| Summary: | 碩士 === 國立臺灣大學 === 工程科學及海洋工程學研究所 === 106 === The counter-rotating streamwise vortex pairs are generated in the free-slip surface boundary layers. The vortical structures, which is also called Langmuir circulations, induce the converging zone at surface and form the streamwisely elongated streaky signatures. In this study, the characteristics of streaky structure on thermographic images of wind wave surface are analyzed. At first, the imaging procession techniques are employed to eliminate the effects of white noises and fine turbulent structures on images. As the positions of streaks are identified, the streak spacings and the corresponding distributions are then derived. The result shows that the mean streak spacing decreases as the friction velocity grows, which is similar to that of the no-slip wall turbulent boundary layer. The non-dimensional mean streak spacing based on friction length, however, increases with the friction velocity, different from that of the no-slip wall turbulent boundary layer, in which the non-dimensional mean streak spacing remains to be constant. Through the method of chi-square goodness-of-fit test, it is found that the distributions of streak spacing are close to lognormal distribution, similar to that of no-slip wall turbulent boundary layer.
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