## Aeroelastic Flutter Energy Harvesting

### Project Objective

This project seeks to investigate, model, and test novel methods of generating electrical power from ambient fluid motion through aeroelastic flutter vibrations of flexible piezoelectric structures. Such a system could be designed for applications ranging from distributed power generation in densely populated urban areas to creating self power in-situ wireless sensors for aerospace applications.

### Project Description

While flow induced vibrations have generally been regarded as dangerous, destructive fluid structure interaction phenomena, these vibration sources may also offer opportunities for novel energy harvesting techniques. The aeroelastic energy harvester designed by the authors consists of a cantilevered piezoelectric bender with a flap connected to the bender tip by a revolute joint, as shown schematically by Figure 1. This arrangement creates a structure with two degrees of freedom, the bending deflection of the bender and the pitch rotation of the flap. The system is designed such that it is subject to a modal convergence flutter instability above a critical wind speed. In this type of instability, the natural frequencies of two or more modes are aerodynamically driven to converge near the onset of flutter, and energy is then transferred from the flow to the structure, causing one of the system poles to become unstable and growing oscillations to occur. The flutter vibrations grow in amplitude until system nonlinearities become significant enough to create limit cycle oscillation.

A semi–empirical nonlinear system modeling that expresses the three-way coupling between the aerodynamic, structural, and electrical aspects of the system has been derived through generalizing the linear aeroelastic wing model to include the effects of large angles of attack and a multimode electromechanical model of the piezoelectric bender. The semi–empirical ONERA model of dynamic stall is employed to calculate the nonlinear aerodynamic forces and moments acting on the fluttering structure at high angles of attack. This model then allows for analysis of the response and interaction of each degree of freedom as well as the electrical output of the system throughout the range of operating wind speeds, as shown by Figures 2 and 3. Wind tunnel experiments provide experimental verification of the model performance.