The Unsteady Aerodynamics of Flapping-Foil Propellers

Abstract

It is the objective of this paper to summarize the authors' recent work on flapping foils. Water tunnel experiments on sinusoidally plunging foils are described which elucidate the change in vortical wake pattern shed from the foil's trailing edge. These experiments were carried out using dye flow visualization and laser-Doppler velocimetry. It is found that the wake pattern is a strong function of the maximum non-dimensional plunge velocity, with the wake topology changing from a typical Kármán vortex street to an inverse Kármán vortex street to an asymmetric wake structure as the non-dimensional plunge velocity increases. These results are partly reproducible with inviscid panel code and Navier-Stokes code predictions. Additional interesting features are obtained if two degrees of freedom are permitted (pitch-plunge motions). Depending on the pitch/plunge amplitudes and the phasing between the two motions, the foil either produces thrust or extracts energy. A water tunnel experiment is described which demonstrates the possibility of power generation from a slowly flowing, shallow river. Additional interesting features are found if two airfoils in close proximity to each other are studied. Experiments with two airfoils arranged in a biplane configuration and oscillating in counter-phase show significant thrust and propulsive benefits in comparison to single flapping foils.