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

利用粒子跟踪测速技术（PTV），在风浪通道上测量了近水面的速度场以及风浪作用下的水面速度。我们的主要目的是正确测量漂流流剖面附近的水面，包括区域以上的波谷。第一年我们用流函数法计算波轨道速度。在忽略旋转部分的情况下，从时间自由面取垂直坐标z，得到了风浪下方实质层的净风漂流垂直廓线，它是波相位角（-180 ° ~+180 °）的函数。还获得了作为波相位角的函数的表面风漂移速度。结果表明，漂移流的垂向剖面随波相的变化而变化。在第二年，我们通过改进程序测量了更接近表面的速度，我们认为在每个阶段的测量速度的总平均值<u>作为漂移电流。并<u>将漂移流与流函数法计算的轨道位速度进行了比较。从时间表面测量的垂向坐标系中，近水面平均漂流流剖面可用Kato（1975）提出的对数分布来表示，粗糙度参数z_0 =0.05cm。由1/60秒的连续图象测量的风浪相速度与Kato（1975）用所得到的参数z_0解计算的相速度符合得很好。

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.