Effect of Trailing-Edge Flexibility on Wake Dynamics in a Wave-Assisted Flapping Foil

Wave-assisted propulsion (WAP) systems convert wave energy into thrust for unmanned surface vehicles using elastically mounted hydrofoils attached beneath the hull. Their performance depends on both wave conditions and hydrofoil design, which together govern the underlying fluid–structure interaction.This study investigates the role of trailing-edge flexibility in enhancing thrust generation by varying the chordwise transition point between rigid and flexible materials along an appended tail. Experiments were conducted in a quiescent wave tank using a NACA 0030 foil with a flat tail, mounted beneath a surface vessel. The tail’s rigidity profile was varied across three configurations: 0% (fully rigid), 50% (half rigid–half flexible), and 100% (fully flexible). The system operated under wave-induced heave motion with a wave amplitude-to-chord ratio of 0.1285, a Strouhal number approaching infinity (St → ∞), and a Reynolds number of 6096 based on maximum trailing-edge velocity.

Foil kinematics were recorded using high-speed GoPro imaging, and wake dynamics were measured via four-dimensional particle tracking velocimetry (4D-PTV). The fully rigid configuration produced closely spaced, weakly convecting vortices, resulting in an unstable and disorganized wake. In contrast, the flexible tails promoted more coherent vortex shedding and stronger downstream jet momentum. The 50% flexible configuration yielded the highest horizontal velocity and most symmetric jet, indicating optimal thrust performance. The fully flexible case outperformed the rigid configuration but exhibited asymmetric motion and wake deflection, reducing directional stability.

This work demonstrates that tuning trailing-edge flexibility can improve WAP system performance. The study was completed as part of a graduate course on Experimental Methods in Fluid Mechanics at the University of Michigan, Ann Arbor, taught by Prof. Anchal Sareen.