Flexible floaters align with the direction of wave propagation

TL;DR

This study models and experimentally verifies that flexible floating strips tend to align with wave direction due to a drift mechanism similar to Stokes drift, with alignment strength decreasing as strip length increases.

Contribution

The paper introduces a diffractionless, inviscid model predicting the orientation of floating strips in waves and confirms it through laboratory experiments.

Findings

Strips tend to align with wave propagation direction.

Alignment decreases with increasing strip length.

Model predictions agree with experimental observations.

Abstract

We investigate the slow, second order motion of thin flexible floating strips drifting in surface gravity waves. We introduce a diffractionless model (Froude-Krylov approximation) that neglects viscosity, surface tension, and radiation effects. This model predicts a mean yaw moment that favors a longitudinal orientation of the strip, along the direction of wave propagation. The physical mechanism for this angular drift is analog to that of the standard linear Stokes drift: it originates from a slight imbalance between the stronger acceleration on the wave crests (that favors the longitudinal orientation) and the wave troughs (that favors the transverse orientation). Laboratory experiments with thin rectangular strips of polypropylene show a systematic rotation of the strips toward the longitudinal orientation, in good agreement with our model. We finally observe that the mean angular velocity toward the stable longitudinal orientation decreases as the strip length increases, an effect likely due to dissipation, which is not accounted for in our inviscid model.