Surface deformations and wave generation by wind blowing over a viscous liquid

TL;DR

This study experimentally examines the initial wave formation on a viscous liquid surface caused by turbulent wind, identifying two deformation regimes and analyzing their characteristics using high-precision optical measurements.

Contribution

It introduces a detailed experimental analysis of wind-induced surface deformations on viscous liquids, revealing two distinct regimes and their transition.

Findings

Low wind speed wrinkles are disorganized and propagate rapidly.

Surface deformations increase linearly with wind speed in the first regime.

Above a certain wind speed, organized quasi-parallel waves form and grow rapidly downstream.

Abstract

We investigate experimentally the early stage of the generation of waves by a turbulent wind at the surface of a viscous liquid. The spatio-temporal structure of the surface deformation is analyzed by the optical method Free Surface Synthetic Schlieren, which allows for time-resolved measurements with a micrometric accuracy. Because of the high viscosity of the liquid, the flow induced by the turbulent wind in the liquid remains laminar, with weak surface drift velocity. Two regimes of deformation of the liquid-air interface are identified. In the first regime, at low wind speed, the surface is dominated by rapidly propagating disorganized wrinkles, elongated in the streamwise direction, which can be interpreted as the surface response to the pressure fluctuations advected by the turbulent airflow. The amplitude of these deformations increases approximately linearly with wind velocity and are essentially independent of the fetch (distance along the channel). Above a threshold in wind speed, the perturbations organize themselves spatially into quasi parallel waves perpendicular to the wind direction with their amplitude increasing downstream. In this second regime, the wave amplitude increases with wind speed but far more quickly than in the first regime.