This work aims at the numerical simulation of compressible fluid/deformable structure interactions. In particular, we have developed a partitioned coupling algorithm between a Finite Volume method for the compressible fluid and a Discrete Element method capable of taking into account fractures in the solid. A survey of existing fictitious domain methods and partitioned algorithms has led to choose an Embedded Boundary method and an explicit coupling scheme. We first showed that the Discrete Element method used for the solid yielded the correct macroscopic behaviour and that the symplectic time-integration scheme ensured the preservation of energy. We then developed an explicit coupling algorithm between a compressible inviscid fluid and an undeformable solid. Mass, momentum and energy conservation and consistency properties were proved for the coupling scheme. The algorithm was then extended to the coupling with a deformable solid, in the form of a semi-implicit scheme. Finally, we applied this method to unsteady inviscid flows around moving structures: comparisons with existing numerical and experimental results demonstrate the excellent accuracy of our method

Die am häufigsten für die Zerkleinerung des Zementklinkers eingesetzte Kugelmühle stellt eine relativ einfache Maschine dar, in der sehr komplizierte dynamische Wechselwirkungen herrschen. Die direkte Messung der Beanspruchungsintensitäten ist heutzutage immer noch eine schwierige Aufgabe. In der vorliegenden Arbeit sind die Spektren der Beanspruchungsgrößen sowie einige spezifische integrale Größen mit der Methode der diskreten Elemente (DEM) untersucht worden. Eine besondere Aufgabe bestand in der Ermittlung der Parameter des viskoelastischen Kontaktmodells und der den Zementklinker charakterisierenden mechanischen Größen. Die reale Abbildung der Stoßvorgänge wurde anhand vereinfachter Modelle der Messprüfstände validiert. Mit den ermittelten Parametern sind numerische Untersuchungen durchgeführt worden, mit denen ihre Auswirkung auf die Zielgrößen bestimmt wurde. In Hinsicht auf eine mögliche konstruktive Weiterentwicklung sind Simulationen mit festgelegten betrieblichen und zu variierenden konstruktiven Parametern durchgeführt worden.

The effect of in-situ or redistributed stress on solute transport in fractured rocks is one of the major concerns for many subsurface engineering problems. However, it remains poorly understood due to the difficulties in experiments and numerical modeling. The main aim of this thesis is to systematically investigate the influences of stress on solute transport in fractured rocks, at scales of single fractures and fracture networks, respectively. For a single fracture embedded in a porous rock matrix, a closed-form solution was derived for modeling the coupled stress-flow-transport processes without considering damage on the fracture surfaces. Afterwards, a retardation coefficient model was developed to consider the influences of damage of the fracture surfaces during shear processes on the solute sorption. Integrated with particle mechanics models, a numerical procedure was proposed to investigate the effects of gouge generation and microcrack development in the damaged zones of fracture on the solute retardation in single fractures. The results show that fracture aperture changes have a significant influence on the solute concentration distribution and residence time. Under compression, the decreasing matrix porosity can slightly increase the solute concentration. The shear process can increase the solute retardation coefficient by offering more sorption surfaces in the fracture due to gouge generation, microcracking and gouge crushing. To study the stress effects on solute transport in fracture systems, a hybrid approach combing the discrete element method for stress-flow simulations and a particle tracking algorithm for solute transport was developed for two-dimensional irregular discrete fracture network models. Advection, hydrodynamic dispersion and matrix diffusion in single fractures were considered. The particle migration paths were tracked first by following the flowing fluid (advection), and then the hydrodynamic dispersion and matrix diffusion were considered using statistic methods. The numerical results show an important impact of stress on the solute transport, by changing the solute residence time, distribution and travel paths. The equivalent dispersion coefficient is scale dependent in an asymptotic or exponential form without stress applied or under isotropic compression conditions. Matrix diffusion plays a dominant role in solute transport when the hydraulic gradient is small. Outstanding issues and main scientific achievements are also discussed. / QC 20111011

Fluid-structure interaction (FSI), occurrent in many areas of engineering and in the natural world, has been the subject of much research using a wide range of modelling strategies. However, problems with high levels of structural deformation are difficult to resolve and this is particularly the case for biomedical flows. A Lagrangian flow model coupled with a robust model for nonlinear structural mechanics seems a natural candidate since large distortion of the computational geometry is expected. Smoothed particle Hydrodynamics (SPH) has been widely applied for nonlinear interface modelling and this approach is investigated here. Biomedical applications often involve thin flexible structures and a consistent approach for modelling the interaction of fluids with such structures is also required. The Lagrangian weakly compressible SPH method is investigated in its recent delta-SPH form utilising inter-particle density fluxes to improve stability. Particle shifting is also used to maintain particle distributions sufficiently close to uniform to enable stable computation. The use of artificial viscosity is avoided since it introduces unphysical dissipation. First, solid boundary conditions are studied using a channel flow test. Results show that when the particle distribution is allowed to evolve naturally instabilities are observed and deviations are noted from the expected order of accuracy. A parallel development in the SPH group at Manchester has considered SPH in Eulerian form (for different applications). The Eulerian form is applied to the channel flow test resulting in improved accuracy and stability due to the maintenance of a uniform particle distribution. A higher-order accurate boundary model is developed and applied for the Eulerian SPH tests and third-order convergence is achieved. The well documented case of flow past a thin plate is then considered. The immersed boundary method (IBM) is now a natural candidate for the solid boundary. Again, it quickly becomes apparent that the Lagrangian SPH form has limitations in terms of numerical noise arising from anisotropic particle distributions. This corrupts the predicted flow structures for moderate Reynolds numbers (O(102)). Eulerian weakly compressible SPH is applied to the problem with the IBM and is found to give accurate and convergent results without any numerical stability problems (given the time step limitation defined by the Courant condition). Modelling highly flexible structures using the discrete element model is investigated where granular structures are represented as bonded particles. A novel vector-based form (the V-Model) is identified as an attractive approach and developed further for application to solid structures. This is shown to give accurate results for quasi-static and dynamic structural deformation tests. The V-model is applied to the decay of structural vibration in a still fluid modelled using Eulerian SPH with no artificial stabilising techniques. Again, results are in good agreement with predictions of other numerical models. A more demanding case representative of pulsatile flow through a deep leg vein valve is also modelled using the same form of Eulerian SPH. The results are free of numerical noise and complex FSI features are captured such as vortex shedding and non-linear structural deflection. Reasonable agreement is achieved with direct in-vivo observations despite the simplified two-dimensional numerical geometry. A robust, accurate and convergent method has thus been developed, at present for laminar two-dimensional low Reynolds number flows but this may be generalised. In summary a novel robust and convergent FSI model has been established based on Eulerian SPH coupled to the V-Model for large structural deformation. While these developments are in two dimensions the method is readily extendible to three-dimensional, laminar and turbulent flows for a wide range of applications in engineering and the natural world.

L'objectif de cette thèse est de mettre en place un cadre numérique qui permet de simuler la réponse de structures en béton armé de taille industrielle soumises à des impacts sévères. Notre modèle repose sur trois ingrédients principaux : 1) la modélisation éléments discrets (ED) du béton, permettant de profiter de la nature « discrète » de cette formulation et de décrire facilement l'apparition et la propagation des discontinuités fortes de la matière ; 2) la modélisation éléments finis (EF) poutre des armatures, donnant la possibilité de représenter toute la complexité du ferraillage que l'on rencontre dans les structures industrielles en béton armé ; 3) un modèle de liaison acier-béton original, que nous avons proposé et mis en œuvre dans le code EUROPLEXUS, et qui constitue le principal apport de cette thèse. Nous avons effectué une étude théorique et numérique sur la stabilité et la précision de ce modèle de liaison, et avons calibré ses paramètres en simulant un essai d'arrachement. En modélisant l'essai de traction d'un tirant (barre en béton contenant une armature), nous avons montré la capacité de notre modèle à reproduire le transfert des efforts entre le béton et l'armature au niveau de l'interface acier-béton. Testé et validé sur des cas-tests élémentaires, le modèle mixte ED-EF a ensuite été appliqué à la simulation de vraies structures en béton armé. Nous avons simulé de manière détaillée quelques essais connus (l'impact mou sur poutre CEA, l'essai Meppen II-4), ce qui a permis de valider l'ensemble de notre approche numérique et de recueillir des éléments pour définir les pistes d'amélioration des modèles que nous avons mis en œuvre. / The aim of this work is to set up a numerical framework to simulate the behaviour of industrial size reinforced concrete structures subjected to severe impacts. Our model is based on three main features : 1) modeling of the concrete with a discrete method to handle easily strong material discontinuities such as initiation and propagation of macro-cracks ; 2) modeling of the reinforcement bars with finite element method to be able to represent complex reinforcement cages of industrial structures ; 3) an original steel-concrete bond model that we proposed and implemented in the dynamic explicit code EUROPLEXUS and that constitutes the main task of this work. We studied theoretically and numerically this model to guarantee its stability and precision during the time integration. We calibrated its parameters by simulating pull-out tests. To verify our model, we simulated a tie-test (a long concrete column which contains a reinforcement bar) and showed that our model ensures the correct transfert of forces between steel and concrete. After testing our model on simple benchmarks, we simulated real reinforced concrete structures subjected to impacts (soft impact on a beam, Meppen test n°II-4), allowing us to validate our numerical approach and to define some perspectives to improve the models we have developped.

Turbulent bedload transport represents the main contribution to the riverbed morphological evolution, and associates the non-trivial collective granular behavior with a turbulent fluid flow. Therefore, its description is both a scientific challenge and a societal issue. The present numerical approach focuses on the granular phase characterization, and considers idealized steady uniform bedload transport, with monodisperse spherical beads and a unidirectional fluid flow. This simplified configuration allows to study the underlying physical mechanisms.A minimal coupled numerical model is proposed, associating a three dimensional discrete element method with a one-dimensional volume-averaged fluid momentum balance resolution. The model is compared with classical experimental results of dimensionless sediment transport rate as a function of the Shields number. The comparison is extended to granular depth profiles of solid volume fraction, solid velocity and sediment transport rate density in quasi-2D bedload transport configurations. Parameter sensitivity analysis evidenced the importance of the fluid-particle phase coupling, and showed a robust agreement of the model with the experiments. The validated model is further used to analyze the granular depth structure in bedload transport. Varying the channel inclination angle and the specific density, it is shown that the classical Shields number and dimensionless sediment transport rate formulations do not take appropriately into account the effects of these two parameters. Analyzing the solid depth profiles and the continuous two-phase flow equations, the neglected fluid flow inside the granular bed is identified as the missing contribution. Its importance is enhanced near the transition to debris flow. A rescaling of the Shields number is proposed and is shown to make all the data collapse onto a master curve when considering the dimensionless sediment transport rate as a function of the modified Shields number. Lastly, the bedload transport granular rheology is characterized by computing locally the stress tensor as a function of the depth. The lowermost part is shown to follow a creeping regime and exhibits signature of non-local effects. The dense granular flow on the top of it, is well described by the mu(I) rheology and is observed to persist up to unexpectedly high inertial numbers. It is characterized by the co-existence of frictional and collisional contributions. The transition from dense to dilute granular flow is controlled by the Shields number, the slope and the specific density. Saltation is observed in the uppermost granular layer. These findings improve the understanding of bedload transport granular mechanisms and challenge the existing granular rheologies. / Turbulent bedload transport represents the main contribution to the riverbed morphological evolution, and associates the non-trivial collective granular behavior with a turbulent fluid flow. Therefore, its description is both a scientific challenge and a societal issue. The present numerical approach focuses on the granular phase characterization, and considers idealized steady uniform bedload transport, with monodisperse spherical beads and a unidirectional fluid flow. This simplified configuration allows to study the underlying physical mechanisms.A minimal coupled numerical model is proposed, associating a three dimensional discrete element method with a one-dimensional volume-averaged fluid momentum balance resolution. The model is compared with classical experimental results of dimensionless sediment transport rate as a function of the Shields number. The comparison is extended to granular depth profiles of solid volume fraction, solid velocity and sediment transport rate density in quasi-2D bedload transport configurations. Parameter sensitivity analysis evidenced the importance of the fluid-particle phase coupling, and showed a robust agreement of the model with the experiments. The validated model is further used to analyze the granular depth structure in bedload transport. Varying the channel inclination angle and the specific density, it is shown that the classical Shields number and dimensionless sediment transport rate formulations do not take appropriately into account the effects of these two parameters. Analyzing the solid depth profiles and the continuous two-phase flow equations, the neglected fluid flow inside the granular bed is identified as the missing contribution. Its importance is enhanced near the transition to debris flow. A rescaling of the Shields number is proposed and is shown to make all the data collapse onto a master curve when considering the dimensionless sediment transport rate as a function of the modified Shields number. Lastly, the bedload transport granular rheology is characterized by computing locally the stress tensor as a function of the depth. The lowermost part is shown to follow a creeping regime and exhibits signature of non-local effects. The dense granular flow on the top of it, is well described by the mu(I) rheology and is observed to persist up to unexpectedly high inertial numbers. It is characterized by the co-existence of frictional and collisional contributions. The transition from dense to dilute granular flow is controlled by the Shields number, the slope and the specific density. Saltation is observed in the uppermost granular layer. These findings improve the understanding of bedload transport granular mechanisms and challenge the existing granular rheologies.

La fracturation hydraulique est au cœur d'un certain nombre de phénomènes naturels et induits et est cruciale pour un développement durable de la production de ressources énergétiques. Compte tenu de son rôle crucial, ce phénomène a été pris en compte au cours des trois dernières décennies par le monde académique. Néanmoins, un certain nombre d'aspects très importants de ce processus ont été systématiquement négligés par la communauté. Deux des plus remarquables sont l'incapacité de la grande majorité des modèles existants à aborder la propagation des fractures hydrauliques dans les massifs rocheux fracturés où l'injection de fluide peut à la fois conduire à la fracturation de la roche intacte et à la réactivation de fractures préexistantes. Un autre aspect essentiel de ce processus est qu'il est intrinsèquement tridimensionnel, ce qui est souvent négligé par les modèles actuellement disponibles. Pour aborder ce problème essentiel, un modèle hydro-mécanique couplé basé sur la méthode des éléments discrets a été développé. La masse rocheuse est ici représentée par un ensemble d'éléments discrets interagissant à travers des lois de contact cohésifs qui peuvent se casser pour former des fissures à l'intérieur du milieu simulé. Ces fissures peuvent se coalescer pour former des fractures. Une méthode de volume fini est utilisée pour simuler l'écoulement de fluide entre les éléments discrets. L'écoulement est calculé en fonction de la déformation de l'espace poreux dans le milieu intact et de l'ouverture des fissures dans les fractures. De plus, les fractures naturelles sont modélisées explicitement de sorte qu'elles peuvent présentées des comportements mécanique et hydraulique différents de ceux de la matrice rocheuse intacte. La simulation des processus de fracturation hydraulique dans un milieu initialement intact en considérant plusieurs points d'injection plus ou moins espacés a permis de mettre en évidence l'évolution spatio-temporelle des fractures hydrauliques et de quantifier l'impact des différentes stratégies d'injection sur des indices représentatifs du volume fracturé, de l'intensité et de la densité des fractures ou encore sur la pression de fluide au niveau du puits. De plus, l'injection dans une fente de perforation non alignée sur le plan de contrainte minimum a génère des fractures hydrauliques non planaires percolantes si la connectivité est faible, ce qui peut être gênant pour la mise en place du proppant. En outre, des interactions fortes prennent place entre des fractures hydrauliques étroitement espacées ont été mises en évidence grâce au le suivi de la orientation de contrainte principale locale et ont révélé l'importance des effets d'ombre de contrainte. Des solutions sont proposées pour optimiser les traitements multiples à partir d'un puits de forage non parfaitement aligné. Enfin, l'interaction entre une seule fracture hydraulique et une seule fracture naturelle de propriétés et d'orientations variables a été étudiée à l'aide du modèle proposé. L'évolution de la fracture hydraulique et la réponse globale de l'échantillon ont été enregistrées d'une manière comparable aux données expérimentales existantes pour établir un pont entre les résultats expérimentaux et numériques. Les fractures naturelles persistantes semblent être des barrières pour la fracture hydraulique si leur conductance est élevée par apport a celle de la matrice ou si leur raideur est faible par rapport a la rigidité du milieu environnant. D'autre part, une faible rigidité dans les discontinuités non persistantes pourrait provoquer une bifurcation de la fracture hydraulique principale. De plus, des angles d'approche élevés et des contraintes différentielles fortes semblent favoriser le croisement de la fracture naturelle alors que des angles faibles engendrent plutôt un glissement ou une dilatation par cisaillement de la partie du plan qui n'est pas affectée par la perturbation de la contrainte. / Hydraulic fracturing is at the core of a number of naturally occurring and induced phenomena and crucial for a sustainable development of energy resource production. Given its crucial role this process has been given increasing attention in the last three decades from the academic world. Nonetheless a number of very significant aspects of this process have been systematically overlooked by the community. Two of the most notable ones are the inability of the vast majority of existing models to tackle at once the propagation of hydraulic fractures in realistic, fractured rocks-masses where hydraulic fracturing is a competing dipole mechanism between fracturing of the intact rock and re-activation of exiting fracture networks. Another essential aspect of this process is that it is intrinsically three-dimensional which is neglected by most models. To tackle this vital problem taking into account these pivotal aspects, a fully coupled hydro-mechanical model based on the discrete element method has been developed. The rock mass is here represented by a set of discrete elements interacting through elastic-brittle bonds that can break to form cracks inside the simulated medium. Theses cracks can coalesce to form fractures. A finite volume scheme is used to simulate the fluid flow in between these discrete elements. The flow is computed as a function of the pore space deformation in the intact medium and of the cracks' aperture in the fractures. Furthermore, the natural fractures are modelled explicitly and present mechanical and hydraulic properties different from the rock matrix. Employing this model in an intact numerical specimen, single fluid injection and multiple closely spaced sequential injections, enabled the description the full spatio-temporal evolution of HF propagation and its impact on quantitative indexes used in description of hydraulic fracturing treatments, such as fractured volume, fracture intensity and down-the-hole pressure for different control parameters and in-situ stress-fields. Moreover, injections from perforation slots which are not well aligned to the minimum stress plane showed possible creation of percolating non-planar hydraulic fractures of low connectivity, which can be troublesome for proppant placement. Also, strong interactions between closely spaced HF were highlighted by tracking the local principal stress rotation around the injection zones, emphasizing the importance of stress shadow effects. Optimization solutions are proposed for multiple treatments from a non-perfectly aligned wellbore. Finally, interaction between a single hydraulic fracture and a single natural fracture of varying properties and orientations was studied using the proposed model. The evolution of the hydraulic fracture and the global response of the specimen were recorded in a way comparable to existing experimental data to bridge the experimental and numerical findings. Persistent natural fractures appeared to be barriers for the hydraulic fracture if their conductance is high compared to the matrix conductivity or if their stiffness is significantly low compared to the rock matrix rigidity. Low stiffness in non-persistent defects might also cause a bifurcation of the main hydraulic fracture due to the local stress field perturbation around the defect and ahead of the hydraulic fracture tip. Furthermore, high approach angles and differential stresses seemed to favour crossing of the natural fracture while low angles enable shear slippage or dilation on the part of the plane which is not affected by the local stress perturbation.

Cette thèse présente un modèle générique d'écrans de filets pare-blocs sous sollicitations dynamiques de type impacts. Ces ouvrages pare-blocs sont décrits comme un assemblage abstrait de leurs principaux constituants. Le modèle développé permet ainsi de représenter la plupart des technologies existantes. Un code de calcul en C++ utilisant une Méthode aux Éléments Discrets est développé afin de réaliser les simulations numériques d'impacts. La description générique des ouvrages est mise en œuvre au niveau du code de calcul grâce à une structuration des données et une programmation orientée objet correspondantes. Le modèle générique est complété par deux modèles mécaniques de constituants. Dans un premier temps, un modèle mécanique de filet à anneaux à 4 contacts est développé. Ce modèle est calibré et validé par une campagne expérimentale réalisée sur des anneaux en acier utilisés dans les écrans de filets. Dans un second temps, un modèle général de câble glissant est développé pour modéliser le phénomène d'effet rideau. Ce modèle démontre de fortes capacités de description des phénomènes de glissement et de très bonnes performances de calcul. Enfin, des essais d'impact en vraie grandeur sur deux écrans de filets de technologies différentes sont utilisés afin de valider le modèle générique. Les simulations numériques de ces essais sur ouvrages réels mettent en évidence la pertinence du modèle développé. Les résultats numériques sont en très bon accord avec les expérimentations et le modèle présente des capacités prédictives pertinentes dans la perspective d'usage en ingénierie. Les déformations, les temps de chargement et l'intensité des efforts sont obtenus avec des erreurs inférieures à 10 %. Des simulations complexes d'impacts répétés sont réalisées pour la première fois et le comportement lors de l'impact et pendant la phase de retour élastique après impact est bien appréhendé par le modèle. / This thesis introduces a generic model of rockfall barriers under impact loading. The structures are described as an abstract assembly of their main components. The developed model thereby enables the consideration of most of the existing technologies. A C++ code based on the Discrete Element Method is developed in order to perform the numerical simulations of impacts. The generic description of the barriers is implemented at the code level using a corresponding data structure and object-oriented programming. The generic model is completed by the mechanical models of two components. First, a mechanical model of 4-contact interlaced ring nets is developed. An experimental campaign is carried out to calibrate and validate the model against steel wire rings used in rockfall restraining nets. Second, a general sliding cable model is developed to account for the curtain effect. This model demonstrates great capabilities in describing sliding phenomena and a low computational cost. Finally, full-scale impact tests performed on two barriers of different technologies are used to validate the generic model. Numerical simulations of the full-scale tests highlight the relevance of the developed model. Numerical results agree finely with experiments and the model exhibits compelling predictive capacities for engineering applications. Deformations, loading time and forces magnitude are all predicted within 10 % relative error. Complex and unprecedented simulations of repeated impacts are carried out and the model is able to reproduce the barrier behavior both during the impact phase and after springback.

This thesis analyses the impact of granular aggregates with structures using experiments and numerical simulations. Original contributions include an insight into multiple factors affecting the loading and damage to the structures, along with study of numerical parameters important for realistic prediction of the interaction between the granular media and structures. It extends the current understanding related to such interactions, with an underlying motivation to guide strategies in order to reduce the structural damage. The response of structures impacted by granular media (sand or soil) is of significant research interest for many applications. One of the applications is for landmine explosions which causes ejection of soil from ground and damage to structures impacted by this ejected soil. Experimentation is done in a laboratory setting where the cylindrical sand slugs are generated at high speed using an impulse provided by a piston. This induces a velocity gradient along the slug, because of which the slug expands during the flight before impacting the target. Deformable as well as rigid flat targets are considered in two orientations relative to the incoming slug: perpendicular (i.e. normal orientation) and inclined at an angle of 45°. The targets are supported by force transducers to capture the loading from the slug. Simulations are performed using a combination of discrete particle and finite element schemes, which enables the analysis of the fully coupled interaction between the flowing granular media and the structure. A contact model involving multiple parameters is used for inter-particle and particle-target contact. Firstly, a numerical analysis is performed to characterise the temporal evolution of slugs and their impact on monolithic beams constrained at the ends. Out of all the parameters used for inter-particle contact definition in discrete particle method, only the contact stiffness is found to effect the velocity gradient in the slug before it impacts the target. Other factor influencing the gradient is the acceleration provided by the piston. A strong dependence of beam deflection on the stand-off distance is observed due to the velocity gradient in the slugs. As the second step, the effect of target surface properties on the transmitted momentum is analysed. Experiments are done by applying coatings of different hardness and roughness on the target surface impacted by sand slugs. For normally oriented targets, the transmitted momentum is observed to be insensitive to the change in surface coating. In contrast, for inclined targets, a significant influence of coatings is observed. Additionally, the momentum transmitted to the inclined targets is always less than that for normal targets. Numerical analysis of this surface effect reveals that assuming the slug particles to be spherical shape in simulations does not capture the particle/target interactions accurately and under-predicts the frictional loading on the target. Following this, a detailed numerical study is done to understand the effect of the shape of particles in the slug. Simple shaped non-spherical particles are constructed by combining spherical sub-particles. With increasing angularity of particles in the slug, the frictional loading on the target is shown to increase. This results in an increase of momentum transmitted to inclined targets. For normally oriented targets however, the particle shape does not affect the overall transmitted momentum, which is a behaviour similar to that observed when studying the effect of target surface properties. In addition, effect of fracture of particles in the slug is analysed by using beam connections between sub-particles that break during the impact with the target. If the fracture results in increasing particle angularity, the transmitted momentum increases, whereas the situation reverses if fracture results on more spherical shaped particles. Lastly, a strategy to reduce the loading on the targets is analysed by using sacrificial coating on the target surface. In experiments, this coating is placed on the rigid target surface using a lubricant at their interface. When impacted by the slug, this coating slides on the target surface, resulting in a reduction of frictional loading on the target. If the friction at the coating/target interface vanishes, the transmitted momentum approaches the theoretical minimum value. Simulations are used to first validate the experimental observations and then to extend the concept of sliding coatings using deformable targets. Both the transmitted momentum and deflections depended on the thickness of the target and coating. When a coating is used, the deflections increase due to reduction in target thickness. It is found that the best strategy to reduce the damage to the target is to use least possible thickness of the coating and minimise the friction at the interface between the coating and the target. The presented work examines many of the factors that affect the loading on the target impacted by granular slugs, in addition to characterising the expansion of slugs before the target impact. The analysed factors include those already known such as target stand-off distance, inclination and unveils others such as target surface properties and granular properties. The numerical analysis discloses important parameters and shows the effect of particle shape, highlighting the shortcomings of widely used spherical particle assumption in the numerical studies. A strategy using a sacrificial coating to reduce damage to the target is also analysed.

Milling is a common unit operation in industry for the purpose of intentional size reduction. Considerable amount of energy is consumed during a grinding process and much of the energy is dissipated as heat and sound, which often makes grinding into an energy-intensive and highly inefficient operation. Despite many attempts to interpret particle breakage during a milling process, the grindability of a material in a milling operation remains aloof and the mechanisms of particle breakage are still poorly understood. Hence the optimisation and refinement in the design and operation of milling are in great need of an improved scientific understanding of the complex failure mechanisms. This thesis aims to provide an in-depth understanding of particle breakage associated with stressing events that occur during milling. A hybrid of experimental, theoretical and numerical methods has been adopted to elucidate the particle breakage mechanics. This study covers from single particle damage at micro-scale to bulk comminution during the whole milling process. The mechanical properties of two selected materials, i.e. alumina and zeolite were measured by indentation techniques. The breakage test of zeolite granules subjected to impact loading was carried out and it was found that tangential component velocity plays an increasingly important role in particle breakage with increasing impact velocity. Besides, single particle breakage via in-situ loading was conducted under X-ray microcomputed tomography (μCT) to study the microstructure of selected particles, visualize the progressive failure process and evaluate the progressive failure using the technique of digital image correlation (DIC). A new particle breakage model was proposed deploying a mechanical approach assuming that the subsurface lateral crack accounts for chipping mechanism. Considering the limitation of existing models in predicting breakage under oblique impact and the significance of tangential component velocity identified from experiment, the effect of impact angle is considered in the developed breakage model, which enables the contribution of the normal and tangential velocity component to be rationalized. The assessment of breakage models including chipping and fragmentation under oblique impact suggests that the equivalent normal velocity proposed in the new model is able to give close prediction with experimental results sourced from the public literature. Milling experiments were performed using the UPZ100 impact pin mill (courtesy by Hosokawa Micron Ltd. UK) to measure the comminution characteristics of the test solids. Several parameters were used to evaluate the milling performance including product size distribution, relative size span, grinding energy and size reduction ratio etc. The collective data from impact pin mill provides the basis for the validation of numerical simulation results. The Discrete Element Method (DEM) is first used to model single particle breakage subject to normal impact loading using a bonded contact model. A validation of the bonded contact model was conducted where the disparity with the experimental results is discussed. A parametric study of the most significant parameters e.g. bond Young’s modulus, the mean tensile bond strength, the coefficient of variation of the strength and particle & particle restitution coefficient in the DEM contact model was carried out to gain a further understanding of the effect of input parameters on the single particle breakage behavior. The upscaling from laboratory scale (single particle impact test) to industrial process scale (impact pin mill) is achieved using Population Balance Modelling (PBM). Two important functions in PBM, the selection function and breakage function are discussed based on the single particle impact from both experimental and numerical methods. An example of predicting product size reduction via PBM was given and compared to the milling results from impact pin mill. Finally, the DEM simulation of particle dynamics with emphasis on the impact energy distribution was presented and discussed, which sheds further insights into the coupling of PBM and DEM.