Tapered foils favor traveling-wave kinematics to enhance the performance of flapping propulsion

Abstract

Structural flexibility, a common feature of fish fins and insect wings, tends to increase propulsion performance compared to equivalent rigid systems in terms of thrust production and efficiency. Most past studies have focused on propulsors with uniform bending stiffness. This study investigates tapered flapping foils subjected to a heaving motion in a quiescent viscous fluid to evaluate the effect of thickness tapering on the hydrodynamic performance of the foils. The experimental results demonstrate that tapered stiffness distributions robustly enhance propulsive performance, associated with stronger vorticity and a downstream-favored orientation of the produced flow.