O estudo do fenômeno de vibrações induzidas pela emissão de vórtices, ou VIV, em risers é um tópico de grande relevância para a engenharia oceânica. Nos últimos anos, diversos resultados vêm sendo apresentados, numéricos e/ou experimentais, buscando o entendimento dos fundamentos deste fenômeno de interação fluido-estrutural, no entanto, sem um entendimento conclusivo e abrangente. Neste contexto, risers também estão sujeitos à ação de correntezas oceânicas e, consequentemente, ao fenômeno de VIV. Neste sentido, o presente estudo traz resultados de uma abordagem experimental para o entendimento do fenômeno de VIV em risers lançados em catenária, configuração que representa grande parte das geometrias adotadas em lançamentos atuais. No entanto, a realização de experimentos com risers em tanque de provas pressupõe a utilização de modelos reduzidos, o que, por si só, representa um grande desafio de pesquisa. Para tanto, desenvolveu-se uma metodologia para a modelagem de risers em escala reduzida, com foco na similaridade entre parâmetros importantes que garantam um comportamento dinâmico equivalente àquele em escala real. Foram, então, realizados experimentos de VIV com catenárias concebidas e construídas com base nesta metodologia, posteriormente ensaiadas em tanque de reboque. Para análise dos resultados, foram utilizadas técnicas como a decomposição modal, que possibilitou a comparação desses com alguns resultados da literatura, mostrando que o fenômeno de VIV na catenária se processa de maneira bastante semelhante ao identificado nos cilindros flexíveis. Além disso, a extensa base de dados experimentais apresentada e discutida nesta tese contribui para futura validação de diversos modelos matemáticos e códigos numéricos de predição do VIV em catenárias, descrevendo com detalhes inúmeros comportamentos imersos neste problema importante de engenharia. / The study of Vortex-induced vibrations in risers is a topic of great relevance in the context of ocean engineering. In recent years, several numerical and experimental results have been presented, addressing the understanding of the fundamentals of this fluid-structure interaction phenomenon, however without a conclusive and comprehensive understanding. In this context, risers are exposed to the action of ocean currents and consequently to the phenomenon of VIV. In this sense, the present study provides results of an experimental approach to understanding the VIV phenomenon on risers launched in catenary configuration, which represents much of the geometries adopted today. However, performing risers experiments in towing-tanks presupposes the use of small-scale models, which represents a major research challenge. Therefore, a methodology for modeling reduced scale risers was developed. The focus of this methodology was on the similarity between certain important parameters that guarantee an equivalent full-scale dynamic behavior. VIV experiments were then performed in the towing tank, with the designed and constructed catenary, based on this methodology. Techniques such as modal decomposition were used for the analysis of the results. This allows the comparison of the results with classical literature paradigms showing that VIV response in the catenary is close to results obtained with flexible cylinders. Furthermore, the large experimental database presented and discussed in the study may contribute to future validation of several mathematical models and numerical codes of VIV prediction in catenary risers, describing in detail several immersed behaviors in this important engineering problem.

O estudo do fenômeno de vibrações induzidas pela emissão de vórtices, ou VIV, em risers é um tópico de grande relevância para a engenharia oceânica. Nos últimos anos, diversos resultados vêm sendo apresentados, numéricos e/ou experimentais, buscando o entendimento dos fundamentos deste fenômeno de interação fluido-estrutural, no entanto, sem um entendimento conclusivo e abrangente. Neste contexto, risers também estão sujeitos à ação de correntezas oceânicas e, consequentemente, ao fenômeno de VIV. Neste sentido, o presente estudo traz resultados de uma abordagem experimental para o entendimento do fenômeno de VIV em risers lançados em catenária, configuração que representa grande parte das geometrias adotadas em lançamentos atuais. No entanto, a realização de experimentos com risers em tanque de provas pressupõe a utilização de modelos reduzidos, o que, por si só, representa um grande desafio de pesquisa. Para tanto, desenvolveu-se uma metodologia para a modelagem de risers em escala reduzida, com foco na similaridade entre parâmetros importantes que garantam um comportamento dinâmico equivalente àquele em escala real. Foram, então, realizados experimentos de VIV com catenárias concebidas e construídas com base nesta metodologia, posteriormente ensaiadas em tanque de reboque. Para análise dos resultados, foram utilizadas técnicas como a decomposição modal, que possibilitou a comparação desses com alguns resultados da literatura, mostrando que o fenômeno de VIV na catenária se processa de maneira bastante semelhante ao identificado nos cilindros flexíveis. Além disso, a extensa base de dados experimentais apresentada e discutida nesta tese contribui para futura validação de diversos modelos matemáticos e códigos numéricos de predição do VIV em catenárias, descrevendo com detalhes inúmeros comportamentos imersos neste problema importante de engenharia. / The study of Vortex-induced vibrations in risers is a topic of great relevance in the context of ocean engineering. In recent years, several numerical and experimental results have been presented, addressing the understanding of the fundamentals of this fluid-structure interaction phenomenon, however without a conclusive and comprehensive understanding. In this context, risers are exposed to the action of ocean currents and consequently to the phenomenon of VIV. In this sense, the present study provides results of an experimental approach to understanding the VIV phenomenon on risers launched in catenary configuration, which represents much of the geometries adopted today. However, performing risers experiments in towing-tanks presupposes the use of small-scale models, which represents a major research challenge. Therefore, a methodology for modeling reduced scale risers was developed. The focus of this methodology was on the similarity between certain important parameters that guarantee an equivalent full-scale dynamic behavior. VIV experiments were then performed in the towing tank, with the designed and constructed catenary, based on this methodology. Techniques such as modal decomposition were used for the analysis of the results. This allows the comparison of the results with classical literature paradigms showing that VIV response in the catenary is close to results obtained with flexible cylinders. Furthermore, the large experimental database presented and discussed in the study may contribute to future validation of several mathematical models and numerical codes of VIV prediction in catenary risers, describing in detail several immersed behaviors in this important engineering problem.

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Depuis l'identification par Brown \& Roshko, en 1974, de structures jouant un rôle majeur dans le mélange d'un écoulement turbulent, la recherche de structures cohérentes a été un des principaux axes d'étude en mécanique des fluides.Les travaux présentés dans ce manuscrit s'inscrivent dans cette voie.La première partie du manuscrit traite ainsi de l'identification de structures cohérentes. Elle se compose de trois chapitres abordant deux techniques d’identification. La Décomposition en mode dynamique (DMD), ainsi que des variantes généralisant son champ applicatif est présenté dans le premier chapitre. Cette méthode propose une représentation par modes spatiaux et temporels d'un ensemble de données. Une méthode pour la sélection de composantes particulièrement représentatives de la dynamique, i.e. présentant de bonnes qualités d'observabilité, se basant sur cette décomposition est également décrite dans ce chapitre.Le deuxième chapitre traite de la détection de structures cohérentes lagrangienne, par suivi de particules. Ces structures permettent d'identifier les frontières matérielles et apportent des éclaircissements sur les mécanismes du mélange au sein de l'écoulement considéré.Ces méthodes sont appliquées, dans le chapitre trois, au cas d’un écoulement incompressible affleurant une cavité ouverte.La seconde partie du manuscrit traite des questions de représentation et discrimination de données scientifiques.Une réponse à la question de la représentation de structures cohérentes a été la mise en place d'outils permettant la visualisation interactive de jeux de données scientifiques, qui dont la présentation fait l'objet du chapitre quatre. En particulier, l'utilisation d'objets tangibles, représentant les données dans le monde réel, permet une exploration plus efficace des ensembles de volumétriques de données scientifiques. La question d'une perception et discrimination efficace de données représentées, e.g. la différentiation entre deux valeurs proches, est abordée dans le cinquième chapitre. / Brown \& Roshko identify the key role, as soon as 1974, of coherent structures in the mixing of a turbulenet jet. Since, the seek for coherent structures has been a major field in fluid mechanics. This manuscript follows this path, and aims at describing algorithm to compute and explore coherent structures from fluid mechanics dataset.The first part of this manuscript is dedicated to present algorithms for the computations of coherent structures from dataset. The first chapter exposes the Dynamical Modes Decomposition, and presents improvements of the method. A criterion to estimate the observability properties of components of the state vector is also presented.The second chapter aims at describing an efficient algorithm to compute Lagrangian Coherent Structures, which are somewhat equivalent to material frontiers in fluid flows, and highlight mixing dynamics.This methods are applied, in a third chapter, to caracterize the dynamics of an open cavity flow.The second parti of this manuscript is dedicated to the representation and discrimination of scientific dataset.The fourth chapter presents metaphors for the interactive exploration of scientific and volumetric dataset. The use of tangible interfaces is investigated. The last chapter deals with the differenciation between represented data, by proposing an algorithm for the differenciation between close but different signals.

L’industrie automobile fournie de plus en plus d’effort pour optimiser l’aérodynamique externe des véhicules afin de réduire son empreinte écologique. Dans ce cadre, l’objectif de ce projet est d’examiner les structures tourbillonnaires responsables de la dégradation de traînée et de proposer une solution de contrôle actif permettant d’améliorer l’efficacité aérodynamique d’un véhicule SUV. Après une étude expérimentale de la maquette POSUV échelle réduite, une analyse modale croisée permet d’identifier les structures périodiques corrélées de l’écoulement qui pilotent la dépression sur le hayon. Une solution de contrôle optimale par jets pulsés sur le parechoc arrière, est obtenue avec un algorithme génétique. Celle-ci permet de réduire la dépression du hayon de 20% et l’analyse croisée des résultats instationnaires avec contrôle montre un changement significatif de la distribution spectrale. Après deux études préliminaires sur la rampe inclinée à 25° et sur le Corps d’Ahmed à 47°, la simulation de POSUV à partir d’un solveur LES, en éléments finis, est validé par rapport aux résultats expérimentaux. L’approfondissement des résultats 3D permet de comprendre les pertes aérodynamiques. La simulation de l’écoulement contrôlé permet également d’identifier les mécanismes du contrôle d’écoulements. / The automotive industry dedicates a lot of effort to improve the aerodynamical performances of road vehicles in order to reduce its carbon footprint. In this context, the target of the present work is to analyze the origin of aerodynamic losses on a reduced scale generic Sport Utility Vehicle and to achieve a drag reduction using an active flow control strategy. After an experimental characterization of the flow past the POSUV, a cross-modal DMD analysis is used to identify the correlated periodical features responsible for the tailgate pressure loss. Thanks to a genetic algorithm procedure, 20% gain on the tailgate pressure is obtained with optimal pulsed blowing jets on the rear bumper. The same cross-modal methodology allows to improve our understanding of the actuation mechanism. After a preliminary study of the 25° inclined ramp and of the Ahmed Body computations, the numerical simulation of the POSUV is corroborated with experiments using the cross-modal method. Deeper investigations on the three-dimensional flow characteristics explain more accurately the wake flow behavior. Finally, the controlled flow simulations propose additional insights on the actuation mechanisms allowing to reduce the aerodynamic losses.

[ES] El principal desafío en los motores turbina de gas empleados en aviación reside en aumentar la eficiencia del ciclo termodinámico manteniendo las emisiones contaminantes por debajo de las rigurosas restricciones. Ésto ha conllevado la necesidad de diseñar nuevas estrategias de inyección/combustión que operan en puntos de operación peligrosos por su cercanía al límite inferior de apagado de llama. En este contexto, el concepto Lean Direct Injection (LDI) ha emergido como una tecnología prometedora a la hora de reducir los óxidos de nitrógeno (NOx) emitidos por las plantas propulsoras de los aviones de nueva generación. En este contexto, la presente tesis tiene como objetivos contribuir al conocimiento de los mecanismos físicos que rigen el comportamiento de un quemador LDI y proporcionar herramientas de análisis para una profunda caracterización de las complejas estructuras de flujo de turbulento generadas en el interior de la cámara de combustión. Para ello, se ha desarrollado una metodología numérica basada en CFD capaz de modelar el flujo bifásico no reactivo en el interior de un quemador LDI académico mediante enfoques de turbulencia U-RANS y LES en un marco Euleriano-Lagrangiano. La resolución numérica de este problema multi-escala se aborda mediante la descripción completa del flujo a lo largo de todos los elementos que constituyen la maqueta experimental, incluyendo su paso por el swirler y entrada a la cámara de combustión. Ésto se lleva a cabo través de dos códigos CFD que involucran dos estrategias de mallado diferentes: una basada en algoritmos de generación y refinamiento automático de la malla (AMR) a través de CONVERGE y otra técnica de mallado estático más tradicional mediante OpenFOAM. Por un lado, se ha definido una metodología para obtener una estrategia de mallado óptima mediante el uso del AMR y se han explotado sus beneficios frente a los enfoques tradicionales de malla estática. De esta forma, se ha demostrado que la aplicabilidad de las herramientas de control de malla disponibles en CONVERGE como el refinamiento fijo (fixed embedding) y el AMR son una opción muy interesante para afrontar este tipo de problemas multi-escala. Los resultados destacan una optimización del uso de los recursos computacionales y una mayor precisión en las simulaciones realizadas con la metodología presentada. Por otro lado, el uso de herramientas CFD se ha combinado con la aplicación de técnicas de descomposición modal avanzadas (Proper Orthogonal Decomposition and Dynamic Mode Decomposition). La identificación numérica de los principales modos acústicos en la cámara de combustión ha demostrado el potencial de estas herramientas al permitir caracterizar las estructuras de flujo coherentes generadas como consecuencia de la rotura de los vórtices (VBB) y de los chorros fuertemente torbellinados presentes en el quemador LDI. Además, la implementación de estos procedimientos matemáticos ha permitido tanto recuperar información sobre las características de la dinámica de flujo como proporcionar un enfoque sistemático para identificar los principales mecanismos que sustentan las inestabilidades en la cámara de combustión. Finalmente, la metodología validada ha sido explotada a través de un Diseño de Experimentos (DoE) para cuantificar la influencia de los factores críticos de diseño en el flujo no reactivo. De esta manera, se ha evaluado la contribución individual de algunos parámetros funcionales (el número de palas del swirler, el ángulo de dichas palas, el ancho de la cámara de combustión y la posición axial del orificio del inyector) en los patrones del campo fluido, la distribución del tamaño de gotas del combustible líquido y la aparición de inestabilidades en la cámara de combustión a través de una matriz ortogonal L9 de Taguchi. Este estudio estadístico supone un punto de partida para posteriores estudios de inyección, atomización y combus / [CA] El principal desafiament als motors turbina de gas utilitzats a la aviació resideix en augmentar l'eficiència del cicle termodinàmic mantenint les emissions contaminants per davall de les rigoroses restriccions. Aquest fet comporta la necessitat de dissenyar noves estratègies d'injecció/combustió que radiquen en punts d'operació perillosos per la seva aproximació al límit inferior d'apagat de flama. En aquest context, el concepte Lean Direct Injection (LDI) sorgeix com a eina innovadora a l'hora de reduir els òxids de nitrogen (NOx) emesos per les plantes propulsores dels avions de nova generació. Sota aquest context, aquesta tesis té com a objectius contribuir al coneixement dels mecanismes físics que regeixen el comportament d'un cremador LDI i proporcionar ferramentes d'anàlisi per a una profunda caracterització de les complexes estructures de flux turbulent generades a l'interior de la càmera de combustió. Per tal de dur-ho a terme s'ha desenvolupat una metodología numèrica basada en CFD capaç de modelar el flux bifàsic no reactiu a l'interior d'un cremador LDI acadèmic mitjançant els enfocaments de turbulència U-RANS i LES en un marc Eulerià-Lagrangià. La resolució numèrica d'aquest problema multiescala s'aborda mitjançant la resolució completa del flux al llarg de tots els elements que constitueixen la maqueta experimental, incloent el seu pas pel swirler i l'entrada a la càmera de combustió. Açò es duu a terme a través de dos codis CFD que involucren estratègies de mallat diferents: una basada en la generación automàtica de la malla i en l'algoritme de refinament adaptatiu (AMR) amb CONVERGE i l'altra que es basa en una tècnica de mallat estàtic més tradicional amb OpenFOAM. D'una banda, s'ha definit una metodologia per tal d'obtindre una estrategia de mallat òptima mitjançant l'ús de l'AMR i s'han explotat els seus beneficis front als enfocaments tradicionals de malla estàtica. D'aquesta forma, s'ha demostrat que l'aplicabilitat de les ferramente de control de malla disponibles en CONVERGE com el refinament fixe (fixed embedding) i l'AMR són una opció molt interessant per tal d'afrontar aquest tipus de problemes multiescala. Els resultats destaquen una optimització de l'ús dels recursos computacionals i una major precisió en les simulacions realitzades amb la metodologia presentada. D'altra banda, l'ús d'eines CFD s'ha combinat amb l'aplicació de tècniques de descomposició modal avançades (Proper Orthogonal Decomposition and Dynamic Mode Decomposition). La identificació numèrica dels principals modes acústics a la càmera de combustió ha demostrat el potencial d'aquestes ferramentes al permetre caracteritzar les estructures de flux coherents generades com a conseqüència del trencament dels vòrtex (VBB) i dels raigs fortament arremolinats presents al cremador LDI. A més, la implantació d'estos procediments matemàtics ha permès recuperar informació sobre les característiques de la dinàmica del flux i proporcionar un enfocament sistemàtic per tal d'identificar els principals mecanismes que sustenten les inestabilitats a la càmera de combustió. Finalment, la metodologia validada ha sigut explotada a traves d'un Diseny d'Experiments (DoE) per tal de quantificar la influència dels factors crítics de disseny en el flux no reactiu. D'aquesta manera, s'ha avaluat la contribución individual d'alguns paràmetres funcionals (el nombre de pales del swirler, l'angle de les pales, l'amplada de la càmera de combustió i la posició axial de l'orifici de l'injector) en els patrons del camp fluid, la distribució de la mida de gotes del combustible líquid i l'aparició d'inestabilitats en la càmera de combustió mitjançant una matriu ortogonal L9 de Taguchi. Aquest estudi estadístic és un bon punt de partida per a futurs estudis de injecció, atomització i combustió en cremadors LDI. / [EN] Aeronautical gas turbine engines present the main challenge of increasing the efficiency of the cycle while keeping the pollutant emissions below stringent restrictions. This has led to the design of new injection-combustion strategies working on more risky and problematic operating points such as those close to the lean extinction limit. In this context, the Lean Direct Injection (LDI) concept has emerged as a promising technology to reduce oxides of nitrogen (NOx) for next-generation aircraft power plants In this context, this thesis aims at contributing to the knowledge of the governing physical mechanisms within an LDI burner and to provide analysis tools for a deep characterisation of such complex flows. In order to do so, a numerical CFD methodology capable of reliably modelling the 2-phase nonreacting flow in an academic LDI burner has been developed in an Eulerian-Lagrangian framework, using the U-RANS and LES turbulence approaches. The LDI combustor taken as a reference to carry out the investigation is the laboratory-scale swirled-stabilised CORIA Spray Burner. The multi-scale problem is addressed by solving the complete inlet flow path through the swirl vanes and the combustor through two different CFD codes involving two different meshing strategies: an automatic mesh generation with adaptive mesh refinement (AMR) algorithm through CONVERGE and a more traditional static meshing technique in OpenFOAM. On the one hand, a methodology to obtain an optimal mesh strategy using AMR has been defined, and its benefits against traditional fixed mesh approaches have been exploited. In this way, the applicability of grid control tools available in CONVERGE such as fixed embedding and AMR has been demonstrated to be an interesting option to face this type of multi-scale problem. The results highlight an optimisation of the use of the computational resources and better accuracy in the simulations carried out with the presented methodology. On the other hand, the use of CFD tools has been combined with the application of systematic advanced modal decomposition techniques (i.e., Proper Orthogonal Decomposition and Dynamic Mode Decomposition). The numerical identification of the main acoustic modes in the chamber have proved their potential when studying the characteristics of the most powerful coherent flow structures of strongly swirled jets in a LDI burner undergoing vortex breakdown (VBB). Besides, the implementation of these mathematical procedures has allowed both retrieving information about the flow dynamics features and providing a systematic approach to identify the main mechanisms that sustain instabilities in the combustor. Last, this analysis has also allowed identifying some key features of swirl spray systems such as the complex pulsating, intermittent and cyclical spatial patterns related to the Precessing Vortex Core (PVC). Finally, the validated methodology is exploited through a Design of Experiments (DoE) to quantify the influence of critical design factors on the non-reacting flow. In this way, the individual contribution of some functional parameters (namely the number of swirler vanes, the swirler vane angle, the combustion chamber width and the axial position of the nozzle tip) into both the flow field pattern, the spray size distribution and the occurrence of instabilities in the combustion chamber are evaluated throughout a Taguchi's orthogonal array L9. Such a statistical study has supposed a good starting point for subsequent studies of injection, atomisation and combustion on LDI burners. / Belmar Gil, M. (2020). Computational study on the non-reacting flow in Lean Direct Injection gas turbine combustors through Eulerian-Lagrangian Large-Eddy Simulations [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/159882 / TESIS

La nécessaire représentation en échelle du monde nous amène à expliquer pourquoi la théorie des ondelettes en constitue le formalisme le mieux adapté. Ses performances sont comparées à d'autres outils : la méthode des étendues normalisées (R/S) et la méthode par décomposition empirique modale (EMD).La grande diversité des bases analysantes de la théorie des ondelettes nous conduit à proposer une approche à caractère morphologique de l'analyse. L'exposé est organisé en trois parties.Le premier chapitre est dédié aux éléments constitutifs de la théorie des ondelettes. Un lien surprenant est établi entre la notion de récurrence et l'analyse en échelle (polynômes de Daubechies) via le triangle de Pascal. Une expression analytique générale des coefficients des filtres de Daubechies à partir des racines des polynômes est ensuite proposée.Le deuxième chapitre constitue le premier domaine d'application. Il concerne les plasmas de bord des réacteurs de fusion de type tokamak. Nous exposons comment, pour la première fois sur des signaux expérimentaux, le coefficient de Hurst a pu être mesuré à partir d'un estimateur des moindres carrés à ondelettes. Nous détaillons ensuite, à partir de processus de type mouvement brownien fractionnaire (fBm), la manière dont nous avons établi un modèle (de synthèse) original reproduisant parfaitement la statistique mixte fBm et fGn qui caractérise un plasma de bord. Enfin, nous explicitons les raisons nous ayant amené à constater l'absence de lien existant entre des valeurs élevées du coefficient d'Hurst et de supposées longues corrélations.Le troisième chapitre est relatif au second domaine d'application. Il a été l'occasion de mettre en évidence comment le bien-fondé d'une approche morphologique couplée à une analyse en échelle nous ont permis d'extraire l'information relative à la taille, dans un écho rétrodiffusé d'une cible immergée et insonifiée par une onde ultrasonore / The necessary scale-based representation of the world leads us to explain why the wavelet theory is the best suited formalism. Its performances are compared to other tools: R/S analysis and empirical modal decomposition method (EMD). The great diversity of analyzing bases of wavelet theory leads us to propose a morphological approach of the analysis. The study is organized into three parts. The first chapter is dedicated to the constituent elements of wavelet theory. Then we will show the surprising link existing between recurrence concept and scale analysis (Daubechies polynomials) by using Pascal's triangle. A general analytical expression of Daubechies' filter coefficients is then proposed from the polynomial roots. The second chapter is the first application domain. It involves edge plasmas of tokamak fusion reactors. We will describe how, for the first time on experimental signals, the Hurst coefficient has been measured by a wavelet-based estimator. We will detail from fbm-like processes (fractional Brownian motion), how we have established an original model perfectly reproducing fBm and fGn joint statistics that characterizes magnetized plasmas. Finally, we will point out the reasons that show the lack of link between high values of the Hurst coefficient and possible long correlations. The third chapter is dedicated to the second application domain which is relative to the backscattered echo analysis of an immersed target insonified by an ultrasonic plane wave. We will explain how a morphological approach associated to a scale analysis can extract the diameter information