Integrated Actuation And Energy Harvesting In Prestressed Piezoelectric Synthetic Jets

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

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

Piezoelectric Composites as Bender Actuators

ABSTRACT Lead Zirconate Titanate, PZT, layered into a composite with different materials, produces pre-stressed, curved, devices capable of enhanced displacement. This study focuses on Thunder and Lipca which are built using different combinations of constituent materials. Thunder devices consist of layers of aluminum, PZT, and stainless steel bonded with a hot-melt adhesive. Lipca devices consist of carbon and fiberglass layers with a PZT layer sandwiched in between them. Measuring out-of-plane displacement under load as a function of temperature is used to evaluate field-dependent stiffness. Results show that Lipca devices have higher stiffness than Thunder at 24 • C, but lower at other temperatures

Pressure Loading of Piezo Composite Unimorphs

ABSTRACTOver the past decade synthetic jets have emerged as a promising means of active flow control. They have the ability to introduce small amounts of energy locally to achieve non-local changes in the flow field. These devices have the potential of saving millions of dollars by increasing the efficiency and simplifying fluid related systems. A synthetic jet actuator consists of a cavity with an oscillating diaphragm. As the diaphragm oscillates, jets are formed through an orifice in the cavity. This paper focuses on piezoelectric synthetic jets formed using two types of active diaphragms, Thunder® and Lipca. Thunder® is composed of three layers; two metal layers, with a PZT-5A layer in between, bonded with a polyimide adhesive. Lipca is a Light WeIght Piezo Composite Actuator, formed of a number of carbon fiber prepreg layers and an active PZT-5A layer. As these diaphragms oscillate, pressure differences within the cavity as well as average maximum jet velocities are measured. These parameters are measured under load and no-load conditions by controlling pressure at the back of the actuator or the passive cavity. Results show that the average maximum jet velocities measured at the exit of the active cavity, follow a similar trend to the active pressures for both devices. Active pressure and jet velocity increase with passive pressure to a maximum, and then decrease. Active pressure and the jet velocity peaked at the same passive cavity pressure of 18kPa for both diaphragms indicating that the same level of pre-stresses is present in both actuators even though Lipca produces approximately 10% higher velocities than Thunder®.</jats:p