mechanisms and of structural features of the grain network. Acoustic measurements in simple shear experiments of glass beads reveal distinct characteristics of AE associated with different source mechanisms. These findings offer new capabilities for non-invasive interrogation of micromechancial interactions and linkage to a stochastic model of shear zone mechanics. Certain statistical features of restructuring events and associated energy release during shearing were predicted with a conceptual fiber-bundle model (FBM). In the FBM the collective behavior of a large number of basic mechanical elements (representing e.g. grain contacts), termed fibers, reproduces the reaction of disordered materials to progressive loading. The failure of fibers at an individual threshold force corresponds to slipping of a particle contact or a single rearrangement event of the granular network. The energy release from model fiber breakage is the equivalent to elastic energy from abrupt grain rearrangement events and provides an estimate of the energy available for elastic wave generation. The coupled FBM–AE model was in reasonable agreement with direct shear experiments that were performed on large granular assemblies. The results underline the potential of using AE as a diagnostic tool to study micro-mechanical interactions, shear failure and mobilization in granular material., Granular Matter, 16 (5), ISSN:1434-5021, ISSN:1434-7636

International audience; The physical laws governing the morphogenesis of biological tissues remain largely misunderstood. In particular, the role of the mechanical interactions occurring in this process needs to be better understood and studied. Inner follicular cells surrounding the oocytes of Ciona intestinalis form an epithelial monolayer resulting from an accretion process (without mitosis or apoptosis). This epithelium is elementary and useful for morphogenesis studies: the cells exhibit polygon packing with a specific but non-systematically repeatable topology (i.e. the distribution of pentagons, hexagons and heptagons changes). To understand the role of mechanical forces in tissue formation, we propose an innovative ''2D spherical'' model based on the physics of divided media. This approach simulates the cellular mechanical behavior and epithelium structuration by allowing cells to adopt a large variety of shapes and to self-organize in response to mechanical interactions. The numerical parameters considered in the model are derived from experimental data in order to perform pertinent and realistic simulations. The results obtained are compared to biological observations using the same counting method to characterize epithelium topology. Numerical and experimental data appear close enough to validate the model. It is then used for exploratory studies dealing with ''Tissue Development Speed'' variation, which is not easily attainable by experimentation. We show that the formation speed of the tissue influences its topology and hence its packing organization.

We describe here how to design a triaxial shear setup suitable for both stress and structure measurements on granular packings. Our novel setup makes use of refractive index matched scanning techniques. It can thus be used to study any kind of material compatible with that method. We describe in particular how to use hydrogel spheres, which we have found to have properties which make them particularly well suited for stress and structure measurements of granular packings. We discuss here some mechanical properties of hydrogel spheres and methods to use them in refractive index matched scanning devices such as the triaxial shear setup discussed.

We have designed a novel experimental apparatus to probe the mechanics of sheared quasi two-dimensional frictional granular materials with tunable friction from the supporting base of the apparatus. The experiment consists of a floating layer of photoelastic disks, which is subject to deformation. Forces on the particles are measured at the particle scale, using their photoelastic properties. This novel setup makes the study of the role of basal friction on sheared granular media possible.

Complex grain-boundary structures, such as the 1-2 nm thick intergranular glassy films (IGF), play a prominent role in the failure behavior of nano-phased ceramics. The IGF plays the role of an imperfection and serves as the location of strain localization and failure. We have recently performed “theoretical” mechanical loading experiments on very large atomic models of IGF in silicon nitride using ab initio simulation to obtain their failure behavior. The ab initio simulations yield characteristic post-peak softening accompanied by strain localization zone. In this paper a micro-structural granular mechanics based higher-order continuum theory is applied to model the failure behavior of these types of material systems. The results obtained from the ab initio simulations are compared with those predicted by the higher order continuum theory.

Polymer processing entails several operations involving the handling of granular materials. We present here a review of some recent advances in the field which are of relevance to polymer processing. The stress distribution in static material is considered with application to particulates in bins and hoppers. A new model for description of the rheology of dense granular flows is reviewed. A model for surface granular flows with application to rotating cylinders is discussed. Finally the mixing and segregation is tumbling mixers is reviewed.