This thesis presents the results of an experimental study that investigates the effects of steady vortex generator jets (VGJs) integrated to a low pressure turbine blade to control the laminar separation bubble occurring on the suction surface of the blade at low Reynolds numbers. The injection technique involves jets issued from the holes located near the suction peak of the test blade which is in the middle of a five-blade low speed linear cascade facility. Three injection cases are tested with different blowing ratio values ranging from low to high. Surface pressure and particle image velocimetry (PIV) measurements are performed. The results show that steady VGJ is effective in eliminating the laminar separation bubble. Also it is observed that to have fully developed attached boundary layer, blowing ratio should be chosen accordingly since a very thin separation zone still exists at low blowing ratios.

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Cette thèse décrit le développement d'actionneurs piézoélectriques miniaturisés pour le contrôle actif d'écoulement à haute vitesse. L'objectif est de retarder la transition laminaire turbulente par le contrôle actif des ondes de Tollmien-Schlichting par introduction d'ondes artificielles à phase inversée. Le premier concept étudié dans cette thèse est un actionneur amplifié mécaniquement avec une structure de type levier élastique situé au-dessus d'une céramique piézoélectrique. Les avantages de cet actionneur sont le contrôle direct, la fonction de transfert linéaire, et le temps de réponse très rapide, ce qui facilite l'intégration dans un système asservi. Le deuxième concept est basé sur l'utilisation d'une membrane conique actionnée par un anneau piézoélectrique dans un mode propre axisymétrique. La géométrie conique focalise les ondes générées par l'anneau piézoélectrique vers le centre ou les amplitudes sont le plus importantes. Pour le control des ondes de Tollmien-Schlichting, la modulation d'amplitude est appliquée pour atteindre une gamme de fréquence large ou la fréquence porteuse est délivrée par les modes propres. Cet actionneur à l'avantage d'utiliser une membrane robuste fermée et une intégration simple. L'effet des actionneurs sur l'écoulement a été mesuré avec des capteurs film chaud dans une soufflerie en boucle ouverte. Pour les dimensions des actionneurs adaptés sur l'écoulement, l'effet désiré pour le contrôle actif de la transition a été observé. Enfin, il a été démontré que la modulation d'amplitude est une alternative intéressante si la fréquence porteuse est sélectionnée de manière optimale pour qu'elle n'agisse pas sur l'écoulement (> 40kHz).

A realização deste trabalho tem como objetivo fundamental estudar os efeitos dos atuadores de jato sintético no escoamento de uma camada limite desenvolvida sobre uma placa plana e um aerofólio hipotético. A interação dos jatos sintéticos com um escoamento transversal pode conduzir a uma aparente modificação da forma aerodinâmica de corpos rombudos e, dessa forma, fornecer uma maneira de controle da separação na camada limite. Estudos recentes demonstram que tipos diferentes de escoamentos podem ser produzidos pelo atuador dependendo da oscilação da membrana. Um método numérico para solução das equações de Navier-Stokes incompressíveis bidimensionais na formulação vorticidade-velocidade é utilizado neste trabalho. As equações governantes são discretizadas utilizando-se métodos de diferenças finitas compactas de sexta ordem para as derivadas espaciais. A equação de Poisson para a componente da velocidade normal é resolvida por um método iterativo de sobre-relaxação em linhas sucessivas usando um esquema com malha composta para acelerar a convergência. Os resultados de simulações com diferentes valores de freqüência, amplitude e comprimento de fenda foram verificados através de uma análise de Fourier temporal. Através desta análise é verificado qual a melhor situação para se atrasar a separação da camada limite / This work has as a fundamental objective the study of the effects of synthetic jet actuators on the boundary layer flow on a flat plate and on a hypothetical airfoil. The interaction of synthetic jets with transverse flow can lead to an apparent modification in the aerodynamic shape of blunt bodies and, in that way, supply a means of control of transition within the boundary layer. Recent studies demonstrate that different types of flow may be produced by the actuator, depending on the amplitude of oscillation of the membrane. A numerical method for the solution of two-dimensional incompressible Navier-Stokes equations written in vorticity-velocity formulation is used in this work. The spatial derivatives are discretized with a sixth order compact finite differences scheme. The Poisson equation for the normal velocity component is solved by an iterative line successive over relaxation method and uses a multigrid full approximation scheme to accelerate the convergence. The results of simulations with different values of frequency, amplitude and slot length were verified through a temporal Fourier analysis. By way of this analysis it is verified which are the better parameters for the controlled delay of boundary layer separation

A realização deste trabalho tem como objetivo fundamental estudar os efeitos dos atuadores de jato sintético no escoamento de uma camada limite desenvolvida sobre uma placa plana e um aerofólio hipotético. A interação dos jatos sintéticos com um escoamento transversal pode conduzir a uma aparente modificação da forma aerodinâmica de corpos rombudos e, dessa forma, fornecer uma maneira de controle da separação na camada limite. Estudos recentes demonstram que tipos diferentes de escoamentos podem ser produzidos pelo atuador dependendo da oscilação da membrana. Um método numérico para solução das equações de Navier-Stokes incompressíveis bidimensionais na formulação vorticidade-velocidade é utilizado neste trabalho. As equações governantes são discretizadas utilizando-se métodos de diferenças finitas compactas de sexta ordem para as derivadas espaciais. A equação de Poisson para a componente da velocidade normal é resolvida por um método iterativo de sobre-relaxação em linhas sucessivas usando um esquema com malha composta para acelerar a convergência. Os resultados de simulações com diferentes valores de freqüência, amplitude e comprimento de fenda foram verificados através de uma análise de Fourier temporal. Através desta análise é verificado qual a melhor situação para se atrasar a separação da camada limite / This work has as a fundamental objective the study of the effects of synthetic jet actuators on the boundary layer flow on a flat plate and on a hypothetical airfoil. The interaction of synthetic jets with transverse flow can lead to an apparent modification in the aerodynamic shape of blunt bodies and, in that way, supply a means of control of transition within the boundary layer. Recent studies demonstrate that different types of flow may be produced by the actuator, depending on the amplitude of oscillation of the membrane. A numerical method for the solution of two-dimensional incompressible Navier-Stokes equations written in vorticity-velocity formulation is used in this work. The spatial derivatives are discretized with a sixth order compact finite differences scheme. The Poisson equation for the normal velocity component is solved by an iterative line successive over relaxation method and uses a multigrid full approximation scheme to accelerate the convergence. The results of simulations with different values of frequency, amplitude and slot length were verified through a temporal Fourier analysis. By way of this analysis it is verified which are the better parameters for the controlled delay of boundary layer separation

Helicopter performance is greatly influenced by its drag. Pylons, fuselage, landing gear, and especially the rotor hub of a helicopter experience large separated flow regions, even under steady level flight conditions the vehicle has been designed for, contributing to the helicopter drag. Several passive and active flow control concepts have been studied for reducing helicopter drag. While passive flow control methods reduce drag, they do so at one optimized design condition. Therefore, passive drag reduction methods may not work for helicopters that operate under widely varying flight conditions. Active flow control (AFC) methods overcome this disadvantage and consequently are widely being pursued. The present investigator has studied some of these AFC methods using computational fluid dynamics (CFD) techniques and has found synthetic (or pulsed) jets as one of the more effective drag reduction devices. Two bluff bodies, representative of helicopter components, have been studied and the mechanism behind drag reduction has been analyzed. It was found that the increase in momentum due to the jet, and a resultant reduction in the separated flow region, is the main reason for drag reduction in these configurations. In comparison with steady jets, synthetic jets were found to use less power for a greater drag reduction. The flow inside these synthetic jet devices is incompressible. It is computationally inefficient to use compressible flow solvers in incompressible regions. In such regions, using Lattice Boltzmann equations (LBE) is more suitable compared to solving the incompressible Navier-Stokes equations. The length scales close to the synthetic jet devices are very small. LBE may be used to better resolve these small length scale regions. However, using LBE throughout the whole domain would be computationally expensive since the grid spacing in the LBE solver has to be of the order of the mean free path. To address this need, a coupled Lattice Boltzmann-Navier-Stokes (LB-NS) methodology has been developed. The LBE solver has been successfully validated in a standalone manner for several benchmark cases. The solver has also been shown to be of second order accuracy. This LBE solver has been subsequently coupled with an existing Navier-Stokes (NS) solver. Validation of the coupled methodology has been done for analytical problems with known closed form solution. This LB-NS methodology is further used to simulate the flow past a cylinder where synthetic jet devices have been used to reduce drag. The LBE solver is used in the cavity of the synthetic jet nozzle while the NS solver is employed in the rest of the domain. The cylinder configuration was chosen to demonstrate drag reduction on helicopter hub shape geometries. Significant drag reduction is observed when synthetic jets are used, compared to the baseline no flow control case.

V této diplomové práci byla provedena série numerických výpočtů proudění kolem křídla s aktivním řízením proudu. Výpočty jsou provedeny pro různé úhly náběhu křídla s profilem NACA 0015. Křídlo s zařízením pro aktivní řízení proudu bylo testováno v podmínkách s Machovým číslem M=0,21 a Re= 2500000. Bylo zkoušeno více možných konfigurací s cílem nalézt nejúčinější variantu, která bude zároveň stále energeticky efektivní. Vybraný přístup k aplikaci aktivního řízení na křídle se od ostatních liší. Použito je velkých ploch pro vyfukování vzduchu o nízké rychlosti a zároveň v souvislosti s tím je studována energetická účinnost. Snížení odporu a zvýšení vztlaku je dosaženo změnou řídících veličin. Při určitých specifických podmínkách je zařízení schopno při velmi vysoké energetické účinnosti dosáhnout pro všechny úhly náběhu výrazného snížení odporu, zvýšení vztlaku křídla, nebo obojího zároveň. Maximální pokles odporu křídla na malých úhlech náběhu přesahuje 40% z celkového odporu křídla a stále s dodržením energetické účinnosti.

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