On the wave boundary layer above wind waves: influence of surfactants

This study aims to investigate the wave boundary layer and the turbulent
airflow above wind waves on slick-free and slick-covered water surfaces. To realize
this, we carried out laboratory measurements of the airflow in a wind-wave
tank, where we deployed three surfactants of different visco-elastic properties,
each at five wind speeds ranging from 4 ms⁻¹ to 8 ms⁻¹. For measurements
over slick-free water surfaces, we chose wind speeds, at which we observed the
same peak wave frequencies as in the presence of the surfactants. We measured
high-resolution single-point profiles of the horizontal and vertical velocity
components at different heights above the water surface using a Laser-Doppler-
Velocimeter (LDV), wave heights using a wire gauge, and wave slopes using
a laser slope gauge. Both wave field parameters were recorded simultaneously
with the airflow measurements to investigate the influences of the small-scale
wave field on the wave boundary layer. In the airflow turbulence spectra, we
found a clear maximum corresponding to the dominant wave frequencies reflecting
the influence of the waves on the airflow. However, depending on wind
speed and the surfactants’ damping behaviour, the maximum differs in both its
strength and its height above the wavy surface, the latter being interpreted as
the wave boundary layer height. The LDV achieved mean data rates exceeding
2 kHz; hence, it resolved the small-scale turbulence, which manifests in the
high-frequency part of the turbulence spectra. For the slick-free cases, we observed
a linear decrease in turbulence with increasing height above the surface,
and increasing turbulence with increasing friction velocity u∗, which depends
on the wind speed and wind-wave interactions. However, we did not find clear
trends at any wind speed when the water surface was covered by a surfactant.
Here, the turbulence increases with increasing height above the water surface for
higher friction velocities. Thus, the surfactants dampen not only the waves, but
they also reduce the turbulence in the airflow directly above the waves, within
the wave boundary layer.