Experiments In Direct Energy Extraction Through Flapping Foils

Work with fish has shown their ability to extract energy from unsteady flows and vortical structures. We demonstrate experimentally that foils performing a sinusoidal sway (heave) and yaw (pitch) motion, similar to a fish, with continuously controlled parameters such as yaw amplitude, Strouhal number, and phase angle between sway and yaw, can efficiently extract energy from unsteady flows. Aluminum NACA 0012 foils with aspect ratios of 4.1, 5.9, and 7.9 were tested at Reynolds numbers ~13,800. Overall efficiencies of up to 43 ± 3% are achieved with simple sinusoidal motions.