Objective:
- • To analyze, design, optimize, and demonstrate a bench-top scale aeroelastic power harvester to operate at typical atmospheric wind speeds. Such a system would provide an alternative method of wind powered electricity generation for applications where traditional wind turbines are impractical and may offer improved efficiency.
Introduction
Aeroelastic flutter of airfoils is typically considered a dangerous phenomenon that must be avoided by designing high stiffness airfoil structures and by restricting the flight envelope of aircraft. Approaching the onset of aeroelastic flutter, energy from a fluid flow is transferred to the airfoil structure leading to coupled bending and torsional vibration of the foil. The amplitudes of these vibrations grow exponentially and can lead to catastrophic failure if the damping in the system is insufficient to limit their magnitudes to safe levels. However, this phenomenon also provides potential for useful applications. In particular, an aeroelastic fluttering airfoil is being investigated as an energy harvesting mechanism.
Description
Modeling the system requires representing the inertial, damping and stiffness properties of the structure, as well as the aerodynamic characteristics of the vibrating structure, and the circuit elements of the power harvesting electronics. The airfoil structural system has been first modeled using a simple two degree of freedom, lumped mass representation of a typical airfoil section in order to gain intuition about the dynamic effects of adding electromagnetic or piezoelectric damping. The lift distribution and pitching moment acting on the airfoil are heavily influenced by unsteady fluid flow effects. These effects were incorporated using a time domain, finite-state, induced-flow theory model for inviscid and incompressible flows.
Figure 1: Two DOF airfoil section model with important parameters labeled.
The flutter problem can be analyzed using the "p method" to solve for the system eigenvalues and determine the flow speed and frequency of oscillation at the flutter boundary. A state space representation of the aeroelastic and electromechanical system has also been implemented. This allows time domain simulation of the system and facilitates actively controlling the power extraction mechanism to maximize the energy harvesting performance while ensuring the strains in the airfoil will not exceed safe limits. By cycling the energy extraction on and off based on the amplitude of the vibrations, the operating wind range of the harvester device can be greatly expanded compared to continuously harvesting power from the system. This also allows the oscillation amplitudes to be bounded to a prescribed level.
Figure 2: Simulated response and energy output of the aeroelastic power harvester.
Efforts are currently underway to optimize system performance by maximizing the energy output and operating range of the aeroelastic energy harvester. In addition, preliminary wind tunnel testing of the system is in progress.