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

<p>This study aims to investigate the wave boundary layer and the turbulent<br>airflow above wind waves on slick-free and slick-covered water surfaces. To realize<br>this, we carried out laboratory measurements of the airflow in a wind-wave<br>tank, where we deployed three surfactants of different visco-elastic properties,<br>each at five wind speeds ranging from 4 ms^&#8722;1 to 8 ms^&#8722;1. For measurements<br>over slick-free water surfaces, we chose wind speeds, at which we observed the<br>same peak wave frequencies as in the presence of the surfactants. We measured<br>high-resolution single-point profiles of the horizontal and vertical velocity<br>components at different heights above the water surface using a Laser-Doppler-<br>Velocimeter (LDV), wave heights using a wire gauge, and wave slopes using<br>a laser slope gauge. Both wave field parameters were recorded simultaneously<br>with the airflow measurements to investigate the influences of the small-scale<br>wave field on the wave boundary layer. In the airflow turbulence spectra, we<br>found a clear maximum corresponding to the dominant wave frequencies reflecting<br>the influence of the waves on the airflow. However, depending on wind<br>speed and the surfactants&#8217; damping behaviour, the maximum differs in both its<br>strength and its height above the wavy surface, the latter being interpreted as<br>the wave boundary layer height. The LDV achieved mean data rates exceeding<br>2 kHz; hence, it resolved the small-scale turbulence, which manifests in the<br>high-frequency part of the turbulence spectra. For the slick-free cases, we observed<br>a linear decrease in turbulence with increasing height above the surface,<br>and increasing turbulence with increasing friction velocity u&#8727;, which depends<br>on the wind speed and wind-wave interactions. However, we did not find clear<br>trends at any wind speed when the water surface was covered by a surfactant.<br>Here, the turbulence increases with increasing height above the water surface for<br>higher friction velocities. Thus, the surfactants dampen not only the waves, but<br>they also reduce the turbulence in the airflow directly above the waves, within<br>the wave boundary layer.</p>