Compte tenu de la vulnérabilité de la population rurale de la région sahélienne aux aléas pluviométriques, et devant les ambitions de certains acteurs d’utiliser le levier de l’usage des terres pour contribuer à l’atténuation du changement climatique, il est important de comprendre les facteurs contribuant à la variabilité de la couverture végétale au Sahel.Une synthèse de la littérature expliquant l’évolution récente de la végétation au Sahel est donc d’abord présentée. Les études s’intéressant au paradigme qui souligne l’impact de l’usage des terres sur les précipitations en Afrique de l’Ouest évaluent principalement ces effets par le couplage de modèles dynamiques globaux de végétation – DGVM – avec des modèles de circulation générale. C’est à l’amélioration d’un tel DGVM, ORCHIDEE, développé à l’Institut Pierre Simon Laplace, que le reste du travail cherche à contribuer.Comme d’autres études ont montré qu’il était possible d’utiliser en première approximation les steppes pâturées et les jachères pour décrire le comportement global de la surface sahélienne, les écarts entre modèle et mesures sont caractérisés pour une jachère située à proximité de Wankama (Niger). Plus précisément, les forces et faiblesses de la paramétrisation et de la structure par défaut du modèle sont diagnostiqués, et l’importance de la réduction d’erreur permise par l’optimisation de certains des paramètres est donnée. En particulier, l’emploi d’une résolution aux différences finies de la diffusion de l’eau dans la colonne de sol est évalué, dans la mesure où cela permet de mieux simuler la réponse rapide du flux évaporatoire aux événements pluvieux que le schéma conceptuel utilisé par défaut dans ORCHIDEE.Le réalisme du modèle est également mesuré à l’échelle régionale, par la comparaison d’observations de NDVI GIMMS_3G à la couverture végétale simulée par le modèle en réponse à différents forçages climatiques . Si les modifications introduites au cours du travail ne permettent pas de mieux décrire les tendances de la végétation au cours des dernières décennies, tirer partie des leçons du présent travail pourra se révéler utile. Il en est de même des conclusions de l’étude de la transitivité des biais conditionnels du modèle réalisée avec Tao Wang et présentée en annexe B. / The evolution of the land-surface conditions is often assessed through the use of “dynamic global vegetation models”, as is shown in a review of the current understanding of the factors of variability and of the recent evolution of the vegetation cover in the Sahel. Such models are also coupled to atmospheric general circulation models to evaluate the land feedback on precipitation in monsoonal climates.Thus, the improvement of the skills of such surface models to simulate the radiative and turbulent fluxes between the land of surface and the atmosphere in the Sahel over a range of scales from hourly to multi-annual has a potential to have significant implications. This is especially true considering the vulnerability of the rural population of the region, which largely relies on rainfed agriculture and the interest on the evolution of the carbon stocks of ecosystems in the context of climate change. Such a work on the ORCHIDEE model is presented here. In complement to croplands, rangelands and fallows represent a large share of the sahelian landscapes and have intermediate characteristics between erosion glacis and acacia bushes. As such, their evolution (in terms of albedo, roughness length,…) may be used to study the Sahel ecosystem behaviour as a first approximation. Differences between model outputs and field observations are quantified for a fallow close to Wankama (Niger). More precisely, some of the drawbacks of the standard parametrisation and structure of the model are diagnosed, and the range of reduction of the model-observation mismatch that results from optimizing some of the parameters are given (plant phenology,…). In particular, the use of a finite difference resolution of the soil water diffusion is considered as it enables to better simulate the fast response of evaporative fluxes to rainfall than the conceptual scheme routinely used in ORCHIDEE. The benefits of the use of such a “physical” hydrological scheme on the different outputs of the surface scheme is evaluated.The realism of the model is also measured at the regional scale, through a comparison with GIMMS_3G NDVI time series over West Africa. If the modifications that have been introduced in the model don’t improve its ability to describe the vegetation cover trends over the last decades in the region, several lessons can be kept from the analysis that has been realised, especially from the work on the transitivity of state-dependant model biases that has been conducted with Tao Wang and which is presented in annex B.

No description available.

Thermal cracking is an industrial process sensitive to both temperature and pressure operating conditions. The use of internally ribbed reactors is a passive method to enhance the chemical selectivity of the process, thanks to a significant increase of heat transfer. However, this method also induces an increase in pressure loss, which is damageable to the chemical yield and must be quantified. Because of the complexity of turbulence and chemical kinetics, and as detailed experimental measurements are difficult to conduct, the real advantage of such geometries in terms of selectivity is however poorly known and difficult to assess. This work aims both at evaluating the real benefits of internally ribbed reactors in terms of chemical yields and at proposing innovative and optimized reactor designs. This is made possible using the Large Eddy Simulation (LES) approach, which allows to study in detail the reactive flow inside several reactor geometries. The AVBP code, which solves the Navier-Stokes compressible equations for turbulent flows, is used in order to simulate thermal cracking thanks to a dedicated numerical methodology. In particular, the effect of pressure loss and heat transfer on chemical conversion is compared for both a smooth and a ribbed reactor in order to conclude about the impact of wall roughness in industrial operating conditions. An optimization methodology, based on series of LES and Gaussian process, is finally developed and an innovative reactor design for thermal cracking applications, which maximizes the chemical yield, is proposed

Cette thèse est consacrée à l'étude des propriétés aéroacoustiques de jets supersoniques impactant une paroi par simulation des grandes échelles. Ces simulations sont réalisées à partir des équations de Navier-Stokes 3-D instationnaires compressibles exprimées pour des coordonnées cartésiennes ou cylindriques. Afin de résoudre ces équations, des schémas numériques de différenciation spatiale et d'intégration temporelle peu dispersifs et peu dissipatifs sont utilisés. Les écoulements étudiés étant supersoniques, une procédure de capture de choc est également implémentée afin de supprimer les oscillations de Gibbs de part et d'autre des chocs.Dans un premier temps, un jet rond libre et quatre jets ronds impactant une paroi avec un angle de 90 degrés sont simulés sur des maillages cylindriques. Ces jets sont supersoniques, sous-détendus, et sont caractérisés par un nombre de Reynolds calculé à partir du diamètre du jet de Re=60.000, et par un nombre de Mach parfaitement détendu de Mj=1.56. Les résultats du jet libre sont tout d'abord présentés. Ils sont comparés aux résultats de plusieurs études expérimentales et de modèles afin de valider l'approche numérique utilisée. Notamment, les différentes composantes acoustiques spécifiques aux jets sous-détendus comme le bruit de choc large-bande et le bruit de screech sont observées et analysées. Les résultats obtenus pour les quatre jets impactant une paroi sont ensuite examinés. Dans ce cas, la présence d'une boucle de rétroaction aéroacoustique entre les lèvres de la buse et la paroi est montrée. Pour finir, le comportement aérodynamique et aéroacoustique des jets est étudié, et comparé à différentes études numériques et expérimentales de la littérature. Quatre jets plans supersoniques idéalement détendus impactant une paroi avec un angle de 90 degrés sont ensuite calculés. Ils ont un nombre de Reynolds évalué à partir de la hauteur de la buse de Re=50.000 et un nombre de Mach de Mj=1.28. Une boucle de rétroaction aéroacoustique entre la buse et la paroi est de nouveau mise en évidence. Une combinaison de modèles associant un modèle d'onde stationnaire aérodynamique-acoustique et un modèle de stabilité de jet plan 2-D avec des couches de mélange infiniment minces est alors proposée. Ce modèle permet de déterminer à la fois les fréquences les plus probables de la boucle de rétroaction aéroacoustique et leurs natures plane ou sinueuse.Enfin, les simulations de deux jets plans supersoniques impactant une paroi avec des angles de 60 et 75 degrés sont réalisées grâce à l'utilisation de deux maillages cartésiens, par une méthode de recouvrement de maillages. Les modifications des propriétés de la boucle de rétroaction aéroacoustique lorsque l'angle d'impact dévie de 90 degrés sont ainsi étudiées. / In this PhD work, supersonic impinging jets are simulated using large-eddy simulation in order to investigate their aerodynamic and acoustic fields. In practice, the unsteady compressible Navier-Stokes equations are solved on Cartesian or cylindrical meshes. Low-dissipation and low-dispersion numerical methods are used for spatial differentiation and time integration. As the jets are supersonic, a shock-capturing filtering is also applied in order to avoid Gibbs oscillations near shocks.A free round jet and four round jets impinging normally on a flat plate are first simulated on cylindrical meshes. They are underexpanded, and have a Reynolds number based on the nozzle diameter of Re=60.000 and a fully expanded Mach number of Mj=1.56. The results for the free jet are first presented. They are compared with experimental results and predictions given by models in order to validate the numerical setup. Acoustic components specific to underexpanded jets such as broadband shock-associated noise and screech noise are obtained. The results for the four impinging jets are then examined. An aeroacoustic feedback mechanism establishing between the nozzle lips and the flat plate is found to generate tones. Finally, the flow and acoustic properties of the jets are studied and compared with numerical and experimental data.Four ideally expanded jets impinging normally on a flat plate are then simulated. They have a Reynolds number based on the nozzle height of Re=50.000 and a Mach number of Mj=1.28. An aeroacoustic feedback mechanism is again observed between the nozzle lips and the flat plate. A combination of models based on an aeroacoustic feedback model and a vortex sheet model of the jet is then proposed. The model appears able to predict the most likely tone frequencies of the feedback mechanism, and the symmetric or antisymmetric nature of the corresponding jet oscillation.Finally, two ideally expanded jets impinging on a flat plate with angles between the jet direction and the plate of 60 and 75 degrees are simulated using two Cartesian meshes. The effects of the angle of impact on the properties of the aeroacoustic feedback mechanism are finally studied.

Escoamentos turbulentos têm sido por muitos anos o objetivo de importantes estudos para descobrir sua dinâmica. Dentre as características mais significativas, destaca-se a multiplicidade de escalas que os caracterizam, desde as maiores estruturas (baixas freqüências) controladas pela geometria que as geram, até as menores estruturas (altas freqüências) limitadas pela viscosidade do fluido. Uma idéia importante é o conceito de vórtices que está ligado a melhorias nas técnicas de visualização, tanto em laboratório quanto em experimentos numéricos. Estes vórtices têm um importante papel em numerosas aplicações tecnológicas, sendo necessário entender a dinâmica da organização de seus movimentos para controlar mecanicamente sua produção ou supressão. Neste contexto, a análise de um misturador de ar em um escoamento em tê é o principal objetivo de estudo deste trabalho. A geometria em tê é bastante simples, mas propicia o aparecimento de um escoamento com passagem de vórtices. Testes experimentais do escoamento, com duas entradas de ar com temperaturas diferentes, foram realizados no Laboratório de Engenharia Térmica e Fluidos da Escola de Engenharia de São Carlos da Universidade de São Paulo (LETeF - EESC - USP). As medidas de temperatura foram obtidas com termopares instalados ao longo da tubulação. Com o software CFX foram realizados estudos com métodos numéricos LES e DES aplicados ao escoamento. Estes resultados computacionais foram comparados com os dados experimentais, através da análise tempo-freqüência. Estudos preliminares do escoamento mostram regiões com passagem de vórtices, e a habilidade da técnica de análise tempo-freqüência em caracterizar a existência e a forma destas estruturas turbulentas. / Turbulent flows have been the objective of important studies to discover its dynamics. One important characteristic of these flows is the multiplicity of scales, since the large structures (low frequencies) controlled by the geometry that generates them, until the small structures (high frequencies) limited by the fluid viscosity. An important idea is the concept of vortices that it is associated with the improvements in the visualization techniques, in laboratory or numerical experiments. These vortices have an important function in many technological applications. In each one of these fields, it is necessary to understand the dynamics of its movements to control the mechanisms for producing or suppressing these vortices. In this context, the analysis of an air mixing in a tee junction is the main objective of this work. The tee geometry is sufficiently simple, but contributes for the appearance of a flow with vortices transition. Experimental tests with two different air temperatures inlets were done at the Thermal and Fluid Engineering Laboratory of the University of São Paulo at São Carlos (LETeF - EESC - USP). The measures of temperature were acquired with thermocouples installed along the pipe. Numerical studies with LES and DES methods using CFX software were applied to the flow. These computational results were compared with the experimental data through the time-frequency analysis. Preliminary studies of the flow show vortices transition regions and the ability of time-frequency technique in describing the existence and shape of turbulent structures.

Os tubos de Geradores de Vapor em Reatores Nucleares do tipo PWR são submetidos a diferentes níveis de tensões e carregamento em altas temperaturas, reduzindo sua vida útil devido o surgimento de defeitos e corrosão. A inspeção por Correntes Parasitas é um ensaio não destrutivo usado para diagnosticar defeitos de corrosão e descontinuidades na superfície externa e interna em tubos de trocadores de calor. Esses tubos estão sujeitos a danos por diferentes mecanismos de degradação mecânica e química, tais como trincas por fadiga e corrosão sob tensão. Os sinais de inspeção por Correntes Parasitas são afetados por diferentes ruídos dificultando sua análise pelo inspetor. Esse trabalho apresenta os resultados da análise dos sinais de Correntes Parasitas usando a Transformada de Hilbert-Huang (THH) funcionando como filtro de ruídos (De-noising), como uma técnica alternativa de processamento e análise de sinais. A Transformada de Hilbert-Huang teve esse nome atribuído pela agência espacial norte-americana (NASA) para o resultado da reunião de dois processos, um método de decomposição empiricamente modal (Empirical Mode Decomposition EMD), seguido da análise espectral de Hilbert (Hilbert Spectral Analysis HSA). Os sinais de inspeção por correntes parasitas possuem características de transiente, não estacionário e não linear. A transformada de Hilbert-Huang aplicada neste trabalho forneceu dois recursos alternativos em processamento de sinais; o pré-processamento que funcionou como filtro de ruídos, e outro de análise de sinais, responsável pela identificação das características tempo-frequência-energia do sinal. / The nuclear power plant steam generator tubes are subjected to different levels of stress and loading at high temperatures, reducing its lifetime due to the development of defects and corrosion. The Eddy Current Testing (ECT) is a nondestructive testing used to diagnose defects of corrosion and discontinuities in the inner and outer surface of heat exchanger tubes. These tubes are subject to failure by different mechanisms of chemical and mechanical degradation such as fatigue and stress corrosion crack. The ECT signals are affected by different noises making the analysis a difficult task to the inspector. This dissertation presents the results of the main characteristics from the ECT signals using the Hilbert-Huang Transform (HHT) as an alternative method for the processing and signal analysis. The Hilbert-Huang Transform has its name given by the American National Aeronautics and Space Administration, NASA, as the result of Empirical Mode Decomposition (EMD) and the Hilbert Spectral Analysis (HSA) methods. The Eddy Current signals are transient, nonstationary and nonlinear. The Hilbert-Huang Transform applied in this work provided two alternative proceedings in signal processing, one in signal pre-processing acting as noise filter (De-noising) and another as signal analysis, which identifies the characteristics of signal time-frequency-energy.

O presente trabalho versa sobre a modelagem de uma chama turbulenta difusiva usando a técnica de simulação de grandes estruturas turbulentas (LES), juntamente com o modelo termo-químico de folha de chama (flame sheet model) e o conceito de fração de mistura como escalar conservativo. Este trabalho também é pioneiro de utilização de LES com reação química no Brasil, podendo colaborar para o desenvolvimento desta técnica na área de combustão. O trabalho consiste na construção e validação das rotinas computacionais de um código CFD, baseado em LES e com flexibilidade para uma futura utilização de cinética química detalhada de combustão (EDC/ISAT), para casos complexos onde modelos mais simples, como a fração de mistura, são falhos. O programa será validado em uma chama de difusão turbulenta não-confinada de metano (CH4), para a qual existem dados experimentais na literatura [61,62] e utilizados pela comunidade acadêmica em excelência (Stanford, TU-Darmstadt, Imperial College, Cornell University etc). As características da implementação numérica do código permitirão sua expansão futura para outras aplicações em: queima de combustíveis líquidos, combustão em câmaras fechadas e fornalhas com a inclusão de modelo de radiação. / The present work is about modeling a diffusive turbulent flame using the Large-Eddy Simulation approach (LES) and the Flame Sheet model as the chemical model with the mixture fraction concept as the conservative scalar. This work is pioneer in the sense of using LES and reactive flow in Brazil, making possible the development for LES techniques in the combustion area. The work is intended to construct and validate a CFD code based on LES and with future flexibility for a more detailed combustion chemical model (EDC/ISAT) for complex flows, where simple models are failed, like the mixture fraction. The program will be validated for a turbulent diffusion methane (CH4) flame not confined, which there are some experimental data on the specialized literature [61,62], and commonly used by the academic community (Stanford, TU-Darmstadt, Imperial College, Cornell University etc). The features of the numerical code implementation will make possible future expansion of its use in other applications: liquid fuel burning, combustion chambers and ovens with the radiation model inclusion.

Escoamentos turbulentos têm sido por muitos anos o objetivo de importantes estudos para descobrir sua dinâmica. Dentre as características mais significativas, destaca-se a multiplicidade de escalas que os caracterizam, desde as maiores estruturas (baixas freqüências) controladas pela geometria que as geram, até as menores estruturas (altas freqüências) limitadas pela viscosidade do fluido. Uma idéia importante é o conceito de vórtices que está ligado a melhorias nas técnicas de visualização, tanto em laboratório quanto em experimentos numéricos. Estes vórtices têm um importante papel em numerosas aplicações tecnológicas, sendo necessário entender a dinâmica da organização de seus movimentos para controlar mecanicamente sua produção ou supressão. Neste contexto, a análise de um misturador de ar em um escoamento em tê é o principal objetivo de estudo deste trabalho. A geometria em tê é bastante simples, mas propicia o aparecimento de um escoamento com passagem de vórtices. Testes experimentais do escoamento, com duas entradas de ar com temperaturas diferentes, foram realizados no Laboratório de Engenharia Térmica e Fluidos da Escola de Engenharia de São Carlos da Universidade de São Paulo (LETeF - EESC - USP). As medidas de temperatura foram obtidas com termopares instalados ao longo da tubulação. Com o software CFX foram realizados estudos com métodos numéricos LES e DES aplicados ao escoamento. Estes resultados computacionais foram comparados com os dados experimentais, através da análise tempo-freqüência. Estudos preliminares do escoamento mostram regiões com passagem de vórtices, e a habilidade da técnica de análise tempo-freqüência em caracterizar a existência e a forma destas estruturas turbulentas. / Turbulent flows have been the objective of important studies to discover its dynamics. One important characteristic of these flows is the multiplicity of scales, since the large structures (low frequencies) controlled by the geometry that generates them, until the small structures (high frequencies) limited by the fluid viscosity. An important idea is the concept of vortices that it is associated with the improvements in the visualization techniques, in laboratory or numerical experiments. These vortices have an important function in many technological applications. In each one of these fields, it is necessary to understand the dynamics of its movements to control the mechanisms for producing or suppressing these vortices. In this context, the analysis of an air mixing in a tee junction is the main objective of this work. The tee geometry is sufficiently simple, but contributes for the appearance of a flow with vortices transition. Experimental tests with two different air temperatures inlets were done at the Thermal and Fluid Engineering Laboratory of the University of São Paulo at São Carlos (LETeF - EESC - USP). The measures of temperature were acquired with thermocouples installed along the pipe. Numerical studies with LES and DES methods using CFX software were applied to the flow. These computational results were compared with the experimental data through the time-frequency analysis. Preliminary studies of the flow show vortices transition regions and the ability of time-frequency technique in describing the existence and shape of turbulent structures.

Os tubos de Geradores de Vapor em Reatores Nucleares do tipo PWR são submetidos a diferentes níveis de tensões e carregamento em altas temperaturas, reduzindo sua vida útil devido o surgimento de defeitos e corrosão. A inspeção por Correntes Parasitas é um ensaio não destrutivo usado para diagnosticar defeitos de corrosão e descontinuidades na superfície externa e interna em tubos de trocadores de calor. Esses tubos estão sujeitos a danos por diferentes mecanismos de degradação mecânica e química, tais como trincas por fadiga e corrosão sob tensão. Os sinais de inspeção por Correntes Parasitas são afetados por diferentes ruídos dificultando sua análise pelo inspetor. Esse trabalho apresenta os resultados da análise dos sinais de Correntes Parasitas usando a Transformada de Hilbert-Huang (THH) funcionando como filtro de ruídos (De-noising), como uma técnica alternativa de processamento e análise de sinais. A Transformada de Hilbert-Huang teve esse nome atribuído pela agência espacial norte-americana (NASA) para o resultado da reunião de dois processos, um método de decomposição empiricamente modal (Empirical Mode Decomposition EMD), seguido da análise espectral de Hilbert (Hilbert Spectral Analysis HSA). Os sinais de inspeção por correntes parasitas possuem características de transiente, não estacionário e não linear. A transformada de Hilbert-Huang aplicada neste trabalho forneceu dois recursos alternativos em processamento de sinais; o pré-processamento que funcionou como filtro de ruídos, e outro de análise de sinais, responsável pela identificação das características tempo-frequência-energia do sinal. / The nuclear power plant steam generator tubes are subjected to different levels of stress and loading at high temperatures, reducing its lifetime due to the development of defects and corrosion. The Eddy Current Testing (ECT) is a nondestructive testing used to diagnose defects of corrosion and discontinuities in the inner and outer surface of heat exchanger tubes. These tubes are subject to failure by different mechanisms of chemical and mechanical degradation such as fatigue and stress corrosion crack. The ECT signals are affected by different noises making the analysis a difficult task to the inspector. This dissertation presents the results of the main characteristics from the ECT signals using the Hilbert-Huang Transform (HHT) as an alternative method for the processing and signal analysis. The Hilbert-Huang Transform has its name given by the American National Aeronautics and Space Administration, NASA, as the result of Empirical Mode Decomposition (EMD) and the Hilbert Spectral Analysis (HSA) methods. The Eddy Current signals are transient, nonstationary and nonlinear. The Hilbert-Huang Transform applied in this work provided two alternative proceedings in signal processing, one in signal pre-processing acting as noise filter (De-noising) and another as signal analysis, which identifies the characteristics of signal time-frequency-energy.

By applying pattern recognition analysis, the organised eddy structures in the near and intermediate fields (10-40 jet diameters) of a numerically simulated turbulent plane jet have been investigated. The Reynolds number based on the jet width is 1, 600. The results of the nearly full simulation show good agreement with the experimental measurements in the near and intermediate regions (< 40 diameters) of the jet. Patterns of organised eddy structures have been extracted from the three-dimensional simulation databases. Ensemble averaged fields of three velocity components and pressure in the spanwise and transverse planes suggest the existence of inclined vortex pairs. These highly three-dimensional vortical structures are confined to one side of the jet centreplane and approximately align with the principal axis of rate-of-strain. Some of the topological features of these averaged eddy structures have been examined.