The primary goal of this research is to assess the effect of synthetic jets on flow separation and provide a feedback control strategy for flow separation using synthetic jets. The feedback control synthesis is conducted based upon CFD simulation for a rounded backward-facing step. The results of the synthetic jet experiments on an airfoil showed that synthetic jets have the potential for controlling the degree of flow separation beyond delaying the onset of flow separation. In the simulation, while the jet is ejected slightly upstream from the separation point, the feedback pressure signal is acquired at a downstream wall point where the vortex is fully developed. Due to the uniqueness of synthetic jets, i.e. "zero-net-mass flux", the profile of synthetic jet velocity cannot be arbitrarily generated. The possible control variables are the magnitude or frequency of the oscillating jet velocity. Consequently, the fluidic system in simulation consists of the actuator model and the NARMAX (Nonlinear Auto Regressive Moving Average with eXogenous inputs) flow model. This system shows a strong nonlinear pressure response to the input jet frequency. Low-pass filtering of the pressure response, introduced for pressure recovery, facilitates a quasi-linear approximation of the system in the frequency domain using the describing function method. The low-pass filter effectively separates the pressure response into two frequency bands. The lower frequency band below the filter pass frequency includes the quasi-linear response targeted by the feedback control and the higher band above the filter stop frequency contains the attenuated higher harmonics, which are treated as nonlinear disturbances. This quasi-linear approximation is utilized to design a PI controller for the fluidic system including the synthetic jet. To ensure one-to-one correspondence of the jet frequency and the filtered pressure response, the upper bound of the jet frequency is set at the frequency of the maximum pressure. The response of the resulting closed loop feedback control system, comprised of a PI controller, low-pass filter, SJA model and NARMAX model, is shown to track the desired pressure command with an improvement in the transient response over the open-loop system.

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.

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.

Plasma synthetic jet actuators are investigated experimentally, in which the geometrical design of single dielectric barrier discharge (SDBD) plasma actuators is modified to produce zero-mass flux jets similar to those created by mechanical devices. The SDBD plasma actuator consists of two rectangular electrodes oriented asymmetrically and separated by a layer of dielectric material. Under an input of high voltage, high frequency AC or pulsed DC, a region of plasma is created in the interfacial air gap on account of electrical breakdown of the ambient air. A coupling between the electric field in the plasma and the neutral air near the actuator is introduced, such that the latter experiences a net force which results in a horizontal wall jet. This effect of the actuator has been demonstrated to be useful in mitigating boundary layer separation in aerodynamic flows. To increase the impact that a plasma actuator may have on the flow field, this research investigates the development and characterization of a novel flow control device, the plasma synthetic jet actuator, which tailors the residual air in the form of a vertical jet resembling conventional continuous and synthetic jets. This jet can be either three dimensional using annular electrode arrays, or nearly two dimensional using two rectangular strip exposed electrodes and one embedded electrode. Detailed measurements on the isolated plasma synthetic jet reveal that pulsed operation of the actuator results in the formation of multiple counterrotating vortical structures in the flow field. The output jet velocity and momentum are found to be higher for unsteady pulsing as compared to steady operation. In the case of flow over a flat plate, the actuator is observed to create a localized interaction region within which the baseline flow direction and boundary layer characteristics are modified. The efficiency of the actuator in coupling momentum to the neutral air is found to be related to the plasma morphology, pulsing frequency, actuator dimension, and input power. An analytical scaling model is proposed to describe the effects of varying the above variables on the output jet characteristics and actuator efficiency, and the experimental data is used for model validation.

With the looming energy crisis compounded by the global economic downturn there is an urgent need to increase energy efficiency and to discover new energy sources. An approach to solve this problem is to improve the efficiency of aerodynamic vehicles by using active flow control tools such as synthetic jet actuators. These devices are able to reduce fuel consumption and streamlined vehicle design by reducing drag and weight, and increasing maneuverability. Hence, the main goal of this dissertation is to study factors that affect the efficiency of synthetic jets by incorporating energy harvesting into actuator design using prestressed piezoelectric composites. Four state-of-the-art piezoelectric composites were chosen as active diaphragms in synthetic jet actuators. These composites not only overcome the inherent brittle and fragile nature of piezoelectric materials but also enhance domain movement which in turn enhances intrinsic contributions. With these varying characteristics among different types of composites, the intricacies of the synthetic jet design and its implementation increases. In addition the electrical power requirements of piezoelectric materials make the new SJA system a coupled multiphysics problem involving electro–mechanical and structural–fluid interactions. Due to the nature of this system, a design of experiments approach, a method of combining experiments and statistics, is utilized. Geometric and electro-mechanical factors are investigated using a fractional factorial design with peak synthetic jet velocity as a response variable. Furthermore, energy generated by the system oscillations is harvested with a prestressed composite and a piezo-polymer. Using response surface methodology the process is optimized under different temperatures and pressures to simulate harsh environmental conditions. Results of the fractional factorial experimental design showed that cavity dimensions and type of signal used to drive the synthetic jet actuator were statistically significant factors when studying peak jet velocity. The Bimorph (~50m/s) and the prestressed metal composite (~45m/s) generated similar peak jet velocities but the later is the most robust of all tested actuators. In addition, an alternate input signal to the composite, a sawtooth waveform, leads to jets formed with larger peak velocities at frequencies above 15Hz. The optimized factor levels for the energy harvesting process were identified as 237.6kPa, 3.7Hz, 1MΩ and 12°C and the power density measured at these conditions was 24.27µW/mm3. Finally, the SJA is integrated with an energy harvesting system and the power generated is stored into a large capacitor and a rechargeable battery. After approximately six hours of operation 5V of generated voltage is stored in a 330µF capacitor with the prestressed metal composite as the harvester. It is then demonstrated that energy harvested from the inherent vibrations of a SJA can be stored for later use. Then, the system proposed in this dissertation not only improves on the efficiency of aerodynamic bodies, but also harvests energy that is otherwise wasted.

A numerical study of axisymmetric synthetic jet flow was conducted. The synthetic jet cavity was modeled as a rigid chamber with a piston-like moving diaphragm at its bottom. The Shear-Stress-Transportation (SST) k-omega and #61559; turbulence model was employed to simulate turbulence. Based on time-mean analysis, three flow regimes were identified for typical synthetic jet flows. Typical vortex dynamics and flow patterns were analyzed. The effects of changes of working frequency, cavity geometry (aspect ratio), and nozzle geometry were investigated. A control-volume model of synthetic jet cavity was proposed based on the numerical study, which consists of two first-order ODEs. With appropriately selected parameters, the model was able to predict the cavity pressure and average velocity through the nozzle within 10% errors compared with full simulations. The cavity model can be used to generate the boundary conditions for synthetic jet simulations and the agreement to the full simulation results was good. The saving of computational cost is significant. It was found that synthetic jet impingement heat transfer outperforms conventional jet impingement heat transfer with equivalent average jet velocity. Normal jet impingement heat transfer using synthetic jet was investigated numerically too. The effects of changes of design and working parameters on local heat transfer on the impingement plate were investigated. Key flow structures and heat transfer characteristics were identified. At last, a parametric study of an active heat sink employing synthetic jet technology was conducted using Large Eddy Simulation (LES). Optimal design parameters were recommended base on the parametric study.

The effect of direct, small-scale excitation on the evolution of a plane shear layer which forms at the edge of a backward facing step is investigated experimentally using high resolution particle image velocimetry and hot-wire anemometry. Actuation is effected at frequencies that are over an order of magnitude higher than the characteristic (or natural) formation frequency of the layer by a spanwise array of piezoelectrically-driven synthetic jet actuators that are placed near the edge of the step. The actuation has significant effects on the evolution of both large- and small-scale motions within the shear layer inducing an increase in small-scale dissipation and simultaneous suppression of turbulence production. While the fundamental instabilities that lead to the formation of large scale motions are typically suppressed, low-frequency amplitude-modulation of the actuation signal allows the formation of large scale motions and entrainment which, in concert with the small-scale actuation, lead to enhancement of the turbulent shear stresses throughout the shear layer. Amplitude modulation is also used to assess the effect of flow transients that are induced by step or low duty cycle actuation. The present findings suggest strategies for controlled suppression or enhancement of mixing in the near field of the shear layer.

In this work, an active flow control method is studied numerically by using a synthetic jet over a NACA 0015 airfoil. Unsteady, turbulent flows over the NACA 0015 airfoil are computed using a Navier-Stokes solver. The Spalart-Allmaras turbulence model is employed in all computations. Unsteady flow solutions are computed in parallel using Parallel Virtual Machine library routines in a computer cluster. The synthetic jet is implemented to the flow solver as a boundary condition. Response Surface Methodology is employed for the optimization of synthetic jet parameters at various angles of attack. The synthetic jet parameters / the jet velocity, the jet location, the jet angle and the jet frequency are optimized to maximize the lift to drag ratio. The optimization study is performed for a constant value of jet power coefficient. The jet slot size is used as a dependent parameter in the optimization studies. The optimization study has shown that the jet velocity and the jet location are the dominant synthetic jet parameters. The optimum synthetic jet angle is observed to be increasing as the angle of attack increases. The optimum jet location is observed to be moving through the leading edge as angle of attack increases for the separated flows. It is observed that the application of the synthetic jet delays the flow separation on the suction side of the airfoil and increases the lift to drag ratio significantly especially at post stall angles of attack. The application of the synthetic jet is observed to be less effective for attached flows.

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.

The stringing regulation on greenhouse gases emissions coupled with the rising fuel price and the growth in aviation transportation have imposed increasing demands on the aircraft industry to develop revolutionary technologies to meet such challenges. Methods of delaying flow separation on aircraft high lift systems have been sought which can lead to an increase in the aircraft performance and ultimately a reduction in aircraft operational costs and its impact on the environment. Synthetic jet actuators are a promising method of delivering flow control for aircraft applications due to their ability to inject momentum to an external flow without net mass flux and their potential in being integrated in MEMS through micro-fabrication with relative ease. It has been demonstrated in many laboratory experiments that synthetic jets are capable of delaying flow separation on aerodynamic bodies of various shapes. However, currently the operating conditions of synthetic jets are mostly chosen by trial-and-error, and thus the flow control effectiveness varies from one experiment to another. In order to deliver an effective flow separation control which achieves a desired control effect at minimum energy expenditure, a better understanding of the fluid mechanics of the behaviour of synthetic jets and the interaction between synthetic jets and a boundary layer are required. The aims of the present research were to achieve such a goal through a series of purposely designed numerical simulations. Firstly, synthetic jets issued from a circular orifice into quiescent air were studied to understand the effect of dimensionless parameters on the formation and the extent of roll-up of vortex rings. The computational results confirmed that the Stokes number determines the strength of vortex roll-up of a synthetic jet. Based on the computational results, a parameter map was produced in which three different operational regimes of synthetic jets were indentified and a criterion for vortex roll-up was also established. A circular synthetic jet issued into a zero-pressure-gradient laminar boundary layer was then investigated. The capability of FLUENT in modelling the key characteristics of synthetic jets was validated using experimental data. The formation and evolution of coherent structures produced by the interaction between synthetic jets and a boundary layer, as well as their near-wall effect in terms of the wall shear stress, were examined. A parameter map illustrating how the appearance of the vortical structures and their corresponding shear stress patterns vary as the synthetic jet operating condition changes was established. In addition, the increase in the wall shear stress relative to the jet-off case was calculated to evaluate their potential separation control effect.Finally, the study moved one step forward to investigate the flow separation control effect of an array of three circular synthetic jets issued into a laminar boundary layer which separates downstream on an inclined plate. The impact of synthetic jets on the boundary layer prior to separation and the extent of flow separation delay on the flap, at a range of synthetic jet operating conditions, were examined and the correlation between them was investigated. Furthermore, the optimal operating conditions for this synthetic jet array in the current study were identified by considering both the flow control effect and the actuator power consumption. The characteristics of the corresponding vortical structures were also examined.The findings from this work have produced some further insights of the behaviour and the interaction between synthetic jets and a boundary layer, which will be useful for ensuring an effective application of synthetic jets in practical settings.