The performance and broader application of dielectric barrier discharge (DBD) plasma actuators are restricted by the manufacturing methods currently employed. In the current work, two methodologies are proposed to build robust plasma actuators for active flow control; a protective silicone oil (PDMS) treatment for hand-cut and laid tape-based actuators and a microfabrication technique for glass-based devices. The microfabrication process, through which thin film electrodes are precisely deposited onto plasma-resistant glass substrates, is presented in detail. The resulting glass-based devices are characterized with respect to electrical properties and output for various operating conditions. The longevity of microfabricated devices is compared against silicone-treated and untreated hand-made devices of comparable geometries over 60 hours of continuous operation. Both tungsten and copper electrodes are considered for microfabricated devices. Human health effects are also considered in an electromagnetic field study of the area surrounding a live plasma actuator for various operating conditions.

The performance and broader application of dielectric barrier discharge (DBD) plasma actuators are restricted by the manufacturing methods currently employed. In the current work, two methodologies are proposed to build robust plasma actuators for active flow control; a protective silicone oil (PDMS) treatment for hand-cut and laid tape-based actuators and a microfabrication technique for glass-based devices. The microfabrication process, through which thin film electrodes are precisely deposited onto plasma-resistant glass substrates, is presented in detail. The resulting glass-based devices are characterized with respect to electrical properties and output for various operating conditions. The longevity of microfabricated devices is compared against silicone-treated and untreated hand-made devices of comparable geometries over 60 hours of continuous operation. Both tungsten and copper electrodes are considered for microfabricated devices. Human health effects are also considered in an electromagnetic field study of the area surrounding a live plasma actuator for various operating conditions.

The Dielectric Barrier Discharge (DBD) plasma actuators have received a significant attention of many researchers in the last few decades. The main focus of these studies has been on the flow control areas such as turbulent boundary layer separation and turbulent skin friction reduction. Little attention has been paid on the effect of the DBD plasma actuators on the aerodynamic noise reduction. In this regard, the aim of the current work is to investigate the effect of the DBD plasma actuator driven at relatively low voltages on vortex-induced noise. The first part of the current work includes an extensive assessment of the effect of the DBD plasma actuator on the narrowband tonal noise radiated from a flat plate with blunt trailing edge and an airfoil (NACA 0012) with blunt and cut-in type serrated trailing edge. The measurements were carried out at Reynolds numbers between 0.75 x 10 to the power of 5 and 4 x 10 to the power of 5. It is found that the DBD plasma actuator effectiveness depends on the direction of the generated electric wind. For example, a high reduction in the narrowband tonal noise level is achieved when a direct streamwise electric wind is injected into the wake region. However, using a plasma actuator, which can induce streamwise vortices into the wake region, shows more superior noise reduction capability at lower voltages. Flow measurement results revealed that the mechanism responsible for the narrowband tonal noise reduction when the electric wind is directly injected into the wake is not due the momentum injection into the wake deficit. Rather, the streamwise jet isolates the two separated shear layers and prevents the interaction between them. On the other hand, it is found that the break-up of the spanwise coherence of the vortex shedding is responsible for the significant reduction in the tonal noise level when the spanwise actuation is used. The second part of the current work comprises the effect of the DBD plasma actuator on both the narrowband tonal noise and interaction broadband noise radiated from both single and tandem cylinder, respectively. The experiments were conducted at subcritical Reynolds number ReD = 1.1 x 10 to the power of 4. The actuators were positioned at different azimuthal angles 27° ≤ θj ≤ 153°. For the single cylinder case, the acoustic results show the DBD plasma actuator that is positioned at θj = 133° leads to a more reduction in the narrowband tonal noise level when compared to the other angles. It is found that the streamwise jet produced by the plasma actuators plays an important role in prevention of the interaction between the shear layers that separates from the cylinder. For the tandem cylinders case, the acoustic results show that the simultaneous actuation of both the upstream and the downstream cylinders leads to more reduction in both the narrowband tonal noise and the interaction broadband noise level compared with the case where only the upstream or the downstream cylinder is actuated. The mechanism responsible for this noise reduction is found to be mainly due to the streamwise jet induced by the upstream cylinder activation, which delays the vortex shedding formation and reduces the turbulence intensity in the near wake region. On the other hand, the plasma induced jet against the main-flow direction works as a virtual fluidic barrier which displaces the wake produced by the upstream cylinder away from the downstream cylinder.

Le contrôle actif d’écoulement est une voie envisagée actuellement pour améliorer les caractéristiques aérodynamiques des véhicules aériens ou terrestres. La diminution de la traînée (force opposée au mouvement) est notamment visée, ce qui permettrait de baisser la consommation en énergie entraînant ainsi une réduction des émissions polluantes. Depuis une dizaine d’années, les actionneurs plasmas sont utilisés comme dispositifs de contrôle. À Orléans, ils sont basés sur l’utilisation d’une décharge à barrière diélectrique créant à sa surface un plasma qui induit un écoulement de quelques km h-1 : le vent ionique. L’actionneur plasma est caractérisé avec l’étude des différents régimes de décharge. Celui où des arcs énergétiques apparaissent est analysé. La température de surface de l’actionneur est également étudiée en fonction de plusieurs paramètres. Elle reste inférieure à 100 °C, ce qui confirme que les effets des actionneurs plasmas sur un écoulement ne sont dus qu’au vent ionique. Une caractérisation du vent ionique permet aussi de confirmer le lien entre le vent ionique et l’extension du plasma : deux phases distinctes existent, pendant lesquelles il est créé. Le contrôle de la transition d’une couche limite de Blasius est effectué sur une géométrie de type plaque plane. En fonction de la position de l’actionneur ou de la puissance consommée, le recul, le déclenchement précoce ou le cas sans effet est obtenu. Le mécanisme d’action est identifié et est lié à une excitation de l’écoulement qui devient plus ou moins sensible aux ondes de Tollmien-Schlichting. La fréquence de fonctionnement de l’actionneur apparaît comme le paramètre principal pour ce type de contrôle. Une nouvelle géométrie d’actionneur est proposée et caractérisée. La décharge conserve des propriétés identiques au cas classique et le sondage du vent ionique par un moyen de mesure 3D permet de montrer le gain en vitesse et l’existence de structures 3D susceptibles de contrôler plus efficacement un écoulement. / Active flow control is a route currently being considered to improve aerodynamic performances of vehicles (airplanes or cars). Drag reduction (force opposite to motion) is particularly concerned and provides reduction of energy consumption of vehicles what induces low exhaust gases emissions. Plasma actuators are used as control devices since a decade. In Orléans, they are based on the use of surface dielectric barrier discharge which creates plasma on its surface and induces a flow of few km h-1 : the ionic wind. The plasma actuator is characterized with the study of different discharge regimes. One in which sparks occur is analyzed. The actuator surface temperature is also studied in function of several parameters. Surface temperature remains below 100 °C what confirms effects of plasma actuators on the flow are only due to the ionic wind. Characterization of ionic wind has also confirmed the link between induced flow and plasma spread over actuator surface : two distinct phases exist where a flow is every time induced. Transition control of a Blasius boundary layer is performed on flat plate geometry. Depending on plasma actuator position or power consumption, promotion, delay or neutral case are obtained on transition location. The mechanism of action is identified and linked to an excitation of the flow that becomes more or less sensitive to Tollmien-Schlichting waves. The operating frequency also appears as the main parameter for this type of control. New actuator geometry is proposed and characterized. The discharge keeps identical properties to a classical configuration of plasma actuator. Ionic wind measurements by means of a 3D device allow us to show the gain in speed and presence of 3D structures in the induced flow topology what is more effectively to control external flows.

No description available.

The flow over a set of tandem cylinders at a moderate Reynolds numbers (Re), and with different separation lengths has been studied. Two dimensional (2D) and three-dimensional (3D) plasma actuators were used to control the flow over the leading cylinder to change the vortex shedding, and subsequently the flow on the second cylinder. The 3D plasma actuator was segmented along the length of the cylinder with a spacing of λ = 4 while the 2D actuator simply ran straight down the span of the cylinder. Particle image velocimetry (PIV) measurements were used to investigate the flow along the central plane in the wake of the cylinders. The image pairs were processed into velocity grids which were then averaged. Plots of the shear, vorticity, and turbulent kinetic energy were created. These plots are used to understand how the character of vortex shedding from the upstream cylinder changes the same from the downstream one.

La réduction de la résistance à l’avancement par des techniques de contrôle active sera une nécessité d’importance croissante dans les années à venir pour le secteur des transports. Cette thèse fait partie d’une nouvelle voie de recherche consistant à utiliser des décharges plasmas à pression atmosphérique pour modifier les écoulements en vue d’améliorer les performances aérodynamiques (diminution de la traînée et augmentation de la portance). L’actionneur retenu est une décharge à barrière diélectrique surfacique avec un diélectrique composé d’une combinaison de deux matériaux polymères souples. L’écoulement induit par l’actionneur plasma, appelé vent ionique, est d’abord étudié expérimentalement dans l’air au repos en fonction du régime de la décharge. Une loi empirique liant la vitesse du vent ionique à la tension et à la fréquence de l’alimentation est proposée. De cette loi se dégagent une vitesse et une longueur caractéristiques utilisées pour prédire les principales propriétés du jet de paroi induit en aval de la décharge (lois d’échelle). L’efficacité de l’actionneur est ensuite testée en soufflerie subsonique pour trois configurations aérodynamiques de complexité croissante : couche limite laminaire non décollée (plaque plane), couche limite turbulente décollée (profil épais) et couche limite décollée 3D (corps de Ahmed, lunette inclinée à 25°). Des résultats significatifs tant sur le plan de la modification de la topologie de l’écoulement que de l’amélioration des efforts aérodynamiques sont obtenus pour des nombres de Reynolds jusqu’à 1,7.106 (35 m/s). / Active flow control is a key issue to meet the environmental requirements for the next generations of aircrafts and cars. The aim of this work is to study subsonic airflows control by plasma actuators in order to improve aerodynamic performances (drag reduction and lift enhancement). The actuator consists in using a surface dielectric barrier discharge with a thin dielectric made by a combination of two polymer materials. The first part of this work is dedicated to the experimental study of the flow induced by the actuator in quiescent air according to the regime of the discharge. An empirical law is proposed in order to determine the velocity of the ionic wind as a function of the high voltage and the frequency of the actuator. From this law the main properties of the wall jet induced by the discharge are predicted. In the second part of the study the actuator is used for wind tunnel experiments for three configurations of increasing complexity: laminar boundary layer (flat plane), separated turbulent boundary layer (wing profile) and 3D separated boundary layer (Ahmed body with rear window tilted at 25°). Significant results are obtained for Reynolds numbers up to 1.7.106 (35 m/s).

The goal of this thesis was to both theoretically and experimentally show the effect of a plasma actuator for flow separation control.  In the theoretical part a solver was implemented in MATLAB code, to solve the governing equations describing the plasma actuator.  The experimental part included PIV (Particle Image Velocimetry) measurements of the velocity field induced by the plasma actuator, visualization of the effect in a wind tunnel and the development of a simple model of the plasma actuator based on the empirical result whose purpose is to be used in CFD (Computational Fluid Dynamics). The PIV measurements were performed with an acceptable result even though a lot of disturbance occurred in and near the plasma region.  The empirical result was used to develop the empirical plasma actuator model for CFD, which showed some interesting result.  The model implies that the induced force by the plasma actuator grows exponential with the applied peak-to-peak voltage.   The model was also used to predict airfoil performance with plasma actuators which showed an increase of the lift coefficient on a NACA0012 with a chord length of 0.1m.  Simulations were done for free-stream velocities up to 20m/s with three different configurations, without plasma actuator for comparison, with one actuator at the quarter-chord and one with three actuators on the airfoil.  With three actuators the increase of the lift coefficient was 108 percent at 5m/s and 14 percent at 20m/s. The simulations with one actuator were only performed up to 10m/s were the effect of the actuator still could be seen but for higher velocities the effect would probably be minor. The wind tunnel experiment clearly showed the effect and the advantages of utilizing plasma actuators for flow separation control.  The experiment showed that a single plasma actuator placed at the quarter chord of a fully stalled NACA0012 airfoil with a chord length of 0.1m, at approximately 20 degrees angle of attack and with a free-stream velocity of 1.5m/s, was able to reattach the flow behind the actuator. The result of the theoretical part was inconclusive, the code could not run with the appropriate voltage and frequency of the plasma actuator.  Some result was however obtained, implying that the time-average force induced by the plasma actuator was in the expected direction.  The theoretical model is however considered to have potential, the major problems concern the code which requires further development.

Plasma actuators are devices which function by creating a discharge in air at atmospheric conditions. These devices have been demonstrated to effectively delay flow separation and enhance the lift- drag characteristics of wing sections. They have also been shown to have potential applications in controlling dynamic stall, flow separation control over turbine blades, flow vectoring, boundary layer manipulation and bluff body flow control. This study examines the characteristics of the plasma actuator, its working and the optimization of its configuration for its use as a lift enhancing device. A single actuator connected to a high-voltage, high-frequency power supply was studied in quiescent conditions. It was demonstrated by means of flow visualization experiments and hot-wire anemometry that the plasma actuator functions by inducing a flow, thus behaving as a source of momentum flux in any system that it is introduced into. Further, it was inferred that the flow induced is a wall jet and that the magnitude of the velocity achieved is maximum within a few millimeters of the surface of the actuator. A parametric investigation of the actuator was conducted next. The variation of the peak velocity induced in quiescent conditions with the variation of configuration parameters was studied by means of photographic studies and hot-wire anemometry. These experiments indicated that there is a strong correlation between the visible extent of the plasma along the direction of the induced _ow (plasma width) and the peak velocity achieved. The peak velocity achieved is found to increase with the increase in the plasma width as long as the discharge created is in the uniform glow discharge regime. The development of localized high intensity streamers, which destroy the uniformity of the plasma, lead to a loss in the peak velocity. Hot-wire tests indicated that the peak velocity increases with a decrease in the spanwise overlap of the electrodes, with the other parameters kept constant. Also, in the uniform glow discharge regime, the velocity increases with the increase in the thickness of the dielectric placed between the two electrodes. After a particular optimum thickness, further increase of the thickness leads to formation of streamers. The velocity increased with a decrease in streamwise overlap, with the maximum being reached for a overlap of approximately 2mm, after which it remained a constant. It was observed that the absence of overlap leads to a loss of uniformity of the discharge created. The velocity was found to be independent of the variations in the electrode widths. Particle Image Velocimetry (PIV) was conducted to study the characteristics of the jet produced. It was observed that when the actuator is switched on, a low pressure region is created near the surface of the actuator, vertically above it, leading to a flow towards this region from above the actuator. Furthermore, a vortex is shed, which is convected downstream, after which a wall jet is established close to the dielectric surface.

No description available.