Date of Award

5-2016

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Mechanical Engineering

Committee Member

Dr. Mohammed F. Daqaq, Committee Chair

Committee Member

Dr. Gang Li

Committee Member

Dr. Nicole Coutris

Committee Member

Dr. Phanindra Tallapragada

Abstract

Fluid-structure coupling mechanisms such as galloping and wake galloping have recently emerged as effective methods to develop scalable flow energy harvesters (FEHs) that can be used to power remote sensors and sensor networks. The oper-ation concept of these devices is based on coupling the pressure forces culminating from the motion of the fluid past a mechanical oscillator to its natural modes of vibration. As a result, the mechanical oscillator undergoes large-amplitude motions that can be transformed into electricity by utilizing an electromechanical transduc-tion mechanism, which is generally piezoelectric or electromagnetic in nature. Due to their scalability and design simplicity, FEHs are believed to be more effective for micro-power generation than their traditional rotary-type counterparts whose efficiency is known to drop significantly as their size decreases. Furthermore, FEHs can be used to harvest energy from unsteady flow patterns which permits targeting a niche market that traditional rotary-type generators do not address. In the open literature, galloping FEHs have always been designed to possess a linear restoring force. This dissertation considers the design and performance analysis of galloping FEHs with a nonlinear restoring force. Specifically, the objective of this dissertation is three fold. First, it assesses the influence of stiffness nonlinearities on the performance of galloping FEHs under steady and laminar flow conditions. Second, it studies the influence of the nonlinearity on the response of a wake gal-loping FEH to single- and multi-frequency Von Karman vortex streets. Third, for known flow characteristics, the dissertation provides directions for how to choose the restoring force of the harvester to maximize the output power. To achieve the objectives of this dissertation, a nonlinear FEH which consists of a thin piezoelectric cantilever beam augmented with a square-sectioned bluff body at the free end is con-sidered. Two magnets located near the tip of the bluff body are used to introduce the nonlinearity which strength and nature can be altered by changing the distance between the magnets. For a steady laminar flow, three types of nonlinear restoring forces are compared: bi-stable, mono-stable hardening, and mono-stable softening. To study the influ-ence of the restoring force on the performance, a physics-based nonlinear lumped-parameter aero-electromechanical model adopting the quasi-steady assumption for aerodynamic loading is developed. A closed-form solution of the nonlinear response is obtained by employing a multiple-scales perturbation analysis using the Jacobi el-liptic functions. The attained solution is validated experimentally using wind tunnel tests performed at different wind speeds for the three types of restoring forces con-sidered. The validated solution is then used to study the influence of the nonlinearity on the harvesters response. In general, it is shown that, under optimal operating conditions, a harvester designed with a bi-stable restoring force outperforms the other designs. For single- and multi-frequency vortex streets, only linear and bi-stable restoring forces were considered and compared. A nonlinear lumped-parameter model adopt-ing the common uncoupled single-frequency force model for aerodynamics loading is developed and solved using the method of multiple scales. The model is validated against experimental data obtained in a wind tunnel. It is demonstrated that when subjected to a single-frequency periodic wake, the broadband characteristics of wake-galloping FEHs can be dramatically improved by incorporating a bi-stable restoring force. This has the influence of reducing the harvester’s sensitivity to variations in the wind speed around the nominal design value. It is also demonstrated that the shape of the potential function has a considerable influence on the performance of the bi-stable wake galloping FEH. Specifically, it is shown that, for shallower poten-tial wells and smaller separation distances between the wells, the harvester starts performing large inter-well motions at lower wind speeds, but the resulting inter-well motions are generally smaller. On the other hand, for deeper potential wells and larger separation distances between the wells, the harvester starts performing large inter-well motions at higher wind speeds, but the magnitude of the resulting inter-well motions are generally larger. The dissertation also compared the performance of linear and bi-stable wake-galloping FEHs under a multi-frequency vortex street. Results demonstrated that the bi-stable system outperforms the linear harvester as long as the vortices have sufficient time to interact and build a multi-frequency vortical structure. Maximum voltage levels were generated at locations where the interacting vortices result in powerful modes close to the harvesters natural frequency.

Share

COinS