This thesis presents a computational framework that models the coupled behavior between sand deposits and their surrounding fluid. Particle deposits that form in gas turbine engines and industrial burners, can change flow dynamics and heat transfer, leading to performance degradation and impacting durability. The proposed coupled framework allows insight into the coupled behavior of sand deposits at high temperatures with the flow, which has not been available previously. The coupling is done by using a CFD-DEM framework in which a physics based collision model is used to predict the post-collision state-of-the-sand-particle. The collision model is sensitive to temperature dependent material properties of sand. Particle deposition is determined by the particle's softening temperature and the calculated coefficient of restitution of the collision. The multiphase treatment facilitates conduction through the porous deposit and the coupling between the deposit and the fluid field. The coupled framework was first used to model the behavior of softened sand particles in a laminar impinging jet flow field. The temperature of the jet and the impact surface were varied(T^* = 1000 – 1600 K), to observe particle behavior under different temperature conditions. The Reynolds number(Rejet = 20, 75, 100) and particle Stokes numbers (Stp = 0.53, 0.85, 2.66, 3.19) were also varied to observe any effects the particles' responsiveness had on deposition and the flow field. The coupled framework was found to increase or decrease capture efficiency, when compared to an uncoupled simulation, by as much as 10% depending on the temperature field. Deposits that formed on the impact surface, using the coupled framework, altered the velocity field by as much as 130% but had a limited effect on the temperature field. Simulations were also done that looked at the formation of an equilibrium deposit when a cold jet impinged on a relatively hotter surface, under continuous particle injection. An equilibrium deposit was found to form as deposited particles created a heat barrier on the high temperature surface, limiting more particle deposition. However, due to the transient nature of the system, the deposit temperature increased once deposition was halted. Further particle injection was not performed, but it can be predicted that the formed deposit would begin to grow again. Additionally, a Large-Eddy Simulation (LES) simulation, with the inclusion of the Smagorinsky subgrid model, was performed to observe particle deposition in a turbulent flow field. Deposition of sand particles was observed as a turbulent jet (Re jet=23000,T_jet^*= 1200 K) impinged on a hotter surface(T_surf^*= 1600 K). Differences between the simulated flow field and relevant experiments were attributed to differing jet exit conditions and impact surface thermal conditions. The deposit was not substantive enough to have a significant effect on the flow field. With no difference in the flow field, no difference was found in the capture efficiency between the coupled and decoupled frameworks. / Master of Science / Particle deposits can form in a wide range of environments leading to altered performance. In applications, such as jet engines, particles are heated to critically high temperatures. At these high temperatures, the particles can soften, and begin to exhibit characteristics of both a liquid and a solid. Overtime as these softened particles aggregate on a wall, a deposit will begin to form. These deposits alter the geometry resulting in changes in fluid temperature and velocity. This change in fluid behavior will affect the rate of particle deposition that happens in the future. There has been limited work that has looked at the coupled behavior between a deposit and its surrounding fluid, experimentally or computationally. The purpose of this research was to develop a framework that models the deposition of softened particles, and the coupled behavior between deposits and the fluid. This research was able to show that the presence of a deposit could change its surrounding fluid’s velocity and temperature significantly. Differences in the rate of particle deposition also occurred when a deposit had formed on a surface. These results show the importance of capturing the relationship between deposits and the surrounding fluid. With further development, this proposed framework can provide insight into altered gas turbine performance and can lead to improved maintenance plans.

Thesis (MEng)--University of Stellenbosch, 2000. / ENGLISH ABSTRACT: The main goal of the thesis is to validate the ability of discrete element methods (DEM) to predict forced granular flow. Granular flow occurs in a broad spectrum of industrial applications. The thesis focuses on earthmoving processes typical of the mining and agricultural industries. Existing soil mechanics soil-tool models are also investigated and general flow behaviour in and around blades and buckets are established. Soil mechanics theories are used to predict the draft forces on a flat blade moving through granular material. Com and wheat grains are used as material. The rupture (slip) lines in front of the blade are predicted by soil mechanics and compared to experimental results. A two-dimensional test bench is used to visualise the flow of the granular material. Forces and moments that act on the tools are measured. DEM can be used to model industrial granular flow with large displacements. Two types of earthmoving equipment are simulated. The first is a flat blade and the second is a bucket. The forces on these tools are determined using DEM and compared to experimental results. The ability of DEM to predict material compressibility, the flow of material in and around the tools, the rupture lines and the bucket fill rate are investigated. A particle relative displacement method is used to determine the rupture lines. / AFRIKAANSE OPSOMMING: Die hoofdoel van die tesis is om die vermoë van diskrete-element-metodes (DEM) om geforseerde partikelvloei te voorspel, te ondersoek. Partikelvloei word aangetref in 'n breë spektrum van industriële toepassings. Die tesis fokus op grondverskuiwing soos aangetref in myn- en landbouprosesse. Bestaande grondmeganika-modelle word ook ondersoek, asook die algemene gedrag van partikelvloei in en rondom lemme en bakke. Die grondmeganika-modelle word hoofsaaklik gebruik om die kragte op lemme te voorspel. Glip (skuif)-vlakke word ondersoek en vergelyk met eksperimentele resultate. 'n Twee-dimensionele toetsbank word gebruik om die vloei waar te neem. Die kragte en momente op die toerusting word ook gemeet. Mielie- en koringpitte word as materiaal gebruik. DEM kan gebruik word om industriële partikelvloei met groot verplasings te modelleer. Twee tipes toerusting word gesimuleer. Die eerste is 'n plat lem en die tweede 'n bak. Die kragte en momente op dié toerusting word bepaal m.b.V. DEM en dan vergelyk met die eksperimentele resultate. Die vermoë van DEM om materiaalsamedrukking, vloeipatrone, glipvlakke en bakvul-tempo's te voorspel word ondersoek. 'n Partikelrelatiewe- verplasings-metode word gebruik om die glipvlakke te voorspel.

Cette étude de la compaction d’un empilement granulaire par vibrations horizontales a été réalisée dans le cadre d’un partenariat avec le groupe MERSEN pour une application au sable contenu dans ses fusibles. L’objectif scientifique est de développer la compréhension des mécanismes mis en jeu dans un milieu granulaire vibré horizontalement. Deux approches ont été utilisées en parallèle, l’une expérimentale, l’autre par simulation numérique. L’approche expérimentale a été réalisée sur des grains de silice de diamètre moyen 500 m. Un récipient de quelques centimètres est soumis à un mouvement sinusoïdal de fréquence comprise entre 20 et 200 Hz avec des accélérations allant jusqu’à 10 g. Le dispositif instrumenté permet la mesure instantanée de la force et de l’accélération, la vitesse des grains aux parois (PIV) ainsi que la densité globale du milieu. L’approche numérique est basée sur la méthode des éléments discrets (DEM). Des méthodes spécifiques d’analyse des résultats ont été développées pour assurer la comparaison avec l’expérience. Elles permettent d’obtenir des informations qui ne sont pas accessibles expérimentalement comme les densités, vitesses et contraintes locales dans l’ensemble de l’empilement. Dans le cas d’un récipient ouvert, la simulation permet de retrouver les résultats expérimentaux : rouleaux de convections, seuils de comportement, influence de l’accélération… Des résultats originaux ont été établis dans les expériences et les simulations comme une croissance de la vitesse des grains avec la longueur du récipient. La simulation a aussi permis d’obtenir des résultats spécifiques comme l’influence du coefficient de friction sur le sens des rouleaux et la caractérisation des contraintes au sein de l’empilement. L’écoulement des grains à travers des orifices circulaires de différentes dimensions a été également étudié. Le comportement du sable dans un récipient fermé (milieu confiné) a été étudié au cours d’un remplissage progressif. Des différences significatives ont été constatées lorsque le taux de remplissage devient élevé. Des mesures d’accélération et de force sur l’ensemble du dispositif ont permis de définir et de mesurer une masse apparente et l’énergie dissipée par le dispositif. Des modèles descriptifs ont permis de comprendre les comportements observés. Ces résultats sont à l’origine d’un brevet déposé par le groupe MERSEN sur le contrôle du remplissage. / This work on the densification of a granular medium under horizontal vibrations was realised in the context of an industrial study on sand in fuses, undertaken by the firm MERSEN. The scientific purpose is the understanding of the mechanisms involved in horizontally vibrated granular media. Both experiments and computer simulations were used. Experiments were conducted with 500 µm silica grains. Sinusoidal accelerations up to 10 g were applied to a container of a few centimetres using frequencies between 20 and 200 Hz. The experimental device enables instantaneous force, acceleration, grain velocity on the walls (PIV) and mean bulk density to be measured. Discrete element method (DEM) was used for the computer simulations. Specific techniques were developed to analyse the results and compare them with experiments. Computer simulations provided data which are not experimentally available such as local values of density, velocities and stress inside the particle packing. In the case of an open container, simulation results are in good agreement with experimental ones (convection rolls, thresholds, effect of acceleration...). Original results are obtained in both experiments and simulations, such as a grain velocity increase with the container length. Simulation provided specific results such as the effect of friction coefficients on the direction of rotation of convection rolls and the characterization of stress inside the particle packing. The flowability of grains across circular holes of various sizes was also studied. The behaviour of sand in a closed container (confined medium) was studied during a progressive filling. Significant differences were observed when the filling rate becomes high. The dissipated energy and the apparent mass of the vibrated device were defined and measured using acceleration and force measurements. Specific models were built to analyse and understand the observed behaviour. Some results on the filling control were patented by MERSEN.

L'objectif de ce travail de recherche est d'étudier numériquement le transport et le dépôt de particules dans des milieux poreux à l'échelle des pores.Premièrement, un couplage entre la méthode de Boltzmann sur réseau (LBM) et la méthode des éléments discrets (DEM) est réalisé et utilisé pour simuler l'écoulement d'un fluide chargé en particules. La LBM est utilisée pour décrire l'écoulement du fluide autour des fibres tandis que la DEM est utilisée pour traiter la dynamique des particules. Ce couplage est bidirectionnel dans le sens où le mouvement des particules affecte le flux de fluide et réciproquement. Ce modèle nous a permis de prédire l'efficacité de capture et la chute de pression à l'étape initiale du processus de filtration. Le facteur de qualité est également calculé pour déterminer la qualité de filtration.Ensuite, on se focalise sur l'étude de l'efficacité de la capture de fibres de formes de section transversale différentes (circulaire, losange et carrée). Les résultats issus de nos simulations du processus de filtration de la fibre circulaire concordent bien avec les corrélations empiriques. L'impaction des particules sur la face avant de la fibre de forme carrée est plus importante que dans les cas de fibre de formes circulaire et losange. Cependant, en raison d'une chute de pression plus faible, la fibre de section losange présente une meilleure qualité de filtration. Ensuite, les variations du facteur de qualité dues à l'angle d'orientation et au rapport d'aspect des fibres ont été étudiées numériquement pour la forme rectangulaire. Pour chaque cas, on a déterminé la valeur optimale de la zone au vent pour laquelle le facteur de qualité est maximal. La comparaison des valeurs du facteur de qualité obtenues pour les différentes formes de fibre monte une meilleure performance pour la fibre de section carrée orientée avec un angle de π/4.Enfin, l'influence de l'arrangement des fibres sur la qualité de la filtration est analysée en considérant la configuration en quinconce pour les différentes formes. Les simulations conduites pour différentes tailles de particules et différentes valeurs de la densité (particule/air) montent que la fibre de section losange est plus performante en termes de facteur de qualité pour les particules de grande taille et pour les valeurs de densité élevée. La présente étude fournit des pistes pour optimiser le processus de filtration et prédire la qualité de filtration. / The objective of the present research was to numerically investigate the transport and deposition of particles in porous media at the pore scale. Firstly, a developed coupled lattice Boltzmann method (LBM) and discrete element method (DEM) is used to simulate the fluid-particle flow. LBM is employed to describe the fluid flow around fibers whereas DEM is used to deal with the particle dynamics. The corresponding method is two-way coupling in the sense that particle motion affects the fluid flow and reciprocally. It allowed us to predict the capture efficiency and pressure drop at the initial stage of filtration process. The quality factor is also calculated for determining the filtration performance. Secondly, we focus on the study the capture efficiency of single fiber with circular, diamond and square cross-section, respectively. The results of LBM-DEM for filtration process of single circular fiber agree well with the empirical correlation. The impaction of particles on the front side of square-shaped fiber is more favorable than those on circular and diamond cases. However, diamond fiber exhibits a good filtration performance. Then the variations of quality factor due to the different orientation angle and aspect ratio of rectangular fiber were studied using LBM-DEM. For each case, we have found the optimal value of the windward area to which corresponds a maximum value of the quality factor. The comparison of the performance of the different forms of fibers shows that the largest quality factor is obtained for square fiber oriented with angle π/4.Finally, the influence of the arrangement of fiber on filtration performance is analyzed by considering the staggered configuration. Simulations conducted for several particle size and density show that the diamond with staggered array performs better for large particles and high particle-to-fluid density ratio in terms of quality factor. The present study provide an insight to optimize the filtration process and predict filtration performance.

Cette thèse porte sur le problème fondamental du mottage des poudres suite aux mécanismes de transition de phase. Le projet vise à étudier l'impact des facteurs intrinsèques (structure moléculaire des matériaux, propriétés physiques et/ou physicochimiques, etc.) ou des facteurs environnementaux (conditions de stockage ou paramètres de procédé) sur la stabilité de la structure des poudres. Plus précisément, notre étude a mis en évidence le rôle prépondérant du phénomène de cristallisation et des transitions entre les différents polymorphes du lactose. L'accent a été mis sur le rôle des phénomènes de cristallisation et de la transition de phase dans l'apparition du mottage des poudres de lactose. Deux cas ont particulièrement retenu notre attention: (1) des poudres de lactose monohydrate contenant une fraction de particules amorphes et (2) des échantillons de poudre anhydre composés des anomères α et β du lactose. Dans les deux cas, le mottage a été induite par l'exposition des échantillons à l'air humide, soit dans un dispositif de sorption dynamique de vapeur (SPS), soit par des tests accélérés utilisant deux appareils conçus et réalisés dans notre laboratoire (CLAIR & OLAF). Nos résultats ont montré que, dans les deux cas, la principale cause de prise en masse était la formation de lactose monohydrate, qui est la forme la plus stable parmi tous les polymorphes de lactose. Cependant, les mécanismes élémentaires, les étapes limites et la cinétique du processus de transformation étaient différents dans chaque cas. Les paramètres les plus déterminants étaient l’humidité relative et la température alors que la pression n’a pas eu d’effet significatif. La résistance mécanique des échantillons mottés était étroitement liée au taux et à la cinétique de cristallisation. Enfin, des simulations numériques basées sur la méthode des éléments discrets (DEM) de la résistance mécanique des échantillons mottés ont été réalisées. Le modèle permet de décrire le comportement des échantillons mottés soumis à des contraintes mécaniques de compression ou de traction. / This PhD study focuses on the fundamental problem of powder caking due to phase transition mechanisms. The project aims to study the impact of intrinsic factors (molecular structure of materials, physical and/or physicochemical properties, etc.) or environmental factors (storage conditions or process parameters) on the stability of the structure of powders. More precisely, our study has highlighted the preponderant role of the crystallization phenomenon and the transitions taking place between the different polymorphs of lactose. Emphasis was placed on the role of crystallization phenomena and phase transition on the advent of lactose powder caking. Two cases attracted particular attention: (1) lactose monohydrate powders containing a fraction of amorphous particles and (2) anhydrous powder samples composed of ð and anomers of lactose. In both cases, the caking was induced by exposure of the samples to moist air, either in a Dynamic Vapor Sorption device (SPS) or in accelerated caking tests using two home-made equipment (CLAIR & OLAF). Our results showed that in both cases, the main cause of caking was the formation of lactose monohydrate, which is the most stable form among all lactose polymorphs. However, the elementary mechanisms, the limiting steps and the kinetics of the transformation process were different in each case. The more influencing parameters were the relative humidity and the temperature whereas the pressure has no significant effect. The yield stress of caked samples was closely linked with crystallization extent and kinetics. Finally, numerical simulations based on Discrete Element Method (DEM) of mechanical resistance of caked samples were performed using the "beam model". The model allows describing the behavior of the caked samples subjected to compressive or tractive mechanical stresses.

Ce travail de thèse s'inscrit dans le cadre de recherches sur le retraitement futur du combustible nucléaire usé et notamment sur la gestion poussée des radionucléides à vie longue tels que les actinides mineurs. Il concerne la fabrication de pastilles céramiques de Couvertures Chargées en Actinides Mineurs (CCAM) dédiées à la transmutation pour les réacteurs à neutrons rapides. Les pastilles céramiques utilisées dans le milieu nucléaire sont classiquement fabriquées en utilisant les procédés issus de la métallurgie des poudres. En raison de la pulvérulence des précurseurs poudres utilisés et de la forte radioactivité des actinides mineurs et notamment de l’américium, un procédé « sans poudre » innovant a été proposé. Ce procédé prévoit la fabrication de pastilles céramiques en utilisant des précurseurs oxydes non plus sous forme de poudre mais sous forme de microsphères (Calcined Resin Microspheres Pelletization, CRMP). Le principe de ce procédé consiste à compacter sous forme de pastilles des microsphères d’oxyde puis à fritter le comprimé obtenu.La première partie de cette étude concerne la synthèse et la caractérisation de précurseurs oxyde sous la forme de microsphères sub-millimétriques d’oxyde de lanthanides (simulants les actinides) par le procédé aux résines. Différents lots de microsphères ont pu être synthétisés afin de mieux appréhender l’influence de certains paramètres de synthèse, tel que la température de calcination, le tri en taille de la résine ou encore la teneur en lanthanides dans le cas des oxydes mixtes ont été investigués dans le but de déterminer leur impact sur les propriétés microstructurales et mécaniques de microsphères d’oxyde.L'étude aborde dans un deuxième temps la mise en forme des microsphères d’oxyde à travers une approche à la fois expérimentale et numérique. L’approche numérique utilise la méthode éléments discrets (Discrete Element Method, DEM), bien adaptée pour ces milieux granulaires. Les microsphères d’oxyde, prises individuellement, sont caractérisées mécaniquement notamment à travers la mesure de leur résistance à l'écrasement. Une mise en relation des conditions de synthèse et des tenues mécaniques des microsphères a été entreprise afin de comprendre l'impact de ces paramètres de synthèse sur le comportement mécanique du matériau. La compaction en matrice de lots de ces microsphères sous forme de pastille a été étudiée. En particulier, la compressibilité d'un certain nombre de microsphères a été analysée expérimentalement puis simulée par la DEM en mesurant la densification de la pastille en cru en fonction de la contrainte axiale appliquée et en décrivant l’évolution de sa microstructure. / One option envisioned for the future management of high level nuclear waste is the transmutation of minor actinides into short-lived fission products in sodium fast reactor. This route requires the development of pellet fabrication processes to prepare Minor Actinide Bearing Blanket (MABB) for the transmutation of americium.Currently, those ceramic pellets are produced by powder metallurgy processes involving numerous grinding and milling steps that generate very fine and highly contaminating and irradiating particles. A viable option for reducing the amount of those fine particles would be to develop a dustless process by working on much coarser particles. In this context, this study is concerned with the pelletization of porous and spherical lanthanides oxide precursors (surrogates of actinides). The present work uses both experimental data and numerical simulations to optimize the pelletization step. The final aim is to obtain, after sintering, homogeneous, dense and undistorted ceramic pellets.Firstly, this study concerns the synthesis and characterisation of these oxide microspheres precursors by the Weak Acid Resin process, which consists in loading beads of ion exchange resin with lanthanides cations and mineralizing the metal loaded resin leads into sub-millimetric-sized oxide microspheres. Comprehensive characterisations of the microstructure and mechanical properties of oxide microspheres have been carried out to better understand their behaviour into the matrix when producing pellets.Secondly, the mechanical properties of a single microsphere were investigated in order to better understand its behaviour during compaction steps. They were also analysed using multi-scale simulations based on the Discrete Element Method (DEM), which is well suited for such particulate materials. In a second approach, compaction studies were carried out in a three parts die to characterize the mechanical behaviour under pressure of a large number of oxide precursors. The behaviour of several microspheres in the matrix was finally simulated using DEM in order to describe interactions between microspheres and to have a better understanding of their evolution during pressing.

With the developments in computer technology, the numerical methods are used widely in geotechnical engineering. Finite element and finite difference are the most common methods used to simulate the behavior of soil and rock. Although the reliability of these methods are proven in several fields of application over the years, they are not equally satisfactory in every case and require sophisticated constitutive relations to model the discontinuous behavior of geomaterials since they assume the material is continuum or the location of discontinuum is predictable. The Discrete Element Method (DEM) has an intensive advantage to simulate discontinuity. This method is relatively new and still under development, yet it is estimated that it will replace of the continuum methods largely in geomechanics in the near feature. In this thesis, the theory and background of discrete element method are introduced, and its applicability in bearing capacity calculation of shallow foundations is investigated. The results obtained from discrete element simulation of bearing capacity are compared with finite element analysis and analytical methods. It is concluded that the DEM is a promising numerical analysis method but still have some shortcomings in geomechanical applications.

Elasto-plastic shock waves in high-velocity compaction of spherical metal particles are the focus of this thesis which consists of four papers (A-D). The compaction process is modeled by a discrete element method while using elastic and plastic loading, elastic unloading and adhesion at contacts. Paper A investigates the dynamic compaction of a one-dimensional chain of homogenous particles. The development of the elasto-plastic shock waves, its propagation and influence on the compaction process are examined. Simulations yield information on the contact behavior, velocity of the particle and its deformation during dynamic compaction. Effects of changing loading parameters on the compaction process are also discussed. Paper B addresses the non-homogeneity in a chain having; particles of different sizes and materials, voids between the particles and particles with/without adhesion between them. Simulations show transmission and reflection of elasto-plastic shock wave during compaction process. The particle deformation during incident and reflected shocks and particle velocity fluctuations due to voids between particles are simulated. The effects of adhesion on particles separation during unloading stage are also discussed. Paper C develops a simulation model for a high-velocity compaction process with auxiliary pistons, known as relaxation assists, in a compaction assembly. The simulation results reveals that the relaxation assists offer; smooth compaction during loading stage, prevention of  the particle separation during unloading stage and conversion of  higher kinetic energy of hammer into particles deformation.  Furthermore, the influence of various loading elements on compaction process is investigates. These results support the findings of experimental work. Paper D further extends the one-dimensional case of Paper A and B into two-dimensional assembly of particles while adding friction between particles and between particles and container walls. Three particular cases are investigated including closely packed hexagonal, loosely packed random and a non-homogenous assembly of particles of various sizes and materials. Consistent with the one-dimensional case, primary interest is the linking of particle deformation with the elasto-plastic shock wave propagation. Simulations yield information on particle deformation during shock propagation and change in overall particles compaction with the velocity of the hammer. The force exerted by particles on the container walls and rearrangement of the loosely packed particles during dynamic loading are also investigated. Finally, the effects of presence of friction and adhesion on both overall particles deformation and compaction process are simulated. / QC 20110311

Ce travail concerne l'analyse de la vulnérabilité des structures de protection et des ouvrages sensibles en béton soumis à des actions dynamiques sévères (impacts, explosions) dues à des risques anthropiques d'origine accidentelle ou non. L'objet est la mise au point d'outils prévisionnels de simulation capables de décrire de manière objective le comportement dynamique du béton. Pour cela, une approche numérique novatrice reposant sur la méthode des Eléments Discrets (MED) est développée. Une première partie de cette thèse concerne la simulation des essais quasi-statiques de compression et traction uniaxiales. Une loi de transfert de moment (LTM) a été introduite pour pallier au problème de fragilité en compression simple. Ensuite, la procédure d'identification des paramètres du modèle modifié a été optimisée pour bien reproduire le comportement macroscopique du béton. Enfin, le modèle a été validé en représentant correctement le comportement quasi-statique de plusieurs types de béton. La deuxième partie du travail traite la simulation des essais de traction dynamique du béton aux barres de Hopkinson. Les résultats ont montré la nécessité de prendre l'effet de vitesse de déformation dû au matériau pour bien reproduire le comportement expérimental. Ensuite, Les paramètres du modèle permettant de reproduire cet effet de vitesse ont été identifiés. Enfin, des essais avec des taux de déformation très élevés ont été simulés et les résultats numériques ont été en accord avec le comportement observé expérimentalement. / This work concerns the analysis of the vulnerability of sensitive concrete structures subjected to severe dynamic actions such as impacts due to natural hazards or human factors. The object is to develop a numerical tool that can describe objectively the dynamic behaviour of concrete. Then, a 3D discrete element method (DEM) was developed and used to perform the analysis. The first part of this thesis focuses on the simulation of quasi-static uniaxial compression and traction tests. A moment transfer law (MTL) was introduced to overcome the problem of brittle compressive behavior. Then, the identification procedure of the modified DEM model has been optimized in order to reproduce very well the macroscopic behaviour of concrete. Finally, the model has been validated by representing properly the real quasi-static behavior of different types of concrete. The second part of the study deals with the simulation of the dynamic Hopkinson traction bar tests of concrete. The results showed that a local rate effect has to be introduced to reproduce the strain rate dependency, which would then be a material-intrinsic effect. Then, the parameters of the model have been identified. Finally, simulations were run at high strain rates and showed consistent results with respect to experimental behaviour.

This thesis reports recent numerical investigation of the rheological behavior of fresh concrete using the Distinct Element Method (DEM). Some relevant questions of the concrete rheology e.g. the influence of the concrete composition on the rheological behavior of the fresh concrete, the experimental determination of the Bingham rheological constants as well as the use of these constants in the numerical simulation were discussed thoroughly. An important topic of the performed investigation was the development of the numerical model for fresh concrete which enables simple, fast and stable predictive simulation of different technological operations with fresh concrete. Firstly, in a literature survey, the state-of-the-art of the numerical simulation of fresh concrete was presented and critically discussed in order to show advantages and disadvantages of other methods and modeling approaches. Open (unsolved) questions were highlighted and the basis for their investigation is created within this thesis. Fundamental concepts of the rheology were then presented and the basic rheological models of viscoelastic materials were considered; the rheological behaviors of different types of concretes were presented and its influencing factors were discussed. Additionally main methods for scientific investigation and testing of the fresh concrete were shown. The test methods were critically discussed in order to select the test, which has been used as a reference experimental test for the numerical simulations. Chosen reference experimental test was the slump flow test. The slump flow test was thoroughly analyzed and an analytical solution was developed which helps to interpret the results of measurements and provides a link between rheological constants and measured quantities. In a further step an extensive experimental program was carried out in order to investigate the rheological behavior of fresh concrete and get the input data for numerical simulation. Firstly, the experiments on macrolevel were performed. Here the rheological behavior of the fresh concrete flow in different tests was investigated (slump and slump flow tests, L-Box). Further, the experiments on mesolevel with polymer on Carbopol basis and mortar were developed and performed in order to investigate the interaction between distinct particles suspended in a fluid matrix. The necessary material parameters, especially those representative of the fluid suspension micromechanical behavior, i.e. the force-displacement relationship, yield force and bond strength, were determined by these experiments. The slump flow test was used as the basic test to calibrate the model for fresh concrete (key data: slump value, slump flow diameter (for concretes with a soft consistency) and the time of spreading). Thus, the decisive phenomena of the fresh concrete flow were highlighted, control points for a contact model were selected and the initial input data for the development of the contact model was obtained. Next, the user-defined contact model was developed and implemented into the Particle Flow Code ITASCA. The contact model was completely described and its limitations discussed. Then, the set of numerical tools was developed, which enable simplified and stable numerical simulation of the fresh concrete with particular behavior, i.e. automatic generation of the concrete with given particle grading, amount of fibers and air, automatic recalculation of the micromechanical parameters of the contact model from given initial yield stress and plastic viscosity. The model was calibrated by slump flow test simulations and validated by corresponding analytical approach. Further, the role of different model parameters was investigated by simulating the slump flow test. Furthermore, for verification of the model several additional experiments were simulated, i.e. L-Box and LCPC-box test. The results of modeling were compared with experimental results and discussed in detail. All numerical simulations provide qualitatively as well as quantitatively correct results and hence adequately represent the phenomena observed in real experiments. The thesis closes with general conclusions and outlook of the work. In the future, the developed contact model and tools of the “Virtual concrete laboratory” could be modified in order to extend the potential of the laboratory to cover such properties as thixotropic behavior of fresh concrete or simulating hardening of the concrete and behavior of the hardened concrete.