No description available.

No description available.

The diploma thesis summarizes the topics from hydromechanics taught at Czech grammar schools (and other Czech secondary schools). Emphasis is placed on the careful derivation of all laws and equations according to the basic principles stated in the thesis (Pascal's law, Archimedes' law, Bernoulli's equation, Euler's laws of motion). The aim of the author was to make all derivations in the thesis to not exceed the complexity of secondary school mathematics and physics. A new approach of teaching hydromechanics based on Euler's laws of motion is outlined. Furthermore, each of the hydromechanics topics is accompanied by (sometimes more or less unconventional) examples and experiments. 1

Este trabalho lida com o escoamento incompressível ao redor de pares de cilindros rígidos e imóveis, posicionados de forma alinhada em relação a uma corrente uniforme. São dois os objetivos desta pesquisa: o primeiro é estabelecer relações de causalidade entre características físicas do escoamento e as mudanças observadas nas forças e no campo fluido com as variações do número de Reynolds (Re) e do espaçamento entre os corpos; e o segundo consiste em compreender a influência mútua entre tridimensionalidades e interferência no escoamento. Utilizou-se o método de elementos espectrais para realizar simulações bi e tridimensionais do escoamento. Os espaçamentos entre centros (lcc) analisados vão de 1,5 a 8 diâmetros, e eles são comparados com o caso de um cilindro isolado. A faixa de Re vai de 160 a 320, compreendendo a transição na esteira. O foco foi dado nas instabilidades de pequena escala (modos A e B). Dados referentes ao número de Strouhal, coeficiente de arrasto médio, flutuação do coeficiente de sustentação e correlação axial são apresentados. Com auxílio de visualizações do escoamento, são propostos mecanismos que explicam o fenômeno de interferência, refletido no comportamento das curvas. Os resultados mostraram que simulações bidimensionais são insuficientes para se prever a combinação (Re, lcc) de inversão do arrasto. Verificou-se também que, quando o espaçamento é menor do que o crítico, o processo de transição na esteira se dá de forma diferente do observado para um cilindro isolado. / This work deals with the incompressible flow around pairs of rigid and immovable circular cylinders in tandem arrangements. There are two goals in this research: the first one is to find causality relationships between physical characteristics of the flow and the changes that are observed in the forces and in the flow field with the variation of the Reynolds number (Re) and the distance between the bodies; and the second one is to comprehend the mutual influence between three-dimensional structures and interference. The spectral element method was employed to carry out two- and three-dimensional simulations of the flow. The centre-to-centre distance (lcc) of the investigated configurations varies between 1.5 and 8 diameters, and they are compared to the isolated cylinder case. The Re range goes from 160 to 320, covering the transition in the wake. We focused in the small scale instabilities (modes A and B). Data of Strouhal number, mean drag coefficient, RMS of the lift coefficient and axial correlation are presented. With aid of flow visualizations, we propose mechanisms to explain the interference phenomenon, which is reflected in the behaviour of the graphics. The results show that two-dimensional simulations are not sufficient to predict the (Re, lcc) pair correspondent to the drag inversion point. We also verified that, in the cases where lcc is lower than the critical spacing, the transition in the wake happens in a way different from the one observed in the flow around a single cylinder.

No description available.

No description available.

Este trabalho lida com o escoamento incompressível ao redor de pares de cilindros rígidos e imóveis, posicionados de forma alinhada em relação a uma corrente uniforme. São dois os objetivos desta pesquisa: o primeiro é estabelecer relações de causalidade entre características físicas do escoamento e as mudanças observadas nas forças e no campo fluido com as variações do número de Reynolds (Re) e do espaçamento entre os corpos; e o segundo consiste em compreender a influência mútua entre tridimensionalidades e interferência no escoamento. Utilizou-se o método de elementos espectrais para realizar simulações bi e tridimensionais do escoamento. Os espaçamentos entre centros (lcc) analisados vão de 1,5 a 8 diâmetros, e eles são comparados com o caso de um cilindro isolado. A faixa de Re vai de 160 a 320, compreendendo a transição na esteira. O foco foi dado nas instabilidades de pequena escala (modos A e B). Dados referentes ao número de Strouhal, coeficiente de arrasto médio, flutuação do coeficiente de sustentação e correlação axial são apresentados. Com auxílio de visualizações do escoamento, são propostos mecanismos que explicam o fenômeno de interferência, refletido no comportamento das curvas. Os resultados mostraram que simulações bidimensionais são insuficientes para se prever a combinação (Re, lcc) de inversão do arrasto. Verificou-se também que, quando o espaçamento é menor do que o crítico, o processo de transição na esteira se dá de forma diferente do observado para um cilindro isolado. / This work deals with the incompressible flow around pairs of rigid and immovable circular cylinders in tandem arrangements. There are two goals in this research: the first one is to find causality relationships between physical characteristics of the flow and the changes that are observed in the forces and in the flow field with the variation of the Reynolds number (Re) and the distance between the bodies; and the second one is to comprehend the mutual influence between three-dimensional structures and interference. The spectral element method was employed to carry out two- and three-dimensional simulations of the flow. The centre-to-centre distance (lcc) of the investigated configurations varies between 1.5 and 8 diameters, and they are compared to the isolated cylinder case. The Re range goes from 160 to 320, covering the transition in the wake. We focused in the small scale instabilities (modes A and B). Data of Strouhal number, mean drag coefficient, RMS of the lift coefficient and axial correlation are presented. With aid of flow visualizations, we propose mechanisms to explain the interference phenomenon, which is reflected in the behaviour of the graphics. The results show that two-dimensional simulations are not sufficient to predict the (Re, lcc) pair correspondent to the drag inversion point. We also verified that, in the cases where lcc is lower than the critical spacing, the transition in the wake happens in a way different from the one observed in the flow around a single cylinder.

Les phénomènes vibratoires à haute fréquence dans les amortisseurs sont défavorables du point de vue de la qualité sonore du véhicule. Les efforts transmis de l’amortisseur à la caisse, situés dans une bande de fréquences entre 200 Hz et 800 Hz, sont responsables du bruit que l’on appelle « gloglottement ». Ce bruit nait dans l’amortisseur lorsque la voiture roule sur une route dégradée et se transmet à la caisse par voie solidienne. La compréhension du phénomène de gloglottement et sa prédiction nécessitent l’identification des mécanismes physiques qui vont générer des pics d’efforts et des vibrations hautes fréquences. Ceux-ci sont liés aux comportements de l’huile de l’amortisseur, des composants mécaniques internes et des interactions entre ces éléments. Les moyens mis en oeuvre pour cette étude sont d’une part le développement d’un modèle d’amortisseur qui prend en compte la modélisation de la dynamique des clapets, des effets de « stiction » des clapets, du frottement et des relations entre les pertes de charges et les débits. D’autre part, une campagne d’essais a permis de construire le modèle et de le valider. La simulation permet de reproduire les phénomènes physiques observés à la mesure et ainsi d’identifier les mécanismes à l’origine du bruit, comme l’ouverture du clapet à ressort et la « stiction » à l’ouverture du clapet anti-retour. La sensibilité de la réponse vibratoire haute fréquence du modèle vis-à-vis de ses paramètres est évaluée avec la méthode de Morris. De plus, des orientations sont données sur la valeur des paramètres de conception dans le but de minimiser les pics d’efforts générés par l’amortisseur. / High-frequency vibratory phenomena in shock absorbers are not suitable for the car sound quality. Forces (in the frequency range [200-800] Hz) transmitted from the shock absorber to the car body are responsible for the so-called "rattling noise". This structure-borne noise is starting from the shock absorber when the car drives over a rough road and is transmitted to the car body by structural transfer path. In order to understand and predict the phenomena, physical mechanisms generating high-frequency vibrations and peaks in the shock absorber response must be identified. These mechanisms are closely related to oil behaviour, internal mechanical components and interactions between all these elements. The present work consists on the one hand in modelling the shock absorber taking into account valve dynamics, valve stiction, friction and loss of pressure relations in the orifices. On the other hand, experimental shock absorber testing was conducted in order to build the model and to validate it. Model simulations reproduce observed phenomena in the experiments and helped us to identify the mechanisms leading to structure-borne noise, like spring valve opening and check-valve stiction. Sensitivity of the model response due to its input parameters was evaluated with Morris method. Moreover, some guidances are given in order to reduce the level of structure-borne noise generated by the shock absorber.

This master’s thesis deals with the analysis of geometric shapes fuselage amfibie SEAGLE and propose optimal shapes for the improvement of hydrodynamic and aerodynamic properties.

Les freins aéronautiques sont soumis à des instabilités vibratoires induites par le frottement. Il en résulte des vibrations qui présentent un risque pour la structure du frein et de l’atterrisseur et posent des problèmes d’intégration. Safran Landing Systems doit donc répondre à des spécifications avionneurs strictes sur les niveaux des vibrations générées par son équipement. Le respect de ces spécifications est actuellement contrôlé par la réalisation d’essais de freinage longs et coûteux. L’objectif de ces travaux de recherche est de reproduire numériquement ces phénomènes vibratoires via des outils intégrables au processus de conception d’un frein. Le crissement de frein, bien qu’il soit l’objet de recherches depuis le début du XXe siècle, demeure un phénomène assez mal compris, notamment dans l’aéronautique. Des vibrations instables apparaissent régulièrement sur l’ensemble de la plage fréquentielle 0-2 kHz. Au cours de la dernière décennie, une instabilité vibratoire vers 200 Hz dénommée whirl 2 s’est manifestée de manière récurrente et souvent critique sur la plupart des nouveaux freins développés. On cherche donc à mettre en place une méthode permettant de simuler l’apparition et les amplitudes des instabilités vibratoires, notamment du mode de whirl 2. Dans une première partie, on présente des analyses d’essais vibratoires réalisés en conditions opérationnelles et expérimentales. On décrit ensuite la modélisation par la méthode des éléments finis du frein instable au sens de Lyapunov. La stabilité du système linéarisé est étudiée et on montre une corrélation en fréquence et déformée entre le modèle et les essais. Ce modèle éléments finis est trop volumineux en l’état pour permettre la simulation d’amplitudes de vibrations non linéaires. On propose donc dans une seconde partie deux méthodes de réduction adaptées à l’architecture complexe d’un système de freinage aéronautique et permettant la prise en compte du frottement. La première est une méthode semi-analytique qui se révèle très performante jusqu’à 500 Hz. La seconde méthode de réduction mise en oeuvre est la double synthèse modale. Elle est implémentée dans sa version classique, puis une amélioration est proposée avec succès : la double synthèse modale complexe. La troisième partie est consacrée à l’étude de la dynamique non linéaire du whirl 2 par la réalisation d’intégrations temporelles. La simulation des amplitudes de vibration nécessite la prise en compte réaliste du comportement non linéaire du frein. Or, on fait d’abord le constat que, contrairement à une hypothèse communément admise, les non-linéarités de contact situées aux interfaces frottantes ne suffisent pas à expliquer à elles seules la saturation des amplitudes vibratoires constatée expérimentalement. La recherche des phénomènes physiques non linéaires influents nous amène a considérer l’interaction de la structure vibrante avec le circuit hydraulique de commande du frein. La modélisation du couplage hydrodynamique fournit alors des éléments de compréhension inédits et permet de formuler des règles de conception. Enfin on étudie l’impact du frottement sec dans les contacts périphériques des disques de freinage avec la structure. Ce phénomène, jusque là négligé, apparaît largement prépondérant. Des études d’influences, présentant une bonne corrélation avec les essais, permettent de mettre en évidence de manière robuste l’influence du design et des scénarios de freinage sur les amplitudes vibratoires. / These vibrations are a threat for the brake and landing-gear structural integrity and represent an issue in terms of integration. Thus Safran Landing Systems has to comply with aircraft manufacturers’ strict requirements on the vibration amplitude its product is likely to generate. Compliance to these requirements is assessed by long and costly braking test campaigns. The objective of the research presented here is to reproduce by simulation the brake dynamic instabilities with numerical tools that could be integrated in the design process. Brake squeal has been a research topic since the early XXth century. However it remains a rather ill-understood phenomenon, especially in aeronautics. Unstable vibrations regularly appear on the whole 0-2kHz frequency spectrum. In the last decade, an instability located around 200 Hz called whirl 2 persistently appeared on the newly developed wheel and brake assemblies, sometimes exhibiting critical vibration amplitudes. Consequently, Safran Landing Systems wishes to develop numerical tools able to simulate both the occurrence and the amplitudes associated with friction-induced instabilities, especially with the whirl 2 mode. In the first part of this report, an experimental analysis of the brake is conducted, on both laboratory and in operational set-ups. The modelling of the wheel and brake assembly using the finite element method is then described. The system stability in a Lyapunov’s sense is studied and shows good correlation in both frequencies and mode shapes with the experiments. This finite element model is too big to be used to perform the transient simulation of the nonlinear amplitudes. In the second part, two reduction methods, tailored to the complex aircraft brakes architectures, are thus presented. The first method is a semi-analytical. It shows excellent performances up to 500 Hz. The second reduction method is the double modal synthesis, implemented under its classical version. It is then successfully improved and called "complex double modal synthesis". The third part is dedicated to the study of the nonlinear dynamics of the whirl 2 through transient analyses. The nonlinear amplitudes simulation requires taking into account the relevant nonlinear brake behavior. However, it is first observed that, contrary to a commonly accepted hypothesis, the contact nonlinearities located at the friction interfaces cannot single-handedly account for the vibration amplitudes saturation observed in the tests. The need to identify the relevant physical phenomena leads then to consider the interaction between the squealing brake structure and its hydraulic command circuit. The modelling of the hydro-mechanical coupling provides an unprecedented insight and allows to prescribe design rules. Finally, we study the impact of dry friction in the peripheral contacts between the braking discs and the structure. This phenomenon, neglected until now, appears to have a major influence. Sensitivity studies exhibit a good correlation with tests, allowing to highlight, in a robust manner, the impact of brake design and braking scenarii on the nonlinear vibration amplitudes.