The objective of this research is to investigate the energy harvesting as well as propulsion of a flapping wing with passively actuated leading and trailing edges, as inspired by biomimetics. Due to high complexity of continuous flexibility found in flying/swimming animals, we focus on the use of a three component wing model. This model consists of a trailing and leading edges connected to the wing body by torsion rods. The torsion rods provide passive rotational flexibility and the wing body is controlled to undergo flapping kinematics. The flapping kinematics is composed of heaving and pitching motions. A 3-D wing assembly is shown below.
Flapping foil energy harvesters utilize a flapping airfoil or hydrofoil to harvest energy from flowing fluid; the flapping foil motion consists of a translational motion as well as a rotational. These energy harvesters operate in a way such that the fluid does net work on the foil which is opposite to that of bird or insect flight. Flapping foil energy harvesters have notable advantages over their conventional rotating counter parts; they have lower tip speeds which both reduces the structural requirements and the negative impact on the environment in terms of wild life death and noise; the flow physics sensitivity to foil geometry is secondary allowing for very simple foil designs and reduces the manufacturing cost and maintenance necessary; the design is inherently more economical for environments with stringent geometries such as shallow riverbeds. Finally these energy harvesters can operate in a fully activated configuration which is ideal for studying the flow physics, a semi-passive configuration in which the rotational motion is actively controlled and induces the translational motion, and a fully passive regime in which both the rotational and translational motions are coupled and the device is self-sustaining.
What makes this design unique and challenging is that significant flow separation occurs during the foil operation making the flow physics challenging to understand. The foil reaches very large angles of attack and a phenomenon known as dynamic stall occurs where the flow separates and rolls up into a large vortex structure known as a Leading Edge Vortex (LEV). The LEV has a lower pressure core and increases the lift force while forming and moving close to the foil affecting the energy harvested; the strength of the LEV as well as the timing of its formation and detachment all affect the energy harvesting performance. Our lab collects aerodynamic force and Particle Image Velocimetry (PIV) data on an experimental flapping foil device in a wind tunnel allowing us to study how the flow develops for different flapping motions. We also work on low order aerodynamic models, such as discrete vortex methods for design work. Key research focuses include how the assumptions in the models limit the its accuracy as well as how the mechanisms of the model could be adjusted to better suit different operating parameters.
In our work we have designed and fabricated a unique foil that uses torsion springs to activate, through passive loading, dynamic changes of camber and effective angle of attack during heaving and pitching motions. The relative phase angle between effective angle of attack and the induced motion determines performance characteristics. We have fabricated in house a transient force measurement system that can be synchronized with our time resolved, three component PIV system to better understand how the vortex generation, and advection, contribute to phase loading that contributes positively to power output and propulsion. We continue to design and modify our designs to achieve high overall efficiency for a range of applications.