Pitching foil propulsion performance decays near the free surface

摘要

A series of experiments reveal the non-linear relationship between the propulsive performance of a pitching foil and its distance to the free surface. An especially designed robotic device allowed to sinusoidally actuate a symmetric NACA 0021 profile, with its shaft being perpendicular to the free surface, at different separations from it, whilst measuring hydrodynamic loads, power and efficiency. Digital Particle Image Velocimetry (DPIV) was used to study the flow structures around the flapping system, to better understand the performance trends observed. Efficiency dropped more than a 40% when the foil was placed at a distance of approximately 5 to 10% of the span of the system, as a consequence of a large decrease in thrust and a large increase in power, both clearly linked to wake formation.