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With the rapid development of portable electrical devices, the demand for batteries to power these portable devices increases dramatically. However, the development of the battery technology is slow in energy storage capability and cannot meet such requirements. This paper proposed an optimal design for a kind of portable piezoelectric stack energy harvesters, with large force magnification and high energy transmission ratio. Two kinds of design approaches have been taken for the flexure compliant mechanism: physical model based and finite element analysis (FEA) based. Prototypes are fabricated and assembled. Experiments with both static test and dynamic test have been conducted to verify the effectiveness of the proposed design. The measured results show a force magnification ratio of 6.13 times and 21.8 times for the first-stage and the dual-stage harvesters, which are close to the design objective of 7.2 times and 24.4 times. The single stage harvester with 100gram top mass and a piezoelectric stack of dimensions 7x7x32 mm^2 can generate 20.7mW/g 2 at resonance frequency of 160Hz and match resistance of 393Ω. The dual-stage harvester with 100gram top mass achieves a power generation of 487mW/g 2 at resonance frequency of 38.9Hz and match resistance of 818Ω. The proposed two-stage PZT energy harvester can be used to develop portable power regenerators to meet the urgent power requirement in remote area for both civic and military application.

Torsional vibration is present in various applications, such as in oil drilling pipes, engine crankshafts, and wind turbine gearboxes. In the case of oil drilling, ever deeper wells are being drilled and sensors used to gather data from the bottom of the well. Providing an energy source for these sensors is a big challenge. Harvesting energy from torsional vibrations presents a promising solution for powering the sensors on rotational systems. We investigated the concept of torsional vibration energy harvesting using a piezoelectric transducer attached to a shaft at an arbitrary angle with respect to the axis of the shaft. A comprehensive theoretical model considering all the working modes, including d15, d31, and d33 mode, has been developed to express the voltage outputs as functions of the mounting angle. The frequency responses of the voltage outputs over the input torque have also been studied and compared. A finite element model was also implemented to verify the theoretical results and illustrate the voltage distribution within the piezoelectric material under an external torque input.

Skin friction drag plays a significant role in determining the fuel efficiency of a vehicle. Reducing the skin friction thus has implications in a number of applications such as aircraft, ships, and automobiles. In recent years, a large amount of studies have researched various active drag reduction methods. One particular method involves active wall motion with the use of spanwise traveling waves. These out-of-plane traveling waves interact with the vortices in the turbulent boundary layer and weaken the bursting events that are responsible for increased skin friction drag. Computational and experimental studies have shown that these spanwise traveling waves are able to reduce the skin friction by upwards of 13% in turbulent flow. However, previous studies generated traveling waves using bulky actuation setups requiring arrays of discrete actuators that limited the achievable bandwidth and traveling wave patterns. In order for traveling waves to be a practical drag reduction method, a more implementable wave generation method is necessary. A promising traveling wave generation method is known as two-mode excitation. This technique takes advantages of a surface's inherent structural properties to generate steady-state traveling waves in an open-loop fashion. In addition, the waves can be excited at most frequencies and using a small number of low-profile piezoelectric actuators. Previous research into two-mode excitation has primarily focused on one-dimensional beams. Traveling waves have been generated on a two-dimensional surface, but this was done from a fundamental standpoint with the resultant waves propagating in arbitrary directions. Before the two-mode excitation method can be applied for drag reduction, traveling waves must be generated on two-dimensional surfaces with tailorable propagation patterns. The goal of this research is the development and testing of an implementable traveling wave generation method that alters the turbulent boundary layer with the aim of reducing skin friction drag. The first objective is to further develop the two-mode excitation method in order to tailor the traveling waves generated on a two-dimensional plate. Then, these tailored traveling waves are experimentally tested to determine their effect on the turbulent boundary layer. By directly investigating the boundary layer, a more fundamental approach is taken than only focusing on the skin friction drag. Finally, the overall effect of the traveling waves on the boundary layer are compared with standing waves. vii None of this would have been possible without the support of my family. My parents, Robert and Jacqueline, you raised me to be the person I am today. You have always been there with support and encouragement, and I am eternally grateful. I want to thank my brothers, Brian and Robert, and their wives, Katie and Lindsay. You are always there to chat and you provided many late nights of fun. Finally, I want to thank Alexis Gushiken and the endless support she has given. You always know when to motivate, encourage, o...