With the significant potential of microalgae as a major biofuel source of the future, a considerable scientific attention is attracted towards the field of biotechnology and bioprocess engineering. Nevertheless the current photobioreactor (PBR) design methods are still too empirical. With this work I would like to promote the idea of designing a production system, such as a PBR, completely \emph{in silico}, thus allowing for the in silico optimization and optimal control determination. The thesis deals with the PBR modeling and simulation. It addresses two crucial issues in the current state-of-the-art PBR modeling. The first issue relevant to the deficiency of the currently available models - the incorrect or insufficient treatment of either the transport process modeling, the reaction modeling or the coupling between these two models. A correct treatment of both the transport and the reaction phenomena is proposed in the thesis - in the form of a unified modeling framework consisting of three interconnected parts - (i) the state system, (ii) the fluid-dynamic model and (iii) optimal control determination. The proposed model structure allows prediction of the PBR performance with respect to the modelled PBR size, geometry, operating conditions or a particular microalgae strain. The proposed unified modeling approach is applied to the case of the Couette-Taylor photobioreactor (CTBR) where it is used for the optimal control solution. The PBR represents a complex multiscale problem and especially in the case of the production scale systems, the associated computational costs are paramount. This is the second crucial issue addressed in the thesis. With respect to the computational complexity, the fluid dynamics simulation is the most costly part of the PBR simulation. To model the fluid flow with the classical CFD (Computational Fluid Dynamics) methods inside a production scale PBR leads to an enormous grid size. This usually requires a parallel implementation of the solver but in the parallelization of the classical methods lies another relevant issue - that of the amount of data the individual nodes must interchange with each other. The thesis addresses the performance relevant issues by proposing and evaluation alternative approaches to the fluid flow simulation. These approaches are more suitable to the parallel implementation than the classical methods because of their rather local character in comparison to the classical methods - namely the Lattice Boltzmann Method (LBM) for fluid flow, which is the primary focus of the thesis in this regard and alternatively also the discrete random walk based method (DRW). As the outcome of the thesis I have developed and validated a new Lagrangian general modeling approach to the transport and reaction processes in PBR - a framework based on the Lattice Boltzmann method (LBM) and the model of the Photosynthetic Factory (PSF) that models correctly the transport and reaction processes and their coupling. Further I have implemented a software prototype based on the proposed modeling approach and validated this prototype on the case of the Coutte-Taylor PBR. I have also demonstrated that the modeling approach has a significant potential from the computational costs point of view by implementing and validating the software prototype on the parallel architecture of CUDA (Compute Unified Device Architecture). The current parallel implementation is approximately 20 times faster than the unparallized one and decreases thus significantly the iteration cycle of the PBR design process.

Three-dimensional woven composites were developed to improve the through-thickness properties which conventional two-dimensional laminate composites currently lack. However, these textile composites generally show lower in-plane mechanical properties due to fibre crimping, and also encounter modelling difficulties due to the complex geometries. In this thesis, the static and fatigue mechanical behaviour of several types of 3D woven composites were experimentally characterised, the influence of the weave architecture on the mechanical performance was revealed, and meso/macro scale numerical models with improved failure criteria were developed to simulate the tensile behaviour of the 3D woven composites. The mechanical characterisation was conducted on six woven structures under tension, compression, and flexural loading, and were also carried out on two weaves under open-hole quasi-static tensile and fatigue loading. Digital image correlation and thermoelastic stress analysis were used to characterise the strain and damage development during static and fatigue loading. The testing results showed that the angle-interlock weave W-3 had higher in-plane quasi-static properties, lower notch sensitivity, higher fatigue damage resistance, but lower delamination resistance. The meso-scale model was developed on the unit cell of the woven structure and the macro-scale model (mosaic model) was created on the testing samples. Both un-notched and notched tensile behaviour were modelled for the angle-interlock weave W-3 and a one-by-one orthogonal weave W-1, and the difference between the predicted and experimental results was within 16% for the unit cell models and within 21% for the mosaic models. A modified failure criterion was developed to better simulate the damage behaviour of the notched macro-scale model and improved the predicted notched strength by 10-20%. Whilst further experimental investigation and improvement in the modelling techniques are still required, the data presented in this thesis provided an essential update for the current 3D woven composites research, and the presented models offered the potential to predict the damage behaviour of large 3D woven structures.

The subject of this work is the development of accurate frameworks to describe the electrochemical promotion of catalysis (EPOC) phenomenon. EPOC, also known as non-Faradaic electrochemical modification of catalytic activity (NEMCA), refers to the enhancement of the catalytic performance by application of current or potential in a catalyst/support system. Although this technology is of increasing interest nowadays in the field of modern electrochemistry and exhibits a great industrial potential, there are still just a few commercial applications, partly because the addressed phenomenon is not fully understood and has not been modelled to allow robust system design and control. For this purpose, a systematic multi-dimensional, isothermal, dynamic model is developed to address the EPOC phenomenon using the electrochemical oxidation of CO over Pt/YSZ as an illustrative system. The formulated model is based on partial differential equations (PDEs) accounting for the simulation of the mass and charge transport as well as the electrochemical phenomena taking place at the triple phase boundaries (TPBs, where the gas phase, the catalyst and the support are all in contact) implemented through a commercial finite element method (FEM) software (COMSOL Multiphysics). The constructed model is used in conjunction with experimental data for parameter estimation purposes, and a validated model is obtained. The results demonstrate that the effect in such a system is strongly non-Faradaic, with Faradaic rates 3 orders of magnidute lower than the non-Faradaic ones. The formulated model is extended to describe the various processes taking place in the electrochemically promoted CO combustion system at their characteristic length-scales. The proposed framework couples a macroscopic model simulating charge transport as well as electrochemical phenomena occuring at the TPBs implemented through a FEM-package and an in-house developed efficient implementation of the kinetic Monte Carlo method (kMC) for the simulation of reaction-diffusion micro-processes on the catalyst. Dynamic communication of macro- and micro-scopic models at the TPBs results in the construction of an integrated multi-scale system. Comparison between the multi-scale framework and a fully macroscopic model is carried out for several sets of operating conditions and differences between the two models steady-state outputs are presented and discussed. A detailed FEM/kMC model, regardless of accurately simulating the several phenomena at their appropriate length-scales, might not be suitable for large system simulations due to the high computational demand. To address this limitation, a computationally efficient coarse-graining methodology, the so-called gap-tooth method, is implemented. In this scheme the catalytic surface is efficiently represented by a small subset of the spatial domain (tooth) separated by gaps. While kMC simulations within each individual tooth (micro-lattice) are used to predict the corresponding evolution of the micro-processes, intelligent interpolation rules are employed to allow for the exchange (diffusion) of species between consecutive micro-lattices. A validated gap-tooth/kMC scheme is obtained and it is exploited for FEM/gap-tooth/kMC electrochemically promoted CO oxidation simulations achieving high computational savings.

Plutonium Uranium Reduction EXtraction (PUREX) technology is a solvent extraction process used to recover plutonium and uranium from spent nuclear fuel. The solvent system is composed of an aqueous nitric acid phase in contact with an organic phase made up of tributyl phosphate in an organic diluent. During the separation process, the PUREX solvent system is subject to an intense multi-component radiation field (gamma rays, alpha particles, beta particles, neutrons, and fission fragments) rendering it susceptible to radiolytic degradation, which reduces its performance. Despite the PUREX process being used for over sixty years, a complete quantitative mechanistic understanding of the radiolytic degradation processes is not available. Nitrous acid is the most significant radiolytic degradation product of nitric acid, especially as its chemical and physical properties alter the formulation of the PUREX solvent system. Furthermore, nitrous acid exhibits complex redox relationships with a number of actinides, with plutonium being of greatest concern to the performance of the PUREX process. A combination of experimental and computational (stochastic and deterministic) techniques have been used to investigate the radiolysis of the PUREX solvent system's aqueous phase, specifically the radiolytic formation of nitrous acid, and its conjugate base nitrite, as a function of solvent system formulation, absorbed dose (up to 1.7 kGy), and radiation quality (cobalt-60 gamma rays and alpha particles from plutonium and americium alpha decay). The research presented in this thesis focuses on: (i) the experimental radiation chemistry of solutions of nitric acid and sodium nitrate over the range of concentrations 1 × 10-3 to 6 mol dm-3, and (ii) the development of a multi-scale modelling approach for evaluating the radiolysis of aqueous systems in terms of reaction mechanisms. The experimental and modelling studies provide insight into the radiation chemistry of the PUREX solvent system's aqueous phase, mechanistically demonstrating how the radiation chemical yield of nitrous acid and nitrite is dependent upon the interplay between non-homogeneous radiation track chemistry and secondary bulk homogeneous chemistry. This interplay is influenced by low pH, the presence of chemical scavengers and redox active metal ions, and radiation quality. These findings will act as a benchmark for the development of advanced reprocessing schemes, which must seriously consider how modifications in solvent system formulation and fuel composition may affect this dynamic interplay, and ultimately the generation of secondary highly active liquid waste.

<p align="justify">In a context of restoration of historical masonry structures, it is crucial to properly estimate the residual strength and the potential structural failure modes in order to assess the safety of buildings. Due to its mesostructure and the quasi-brittle nature of its constituents, masonry presents preferential damage orientations, strongly localised failure modes and damage-induced anisotropy, which are complex to incorporate in structural computations. Furthermore, masonry structures are generally subjected to complex loading processes including both in-plane and out-of-plane loads which considerably influence the potential failure mechanisms. As a consequence, both the membrane and the flexural behaviours of masonry walls have to be taken into account for a proper estimation of the structural stability.</p> <p align="justify">Macrosopic models used in structural computations are based on phenomenological laws including a set of parameters which characterises the average behaviour of the material. These parameters need to be identified through experimental tests, which can become costly due to the complexity of the behaviour particularly when cracks appear. The existing macroscopic models are consequently restricted to particular assumptions. Other models based on a detailed mesoscopic description are used to estimate the strength of masonry and its behaviour with failure. This is motivated by the fact that the behaviour of each constituent is a priori easier to identify than the global structural response. These mesoscopic models can however rapidly become unaffordable in terms of computational cost for the case of large-scale three-dimensional structures.</p> <p align="justify">In order to keep the accuracy of the mesoscopic modelling with a more affordable computational effort for large-scale structures, a multi-scale framework using computational homogenisation is developed to extract the macroscopic constitutive material response from computations performed on a sample of the mesostructure, thereby allowing to bridge the gap between macroscopic and mesoscopic representations. Coarse graining methodologies for the failure of quasi-brittle heterogeneous materials have started to emerge for in-plane problems but remain largely unexplored for shell descriptions. The purpose of this study is to propose a new periodic homogenisation-based multi-scale approach for quasi-brittle thin shell failure.</p> <p align="justify">For the numerical treatment of damage localisation at the structural scale, an embedded strong discontinuity approach is used to represent the collective behaviour of fine-scale cracks using average cohesive zones including mixed cracking modes and presenting evolving orientation related to fine-scale damage evolutions.</p> <p align="justify">A first originality of this research work is the definition and analysis of a criterion based on the homogenisation of a fine-scale modelling to detect localisation in a shell description and determine its evolving orientation. Secondly, an enhanced continuous-discontinuous scale transition incorporating strong embedded discontinuities driven by the damaging mesostructure is proposed for the case of in-plane loaded structures. Finally, this continuous-discontinuous homogenisation scheme is extended to a shell description in order to model the localised behaviour of out-of-plane loaded structures. These multi-scale approaches for failure are applied on typical masonry wall tests and verified against three-dimensional full fine-scale computations in which all the bricks and the joints are discretised.</p>

Les fibres végétales utilisées comme renfort dans les matériaux composites présentent des propriétés mécaniques spécifiques concurrentielles par rapport à celles des fibres de verre. De plus, elles ont l'avantage d'être renouvelables et recyclables. Toutefois, leur principal inconvénient est leur sensibilité à l'humidité, ce qui a pour conséquence d'induire une baisse des propriétés mécaniques ainsi d'une décohésion de l'interface fibre/matrice. L'objectif principal de cette thèse est d'étudier l'influence de l'humidité sur le comportement hygro-mécanique de fibres de lin. La première partie de mes travaux a été consacrée à la caractérisation des propriétés hygroscopiques et mécaniques de la fibre et du composite. Dans la deuxième partie, deux modèles multi-échelles, l'un analytique et l'autre numérique, ont été développés pour l'estimation des propriétés hygro-mécaniques des fibres élémentaires de lin. Ils utilisent en partie pour données d'entrée, les propriétés identifiées dans la première partie. / Natural fibres used as reinforcement in composite materials present specific mechanical properties, which are comparable to glass fibres. In addition, they have the advantage of being renewable and recyclable. But, their main drawback is their inherent susceptibility to moisture expansion, which has the effect of inducing a decrease in mechanical properties, and of debonding and fracturing interface in the composite. The main aim of this thesis is to study the influence of humidity on hygro-mechanical behavior of flax fibres. The first part of my work was deal with the characterization of mechanical and hygroscopic properties of the fibre and the composite. The second part is dedicated to the development of two multiscale models, one analytical and one numerical. They have been developed for the estimation of hygro-mechanical properties of elementary flax fibres. Properties identified in the first part of the work are used as input data.

The present work applies the hybrid Voronoi cell finite element method (VCFEM) within geomechanics. Coupled seepage and deformation analysis using the VCFEM incorporating body forces allows accurate analysis of earth dams. The development of a novel approach for simulating granular material behaviour using the combined finite-discrete element method (VCFEM-DEM) provides new insights into strain localization in granular materials. Chapter 1 provides background including summary literature reviews for all concepts in the title including seepage analysis, micromechanical and continuum mechanics theory, Voronoi diagrams, finite elements (FEM), discrete elements (DEM) and combined FEM-DEM. Chapter 1 concludes by detailing the contributions of the present work. Chapter 2 presents the VCFEM for seepage analysis. The numerical examples include an investigation of mesh sensitivity and a comparison of conforming shape functions. Polygonal elements with more than four nodes show a decrease in mesh sensitivity in free surface problems, compared with four-node quadrilateral elements. The choice of conforming shape function within the VCFEM analysis did not affect the results. Chapter 3 formulates and applies the VCFEM-DEM, showing that strain localization effects in granular materials are important at all scales. The VCFEM-DEM captures shear banding in biaxial compression tests, demonstrating that global shear strains and inhomogeneities in the shear stress field present after consolidation are early precursors to the failure mode. At the field scale, strain localization can lead to significant non-uniformity in subsurface stress distribution owing to self-weight. Chapter 4 presents the coupled VCFEM for seepage and deformation. A practical example of the design of an earth dam demonstrates the application of general body forces within a hybrid formulation, notably lacking in the literature. Chapter 5 concludes by summarizing the key observations of the present work, and providing direction for future research. The Appendix provides additional details related to numerical integration within the VCFEM. / Thesis / Doctor of Philosophy (PhD) / The focus of the present work is the simulation of geomechanical behaviour at multiple scales. This ranges from simulating the interaction of grains of sand in a laboratory compression test to the seepage of water through and deformation of a large dam constructed of granular material. The simulations use a numerical tool called the Voronoi cell finite element method (VCFEM), which the present work extends to allow accurate analysis of the flow of fluid through a porous medium, deformation of a granular material under load and coupled analysis of these phenomena. The development and testing of this numerical tool for use in geomechanical analysis is itself a contribution. The present work also contains new insights into how localized stresses and strains in a granular material that are present well before the peak strength can have an important influence on the mode of failure.

Yes / The mechanical property enhancement of polymer reinforced with nano-thin clay platelets (of high aspect ratio) is associated with a high polymer-filler interfacial area per unit volume. The ideal case of fully separated (exfoliated) platelets is generally difficult to achieve in practice: a typical nanocomposite also contains multilayer stacks of intercalated platelets. Here we use numerical modelling to investigate how the platelet properties affect the overall mechanical properties. The configuration of platelets is modelled using a statistical interpretation of the Representative Volume Element (RVE) approach, in which an ensemble of "sample" heterogeneous material is generated (with periodic boundary conditions). A simple Monte Carlo algorithm is used to place non-intersecting platelets in the RVE according to a specified set of statistical distributions. The effective stiffness of the platelet-matrix system is determined by measuring the stress (using standard Finite Element analysis) produced as a result of applying a small deformation to the boundaries, and averaging over the entire statistical ensemble. In this work we determine the way in which the platelet properties (curvature, filling fraction, stiffness, aspect ratio) and the number of layers in the stack affect the overall stiffness enhancement of the nanocomposite. Thus, we bridge the gap between behaviour on the macroscopic scale with that on the scale of the nano-reinforcement, forming part of a multi-scale modelling framework.

Malgré l'intérêt croissant que représente dans le domaine de l'emballage alimentaire la conception raisonnée de structures composites aux propriétés de transfert contrôlées, la compréhension des transferts de gaz et de vapeurs avec l'ajout de particules dans des polymères reste complexe. En vue d'apporter un nouvel éclairage à ce verrou scientifique, les travaux de thèse se sont focalisés sur les trois parties suivantes : - contribuer à une meilleure compréhension des transferts de matière dans les composites. Pour ce faire, une analyse exhaustive des données expérimentales de transfert de gaz et de vapeurs disponibles dans la littérature a été menée pour les nano- et micro-composites et une comparaison de ces données a été réalisée avec des modèles de tortuosité, basés sur des paramètres géométriques ; - comprendre et modéliser la perméabilité dans des composites avec deux phases perméables. Pour cela, les transferts de vapeur d'eau dans un composite (fibre de paille/bio-polyester) chargé avec des particules perméables ont été mesurés et décrits en détail, et une comparaison de ces données avec des modèles analytiques issus d'autres champs disciplinaires, prenant en compte la perméabilité dans la particule et dans la matrice, a été menée. Cette étude a mis en avant le manque de modèles adaptés pour la prédiction de la perméabilité dans les composites contenant des particules perméables ; - développer une nouvelle approche multi-échelle pour la prédiction de la perméabilité dans des composites prenant en compte les propriétés de transfert dans les particules et dans la matrice polymérique avec une représentation 2D de la structure du composite. Afin d'atteindre un niveau satisfaisant de validation du modèle, la détermination des paramètres expérimentaux tels que la diffusion dans les particules doit être améliorée. Cette nouvelle approche de modélisation ouvre la voie à la création d'outils d'ingénierie inverse pour le design de structures composites, ajustés aux besoins des aliments en termes de propriétés barrières. / Despite the global growing interest in the food packaging field for the design of tailored composite structures with controlled mass transfer properties, the understanding of the modulation of the mass transfer properties with the incorporation of particles in polymer still remains very complex. In order to throw light on this scientific problem, the thesis work was focused on the following parts: - providing a better understanding of mass transfer in composites. In this purpose an analysis of all experimental gas and vapour permeability data available in the literature has been carried out in nano- and micro- composites and a comparison of these data with predictions from tortuosity models based on few geometrical inputs has been achieved; - performing a detailed study of water vapour mass transfer in composites (wheat straw fibres/bio-polyester). These data were compared with the prediction of bi-phasic analytical models coming from other disciplinary fields. This part of the work has highlighted the lack of comprehensive and complete models for the prediction of permeability in composite with permeable particles; - developing of an innovative multi-scale approach for the prediction of mass transfer in bi-phasic composites considering both the particle and the polymer matrix properties with realistic 2D geometry of the composite structures has been proposed. For the sake of reaching a satisfactory validation level of the model, some experimental improvements are still needed to increase the accuracy of input parameters such as diffusivity of the particles.This new modelling approach open the way for the creation of a reverse-engineering toolbox for the design of tailor made composites structures, tightly adjusted to barrier properties requirements of the packed food.

Ce travail a pour objectif d'analyser l'utilisation des fibres de lin en substitution aux fibres de verre dans les composites destinés au secteur du nautisme. Cette substitution nécessite une meilleure compréhension du cycle de vie des composites depuis la sélection des matériaux, incluant le procédé de mise en œuvre et jusqu'à leur vieillissement hydrique, principalement sous conditions réelles en service (exposition à l'eau, à la température et aux sollicitations mécaniques). Une étude préliminaire a été consacrée à la sélection des matériaux (tissus de fibre de lin et résine polyester) et à la comparaison entre deux procédés de fabrication des composites (infusion sous vide et thermocompression) sur la base de leurs propriétés morphologiques et mécaniques. Le comportement au vieillissement de composites renforcés de fibres de lin (CRFL) et de composites renforcés de fibres de verre (CRFV) est ensuite étudié. Un ensemble de moyens originaux ont été développés afin de suivre les évolutions de la morphologie (prise en eau, gonflement) et des propriétés mécaniques (statiques et dynamiques). Tout d'abord, le vieillissement hydrothermique des composites à base de fibre de lin et de verre est caractérisé jusqu'à leur saturation en eau. Cette étape est suivie de l'étude de la dessiccation afin de contrôler la réversibilité des propriétés physiques et mécaniques. Parallèlement, une étude particulière de ce travail a été consacrée au traitement des fibres afin de réduire leur hydrophilicité dans le composite. Le couplage hydro-thermo-mécanique est alors analysé en imposant une sollicitation de fluage au biocomposite en immersion. L'application de cette méthodologie révèle l'influence significative des sollicitations couplées. Contrairement à toute attente, la superposition d'une charge au vieillissement hydrothermique ralentit la baisse des propriétés élastiques en comparaison de la somme des effets dus aux vieillissements non-couplés. Enfin un modèle de calcul par éléments finis a été mis au point afin de prédire la diffusion hydrique au sein d'un matériau. Le modèle 2D développé intègre la morphologie réelle du composite et particulièrement l'organisation des fibres de lin dans le composite à plusieurs échelles. Cette modélisation représente la première étape dans la prédiction du comportement évolutif des biocomposites pour des conditions de vieillissement en service. / This work aims to address a complete analysis of the use of flax fibres to substitute glass fibres in composite materials designed for nautical applications. This substitution requires a better understanding of the composites life cycle: from materials selection and processing to its hydric ageing, especially under real conditions (exposition to water, temperature and mechanical loadings).A preliminary study is devoted to the selection of materials (flax fibre fabrics and polyester resin) and to the comparison between two methods for manufacturing composites (vacuum infusion and compression moulding) through their mechanical and the morphological properties.The ageing behaviour of flax fibre reinforced composites (FFRC) and glass fibre reinforced composites (GFRC) is then studied. A set of original methods have been implemented to monitor the evolving morphology (water uptake, swelling) and mechanical properties (statics and dynamics). First of all, hydrothermal ageing of glass and flax fibre composites is characterized until water saturation. This step is followed by the study of a drying phase in order to verify the reversibility of physical and mechanical properties. In parallel, a particular issue of this work was devoted to reducing the hydrophilicity of flax fibres into composites.The hydrothermo-mechanical coupling behaviour of biocomposites is then studied by imposing a creep solicitation to biocomposites immersed in water. The application of this methodology highlighted the significant influence of the coupled solicitations. Contrary to all expectations, superimposing a load during a hydrothermal ageing slows the loss of the elastic properties in comparison with the sum of the uncoupled ageing effects.Finally, a finite element model was stablished in order to predict the hydric diffusion within the composite material. The 2D model integrates the real morphology of composites, and particularly the organisation of the flax fibres in the matrix at different scales. This modelling represents the first step in predicting the evolving behaviour of biocomposites exposed to ageing conditions.