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

利用风浪通道，利用粒子跟踪测速仪(PTV)测量了近水面的速度场和风浪作用下的水面速度场。我们的主要目标是准确地测量水面附近的漂流剖面，包括波谷以上的区域。在第一年，我们使用流函数方法计算了波的轨道速度。不考虑它们的旋转部分，得到了风浪下实质层净风漂流的垂直廓线，它是从时间自由面取垂直坐标z的波相角(-180到+180度)的函数。还得到了地面风速随波相角的变化规律。结果表明，漂移电流的垂直分布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，并将每一阶段测量的速度的总平均值<u>视为漂移电流。然后将<u>和漂移电流的差值与流函数法计算的轨道势速度进行比较。用Kato(1975)提出的粗糙度参数z_0=0.05 cm的对数分布，可以很好地描述由时间面测得的垂直坐标在水面附近的平均漂流剖面。每隔1/60秒从连续图像图像上测得的风浪相速度与由Kato(1975)解得到的参数z_0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.