Stabilizing effect of flexibility in the wake of a flapping foil

TL;DR

This study investigates how chord-wise flexibility in a flapping foil influences wake dynamics and propulsion efficiency, revealing that flexibility stabilizes wake symmetry and significantly increases thrust compared to rigid foils.

Contribution

It demonstrates that foil flexibility inhibits wake symmetry breaking and enhances thrust, providing new insights into flexible foil design for improved propulsion.

Findings

Flexible foils suppress wake symmetry breaking.

Thrust can be up to three times greater with flexibility.

Vortex dynamics explain the stabilization and thrust increase.

Abstract

The wake of a flexible foil undergoing pitching oscillations in a low-speed hydrodynamic tunnel is used to examine the effect of chord-wise foil flexibility in the dynamical features of flapping-based propulsion. We compare the regime transitions in the wake with respect to the case of a rigid foil and show that foil flexibility inhibits the symmetry breaking of the reverse B\'enard-von K\'arm\'an wake reported in the literature. A momentum balance calculation shows the average thrust to be up to three times greater for the flexible foil than for the rigid foil. We explain both of these observations by analyzing the vortex dynamics in the very near wake.