Die vorliegende Arbeit widmet sich der, vorwiegend experimentellen, Untersuchung der Wirkung wandparalleler Lorentzkräfte in Strömungsrichtung auf Grenzschichtprofile und Körperumströmungen. Die Themen - Beeinflussung der Grenzschicht an ebenen Platten mit stationären Lorentzkräften - Kontrolle von Strömungsablösungen an Zylindern und symmetrischen Profilen mit stationären Lorentzkräften - Beeinflussung von Zylindernachläufen und abgelösten Tragflügelumströmungen mit zeitlich periodischen Lorentzkräften werden behandelt.

This thesis examines the effects of spatial location, Reynolds number and near exit flow modification on the development region of a round, free, turbulent jet. It is based on the publications generated by the author. The experiments were carried out over the range of Reynolds numbers between 10000 < ReD < 50000, where ReD is calculated based on the jet exit mean velocity and the nozzle exit diameter. The measurements were performed in the near- to intermediate-field region of a free jet defined between 0 ≤ x/D ≤ 30. In order to control the flow near the exit, two wire rings, with square cross-sections, of sides h = 1.5 mm, and outer diameter Dwire = 71.6 mm (positioned in the shear layer and called Rsl) and Dwire = 60 mm (positioned in the potential core and called Rpc) were placed at a stand-off distance downstream of the jet nozzle exit plane x/D = 0.03. Both stationary and flying hot wires were used to investigate the jet flow field. The results showed a considerable reduction in the jet spread rate and turbulence intensity using the passive rings. The reduction in the velocity decay rate was more obvious in the case of Rsl in lower Re; however, it was observed that as Re increases, the velocity decay rate became nearly the same for both cases of Rsl and Rpc. The axial velocity spectra showed the initial shear layer instability (shear layer mode) was suppressed while the jet preferred instability (preferred mode) remained active as the shear layer and potential core were modified. This shows the separation of these modes and is at variance with ideas that appeared in the literature that claimed the dependency of these two modes. / Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2012-04-27 16:06:31.03

Current research in experimental and computational fluid dynamics is focused in the area of flow control. Flow control devices are usually classified as either passive or active. Plasma actuators are active flow control devices that require input from an external power source. Current efforts have modeled the effects of plasma actuators as a body force near the electrode. The research presented herein focuses on modeling the fluid-plasma interaction seen in dielectric barrier discharge plasma actuators as a body force vector in the region above the embedded electrode using computational fluid dynamics (CFD). This body force is modeled as the product of the gradient of the potential due to the electric field and the net charge density. In a passive flow control study, two-dimensional simulations using CFD are done with a smooth and bumpy Eppler 398 airfoil with laminar, transition, and turbulent models in an effort to improve the understanding of the flow over bumpy airfoils and to quantify the advantages or disadvantages of the bumps.

The dynamics of controlled transitory 2- and 3-D attachment of the separated flow over a 2-D airfoil model are investigated in wind tunnel experiments. Pulsed actuation is effected on time scales that are an order of magnitude shorter than the characteristic convective time scale of the base flow by momentary jets that are generated by a spanwise array of combustion-based actuators. The effects of the transitory actuation on the aerodynamic characteristics of the airfoil are assessed using measurements of the global lift force and pitching moment and of streamwise distributions of surface pressure, and planar and stereoscopic particle image velocimetry (PIV) acquired phase-locked to the actuation waveform. A single spanwise-bounded actuation pulse leads to 2-D severing of the separated vorticity layer and the subsequent shedding of a large-scale stall vortex that are followed by momentary attachment of the upstream boundary layer and ultimately re-separation that are accompanied by a strong transitory change in the airfoil's circulation. It is shown that the primary mechanism for the attachment is alteration of the adverse pressure gradient of the separated base flow by local blockage of the momentary jet and.the formation of the large-scale stall vortex. The disparity between the characteristic time scales of flow attachment and subsequent separation [O(Tconv) and O(10Tconv), respectively] is exploited for temporal and spatial extensions of the attachment and enhancement of the global aerodynamic performance using strings of successive actuation pulses. Pulsed actuation effected by an unbounded actuator array leads to spanwise spreading of the induced transitory 3-D flow attachment well beyond the spanwise edges of the actuators. It is shown that 3-D pulsed actuation enhances the accumulation of vorticity over the airfoil and improves its aerodynamic performance compared to 2-D, spanwise-bounded actuation. When the airfoil is undergoing time-periodic pitch oscillations beyond its static stall margin, a sequence of staged 3-D actuation pulses coupled to the airfoil's motion can lead to reduced lift hysteresis and increased pitch stability (lower “negative damping”) that are typically associated with the presence of dynamic stall.

In 2009, based on its commitment to take action on the climate change, the aviation industry accorded a group of objectives to reduce carbon dioxide emissions. Although only 2% of all human-induced carbon dioxide (CO2) emissions are produced by the global aviation industry, the aviation industry is set to grow in the next 30 years. In order to maintain this growth without increasing its negative environmental impact, the future aircraft have to be cleaner and greener. In order to reduce carbon emissions and increase the operative efficiency, novel technologies have been developed and applied on aircraft. One of the recently introduced technologies is the flow control over the wing by employing active flow control methods. Amongst the active flow control methods, synthetic jets have emerged as a developing and promising technology. The latter have been extensively investigated since 1990 in laboratory based investigations. In spite of the fact that many experimental studies have been performed to design synthetic jet actuators for optimal flow control, due to the the vast number of operating parameters involved, and the lack of current measurement technologies, they can be impractical and highly expensive. Hence, there is a need for a systematic analysis to establish the optimal operating conditions with the highest effectiveness at the cost of minimum energy input, and the most suitable orientation of synthetic jet orifices. This would require enhanced comprehension of the inherent features of synthetic jets and their corresponding near wall effects. By using numerical simulations with a commercial CFD software (Star-CCM+), this thesis investigates some features associated with synthetic jet performance that are not fully understood, such as: • The optimal working configuration of a synthetic jet array embedded into a laminar detached boundary layer for flow separation control. • The effect of orifice orientation (inclined and skewed synthetic jets) over normal synthetic jets and their optimal working configuration in an attached laminar boundary layer.

[pt] Principalmente nos últimos anos, diversos pesquisadores tem ressaltado a importância da obtenção de um método para seleção de tamanho de janelas de uma rede de computadores. A dissertação em pauta se propõe a apresentar um algoritmo com este propósito. É analisada a seguir a interação entre os controles de fluxo END TO END e Local em uma cadeia de lances. Finalmente examina-se através de simulação, o comportamento de uma rede de computadores quando são aplicados os controles de fluxo END TO END e Local simultaneamente. / [en] In past years the importance of a method for selection of the window size in a computer network with End-to-End flow control hás been emphasized by several researchers. In this work, an algorithm is presented that perform this task. The analysis of the interaction between End-to-End flow control and Local flow control in a chain of links is also presented. Finally, using computer simulation, the behavior of a computer network when the End-to-End flow control and Local flow control are applied simultaneously is examined.

Thermoacoustic instability is one of the most significant problems faced in the design of some combustion systems. Thermoacoustic oscillations arise due to feedback between acoustic waves and unsteady heat release rate when the fluctuating heat release rate is sufficiently in phase with the unsteady pressure. The primary aim of designers is to design linearly stable thermoacoustic systems in which these dangerous oscillations do not arise. In thermoacoustics, adjoint-based sensitivity analysis has shown promise at predicting the parameters which have the most influence on the linear growth and decay rates as well as oscillation frequency observed during periods of linear growth and decay. Therefore, adjoint-based methods could prove to be a valuable tool for developing optimal passive control solutions. This thesis aims to develop novel experimental sensitivity analysis techniques and provide a first comparison with the predictions of adjoint-based sensitivity analysis. In this thesis experimental sensitivity analysis is performed on (i) a vertical electrically-driven Rijke tube, and (ii) a vertical flame-driven Rijke tube. On the electrically-driven Rijke tube, the feedback sensitivity is studied by investigating the shift in linear growth and decay rates and oscillation frequency observed during periods of linear growth and decay due to the introduction of a variety of passive control devices. On the flame-driven Rijke tube, the base-state sensitivity is studied by investigating how the linear growth and decay rates as well as oscillation frequency during periods of linear growth and decay change as the convective time delay of the flame is modified. Adjoint-based sensitivity analysis gives the shift in linear growth and decay rate and the oscillation frequency when parameters are changed. This thesis provides experimental measurements of the same quantities, for comparison with the numerical sensitivity analysis, opening up new avenues for the development, implementation and validation of optimal passive control strategies for more complex thermoacoustic systems.

The increase in CO2 emissions due to the significant growth of the level of air traffic expected in the next 40 years can be tackled with new technologies able to reduce the skin friction drag of the new generation aircraft. The ACARE (Advisory Council for Aeronautical Research in Europe), within the Flightpath 2015 Visions, has established stringent targets for drag reduction, which can be achieved only with innovative flow control methods. Synthetic jets are a promising method of flow control, especially for their ability to control the flow without the need of a bleed air supply. The application of synthetic jets for flow separation control has been already proven. Their application can also be extended to skin friction drag reduction in a turbulent flow. Indeed, most of turbulence production in a turbulent boundary layer is related to the dynamics of streamwise streaks and vortices in the near-wall region. Synthetic jets can be used to weaken these structures, to reduce turbulence production and consequently skin friction drag. The effectiveness of synthetic jets for skin friction drag reduction in a turbulent boundary layer has already been explored in a few works. However, there is a lack of understanding on the physical mechanism by which this effect is achieved. The aim of this work is to provide further insight on this. A series of experimental investigations are carried out, using three main measurement techniques: Particle Image Velocimetry, Liquid Crystal Thermography and Constant Temperature Anemometry. The effectiveness of a single round synthetic jet in controlling near-wall streamwise streaks and vortices in a laminar environment, in particular those that develop downstream of a circular cylinder, is verified. Turbulent boundary layer forcing is attempted using a synthetic jet array that produces coherent structures of the same scale as the streamwise vortices and streaks of a turbulent boundary layer. The synthetic jet array is able to create regions of lower velocity in the near-wall and of lower skin friction. A possible physical mechanism behind this has been proposed. With a few minor modification, it is believed that the performance of the synthetic jet array could be significantly improved. This can be achieved especially if the array is installed in a feed-forward control unit, which is only briefly explored in this work. In this case the information on the flow field gathered real-time with wall sensors can help to consistently improve the synthetic jet array performance in terms of skin friction drag reduction.

This Master Thesis work cover the simulation of the movement of charged species exposed to a high gradient electric field, the same environment a plasma actuator produces. The final goal is to use the plasma actuator as an active flow control device to decrease the drag of a body moving in air. This report describes how the problem was set up in COMSOL Multiphysics and the resulting volume force achieved. The volume force is the force generated by the plasma actuator that is acting on the air. To understand the effect of a plasma actuator better experimental work was also performed. The experimental work include what effect a plasma actuator has on a wing that has stalled out and measuring the air velocity obtained from a single plasma actuator. The conclusion is that more work has to be performed to make the plasma actuator a more effective flow control device. This type of work is a way to understand how plasma actuators work and in extension will lead to how a plasma actuator will be used effectively. / Detta examensarbete behandlar simuleringen om hur laddade partiklar rör sig när de utsätts för ett elektriskt fält med hög gradient, ett liknande närområde som skapas av en plasma aktuator. Slutmålet är att kunna använda plasma aktuatorer som en aktiv flödeskontroll med avsikt att minimera motsåndet på en stelkropp som rör sig genom luften. Den här rapporten beskriver hur problemet ställs upp i COMSOL Multiphysics och vilken volymkraft som fås utav simulerignen. Volymkraften är den genererade kraft från plasma aktuatorn som aggerar på den omgivande luften. Utöver simuleringen har praktiska experiment gjorts för att förstå effekten från en plasma aktuator bättre. Dessa experiment består av hur en plasma aktuator förändrar luftströmmen över en vinge som redan tappat sin lyftkraft och av att mäta vilken hastighet luften kan nå på grund av en plasma aktuator. Sammanfattnignen är att mer arbete behöver göras för att effektivisera en plasma aktuator om den ska användas för flödeskontrol. Detta arbeta är ett steg i att förstå hur plasma aktuatorer fungerar vilket i förlängningen kommer leda till hur man ska använda en plasma aktuator på bästa sätt.

La réduction de la trainée aérodynamique des véhicules terrestres est un défi actuel dans l’industrie automobile. La région de basse pression du sillage à l’arrière des voitures est responsable d’une part importante de la résistance à l’avancement. Cette étude porte sur le développement de nouvelles stratégies de manipulation de l’écoulement autour de ces géométries dans le but de réduire la traînée. Afin d’atteindre ces objectifs, nous explorons expérimentalement les effets d’un forçage fluidique sur le sillage et la traînée d’un corps au culot droit. Des jets périodiques émis aux bords de fuite du modèle, tangentiellement à l’écoulement principal et avec des fréquences et amplitudes variables sont utilisés pour forcer le sillage. Selon les conditions du contrôle, trois phénomènes sont principalement observés. Premièrement, sur la plage des fréquences comprenant celle du lâcher tourbillonnaire, les jets pulsés sont convectés et modifient l’entrainement de fluide vers la région de recirculation ainsi que l’évolution des instabilités de la couche cisaillée. Cette dynamique complexe a comme conséquence l’augmentation de la trainée du corps. De plus, une résonance subharmonique apparait quand les jets pulsés sont émis avec des fréquences voisines de deux fois la fréquence du mode global. Une importante augmentation de la trainée est alors mesurée et corrélée à une forte amplification des mouvements du sillage. Une augmentation de la fréquence de pulsation se traduit par un effet de vectorisation des couches cisaillées. En outre, une diminution de l’intensité turbulente du sillage proche ainsi qu’une réduction de l’entrainement de fluide le long des couches cisaillées sont mesurés, correspondant à une réduction globale de l’énergie cinétique turbulente de l’écoulement. Le couplage de ces effets est responsable d’une augmentation de la pression au culot et de la réduction de la traînée. Il est important de noter que ces trois régimes d’actuation sont indépendants des modes de brisure de symétrie existant dans ces écoulements, qui sont analysés ici par des études paramétriques de sensibilité aux perturbations. Les aspects physiques de ces phénomènes sont discutés par des mesures de la traînée, de la pression pariétale et de la vitesse avec différentes conditions de l’écoulement et du contrôle. L’addition d’une surface courbée au voisinage du jet pulsé permet de profiter d’un effet Coanda et augmente les réductions de traînée jusqu'à 20%dans le régime instationnaires. De façon générale, l’effet Coanda amplifie non seulement la récupération de la traînée mais préserve aussi les effets de la pulsation haute fréquence sur l’écoulement turbulent. Ces résultats encouragent le développent des actionneurs fluidique pour l’utilisation en aérodynamique des véhicules et fournissent un complément pour notre compréhension sur la traînée des corps non profilés et sa manipulation. / Aerodynamic drag reduction of bluff bodies has become a major challenge for transport vehicles. The massive flowseparation occurring behind cars, buses or trucks is responsible for a large resistance force due to the low-pressure, rearwake flow. The present study aims to develop novel strategies to manipulate the flow past such geometries as well as toassociate its modifications to the corresponding drag changes. In order to achieve this goal, we experimentally investigatethe impact of fluidic actuation on the wake and drag of a square-back bluff body. Wake forcing is performed by theemission of pulsed jets along the blunt trailing-edges of the model, tangentially to the main flow with variable frequencyand velocity. Depending on the forcing conditions, mainly three flow regimes can be identified. First, for a broadbandrange of frequencies comprising the natural wake instabilities, the convection of the jet structures enhances wakeentrainment, shortens the recirculating flow length with an increase of the bluff body drag. Besides, a subharmonicresonance takes place on the flow at forcing frequencies in the vicinity of twice the wake vortex shedding, leading to ahighly unsteady near wake with significant decrease of the bluff body base pressure. It corresponds to an importantincrease of the model’s drag. Further increase of the actuation frequency induces a wake fluidic boat-tailing by shearlayerdeviation. It additionally lowers turbulent intensity and entrainment of high momentum fluid in the shear layer,revealing an overall reduction of the wake fluctuating kinetic energy. The association of both mechanisms is responsiblefor a raise of base pressure and a decrease of the model's drag. These actuation regimes are independent of the symmetrybreaking modes, wake reversals existing in such flows, which are further clarified here by parametric sensitivity analysisusing flow perturbations. The physical features of such regimes are discussed on the basis of drag, pressure and velocitymeasurements at several upstream conditions and control parameters. By adding curved surfaces at the jet outlets totake advantage of the so-called Coanda effect, the effect of periodic actuation can be further reinforced leading to dragreductions of about 20 % in unsteady regime. In general, the unsteady Coanda blowing not only intensifies the basepressure recovery but also preserves the effect of unsteady high frequency forcing on the turbulent field. The presentresults encourage the development of fluidic control in road vehicles' aerodynamics as well as provide a complement toour current understanding of bluff body drag and its manipulation.