As estruturas aeronáuticas estão sujeitas a diversas solicitações, devido principalmente às interações com o escoamento aerodinâmico, que podem causar distúrbios e vibrações, comprometendo seu desempenho. As medidas aerodinâmicas aplicadas em uma aeronave podem ser obtidas por simulações computacionais ou testes experimentais. No entanto, podem existir imperfeições na simulação computacional, como por exemplo, se conseguir reproduzir algumas condições de vôo real. Sendo assim, diversas pesquisas vêm sendo realizadas para solucionar estes problemas. Dentre elas estão os testes experimentais feitos em túnel de vento com modelos de escala real em diversas condições de vôo. Desta forma, a construção de um modelo físico de um aerofólio em escala reduzida e a implementação de sensores a este modelo torna-se uma ferramenta bastante importante para validar resultados teóricos e experimentais. Assim, nesse trabalho realizou-se a construção de um modelo de aerofólio NACA0012, o desenvolvimento de um mecanismo de fixação do modelo ao túnel de vento e a implementação de um controlador de oscilação forçada. O modelo físico realiza oscilações harmônicas, em regime não-estacionário. O objetivo do trabalho foi mapear as medidas de pressão atuantes sobre modelo ensaiado em regime estacionário e não-estacionário e fazer a comparação entre os dois casos. / Aeronautical structures are affected by many loads, most of them given by the aerodynamic flow interactions. These flow interactions may cause vibration leading to structural failure, such as cracks and fatigue. The aerodynamic flow interactions can be measured by experiment or predicted by computational simulation. Otherwise, computational simulations on its own are not reliable and can not reproduce a real flight condition, such as the mean atmospheric turbulence dynamic. Many researches has been done to solve these problems for computational simulations. One of them are the wind tunnel experiments with a full scale models in many flight conditions for posterior comparison. For a smaller wind tunnel, a small scale physical prototype well instrumented becomes an important solution to validate theoretical and experimental results. In the present work the construction of a NACA 0012 airfoil model, the development of a constraint mechanism and the implementation of a forced oscilation control system were done. The physical model oscilates with a given frequency. The aim of present work is to map the pressure measurements actuating on the model, testing it under a steady state condition and a transient condition for posterior comparison of both conditions.

L'éolienne Darrieus connaît un intérêt accru ces dernières années parce qu'elle représente une solution alternative potentielle de production d'électricité dans les milieux urbains. En particulier,une éolienne de forte solidité peut être choisie car certaines de ses propriétés peuvent être avantageuses pour son implantation proche de zones habitées. A l'inverse, certaines difficultés aérodynamiques émergent. Ce type d'éolienne fonctionne à de faibles vitesses réduites pour lesquelles le décrochage dynamique a un rôle très significatif. L'objectif de ce travail de thèse consiste à compléter la connaissance du phénomène de décrochage dynamique sur une éolienne à axe vertical afin d'améliorer les modèles numériques de prédiction existants. Cette étude s'appuie sur une analyse combinée de résultats numériques et expérimentaux. Les simulations numériques sont produites avec une méthode des panneaux bidimensionnelle instationnaire. Les effets de la viscosité sont introduits par des corrections utilisant notamment un modèle semi-empirique de décrochage dynamique. Le travail expérimental s'est concentrée sur la dynamique tourbillonnaire à proximité immédiate du rotor résultante du décrochage dynamique. Le montage se compose d'une éolienne à pale droite placée dans une soufflerie. Des mesures instationnaires de la répartition de pression pariétale le long de la corde et des mesures de champ de vitesse par vélocimétrie par images de particules ont été accomplies. Les résultats révèlent la manière dont les caractéristiques du décrochage dynamique sont conditionnées par la vitesse réduite. Le retard au décrochage, l'intensité de l'effet du tourbillon de décrochage dynamique et sa convection ont été quantifiés. Enfin, un examen critique de l'applicabilité du modèle de Leishman-Beddoes pour simuler efficacement les effets du décrochage dynamique a été réalisé. / The Darrieus wind turbine has entered a period of renewed interest over the last years because it may stand for an alternative solution to produce electricity in urban areas. In particular, high solidity wind turbine can be chosen to take benefit from some of its key properties for use near populated city areas. Conversely, some aerodynamic problems arise. This type of wind turbine operates at low tip speed ratio for which dynamic stall has a very significant role. The goal of this work is to provide valuable data to complement the knowledge of the dynamic stall phenomenon that occurs on a vertical axis wind turbine in order to improve existing numerical models. This study relies on a combined analysis of numerical and experimental results. The numerical simulations are based on a bidimensional unsteady vortex panel method. Effects of viscosity are introduced by adding corrections computed with a semi-empirical dynamic stall model. The experimental work focuses on the dynamics of the shed vortices existing in the vicinity of the rotor as a result of dynamic stall. The set-up consists of a straight-bladed wind turbine tested in a wind tunnel. Unsteady pressure distribution measurements along the chord and velocity fields measurements by particle image velocimetry were carried out. Results indicate how the characteristics of dynamic stall are conditioned by the tip speed ratio. Stall inception delay, magnitude of the dynamic stall vortex effects and its convection velocity were evaluated. Blade/Vortex interaction was analyzed through the observation of the vortical system downstream of the rotor. In addition, a critical review of the suitability of the Leishman-Beddoes model to effectively simulate the effects of dynamic stall was accomplished.

<p>A series of aeroelastic analyses is performed for a flexible high-aspect-ratio wing representative of a high altitude long endurance (HALE) aircraft. Such aircraft are susceptible to dynamic instabilities such as flutter, which can lead to large amplitude limit cycle oscillations. These structural motions are modeled by a representative linear typical section model and by Hodges-Dowell beam theory, which includes leading-order nonlinear elastic coupling. Aerodynamic forces are represented by the ONERA dynamic stall model with its coefficients calibrated to CFD data versus wind tunnel test data. Time marching computations of the coupled nonlinear beam and ONERA system highlight a number of features relevant to the aeroelastic response of HALE aircraft, including the influence of a tip store, the sensitivity of the flutter boundary and limit cycle oscillations to aerodynamic CFD or test data, and the roles of structural nonlinearity and nonlinear aerodynamic stall in the dynamic stability of high-aspect-ratio wings.</p> / Dissertation

Cette thèse de doctorat a été réalisée dans le cadre du projet HARVEST, programme de recherche initié en 2001 au LEGI et consacré au développement d'un nouveau concept d'hydrolienne à axe vertical inspiré des turbines Darrieus pour la récupération de l'énergie cinétique des courants marins et fluviaux. Ce travail s'est focalisé sur la mise en place d'un moyen de mesure par Vélocimétrie par Image de Particules deux dimensions – deux composantes (2D-2C) et deux dimensions – trois composantes (2D-3C). L'objectif est d'une part de constituer une base de données expérimentale pour la validation locale des simulations numériques RANS 2D et 3D menées dans le cadre de travaux précédents, et d'autre part d'améliorer la compréhension des phénomènes hydrodynamiques instationnaires rencontrés dans ces machines et en particulier du décrochage dynamique. La confrontation des mesures expérimentales et des simulations a notamment permis de mettre en évidence les points forts et les limites des modèles numériques dans les différents régimes de fonctionnement de la machine. / This PhD thesis has been carried out within the framework of the HARVEST project. This research program, initiated in 2001 by the LEGI laboratory (Grenoble, France), is devoted to the development of a new marine turbine concept inspired from Darrieus turbines in order to convert kinetic energy of marine, tidal or river currents into electric energy. This work has been focused on the development of an experimental apparatus based on two dimensions – two components (2D-2C) and two dimensions – three components (2D-3C) Particle Image Velocimetry. The objective is to provide an experimental database for the local validation of 2D and 3D RANS computations and to improve our understanding of unsteady hydrodynamics phenomena experienced in this type of turbine and especially of dynamic stall. Comparison between measurements and computations enabled to identify strengths and limitations of numerical models for various operating conditions of this type of turbine.

As estruturas aeronáuticas estão sujeitas a diversas solicitações, devido principalmente às interações com o escoamento aerodinâmico, que podem causar distúrbios e vibrações, comprometendo seu desempenho. As medidas aerodinâmicas aplicadas em uma aeronave podem ser obtidas por simulações computacionais ou testes experimentais. No entanto, podem existir imperfeições na simulação computacional, como por exemplo, se conseguir reproduzir algumas condições de vôo real. Sendo assim, diversas pesquisas vêm sendo realizadas para solucionar estes problemas. Dentre elas estão os testes experimentais feitos em túnel de vento com modelos de escala real em diversas condições de vôo. Desta forma, a construção de um modelo físico de um aerofólio em escala reduzida e a implementação de sensores a este modelo torna-se uma ferramenta bastante importante para validar resultados teóricos e experimentais. Assim, nesse trabalho realizou-se a construção de um modelo de aerofólio NACA0012, o desenvolvimento de um mecanismo de fixação do modelo ao túnel de vento e a implementação de um controlador de oscilação forçada. O modelo físico realiza oscilações harmônicas, em regime não-estacionário. O objetivo do trabalho foi mapear as medidas de pressão atuantes sobre modelo ensaiado em regime estacionário e não-estacionário e fazer a comparação entre os dois casos. / Aeronautical structures are affected by many loads, most of them given by the aerodynamic flow interactions. These flow interactions may cause vibration leading to structural failure, such as cracks and fatigue. The aerodynamic flow interactions can be measured by experiment or predicted by computational simulation. Otherwise, computational simulations on its own are not reliable and can not reproduce a real flight condition, such as the mean atmospheric turbulence dynamic. Many researches has been done to solve these problems for computational simulations. One of them are the wind tunnel experiments with a full scale models in many flight conditions for posterior comparison. For a smaller wind tunnel, a small scale physical prototype well instrumented becomes an important solution to validate theoretical and experimental results. In the present work the construction of a NACA 0012 airfoil model, the development of a constraint mechanism and the implementation of a forced oscilation control system were done. The physical model oscilates with a given frequency. The aim of present work is to map the pressure measurements actuating on the model, testing it under a steady state condition and a transient condition for posterior comparison of both conditions.

L'enveloppe de vol des hélicoptères est limitée par le décrochage dynamique, que le contrôle d'écoulement permet de surmonter. Malgré de nombreuses études, aucun dispositif de contrôle ne s'est révélé utilisable sur un rotor. Conçu par l'ONERA, un Vortex Generator déployable (DVG) s'est révélé efficace pour limiter les effets du décrochage dynamique d'un profil OA209 en oscillation de tangage. Mais l'étude de cet actionneur est nécessaire avant son application. Le travail exposé dans le présent mémoire s'attache à reproduire par simulations numériques l'écoulement du décrochage dynamique contrôlé par DVG afin d'identifier les phénomènes physiques en jeu. Après une première partie bibliographique dédiée au contrôle de décrochage dynamique et aux Vortex Generators, une seconde partie a été consacrée à valider et étudier la simulation numérique du contrôle du décrochage statique. L'écoulement induit a pu être ainsi caractérisé par les interactions tourbillonnaires présentes du fait de l'épaisseur du DVG et qui réduisent l'efficacité du contrôle. Cette analyse a permis d'élaborer une modélisation préliminaire du DVG. Dans une troisième partie, la validation et l'étude de la simulation numérique du contrôle du décrochage dynamique a mis en évidence un effet de contrôle similaire au cas statique, et la décomposition en modes propres orthogonaux de l'écoulement contrôlé a montré une altération du mode lié au tourbillon de décrochage dynamique. Ce travail valide la simulation numérique mise en place, laisse entrevoir des perspectives d'amélioration du dispositif de contrôle et permet la simulation numérique ultérieure du contrôle de décrochage dynamique de voilure tournante. / The helicopter flight envelope is limited by dynamic stall, which can be reduced by flow control actuation. Numerous studies are dedicated to the alleviation of dynamic stall, however no actuator has ever been used on real rotor configuration. A new deployable Vortex Generator-type actuator called DVG has been designed by the ONERA and proved its efficiency over pitch-oscillating dynamic stall control of an OA209 airfoil. However, the control effect must be in-depth analyzed before further application. Therefore, the present works aims at reproducing the dynamic stall control through numerical simulations,in order to investigate the physics involved. After a first part dedicated to the literature review of dynamic stall control and Vortex Generators, a second part considered the validation and investigation of the static stall control. DVG-induced secondary flow has been characterized by its vortex interactions, which reduce the control efficiency because of the DVG thickness. This analysis made a preliminary modeling of the DVG possible. In a third part, validation and investigation of the dynamic stall control have been performed, and the actuation showed strong similarities with the static case. Eventually, Proper Orthogonal Decomposition of the flow brought evidence of Dynamic Stall Vortex mode distortion thanks to DVG presence. This work validates the numerical simulation methodology, lets hope possible improvement of the actuator design and allows further numerical simulations of dynamic stall control over real helicopter blades.

Cette thèse est consacrée à l'étude d'amortisseurs passifs non linéaires innovants pour le contrôle de l'instabilité de flottement sur un profil d'aile à deux degrés de libertés. Lorsqu'un profil d'aile entre en flottement, il oscille de façon croissante jusqu'à se stabiliser sur un cycle limite dont l'amplitude peut être significative et détériorer sa structure. Le contrôle a ainsi deux objectifs principaux : retarder l'apparition de l'instabilité et réduire l'amplitude des cycles limites. Avant d'étudier l'influence des amortisseurs passifs, l'instabilité de flottement, et notamment le régime post-flottement, a été étudié. Une expérience de flottement sur une plaque plane a été menée et sa modélisation, prenant en compte le phénomène de décrochage dynamique, a été réalisée. Concernant le contrôle passif, le premier type d'amortisseur étudié est un amortisseur hystérétique réalisé à l'aide de ressorts en alliage à mémoire de forme. La caractéristique principale de tels amortisseurs est que leur force de rappel étant hystérétique, elle permet de dissiper une grande quantité d'énergie. L'objectif principal est ainsi de réduire l'amplitude des cycles limites provoqués par l'instabilité de flottement. Cet effet escompté a été observé et quantifié expérimentalement et numériquement à l'aide de modèles semi-empiriques. Le second type d'amortisseur utilisé est un amortisseur non linéaire de vibration accordé. Il est composé d'une petite masse connectée au profil d'aile à l'aide d'un ressort possédant une raideur linéaire et une raideur cubique. La partie linéaire de ce type d'amortisseur permet de retarder l'apparition de l'instabilité tandis que la partie non linéaire permet de réduire l'amplitude des cycles limites. L'influence de l'amortisseur non linéaire de vibration accordé a été étudiée analytiquement et numériquement. Il a été trouvé que l'apparition de l'instabilité est significativement retardée à l'aide de cet amortisseur, l'effet sur l'amplitude des cycles limites étant plus modeste. / The aim of this thesis is to study the effect of passive nonlinear absorbers on the two degrees of freedom airfoil flutter. When an airfoil is subject to flutter instability, it oscillates increasingly until stabilizing on a limit cycle, the amplitude of which can be possibly substantial and thus damage the airfoil structure. The control has two main objectives : delay the instability and decrease the limit cycle amplitude. The flutter instability, and the post-flutter regime in particular, were studied first. A flutter experiment on a flat plate airfoil was conducted and the airfoil behavior was modeled, taking into account dynamic stall. Regarding the passive control, the first absorber studied was a hysteretic damper, realized using shape memory alloys springs. The characteristic of such dampers is their hysteretic restoring force, allowing them to dissipate a large amount of energy. Their main goal was thus to decrease the limit cycle amplitude caused by the flutter instability. This expected effect was observed and quantified both experimentally and numerically, using heuristic model. The second absorber studied was a nonlinear tuned vibration absorber. This absorber consists of a light mass attached to the airfoil through a spring having both a linear and a cubic stiffness. The role of the linear part of such absorber was to repel the instability threshold, while the aim of the nonlinear part was to decrease the limit cycle amplitude. It was found, analytically and numerically, that the instability threshold is substantially shifted by this absorber, whereas the limit cycle amplitude decrease is relatively modest.

<p>This thesis reports on a numerical and experimental investigation of the unsteady loading of high solidity vertical axis wind turbines (VAWTs). Two-dimensional, unsteady Reynolds averaged Navier-Stokes simulations of a small scale, high solidity, H-type Darrieus vertical axis wind turbine revealed the dominant effect of dynamic stall on the power production and vibration excitation of the turbine. Operation of the turbine at low blade speed ratios resulted in complex flow-blade interaction mechanisms. These include; dynamic stall resulting in large scale vortex production, vortex impingement on the source blade, and significant flow momentum extraction.</p> <p>To validate the numerical model, a series of full-scale experimental wind tunnel tests were performed to determine the aerodynamic loading on the turbine airfoils, vibration response behaviour, and wake velocity. In order to accomplish this, a complex force measurement and wireless telemetry system was developed. During the course of this investigation, high vibration response of the turbine was observed. This resulted in conditions that made it difficult or impossible to measure the underlying aerodynamic loading. A vibration mitigation methodology was developed to remove the effect of vibration from the measured aerodynamic forces. In doing so, an accurate and complete measurement of the aerodynamic loading on the turbine blades was obtained.</p> <p>Comparison of the two-dimensional numerical model results to the experimental measurements revealed a considerable over-prediction of the turbine aerodynamic force and power coefficients, and wake velocity. From this research, it was determined that the three-dimensional flow effects due to the finite aspect ratio of the turbine and blades, as well as parasitic losses, could be accounted for through the application of inlet velocity and turbine height correction factors. In doing so, the two-dimensional numerical model results could be properly scaled to represent the three-dimensional flow behaviour of the turbine prototype. Ultimately, a validated VAWT design tool was developed.</p> / Doctor of Philosophy (PhD)