Flow control can lead to saving millions of dollars in fuel costs each year by making an aircraft more efficient. Synthetic jets, a device for active flow control, operate by introducing small amounts of energy locally to achieve non-local changes in the flow field with large performance gains. These devices consist of a cavity with an oscillating diaphragm that divides it, into active and passive sides. The active side has a small opening where a jet is formed, whereas and the passive side does not directly participate in the fluidic jet.Research has shown that the synthetic jet behavior is dependent on the diaphragm and the cavity design hence, the focus of this work. The performance of the synthetic jet is studied under various factors related to the diaphragm and the cavity geometry. Four diaphragms, manufactured from piezoelectric composites, were selected for this study, Bimorph, Thunder®, Lipca and RFD. The overall factors considered are the driving signals, voltage, frequency, cavity height, orifice size, and passive cavity pressure. Using the average maximum jet velocity as the response variable, these factors are individually studied for each actuator and statistical analysis tools were used to select the relevant factors in the response variable. For all diaphragms, the driving signal was found to be the most important factor, with the sawtooth signal producing significantly higher velocities than the sine signal. Cavity dimensions also proved to be relevant factors when considering the designing of a synthetic jet actuator. The cavities with the smaller orifice produced lower velocities than those with larger orifices and the cavities with smaller volumes followed the same trend. Although there exist a relationship between cavity height and orifice size, the orifice size appears as the dominant factor.Driving frequency of the diaphragm was the only common factor to all diaphragms studied that was not statistically significant having a small effect on jet velocity. However along with waveform, it had a combined effect on jet velocity for all actuators. With the sawtooth signal, the velocity remained constant after a particular low frequency, thus indicating that the synthetic jet cavity could be saturated and the flow choked. No such saturation point was reached with the sine signal, for the frequencies tested. Passive cavity pressure seemed to have a positive effect on the jet velocity up to a particular pressure characteristic of the diaphragm, beyond which the pressure had an adverse effect. For Thunder® and Lipca, the passive cavity pressure that produced a peak was measured at approximately 20 and 18kPa respectively independent of the waveform utilized. For a Bimorph and RFD, this effect was not observed.Linear models for all actuators with the factors found to be statistically significant were developed. These models should lead to further design improvements of synthetic jets.

This research discusses the mechanical characteristics of laminated piezoelectric actuators that are manufactured at an elevated temperature, to cure the adhesive bonding the layers together, or to cure the layers made of polymeric composite material, and then cooled to a service temperature. Mainly discussed are actuators that are composed of layers of passive materials and a layer of piezoelectric material. THUNDER (THin layer UNimorph ferroelectric DrivER and sensor) and LIPCA (LIghtweight Piezo-composite Curved Actuator) actuators, which consist of layers of metal, adhesive and piezoelectric material, and carbon-epoxy, glass-epoxy and piezoelectric material, respectively, are studied and investigated in detail to understand the thermal effects due to the elevated manufacturing temperature. Owing to the large out-of-plane deformations of the THUNDER actuators as a result of cooling to the service temperature, inclusion of geometric nonlinearities in the kinematic relations is taken into consideration for prediction of the thermally-induced deformations and residual stresses. The deformations and residual stresses are predicted by using a 23-term Rayleigh-Ritz approach and more rigorous, time-consuming, finite-element analyses performed with ABAQUS. The thermally-induced deformations of THUNDER actuators can result in multiple room-temperature manufactured shapes, whereas those of LIPCA actuators (LIPCA-C1 and LIPCA-C2) exhibit single room-temperature manufactured shape. Actuation responses of these actuators caused by a quasi-static electric field applied to the piezoelectric layer are also studied with the Rayleigh-Ritz approach. It is shown that geometrical nonlinearities play an important role in the actuation responses, and these nonlinearities can be controlled by the choice of actuator geometry and the materials in the passive layers. In addition, blocking forces representing load-carrying capability of THUNDER and LIPCA actuators are determined. Support conditions and again geometrical nonlinearities are vital factor in load-resisting performances. Amongst the actuators considered, the actuated deflection and blocking forces are compared. Finally, based on the outcome of this study, new criteria for designing a new type of laminated piezoelectric actuators with improvement of performance characteristics are proposed. / Ph. D.