Large-amplitude oscillations of foils for efficient propulsion

TL;DR

This paper investigates how large-amplitude oscillations of foils improve propulsion efficiency, highlighting the roles of drag and amplitude in optimizing thrust and efficiency through scaling laws and experiments.

Contribution

It introduces scaling relations and experimental insights explaining the benefits of large-amplitude oscillations for propulsion efficiency, emphasizing drag effects.

Findings

Large-amplitude oscillations increase propulsive efficiency.

Drag significantly reduces efficiency, especially at higher amplitudes.

A fundamental tradeoff exists between high thrust and high efficiency.

Abstract

Large-amplitude oscillations of foils have been observed to yield greater propulsive efficiency than small-amplitude oscillations. Using scaling relations and experiments on foils with peak-to-peak trailing edge amplitudes of up to two chord lengths, we explain why this is so. In the process, we reveal the importance of drag, specifically how it can significantly reduce the efficiency, and how this effect depends on amplitude. The scaling relations and experimental data also reveal a fundamental tradeoff between high thrust and high efficiency, where the drag also plays a crucial role.