Fluid-Structure Instability in Internal Flow Energy Harvester

Abstract

Access to reliable power sources in remote locations is a recurring engineering challenge for both large and small applications.  The developing world struggles with power connectivity in remote villages, while sensor networks strain with power limitations of batteries or short-lived turbines.  Energy harvesting based on fluid-induced vibration provides a potential robust alternative for in-situ power generation, furnishing means for a decades long supply of power.  Yet, one of the main challenges in the design of flow energy harvesters is understanding the mechanisms that drive their motion.  Fluid-structure interaction problems often span a large parametric space and require considerable computational resources to resolve the necessary dynamic details for reliable designs.  
 
This thesis aims to address this challenge for a piezoelectric internal flow energy harvester developed in conjunction with NASA Jet Propulsion Laboratory for in-well, deepwater sensor and actuator systems.  Through exploratory experimentation, a configuration consisting of a piezoelectric beam within a converging-diverging channel in axial flow generated considerable power at moderate flow velocities when compared to other devices of the same size.  The current device, though adapted to a more robust configuration based on flextensional actuators, still maintains the same fluid-structure interaction: the instability that ensues forces the system into self-sustained oscillations that produces consistent power output for flow rates above a critical threshold.  
 
To understand and quantify this behavior, we develop an analytical framework based on a leakage-flow type instability, which curtails the shortcomings of expensive numerical simulations once verified.  The formulation consists of a quasi one-dimensional simplification of coupled fluid-structure equations, which are linearized for classical stability analysis.  The stability boundary and critical property predictions are verified through a set of fully coupled fluid-structure immersed boundary direct numerical simulations.  Experiments are carried out in tandem to quantify the dynamics of the harvester, specifically targeting the critical flow rate threshold.   The analytical framework is expanded to include flow in the spanwise direction of the beam, and results to a simplified geometry of the harvester compared with those from experiments.  Agreement between predicted critical values suggest that leakage-flow may be the principal mechanism for fluid-induced vibration within our device.  The model can serve as the foundation of initial exploration of design parameters, and perhaps more powerful devices in future endeavors.