Influence of a Surfactant on Physical Processes Above and Below Wind-Generated Waves in a Wind-Wave Tank

The air and water flow boundary layers are strongly coupled with the wave field and the physical phenomena involved are essentially based on small submillimeter/millimeter scale features and dynamic processes within the first millimeters above and below the SML (Sea surface Micro Layer). The scale at which these complex feed-back mechanisms operate make their study particularly challenging.

Surfactants at the air-sea interface strongly dampen both the dominant gravity-capillary waves and micro-breaking waves and hence dramatically influence the dynamics and associated air-sea fluxes. Even though the general effect of these monolayers on the waves is well known by the scientific community, their influence on the surface dynamics and air-sea fluxes associated still need to be carefully studied.

A series of experiments were conducted at the 26-m long, 1.5-m high, 1-m wide wind-wave tank of the University of Hamburg (Germany), where a measurement system was developed and installed at a fetch of 15.5m. The system offers the possibility to perform high resolution (33 µm/pixel) PIV (Particle Image Velocimetry) to capture the motion in the air-water flows in the SML’s vicinity, and LIF (Laser Induced Fluorescence) to accurately detect the wavy interface, with a resolution of 55 µm/pixel. Experiments were carried out at a reference wind speed of 4.5 m/s, without and with an insoluble surfactant (oleyl alcohol).

In slick free conditions, high vorticity regions are observed under the wave crests. On the air-side, the viscous sublayer detaches from the crest of most of the observed waves, and is being regenerated on the windward side of the directly following wave. However, thanks to the wide 50-cm field of view, some evidence was found that, under specific conditions, the sheltered region past the airflow separation can overcome a wave, hence strongly affecting its growth. After deployment of oleyl alcohol at the water surface, the dominant gravity-capillary waves are strongly dampened, and the capillaries mostly disappeared. Waves are still sheltering the airflow on their leeward side, but no clear airflow separation is being seen, and the enhanced turbulent regions, which were observed below the crest in slick-free conditions, are thinner, more elongated, and less intense. In the waterside, it has also been noticed that with surfactants, some streaks are being ejected away from the wavy interface.