Nonlinearly enhanced vortex induced vibration energy harvesting systems : characterization and chaotic mitigation
Huynh, Bao Huy
Date of Issue2017-12-11
Interdisciplinary Graduate School (IGS)
Energy Research Institute @ NTU (ERI@N)
The world energy supply is still mainly relying on fossil fuels, which are hardly renewable and environmentally destructive. Therefore, technologies to harvest renewable energy have been developed as a solution. Among various renewable energy resources, marine energy is receiving intensive attention due to its cleanliness and worldwide availability. However, the energy supply from this resource is minor due to several critical shortcomings such as high cost of produced power, location dependence and limited working range. As a new alternative to the current marine energy technologies, Vortex Induced Vibration (VIV) energy harvesting systems have been recently developed. The main structure of a VIV system is a cylinder elastically supported by linear springs and perpendicularly immersed into a water flow. Due to the vortex shedding effect, the induced fluid force will excite the structure and consequently lead to vibration of the structure. Utilizable energy can be produced by connecting the vibrating structure to a conversion mechanism. The prominent merits of the VIV technology compared to other renewable energy technologies are its capability to harvest energy from low velocity water flows, relatively high output efficiency and low cost of produced power. The VIV system operates effectively at the water flow velocity that corresponds to its resonance frequency. When the flow velocity deviates from the resonance range, the effective operation is hardly maintained. This situation likely occurs since natural water flows inherently fluctuate in velocity. This challenge motivates the first objective of this thesis, which is to apply the bi-stable and hardening springs in order to widen the resonance range of the system. The investigation is carried out based upon the numerical simulation on a wake oscillator model and experimentally validated by a dedicated cyber-physical force-feedback testing platform. It is found that the bi-stable spring can widen the resonance towards the side of the low velocity flows. On a contrary, the resonance range is extended towards the side of the high velocity flows when the system is enhanced by the hardening spring. Therefore, a combined nonlinear spring is proposed and proven to be able to extend the resonance range towards both sides of the low and high velocity flows. Meanwhile, it is also observed that the bi-stable spring might result in chaotic responses that drastically lower the output power. Therefore, the second objective of this thesis is to quantify chaotic responses of the bi-stable VIV system through calculating the Lyapunov exponent, which is an effective tool to measure the chaotic degree. The quantification is performed in a wide range of the governing parameters to construct a comprehensive bifurcation map that presents the dependency of chaotic responses on these parameters. As the third objective, this thesis focuses on the case where the bi-stable VIV system is designed in such the output power is significantly improved at the low velocity flows and chaotic responses occur when the flow velocity increases. Therefore, the Ott-Grebogi-Yorke (OGY) and time delay feedback controllers are designed to actively stabilize chaotic responses. Both the controllers are shown to be able to stabilize chaotic responses and improve the output power up to 73.5 % compared to that of the uncontrolled chaotic system. This thesis is completed with three main contributions. The first contribution is the characterization of the VIV system enhanced by the hardening and bi-stable springs and the proposal of the combined nonlinear spring. The second contribution is attributed to the quantification of chaotic responses of the bi-stable VIV system. The third contribution is the design and implementation of the OGY and time delay feedback controllers to stabilize chaotic responses of the bi-stable VIV system.