Laboratory study of turbulent momentum and energy fluxes above/below microscale breaking wind waves, and influence of surfactants

Exchanges of momentum and energy across the sea surface microlayer (SML)  are controlled by turbulent dynamics within the first millimeters above/below the wavy water surface. Wind-generated waves, ubiquitous at the ocean surface, strongly influence turbulent processes in the air and water near the surface, especially as the waves grow and (microscale) break. Surface-active substances, commonly found in coastal waters, are known to dampen waves over a wide range of scales. However, the influence of these surfactants on the coupled air-water flow dynamics and associated fluxes remains unclear. Indeed, some of the phenomena involved take place at a sub-millimeter scale, which makes it challenging to investigate the complex mechanisms at stake.

A combination of two experimental techniques (PIV, particle image velocimetry, and LIF, Laser Induced Fluorescence) with a high resolution (33 µm/pixel for the PIV and 55 µm/pixel for the LIF) were used to determine flow motions on both sides of the SML. The complex set-up was installed at a fetch of 15.5m at the 24-m long, 1-m wide, 1-5m high wind wave tank of the University of Hamburg (Germany) which is specially designed for studies with surfactants (Oleyl Alcohol, OLA in this work). Here, we focus on conditions with a reference windspeed of 4.5m/s measured by an ultrasonic anemometer at 64 cm above the water surface.

The wide field of view (51cm) enables us to capture the evolution in time and space of turbulent shear stress above and below individual wind waves. As the waves move through the field of view, steepen and microbreak, high magnitude turbulent shear is produced in the airflow past the wave-crest and can sometimes spread over several wavelengths when intense air-flow separation events occur. A quadrant analysis shows that negative momentum flux (Q1 and Q3) events are usually encountered before wave-crests whereas positive momentum fluxes (Q2 and Q4) events are produced past them on average. In the water, positive turbulent shear stress mainly shows up below the windward side of the waves, while negative turbulent shear is present below their leeward sides. An estimation of the viscous and turbulent energy dissipation integrated over the first centimeter underneath the water surface shows that the production of bound capillary waves enhances the energy dissipation, which becomes more intense as the capillary train grows up.

When surfactants are present, a reduction of sweeps and ejections (Q2 and Q4) past the dampened wave crests is notable and can be associated with the reduced occurrence and intensity of air-flow separation events. In the water, the removal of most capillary waves leads to a reduction in energy dissipation, as well as in the (phase) averaged turbulent kinetic energy below crests.