Propulsion of a flexible foil in a wavy flow: resonance, antiresonance, and destructive self-interference

TL;DR

This study models how a flexible foil interacts with wavy flows, revealing resonance, antiresonance, and interference effects that influence propulsion efficiency and energy extraction.

Contribution

It introduces a linear inviscid model showing how wavy flows affect foil propulsion, including antiresonance and destructive interference phenomena.

Findings

Wavy flow induces antiresonance near phase velocity unity.

Wavy flow acts as a low-pass filter, enhancing thrust and efficiency.

Flow and heaving frequency differences can enable propulsion and energy extraction.

Abstract

Swimming and flying animals demonstrate remarkable adaptations to diverse flow conditions in their environments. In this study, we aim to advance the fundamental understanding of the interaction between flexible bodies and heterogeneous flow conditions. We develop a linear inviscid model of an elastically mounted foil that passively pitches in response to a prescribed heaving motion and an incoming flow that consists of a traveling wave disturbance superposed on a uniform flow. In addition to the well-known resonant response, the wavy flow induces an antiresonant response for non-dimensional phase velocities near unity due to the emergence of non-circulatory forces that oppose circulatory forces. We also find that the wavy flow destructively interferes with itself, effectively rendering the foil a low-pass filter. The net result is that the waviness of the flow always improves thrust and efficiency when the wavy flow is of a different frequency than the prescribed heaving motion. Such a simple statement cannot be made when the wavy flow and heaving motion have the same frequency. Depending on the wavenumber and relative phase, the two may work in concert or in opposition, but they do open the possibility of simultaneous propulsion and net energy extraction from the flow, which, according to our model, is impossible in a uniform flow.