Experimental Design and Analysis of Piezoelectric Synthetic Jets in Quiescent Air

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