Studies of nucleation in freezing nanoparticles usually assume that the embryo of the solid phase is completely wet by the liquid and forms in the core of the droplet. However, recent experiments and computer simulations have suggested that some nanoparticles start nucleating at the liquid-vapor interface of the drop in a pseudoheterogeneous process. The goal of the present work is to propose phenomenological models suitable for the study of surface nucleation in nanoparticle systems that can be used to understand the contributions of the various surface phenomena, such as surface and line tensions, to the nucleation barrier.<p/> The nucleation barrier for the freezing of a 276 atom gold cluster is calculated using Monte Carlo simulation techniques while previous simulation studies of a 456 atom gold cluster are extended in order to find the probability that the embryo forms in the surface or core of the nanoparticle. These calculations confirm that the crystal embryo forms at the liquid-vapor interface. Geometric studies measuring the liquid-solid and solid-vapor surface areas of the embryo suggest that it changes shape as it becomes larger and grows in towards the core of the droplet.<p/> Three phenomenological models that are based on the capillarity approximation and can account for surface nucleation are proposed. These models highlight the importance of accounting for the surface curvature contributions related to the Tolman length and the presence of the three phase contact line in calculating the nucleation free energy barrier. In some cases, the models are able to reproduce the qualitative properties of the free energy barriers obtain from simulation but numerical fits of the models generally result in estimates of the solid-liquid surface tension that are lower than the values expected on the basis of partial wetting in the bulk.<p/> Finally, a semi-phenomenological model approach to nucleation is proposed where the usual phenomenological expression for the free energy barrier is retained, but where the geometric prefactors are obtained from molecular simulation of the embryo. This method is applied to nucleation in the gold cluster and to the freezing of a bulk Lennard-Jones liquid.<p/>

Studies of nucleation in freezing nanoparticles usually assume that the embryo of the solid phase is completely wet by the liquid and forms in the core of the droplet. However, recent experiments and computer simulations have suggested that some nanoparticles start nucleating at the liquid-vapor interface of the drop in a pseudoheterogeneous process. The goal of the present work is to propose phenomenological models suitable for the study of surface nucleation in nanoparticle systems that can be used to understand the contributions of the various surface phenomena, such as surface and line tensions, to the nucleation barrier.<p/> The nucleation barrier for the freezing of a 276 atom gold cluster is calculated using Monte Carlo simulation techniques while previous simulation studies of a 456 atom gold cluster are extended in order to find the probability that the embryo forms in the surface or core of the nanoparticle. These calculations confirm that the crystal embryo forms at the liquid-vapor interface. Geometric studies measuring the liquid-solid and solid-vapor surface areas of the embryo suggest that it changes shape as it becomes larger and grows in towards the core of the droplet.<p/> Three phenomenological models that are based on the capillarity approximation and can account for surface nucleation are proposed. These models highlight the importance of accounting for the surface curvature contributions related to the Tolman length and the presence of the three phase contact line in calculating the nucleation free energy barrier. In some cases, the models are able to reproduce the qualitative properties of the free energy barriers obtain from simulation but numerical fits of the models generally result in estimates of the solid-liquid surface tension that are lower than the values expected on the basis of partial wetting in the bulk.<p/> Finally, a semi-phenomenological model approach to nucleation is proposed where the usual phenomenological expression for the free energy barrier is retained, but where the geometric prefactors are obtained from molecular simulation of the embryo. This method is applied to nucleation in the gold cluster and to the freezing of a bulk Lennard-Jones liquid.<p/>

Grâce à leurs bonnes propriétés mécaniques spécifiques, les matériaux cellulaires architecturés présentent un fort intérêt pour répondre aux problématiques du secteur aéronautique. Cependant, la modélisation d'une structure macroscopique incluant un matériau cellulaire nécessite, soit de modéliser complètement l'architecture à l'échelle mésoscopique - ce qui est coûteux en temps de calcul - soit d'utiliser un Milieu Homogène Equivalent (MHE). Ainsi, cette thèse propose de caractériser un matériau cellulaire modèle constitué d'un empilement de tubes, selon un motif carré ou hexagonal, puis d'identifier un modèle phénoménologique rendant compte du comportement mécanique inélastique du matériau. Dans un premier temps, le matériau est caractérisé sous chargements multi-axiaux à l'aide de simulations éléments finis périodiques en petites déformations. Le comportement homogénéisé en petites déformations est ensuite utilisé pour l'identification d'une Loi Homogène Equivalente (LHE) compressible et anisotrope, qui permet la modélisation de structures sandwichs en remplaçant le coeur cellulaire par son MHE. Une comparaison est réalisée entre les réponses mécaniques des simulations de référence complètement maillées et celles utilisant l'approche par MHE, validant ainsi la pertinence de la méthode multi-échelle de modélisation proposée. La caractérisation en grandes déformations des deux types d'empilement est ensuite menée. D'abord, les effets de bords et les instabilités qui gouvernent le comportement macroscopique sont étudiés. Puis, après une étude du volume élémentaire représentatif des empilements, la caractérisation du comportement inélastique par la technique de l'homogénéisation périodique est réalisée. Le comportement adoucissant en compression de l'empilement hexagonal est ainsi étudié. Finalement, une extension des LHE identifiées en petites déformations est proposée pour rendre compte du comportement en compression du matériau observé en grandes déformations. / Cellular materials have excellent specific properties, which make them attractive for aeronautical applications. However, modelling macroscopic structures including a cellular material is either very costly in terms of computational time if the whole mesoscopic structure is considered or a Homogeneous Equivalent Medium (HEM) has to be used. This Ph.D. dissertation presents, the characterisation of a cellular material built from a stacking of tubes with a square or hexagonal based pattern and the identification of a phenomenological model of their inelastic mechanical behaviour. First, the material is characterised for multi-axial loadings through a periodic finite element model in small deformations for each tube stacking pattern. The macroscopic behaviour is then used to identify a compressible anisotropic Homogeneous Equivalent Law (HEL). Within the infinitesimal strain hypothesis, a comparison is carried out between reference full scale models and HEM based ones of sandwich structures with a cellular core, confirming the relevance of the proposed multi-scale method. Then, the mechanical behaviour of each tube stacking is characterised for large deformations in order to study the influence of the boundary size effects and the instabilities in the core on the macroscopic behaviour of sandwich structures. After a study on the representative volume element, the macroscopic inelastic behaviour is characterised through the periodic homogenisation technique, especially the softening observed in compression for the hexagonal pattern. Finally, an extension of the HELs identified in small deformations is proposed to model the behaviour observed in large deformations.

O presente trabalho analisa criticamente um conjunto de resultados sobre a fundamentação, simulação e aplicabilidade de alguns modelos fenomenológicos para Vibração Induzida por Vórtices, ou VIV. São estudados os modelos de Hartlen&Currie, 1970, de Iwan&Blevins, 1975, e de Facchinetti&de Langre&Biolley, 2004. O estudo foi desenvolvido com os propósitos: fundamentar, com base em resultados teóricos e experimentais existente na literatura, a representatividade do fenômeno de VIV através desta classe de modelos; obter resultados da simulação numérica destes modelos para posterior confrontação com dados experimentais disponíveis na literatura; discutir a possibilidade de aplicação destes modelos na prática da engenharia. A fundamentação dos modelos fenomenológicos é obtida do trabalho de Aranha, 2004, que conclui pela possibilidade teórica de representar os fenômeno de VIV através de um conjunto de equações diferenciais simplificadas. Este fato traz embasamento aos modelos fenomenológicos, que partem deste princípio de simplificação e resultam em equações similares àquelas desenvolvidas por Aranha. Para desenvolver a simulação numérica dos três modelos foi idealizado um programa de computação científica capaz de resolver numericamente as equações diferenciais ordinárias dos modelos, interpolar os resultados da simulação, efetuar a transformada de Fourier dos resuldados interpolados e apresentar em um documento formato PDF um relatório detalhado da simulação. Este relatório permite a análise direta dos dados sem necessidade de pós-processamento, contendo gráficos de séries temporais, espectro de frequência e diversos gráficos usualmente encontrados na literatura especializada de VIV. Por sua vez, para discutir a aplicabilidade dos modelos estudados é feita uma compilação da fenomenologia de VIV e os resultados experimentais são comparados com os resultados da simulação. A comparação dos resultados das simulações com dados experimentais conclui que é bastante limitada a aplicabilidade dos modelos fenomenológicos. Apenas o modelo de Iwan&Blevins foi capaz de reproduzir a amplitude de pico constatada experimentalmente. No contexto mais amplo de VIV, o trabalho observa que o estudo de modelos fenomenológicos voltou a ser uma área de fértil desenvolvimento científico. / The present work develops a critical analysis of a family of results emerging from the systematic study of phenomenological models in Vortex Induced Vibration, VIV. The study covers the theoretical foundation, numerical simulation and the applicability of several fenomenological models already available in the literature. Amongst the models presented here are those from Hartlen&Currie, 1970, Iwan&Blevins, 1975, and Facchinetti&de Langre&Biolley, 2004. The purposes are threefold: based in theoretical and experimental results in the literature, to express the ability of those models in representing de VIV phenomena; to obtain numerical simulations results in order to compare them with experimental data; to address the practical application of the studied models in real engineering cases. The fundamental basis of the phenomelogical models was borrowed from the work of Aranha, 2004, which lead to the feasibility of modelling the VIV phenomena by a set of simplified differential equations. The phenomenological models have been using simplified equations since their beginning and now the work of Aranha can be used to give the phenomenological models a sound basis. The numerical simulator was directly implemented in a scientific program specialy designed for this purpose. The resulting software can numericaly solve the models' differential equations, interpolate the results, transpose the results from time domain to frequency domain and finally present a complete report in PDF format. The report allows the direct analysis of the models dispensing with the need of any further task. In order to address the applicability issues, a fairly complete compilation of the current available experimental data on VIV was done. The compiled data was then compared with the results from the numerical simulation of the models. The main conclusion of this last step is that the phenomenological models analysed are strongly limited and do not represent the VIV characteristics well enough. An exception is made to the Iwan&Blevins models for its capacity to predict the peak vibration amplitude. In a broad sense, this work points the phenomenological modelling of VIV as an active field of research.

O presente trabalho analisa criticamente um conjunto de resultados sobre a fundamentação, simulação e aplicabilidade de alguns modelos fenomenológicos para Vibração Induzida por Vórtices, ou VIV. São estudados os modelos de Hartlen&Currie, 1970, de Iwan&Blevins, 1975, e de Facchinetti&de Langre&Biolley, 2004. O estudo foi desenvolvido com os propósitos: fundamentar, com base em resultados teóricos e experimentais existente na literatura, a representatividade do fenômeno de VIV através desta classe de modelos; obter resultados da simulação numérica destes modelos para posterior confrontação com dados experimentais disponíveis na literatura; discutir a possibilidade de aplicação destes modelos na prática da engenharia. A fundamentação dos modelos fenomenológicos é obtida do trabalho de Aranha, 2004, que conclui pela possibilidade teórica de representar os fenômeno de VIV através de um conjunto de equações diferenciais simplificadas. Este fato traz embasamento aos modelos fenomenológicos, que partem deste princípio de simplificação e resultam em equações similares àquelas desenvolvidas por Aranha. Para desenvolver a simulação numérica dos três modelos foi idealizado um programa de computação científica capaz de resolver numericamente as equações diferenciais ordinárias dos modelos, interpolar os resultados da simulação, efetuar a transformada de Fourier dos resuldados interpolados e apresentar em um documento formato PDF um relatório detalhado da simulação. Este relatório permite a análise direta dos dados sem necessidade de pós-processamento, contendo gráficos de séries temporais, espectro de frequência e diversos gráficos usualmente encontrados na literatura especializada de VIV. Por sua vez, para discutir a aplicabilidade dos modelos estudados é feita uma compilação da fenomenologia de VIV e os resultados experimentais são comparados com os resultados da simulação. A comparação dos resultados das simulações com dados experimentais conclui que é bastante limitada a aplicabilidade dos modelos fenomenológicos. Apenas o modelo de Iwan&Blevins foi capaz de reproduzir a amplitude de pico constatada experimentalmente. No contexto mais amplo de VIV, o trabalho observa que o estudo de modelos fenomenológicos voltou a ser uma área de fértil desenvolvimento científico. / The present work develops a critical analysis of a family of results emerging from the systematic study of phenomenological models in Vortex Induced Vibration, VIV. The study covers the theoretical foundation, numerical simulation and the applicability of several fenomenological models already available in the literature. Amongst the models presented here are those from Hartlen&Currie, 1970, Iwan&Blevins, 1975, and Facchinetti&de Langre&Biolley, 2004. The purposes are threefold: based in theoretical and experimental results in the literature, to express the ability of those models in representing de VIV phenomena; to obtain numerical simulations results in order to compare them with experimental data; to address the practical application of the studied models in real engineering cases. The fundamental basis of the phenomelogical models was borrowed from the work of Aranha, 2004, which lead to the feasibility of modelling the VIV phenomena by a set of simplified differential equations. The phenomenological models have been using simplified equations since their beginning and now the work of Aranha can be used to give the phenomenological models a sound basis. The numerical simulator was directly implemented in a scientific program specialy designed for this purpose. The resulting software can numericaly solve the models' differential equations, interpolate the results, transpose the results from time domain to frequency domain and finally present a complete report in PDF format. The report allows the direct analysis of the models dispensing with the need of any further task. In order to address the applicability issues, a fairly complete compilation of the current available experimental data on VIV was done. The compiled data was then compared with the results from the numerical simulation of the models. The main conclusion of this last step is that the phenomenological models analysed are strongly limited and do not represent the VIV characteristics well enough. An exception is made to the Iwan&Blevins models for its capacity to predict the peak vibration amplitude. In a broad sense, this work points the phenomenological modelling of VIV as an active field of research.

La prédiction de la géométrie d'une pièce mise en forme par déformation plastique grâce à un logiciel de calcul par éléments finis (EF) s'effectue en suivant séquentiellement différentes étapes : la caractérisation thermo-mécanique du matériau, la modélisation de son comportement et son intégration dans un logiciel EF, puis la mise en données et la simulation de l'opération de formage. La phase de modélisation consiste entre autre à identifier quel type de modèle de comportement est le plus approprié pour prédire les réactions du matériau lors de l'opération de formage. Ces modèles sont essentiellement développés dans le cadre de la mécanique des milieux continus (MMC). L'hypothèse forte, si ce n'est centrale, de la MMC consiste à considérer que les variables qui servent à déterminer le comportement du matériau sont continues et dérivables. Cependant, les connaissances les plus élémentaires de métallurgie indiquent que les grandeurs locales dans les matériaux métalliques sont discontinues. La majorité des modèles de comportement mécanique des matériaux métalliques repose sur la définition d'un volume élémentaire représentatif dont la taille est assez grande pour permettre une homogénéisation de la description du comportement en gommant l'influence des hétérogénéités localesL'objet de ces travaux est de montrer que la prise en compte des hétérogénéités locales dans la modélisation du comportement des matériaux métalliques est pertinente et contribue à l'amélioration de la prédiction des simulations d'opérations de mise en forme en élargissant le potentiel prédictif des modèles ainsi construits. Un modèle élasto-plastique prenant en compte les hétérogénéités locales est alors proposé. / To obtain a relevant shape of a formed part during its finite element simulation, several steps are needed: thermo-mechanical caracterization of the material, definition of the most relevant model and integration of this model in the FE software and finally after data converting and computing processes. The modelling step include, among other things, the identification of the most appropriate model to fit the experimental material behaviour. Those models are essentially developped within the framework of continuum mechanics (CM). A strong, if not the main assumption of the CM consists in considering that mechanical description variables are continuous and differentiable. However, the basic knowledge of metallurgy indicates that local data in metallic materials are discontinuous. For metallic materials, the majority of constitutive models are based on the definition of a representative elementary volume (REV). This REV is supposed to be large enough to erase the incidence of local heterogeneities. Then those constitutive models are assumed to be homogeneous.The aim of this work is to show that introducing local heterogeneities in the description of constitutive models is relevant and contribute to improve the simulation accuracy. Those models also provide an enlargement of the simulation predictive potential. Then an elasto-plastic model, based on local heterogeneities description, is proposed.

La génération de seconde harmonique magnétique (mSHG pour magnetic Second Harmonic Generation) est un phénomène physique très sensible apparaissant grâce aux brisures de symétrie aux niveaux des surfaces et interfaces des structures métalliques magnétiques. Elle constitue donc un outil puissant pour explorer ce type d'interfaces et des nanostructures. Dans ce travail, nous nous intéressons aux couplages et interactions entre la mSHG et les ondes électromagnétiques pouvant se propager en surface des matériaux. Un intérêt spécifique est porté sur l’ excitation de (i) plasmon polaritons de surfaces (SPP) dans des films métalliques en structures multicouches, (ii) d'anomalies de diffraction (dîtes de Wood) dans des nanostructures métalliques périodiques. Pour étudier l'influence de l'excitation linéaire et non-linéaire des SPP sur la mSHG, l'intensité du signal réfléchi par génération de seconde harmonique (SH) et le contraste magnétique lié à ce signal ont été mesurés par la technique de l'effet Kerr magnéto-optique transverse (MOKE) en fonction de l'angle d'incidence. Via l'utilisation de sources lasers femtosecondes émettant dans le proche infrarouge, domaine spectral où les variations de la dispersion des SPP et du coefficient d'amortissement sont significatives, nous avons pu distinguer les différentes contributions linéaires et non-linéaires aux processus d'excitation. Ce travail de thèse a ainsi permis de montrer que l’accord de phase entre la mSHG et les ondes électromagnétiques de surface peuvent contribuer très efficacement à l'augmentation des signaux SH et de contraste magnétique associé. / Owing to surface and interface sensitivity, the magnetic Second Harmonic Generation (mSHG) represents a useful tool to probe magnetic interfaces and nanostructures. This work investigates the coupling and interaction of the mSHG with electromagnetic waves propagating along the surface. Two types of surface waves have been studied: (i) surface plasmon polaritons (SPP) at surfaces of metallic thin films and multilayers, and (ii) the diffraction anomaly at the surface of periodically arranged metallic nanostructures. To study influence of linear and nonlinear excitation of surface waves on the mSHG, the reflected second harmonic (SH) intensity and the magnetic SH contrast in the transverse magneto-optical geometry were measured as a function of the angle of incidence. The use of different femtosecond light sources in the near-infrared optical range, where the SPP dispersion and damping exhibit significant variations, made it possible to disentangle linear and nonlinear contributions to the excitation of surface waves. In this thesis, it is proven that phase-matching of the mSHG and surface electromagnetic waves can lead to the enhancement of both the SH yield and the nonlinear magneto-optical signal. These results are important for controlling of the nonlinear magneto-optical response and could impact the development of magnetic storage devices, label-free biosensors and nonlinear magneto-optical switches.