Generation of stripe-like vortex flow by noncollinear waves on the water surface

TL;DR

This study experimentally investigates how noncollinear gravity waves generate stripe-like vortex flows on water surfaces, revealing the relationship between wave parameters, vortex strength, and surface contamination effects.

Contribution

It provides the first experimental validation of a model describing vortex generation by noncollinear waves, including the influence of surface contamination.

Findings

Vortex vorticity is proportional to wave amplitude product.

Vortex strength exceeds Stokes drift predictions.

Surface contamination affects vortex dynamics and wave decay.

Abstract

We have studied experimentally the generation of vortex flow by gravity waves with a frequency of 2.34 Hz excited on the water surface at an angle $2 \theta = arctan(3/4) \approx 36\deg$ to each other. The resulting horizontal surface flow has a stripe-like spatial structure. The width of the stripes L = $\pi$/(2ksin$\theta$) is determined by the wave vector k of the surface waves and the angle between them, and the length of the stripes is limited by the system size. It was found that the vertical vorticity $\Omega$ of the current on the fluid surface is proportional to the product of wave amplitudes, but its value is much higher than the value corresponding to the Stokes drift and it continues to grow with time even after the wave motion reaches a stationary regime. We demonstrate that the measured dependence $\Omega$(t) can be described within the recently developed model that takes into account the Eulerian contribution to the generated vortex flow and the effect of surface contamination. This model contains a free parameter that describes the elastic properties of the contaminated surface, and we also show that the found value of this parameter is in reasonable agreement with the measured decay rate of surface waves.