| Popis: |
This study aims to investigate the wave boundary layer and the turbulentairflow above wind waves on slick-free and slick-covered water surfaces. To realizethis, we carried out laboratory measurements of the airflow in a wind-wavetank, 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 measurementsover slick-free water surfaces, we chose wind speeds, at which we observed thesame peak wave frequencies as in the presence of the surfactants. We measuredhigh-resolution single-point profiles of the horizontal and vertical velocitycomponents at different heights above the water surface using a Laser-Doppler-Velocimeter (LDV), wave heights using a wire gauge, and wave slopes usinga laser slope gauge. Both wave field parameters were recorded simultaneouslywith the airflow measurements to investigate the influences of the small-scalewave field on the wave boundary layer. In the airflow turbulence spectra, wefound a clear maximum corresponding to the dominant wave frequencies reflectingthe influence of the waves on the airflow. However, depending on windspeed and the surfactants’ damping behaviour, the maximum differs in both itsstrength and its height above the wavy surface, the latter being interpreted asthe wave boundary layer height. The LDV achieved mean data rates exceeding2 kHz; hence, it resolved the small-scale turbulence, which manifests in thehigh-frequency part of the turbulence spectra. For the slick-free cases, we observeda linear decrease in turbulence with increasing height above the surface,and increasing turbulence with increasing friction velocity u∗, which dependson the wind speed and wind-wave interactions. However, we did not find cleartrends at any wind speed when the water surface was covered by a surfactant.Here, the turbulence increases with increasing height above the water surface forhigher friction velocities. Thus, the surfactants dampen not only the waves, butthey also reduce the turbulence in the airflow directly above the waves, withinthe wave boundary layer. |
