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Experimental design and analysis for piezoelectric circular actuators in flow control applications

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, while the passive side does not directly participate in the fluidic jet. Over the years, research has shown that synthetic jet behavior is dependent on the active 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. Three diaphragms, manufactured from piezoelectric composites, were selected for this study: Bimorph, Thunder® and Lipca. 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 are used to select the relevant factors in the response variable. The factors are divided into two experimental fractional factorial design matrices, with five and four factors, respectively. Both experiments are chosen to be of resolution V, where main factors are confounded with three-factor interactions. In the first experimental design, the results show that frequency is not a significant factor, while waveform is significant for all the actuators. In addition, the magnitude of the regression coefficients suggests that a model that includes the diaphragm as a factor may be possible. These results are valid within the ranges tested, that is low frequencies and sawtooth and sine waveform as driving signals. In the second experimental design, cavity dimensions are kept constant and four factors including back pressure are considered. In this case, each diaphragm produces different results with only one diaphragm, Thunder, showing a definite relationship between the studied factors. The other two diaphragms do not show conclusive results, indicating that there may be other factors that need to be considered when pressure is a concern. In summary, independently of the diaphragm utilized in a synthetic jet actuator, applied waveform is an important factor when maximizing peak jet velocity. In addition, frequency is found not to be significant in all cases within the limits of the study. This indicates that the diaphragm and the driving signal should be included in any optimization design of a piezoelectric synthetic jet actuator.