Cette étude traite de la dynamique des éruptions volcaniques explosives, depuis les mécanismes de sub-surface jusqu’aux processus d’émission et de dispersion des pyroclastes. A cet effet un radar Doppler sol est utilisé (VOLDORAD), lequel renseigne sur la charge / vitesse des ejectas. Les données sont intégrées avec d’autres techniques géophysiques, et des modèles numériques sont développés afin de simuler les émissions pyroclastiques, générer des signaux radar synthétiques, pour finalement améliorer notre compréhension des processus qui leurs sont sous-jacents. L’Arenal (Costa Rica) est utilisé comme volcan cible, où de fréquentes éruptions de faible magnitude émettent des panaches de cendres et des projections balistiques jusqu’à quelques centaines de mètres au-dessus de l’évent. Dans un premier temps, nous combinons des données sismiques et radar afin d’explorer la relation entre les processus de conduit et les émissions pyroclastiques. Leurs interactions complexes sont interprétées via un modèle conceptuel, lequel décrit les fractures parsemant le bouchon de lave comme responsables du dégazage du système, et en retour des signaux sismiques et radar collectés (ces derniers dépendants de la charge en cendres des émissions de gaz). Par ailleurs, nous investiguons la dynamique des émissions pyroclastiques à travers l’étude de radargrammes Doppler. La distribution spatio-temporelle de la vitesse des ejectas indique l’existence de deux phénomènes aux dynamiques distinctes. Des modélisations numériques permettant la reconstruction de signaux synthétiques indiquent qu’il s’agit de l’émission simultanée de blocs balistiques et de panaches de cendres. Une procédure d’inversion de type Monte Carlo couplée d’un algorithme d’optimisation permet de retrouver les radargrammes synthétiques qui reproduisent au mieux ceux observés. Les résultats apportent des contraintes sur divers paramètres éruptifs, tels que les tailles, trajectoires, vitesses des ejectas et des gaz, ainsi que la vitesse / direction de dispersion des panaches de cendres par le vent. Enfin, nous discutons du potentiel des radars Doppler appliqués à la surveillance opérationnelle des émissions volcaniques. En particulier, la possibilité de quantifier les masses éjectées dans l’atmosphère ou retombant sur les flancs du volcan, fournit des paramètres éruptifs à la source pouvant alimenter les modèles de dispersion de panaches de cendres. / This study investigates the dynamics of explosive volcanic eruptions, from the sub-surface source mechanisms through to the emission dynamics and downwind dispersal of tephra. To this end, we use a ground-based Doppler radar (VOLDORAD) which informs on the loading / velocimetry of the expelled ejecta. Data are integrated with complementary geophysical techniques, and numerical models are developed to simulate pyroclastic emissions, generate synthetic radar data, and in turn enhance our understanding of the underlying dynamical processes. Arenal (Costa Rica) is used as a case study volcano, where frequent mildly-explosive eruptions commonly expel ash plumes and ballistic projections up to a few hundred meters above the vent. Firstly, we combine seismic and radar data to investigate the link between conduit processes and pyroclastic emissions. A conceptual model is proposed to account for their complex interplay, whereby fractures through a rigid lava cap control the system’s degassing, which in turn governs both the seismic and radar signals (the latter depending on the ash load carried by the gas). Secondly, we investigate the dynamics of pyroclastic emissions from the analysis of Doppler radargrams. Time-velocity distribution of the expelled tephra shows the signature of two distinct phenomena. Numerical modeling and computation of synthetic radargrams show that these are consistent with both ballistic projections and ash plume crossing the beam simultaneously, whose respective mass load can be derived. Inverse modeling using a nearneighborhood Monte Carlo procedure was used to find synthetic Doppler radargrams which best matched the observed ones. The results give constrains on eruptive parameters, such as the size, trajectory, exit velocities and source gas velocities of the ballistics, as well as the speed / direction of the ash cloud drifted by trade winds. Lastly, because Doppler radars are powerful tool for real-time allweather monitoring of volcanic activity, we address issues relative to the operational radar monitoring of ash plumes. In particular, the ability to remotely quantify the mass proportions of ejecta either falling on the slopes of the volcano or prone to be ejected into the atmosphere, gives source eruptive parameters which may feed volcanic ash dispersal models.

This study examines wind generated waves during winter storms, their transformation/attenuation near the marsh edge, and the resulting saltmarsh edge erosion. A simple numerical model for wave generation, transmission and marsh edge erosion was developed and validated against observations from Lake Borgne, Louisiana. Results suggest that meteorological conditions modify the local water depth via wind or wave setup and atmospheric pressure, thus exerting a first order control on the location of wave attack, which in turn determines the type of wave forces (shear vs. impact) that dominate the erosion process. Scarp failure follows, at a location determined by water level, creating multiple erosive scarps and terraces. High measured erosion, likely due to marsh edge destabilization followed by subsequent frontal passage forces differential marsh erosion, exposing underlying substrate to further erosion. A conceptual model for marsh edge retreat is developed using these observations and supported further by model predictions.

A hurricane is a combination of sustained winds, low atmospheric pressures and precipitation. Over the past decades, Louisiana has experienced several devastating hurricanes. The east bank of the City of New Orleans is bounded by Lake Pontchartrain to the North and the Mississippi River to the South. Lake Pontchartrain is a brackish system connected to the Gulf of Mexico through Lake Borgne to the East. As a Hurricane enters the Estuary from the Gulf of Mexico, it imposes a sustained surge of a few meters which may lead to flooding in areas which are not protected by levees. These flood water may be saline. Saltwater flooding is an environmental issue in flooded marshlands since saltwater can be fatal to some plants. The response of salinity and storm surge to hurricane duration which represents the forward speed of the storm is numerically modeled.

A 3-D numerical model of the Head of Passes and Bird’s Foot Delta of the Mississippi River- was developed. The model was based on Delft3D and simulates the hydrodynamics and salinity transport for Head of Passes area from RM 6.5 to Gulf of Mexico. The model was calibrated, validated, and used to predict the response of the river to certain stimuli, such as - channel closures, channel modifications and diversions. The model includes West Bay, Southwest Pass, South Pass, Pass-A-Loutre and Main Pass. Three basic cases were developed: existing conditions, closure of Southwest Pass with a levee of 1.5m with dredging of Pass-A-Loutre to 13.7 meter depth, and closure of Southwest Pass and South Pass with dredging of Pass-A-Loutre to 13.7 meter (45 ft) depth. Salinity has been added to the model. It has been proved that salinity intrusion has a significant impact on the model instantaneous discharge. For all passes except Southwest Pass instantaneous discharge decreases almost in half. Closure of Southwest Pass and dredging of Pass-A-Loutre leads to changes in flow speed and distribution. As a result Pass-A-Loutre becomes a main River channel.

<p>The computational modeling of ocean waves and ocean-faring devices poses numerous challenges. Among these are the need to stably and accurately represent both the fluid-fluid interface between water and air as well as the fluid-structure interfaces arising between solid devices and one or more fluids. As techniques are developed to stably and accurately balance the interactions between fluid and structural solvers at these boundaries, a similarly pressing challenge is the development of algorithms that are massively scalable and capable of performing large-scale three-dimensional simulations on reasonable time scales. This dissertation introduces two separate methods for approaching this problem, with the first focusing on the development of sophisticated fluid-fluid interface representations and the second focusing primarily on scalability and extensibility to higher-order methods.</p><p>We begin by introducing the narrow-band gradient-augmented level set method (GALSM) for incompressible multiphase Navier-Stokes flow. This is the first use of the high-order GALSM for a fluid flow application, and its reliability and accuracy in modeling ocean environments is tested extensively. The method demonstrates numerous advantages over the traditional level set method, among these a heightened conservation of fluid volume and the representation of subgrid structures.</p><p> </p><p>Next, we present a finite-volume algorithm for solving the incompressible Euler equations in two and three dimensions in the presence of a flow-driven free surface and a dynamic rigid body. In this development, the chief concerns are efficiency, scalability, and extensibility (to higher-order and truly conservative methods). These priorities informed a number of important choices: The air phase is substituted by a pressure boundary condition in order to greatly reduce the size of the computational domain, a cut-cell finite-volume approach is chosen in order to minimize fluid volume loss and open the door to higher-order methods, and adaptive mesh refinement (AMR) is employed to focus computational effort and make large-scale 3D simulations possible. This algorithm is shown to produce robust and accurate results that are well-suited for the study of ocean waves and the development of wave energy conversion (WEC) devices.</p> / Dissertation

A região de estudo situa-se no Canal de São Sebastião, Litoral Norte do Estado de São Paulo (Brasil), onde se localizam quatro emissários submarinos situados entre as cidades São Sebastião e Ilhabela. Este canal é utilizado para a prática de diversos esportes náuticos, pesca esportiva e artesanal, sendo a avaliação dos efeitos do despejo de efluentes domésticos e industriais nesse corpo d\'água fundamental para assegurar a qualidade do ambiente e garantir a preservação da saúde humana. O objetivo deste estudo foi avaliar, em meses de verão e inverno de um ano típico, a concentração de potenciais contaminantes (coliformes fecais e nutrientes), em condições extremas, e a dispersão das plumas dos efluentes desses emissários. Para isso, foram utilizados módulos hidrodinâmico e de qualidade de água do modelo Delft3D (para o campo distante), bem como o modelo CORMIX (para o campo próximo). Os resultados das simulações hidrodinâmicas foram consistentes com as características citadas em literatura e a partir de parâmetros estatísticos. A modelagem de campo próximo mostrou a influência das correntes no transporte e diluição inicial das plumas dos efluentes. Após o acoplamento, os resultados das simulações de campo distante da modelagem das plumas dos emissários submarinos mostraram que as mesmas não excederam os níveis máximos estabelecidos pelo CONAMA (Nº 274/2000 e 357/2005) ao atingir a costa. Conforme os resultados de monitoramento in situ realizados pela Companhia Ambiental do Estado de São Paulo, as praias locais próximas aos emissários são geralmente classificadas como impróprias para o banho e com má qualidade da água. Assim, o esgoto bruto despejado através dos córregos ou diretamente nas praias pode ser identificado como o responsável pela contaminação ambiental da costa na área de estudo. / The study region is located in the São Sebastião channel, in the north coast of São Paulo (Brazil), where four submarine outfall are located between São Sebastião and Ilhabela cities. This channel is used for the practice of various nautical sports, sport fishing and crafts, and the evaluation of the effects of the disposal of domestic and industrial effluents in this water body is fundamental to ensure the water quality and the preservation of human health. The objective of this study was to evaluate the concentration of potential contaminants (fecal coliforms and nutrients) during the summer and winter of a typical year, in extreme conditions, and the dispersion of effluent plumes from these submarine outfalls. For that, Delft3D model\'s hydrodynamic and water quality modules (far field) were used, as well as the CORMIX model (near field). Results of hydrodynamic simulations were consistent with the characteristics cited in the literature and from statistical parameters. Near field modeling showed the influence of the currents in the transport and initial dilution of the effluent plumes from the submarine outfalls. After coupling, the results of far field modeling showed that they did not exceed the maximum levels established by the National Environmental Council, Resolutions Nº 274/2000 and Nº 357/2005, when reaching the coast. Once the literature indicates that the local beaches near the submarine outfalls are often classified as inappropriate for bathing and with bad water quality. Thus, raw sewage dumped through the streams or directly on the beaches can be identified as responsible for the environmental contamination of the coast in the study area.

Avec la multiplication des antennes radios, due à l’augmentation des abonnés et aux nouveaux services et usages, l’exposition aux ondes électromagnétiques est devenue un sujet interpellant toute une catégorie de personnes, notamment les collectivités locales, les opérateurs, les associations, etc. Dans le domaine de l’exposition des populations aux champs électromagnétiques radiofréquences en milieu urbain, la modélisation numérique a pris depuis quelques années de plus en plus d’importance du fait de la montée en puissance des outils de calcul et de la quantité et qualité des données topographiques disponibles. Elle est utilisée conjointement avec plusieurs types de mesures pour l’analyse et la gestion de risques, comme support d’aide à la décision publique, comme méthode réglementaire et plus largement pour la concertation entre public, industriels, collectivités territoriales, associations, etc. Les résultats de ces méthodes ont donc de plus en plus d’importance. Ce manuscrit permet d'améliorer les connaissances sur la quantification de l'exposition due aux antennes relais en associant mesures et simulations et en prenant en considération la notion de variabilité du champ électrique.Une première partie du travail a permis de déterminer les avantages et les limites des méthodes de calcul, et de même pour les mesures, dans le cadre de l'évaluation de l'exposition à l'échelle urbain. Une seconde partie du travail a consisté à caractériser la variabilité spatiale et temporelle de l'exposition réelle, un point essentiel du fait de la grande variabilité du champ électrique et de la difficulté de trouver un indicateur pertinent pour quantifier l'exposition réelle. / With the increasing number of mobile phone users, new services and mobile applications, the proliferation of radio antennas has raised concerns about human exposure to electromagnetic waves. This is now a challenging topic to many stakeholders such as local authorities, mobile phone operators, citizen and consumer groups, etc. The assessment of human exposure to radiofrequency electromagnetic fields in the urban environment is mainly based on numerical modeling. The improvement of computational tools together with the quantity and quality of available topographic data has strongly promoted this approach. Numerical modeling is now used in combination with several types of measurements to perform risk analysis and management. It is the basis of decision-making tools and local regulations. It is also used during consultations between the public, the industry, local authorities, governments and associations. The results of these methods are therefore increasingly important. This research contributes to gain knowledge about quantifying the human exposure from base stations through the combination of measurements and simulations, while taking into account the concept of variability of the electrical field. A first part of this work establishes the benefits and limitations of computational methods as well as measurements when assessing exposure to electric fields in urban environment. A second part of this work is aimed at characterizing the spatial and temporal variability of the actual exposure, which is a very important requirement because of the high variability of the electrical field and the difficulty of finding a relevant indicator to quantify the real exposure.

L’usinage des pièces en alliages de titane, notamment en Ti6Al4V qui a une faible usinabilité, a été toujours parmi les préoccupations majeures des entreprises du secteur de l’aéronautique. Toutefois, il est difficile, en se basant seulement à des essais expérimentaux, de bien comprendre les mécanismes participants à la formation du copeau. Il est alors nécessaire d’avoir recours à des modélisations numériques fiables permettant d’avoir accès à des grandeurs physiques instantanées et très localisées. Le travail présenté porte sur la fiabilisation de la modélisation de la coupe. Des simulations numériques 2D et 3D ont été mises en place. Le modèle de comportement de Johnson-Cook et le critère énergétique d’évolution d’endommagement ont été utilisés. L’étude préliminaire 2D de l’effet du maillage, notamment la taille, le type et la fonction d’interpolation des éléments finis, a souligné l’importance d’une discrétisation convenable du modèle qui tient compte du coût de calculs. De plus, il a été montré qu’un choix convenable du type de la formulation est crucial. L’effet des coefficients rhéologiques et d’endommagement (initiation et évolution) sur la formation du copeau (morphologie, champ de déformation et de température) a été déterminé. Des essais expérimentaux de la coupe orthogonale du Ti6Al4V à différentes conditions de coupe ont été effectués. La dépendance de la géométrie du copeau et des efforts à la vitesse de coupe et à l’avance a été étudiée. Les résultats expérimentaux ont été utilisés pour la validation des modèles numériques 3D qui permettent une étude fine de la formation du copeau. Cette approche a permis de reproduire fidèlement les phénomènes physiques se produisant au niveau du plan médian de la pièce tout en tenant compte de l’écoulement de la matière sur les bords. Les résultats prédits ont mis en évidence que, même dans le cas d’une coupe orthogonale, la formation du copeau est bien un phénomène 3D. Afin d’augmenter la fiabilité des modèles numériques 3D, une nouvelle loi thermo-viscoplastique a été proposée. Cette loi, identifiée et implémentée dans le logiciel Abaqus® à travers la routine VUMAT©, a été utilisée pour la simulation de l’usinage du Ti6Al4V. Elle a conduit à une amélioration notable des résultats numériques. / Machining of titanium alloys workpieces, especially in Ti6Al4V which has a low machinability, has always been among the major preoccupations of the companies in the aeronautics sector. However, it is difficult, basing only on experimental tests, to well understand the mechanisms involved during the chip formation. In fact, the use of reliable numerical models that allow the access to instantaneous and very localized physical quantities is required. The presented work consists on the increase of the cutting modeling reliability. 2D and 3D numerical simulations have been performed. The Johnson-Cook constitutive model and the damage evolution criterion have been used. The preliminary 2D study focused on the mesh effect, especially the size of the finite element, its type and its interpolation function, has highlighted the importance of a convenient discretization of the model that takes into account the machining computing cost. In addition, it has been shown that a suitable choice of the formulation type is crucial. The effect of the rheological and damage (initiation and evolution) coefficients on the chip formation (morphology, strain and temperature field) has been determined. Experimental orthogonal cutting tests of the Ti6Al4V at different cutting conditions have been performed. The dependency of the chip geometry and the efforts to the cutting speed and the feed rate has been studied. Experimental results have been used in the validation of the 3D numerical models, which allow a deep study of the chip formation process. This approach has allowed an accurately reproduction of the physical phenomena that occurs in the median plan of the workpiece as well as in its sides. The predicted results have highlighted that, even in the case of orthogonal cutting process, the chip formation is a 3D phenomenon. In order to increase the reliability of 3D numerical models, a new thermo-visco-plastic law has been proposed. This law, identified and implemented in the software Abqus® through the subroutine VUMAT©, has been used to model machining process of the Ti6Al4V. It has resulted in a notable improvement of numerical results.

[pt] O presente trabalho avalia a utilização de elementos enriquecidos para obtenção de cargas críticas, freqüências de vibração e seus respectivos modos de peças estruturais bidimensionais (flexão de placas retangulares sujeitas a compressão em seu plano). O método de aproximação empregado foi o de Rayleigh-Ritz voltado para o uso de elementos finitos convencionais enriquecidos com funções de deslocamentos adicionais internas e de contorno. As funções ditas internas são desenvolvidas de forma a não envolver deslocamentos e rotações nodais e no contorno. Já as funções ditas de contorno são concebidas de forma a envolver apenas deslocamentos internos e ao longo de um lado apenas, sem deslocamentos generalizados nodais. Para este estudo foram desenvolvidas duas famílias de funções, uma com termos adicionais trigonométricos e outra com termos adicionais polinomiais. Para o cálculo de cargas críticas e freqüências são utilizadas as matrizes de rigidez elástica, rigidez geométrica e de massa, introduzidas em problemas generalizados de autovalores. Resultados numéricos são obtidos através de procedimentos computacionais utilizando o software Maple. Verifica-se que as funções adicionais trigonométricas, embora mais satisfatórias que as polinomiais quanto à convergência, exigem maior esforço computacional. São comparados resultados de elementos para placas esbeltas (teoria de Kirchhoff), com três e quatro graus de liberdade por nó, onde o quarto grau de liberdade corresponde à derivada mista (torção). Mostra-se que as funções adicionais, não-nodais, requerem o uso do elemento com quatro graus de liberdade por nó, para se ter convergência no cálculo das cargas críticas e freqüências em situações gerais. Outros exemplos abordam preliminarmente a inclusão de efeitos de dano e ortotropia no material, visando a modelagem de lajes comprimidas e pilares com seções retangulares alongadas. Esta modelagem envolvendo combinação de funções adicionais gerais e elementos convencionais representa um passo no desenvolvimento de uma técnica aplicável à combinação de modos globais e localizados de instabilidade / [en] The focus of the present work is to developand evaluate enriched elements used to obtain critical loads, frequencies of vibration and respective modes for two-dimensional structural components (rectangular plates in bending under inplane compressive loading). The Rayleigh-Ritz approximation method has been employed, directed to the use of conventional finite elements enriched by internal and boundary additional displacements functions. The socalled internal functions are do not involve nodal and boundary displacements and rotations. The boundary functions are conceived to include displacements within the element and along one side, without involving any generalized nodal displacements. Two displacement function families were developed, the first with trigonometric additional terms and the second with polynomial additional terms. Critical loads and frequencies, and respective modes, are obtained by the use of elastic stifiness, geometric, and mass matrices, introduced in generalized eigenvalue problems. Numerical results are obtained by computational procedures using Maple software. The trigonometric additional functions, in spite of better convergence properties, demand greater computational effort. The basic elements are classical thin plate elements (Kirchhoff's theory) with three or four degrees of freedom per node, where the fourth degree of freedom corresponds to the mixed derivative (torsion). The results indicate that non- nodal additional functions require the use of elements with four freedom degrees by node to obtain convergence of critical loads and frequencies convergence in general situations. Other examples consist of preliminary approaches to include damage effects, in reinforced orthotropic plates, as modeling columns with wide rectangular sections and compressed slabs. The use of general additional functions combined with conventional elements represents a step on the development of a technique applicable to global and localized instability modes.

[pt] Este trabalho consiste em desenvolver e avaliar modelos clássicos de elementos finitos combinados com funções polinomiais adicionais, para a obtenção de cargas críticas de instabilidade e freqüências naturais de estruturas planas, com seus respectivos modos. O objetivo consiste em buscar uma sistemática confiável para obter estimativas de deformações localizadas em regime próximo ao colapso. Utiliza-se o método dos Elementos Finitos em combinação com o método clássico de Rayleigh-Ritz. Como elemento fundamental para tal estudo, emprega-se o elemento retangular de Barber-Weaver, que contém quatro nós, cada nó com duas translações e duas rotações independentes (equivalentes a uma rotação e uma distorção angular). Esse elemento é enriquecido com funções de deslocamentos adicionais internas e de contorno, em forma de séries polinomiais gerais. Esse conjunto de funções é incorporado nas expressões de energia para levar ao estabelecimento de matrizes de rigidez elástica, geométrica e de massa. Tais matrizes permitem estabelecer problemas generalizados de autovalor para obtenção de cargas críticas e freqüências, e respectivos modos de flambagem e vibração. Para os estudos numéricos comparativos apresentados nos exemplos, são implementadas diversas rotinas usando o software Maple 9.0. Os resultados mostram que a metodologia apresentada pode ser usada no desenvolvimento de uma técnica aplicável à obtenção de modos globais e localizados de instabilidade, quando há a combinação de efeitos não lineares geométricos e de material. / [en] This work consists in developing and evaluating classical of finite element models combined with additional polynomial functions, to obtain critical loads of instability and natural frequencies of plane structures, and respective modes. The objective is to search for a reliable technique to get estimates of localized deformations near to collapse. The Finite Elements method is used in combination with the classic method of Rayleigh-Ritz. As a basic element for such study, the rectangular element of Barber-Weaver is used, containing four nodes, each one with two translations and two independent rotations (equivalents to a rotation and an angular distortion). This element is enriched with additional internal displacement functions and with functions on the boundary, forming general polynomial series. These nodal functions are incorporated in the energy expressions leading to the establishment of elastic stiffness, geometric, and mass matrices. Such matrices allow the establishment of generalized eigenvalue problems to obtain critical loads and frequencies, and the respective modes of buckling and vibration. For the comparative numerical studies presented in the examples, several routines are implemented using software Maple 9.0. The results show that the methodology presented herein can be used in the development of an applicable technique to the ascertainment of instability in global and located modes, when there is a combination of geometric nonlinear and material effects.