In this work, numerical simulations of flow separation control are performed. The sep-aration control technique studied is called 'synthetic jet actuation'. The developed code employs a cell centered finite volume scheme which handles viscous, steady and unsteady compressible turbulent flows. The pulsating zero mass jet flow is simulated by imposing a harmonically varying transpiration boundary condition on the airfoil's surface. Turbulence is modeled with the algebraic model of Baldwin and Lomax. The application of synthetic jet actuators is based in their ability to energize the boundary layer, thereby providing signifcant increase in the lift coefficient. This has been corroborated experimentally and it is corroborated numerically in this research. The performed numerical simulation investigates the flow over a NACA0015 air-foil. For this flow Re = 9??105 and the reduced frequency and momentum coefficient are F+ = 1:1 and C?? = 0:04 respectively. The oscillatory injection takes place at 12.27% chord from the leading edge. A maximum increase in the mean lift coefficient of 93% is predicted by the code. A discrepancy of approximately 10% is observed with corresponding experimental data from the literature. The general trend is, how-ever, well captured. The discrepancy is attributed to the modeling of the injection boundary condition and to the turbulence model.A sensitivity analysis of the lift coefficient to different values of the oscillation parameters is performed. It is concluded that tangential injection, F + ?? O(1) and the utilized grid resolution around the site of injection are optimal. Streamline fields ob-tained for different angles of injection are analyzed. Flow separation and attachment as functions of the injection angle and of the velocity of injection can be observed. It is finally concluded that a reliable numerical tool has been developed which can be utilized as a support tool in the optimization of the synthetic jet operation and in the modeling of its operation.

In this work, numerical simulations of flow separation control are performed. The sep-aration control technique studied is called 'synthetic jet actuation'. The developed code employs a cell centered finite volume scheme which handles viscous, steady and unsteady compressible turbulent flows. The pulsating zero mass jet flow is simulated by imposing a harmonically varying transpiration boundary condition on the airfoil's surface. Turbulence is modeled with the algebraic model of Baldwin and Lomax. The application of synthetic jet actuators is based in their ability to energize the boundary layer, thereby providing signifcant increase in the lift coefficient. This has been corroborated experimentally and it is corroborated numerically in this research. The performed numerical simulation investigates the flow over a NACA0015 air-foil. For this flow Re = 9??105 and the reduced frequency and momentum coefficient are F+ = 1:1 and C?? = 0:04 respectively. The oscillatory injection takes place at 12.27% chord from the leading edge. A maximum increase in the mean lift coefficient of 93% is predicted by the code. A discrepancy of approximately 10% is observed with corresponding experimental data from the literature. The general trend is, how-ever, well captured. The discrepancy is attributed to the modeling of the injection boundary condition and to the turbulence model.A sensitivity analysis of the lift coefficient to different values of the oscillation parameters is performed. It is concluded that tangential injection, F + ?? O(1) and the utilized grid resolution around the site of injection are optimal. Streamline fields ob-tained for different angles of injection are analyzed. Flow separation and attachment as functions of the injection angle and of the velocity of injection can be observed. It is finally concluded that a reliable numerical tool has been developed which can be utilized as a support tool in the optimization of the synthetic jet operation and in the modeling of its operation.

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.

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.

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Pour améliorer les performances aérodynamiques notamment dans le domaine des transports (aéronautique, automobile, ..), le contrôle des écoulements constitue une solution de rupture prometteuse. Il présente à la fois un enjeu majeur pour l'industrie et un défi pour les scientifiques. Les retombées visées par le contrôle (notamment actif) sont d'ordre à la fois économique (réduction de la consommation) et environnemental (diminution des gaz à effet de serre). L'efficacité du contrôle est intimement liée à l'actionneur utilisé. Les actionneurs fluidiques et tout particulièrement le jet synthétique, semblent être une technique prometteuse de progrès. L'objectif de ce travail est le développement, la validation et la mise en œuvre de ce type d'actionneur. Nous avons ainsi conçu un actionneur de type jet synthétique basé sur un haut parleur. Les performances de cet actionneur ont été évaluées dans un milieu au repos en utilisant à la fois l'anémométrie à fil chaud et la PIV. Les effets des différents paramètres ont été analysés (fréquence, amplitude de forçage, …). Une attention particulière a été consacrée à l'étude de l'inclinaison de la fente d'éjection. Trois inclinaisons ont été utilisées (30°, 45° et 90°). Dans chaque cas l'évolution du jet synthétique ainsi que celle des tourbillons ont été caractérisées. Par les mêmes techniques de mesure dans une soufflerie, nous avons étudiés l'interaction du jet synthétique avec un écoulement transverse. Des visualisations par fumée du contrôle du décollement par cet actionneur montrent ses capacités à pouvoir recoller l'écoulement. / To improve the aerodynamic performance particularly in the field of transport (aerospace, automotive, ..), the flow control is a promising solution for rupture. It presents at the same time a major stake for industry and a challenge for the scientists. The fallout aimed by control (particularly active control) are of an at the same time economic (reduction of consumption) and environmental (decrease of greenhouse gases). The effectiveness of control is closely related to the actuator used. The fluidic actuators and particularly the synthetic jet appear to be a promising technique for progress. The objective of this work is the development, validation and implementation of this type of actuator. We thus designed an actuator of the synthetic jet type based on a loudspeaker. The performances of this actuator were estimated in a quiescent environment. Both hot wire anemometer and the Particle Image Velocimetry (PIV) are used. The effects of different parameters were analyzed (frequency, amplitude forcing, ...). Particular attention has been devoted to the study of the inclination of the ejection slot. Three angles were used (30 °, 45 ° and 90 °). In each case the development of synthetic jet and the vortices have been characterized. By the same techniques of measurement in a wind tunnel, we have studied the interaction of the synthetic jet with a transverse flow. Visualizations by smoke control detachment by the actuator to show his ability to pick up the flow.

Parameters governing separation control with sweeping jet actuators over a deflected flap are investigated experimentally on a generic "Multiple Flap Airfoil" (MFA). The model enables an extensive variation of geometric and aerodynamic parameters to aid the scaling of this novel flow control method to full-size applications.Sweeping jets exit from discrete, millimeter-scale nozzles distributed along the span and oscillate from side-to-side. The sweeping frequency is almost linearly dependent on the supplied flowrate per actuator. The measured thrust exerted by a row of actuators agrees well with vectored momentum calculations. Frequency and thrust measurements suggest that the jet velocity is limited to subsonic speeds and that any additional increase in flowrate causes internal choking of the flow.Neither the flowrate nor the momentum input is found to be a sole parameter governing the lift for varying distance between adjacent actuators. However, the product of the mass flow coefficient and the square root of the momentum coefficient collapses the lift onto a single curve regardless of the actuator spacing. Contrary to other actuation methods, separation control with sweeping jets does not exhibit any hysteresis with either momentum input or flap deflection. A comparison between sweeping and non-sweeping jets illustrates the superior control authority provided by sweeping jets. Surface flow visualization on the flap suggests the formation of counter-rotating pairs of streamwise vortices caused by the interaction of neighboring jets.The actuation intensity required to attach the flow increases with increasing downstream distance from the main element's trailing edge and increasing flap deflection. No obvious dependence of the ideal actuation location on actuator spacing, flap deflection, angle of attack, or actuation intensity is found within the tested range. Comparisons between experimental and numerical results reveal that the inviscid flow solution appears to be a suitable predictor for the effectively and efficiently obtainable lift of a given airfoil configuration. The flap size affects the achievable lift, the accompanying drag, and the required flap deflection and actuation intensity. By controlling separation, the range of achievable lift coefficients is doubled without significant penalty in drag even when considering a safety margin for the maximum applicable incidence.

Aujourd’hui, la politique européenne incite les industriels de l’automobile à concevoir des véhicules “propres”, ainsi des normes de plus en plus contraignantes visant notamment à réduire les émissions de gaz à effet de serre ont vu le jour. Les ingénieurs sont donc amenés à envisager des solutions innovantes basées sur le contrôle des écoulements, en particulier celui des décollements puisqu’ils ont généralement des effets délétères sur les performances aérodynamiques. En effet, les phénomènes de décollement, inhérent aux corps épais, sont à l’origine de l’augmentation de la traînée et par conséquent de la consommation de carburant. Les travaux présentés dans ce mémoire sont dédiés au contrôle d’écoulement en vue de la réduction de traînée sur un modèle générique de véhicule automobile appelé “Corps de Ahmed” par une combinaison d’actionneur de type jets ynthétiques. Le mémoire s’articule autour de deux axes principaux. Le premier a permis la caractérisation d’actionneurs de type jets synthétiques, les fréquences et amplitudes optimales au regard de différents critères ont ainsi été identifiées et analysées. Le second a été consacré à étudier la réponse des structures décollés à des perturbations périodiques localisées afin de poursuivre la compréhension de la physique impliquée dans le contrôle de la traînée aérodynamique. / Nowadays, the car manufacturers are forced by the european policy to follow more and more strict rules concerning the design of “clean” automotives, in order to reduce the gas emissions responsible for the greenhouse effect. The engineers have to consider innovative solutions based on flow control, particularly for flow separation which generally has deleterious effects on aerodynamic performance. Indeed, the separation phenomena, inherent to the bluff bodies, are at the origin of thedrag increase and consequently of the fuel consumption. The work presented in this thesis is dedicated to combinated flow separation control from simplified hatchback geometry (Ahmed body) by a synthetic jet. The thesis focuses on two main axes. The first axis has allowed the characterization of synthetic jets actuators, the frequencies and the optimal amplitudes with regard to various criteria were so identified and analyzed. The second axis was dedicated to study the separation flow structures response from local periodic disturbances in order to pursuing the physics knowledge involved in the aerodynamic drag control.

The aerodynamic forces and moments on an axisymmetric body of revolution are controlled in a low-speed wind tunnel by induced local flow attachment. Control is effected by an array of aft-facing synthetic jets emanating from narrow, azimuthally segmented slots embedded within an axisymmetric backward facing step. The actuation results in a localized, segmented vectoring of the separated base flow along a rear Coanda surface and induced asymmetric aerodynamic forces and moments. The observed effects are investigated in both quasi-steady and transient states, with emphasis on parametric dependence. It is shown that the magnitude of the effected forces can be substantially increased by slight variations of the Coanda surface geometry. Force and velocity measurements are used to elucidate the mechanisms by which the synthetic jets produce asymmetric aerodynamic forces and moments, demonstrating a novel method to steer axisymmetric bodies during flight.

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