Study on the vertical structures of a wind drift current near water surface by the use of particle image velocimetry

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Using a wind-wave channel, measurements of velocity field near the water surface as well as the surface velocity under wind-waves were made by means of the particle tracking velocimetry (PTV). Our primary aim is to measure the drift current profile correctly near the water surface including the region above wave trough. In the first year we used the stream function method to calculate the wave orbital velocities. Neglecting the rotational part of them the vertical profiles of the net wind drift current were obtained at the substantial layer below the wind-waves as a function of the wave phase angles (-180 to +180 degree) taking the vertical coordinate z from the temporal free surface. The surface wind drift speeds were also obtained as a function of the wave phase angles. The results show the vertical profiles of the drift currents vary with the wave phases. In the second year we measured the velocities more close to the surface by improving the procedure and we considered the total average of the measured velocity at each phase <u> as a drift current. Then the difference of <u> and the drift current were compared with the orbital potential velocity calculated from the stream function method. The mean drift current profile near the water surface with the vertical coordinate measured from a temporal surface was found to be expressed well by a logarithmic distribution proposed by Kato (1975) with a roughness parameter z_0 =0.05cm. The phase speeds of wind-waves measured from sequential picture images at 1/60 second interval agreed well with the phase speeds calculated from Kato's (1975) solution with the obtained parameter z_0.