Measurements of Wind-Driven Current Near the Water Surface by the Use of the PTV Method

Abstract

Using a wind-wave channel, the measurements of velocity field near the water surface as well as the surface velocity under the wind-waves were made by means of particle tracking velocimetry (PTV). We removed the orbital velocity by using the stream function method (Thais and Magnaudiet, 1995), though we neglected the rotational part. Vertical profiles of the net wind-driven velocities were obtained in a substantial layer below the wind-waves as a function of the wave phase angles with taking the vertical coordinate z from the temporal free surface. The surface wind drift speeds were also obtained as a function of the phase angles. The results show the vertical profiles of the drift currents vary as a function of the wave phases. The mean profile near the water surface was found to be expressed by a logarithmic distribution with z 0 = 0.10cm. In this work the resulting scatter of measured points both at various phases and depths might correspond to turbulence that dominates the gas transfer rate at the air-water interface. The relative magnitude of turbulence in the aqueous layer near the water surface at various wave phases can be seen qualitatively.