| Summary: | 碩士 === 國立中興大學 === 土木工程學系所 === 96 === The objective of the study is to investigate the characteristic of bottom boundary layer flow induced by a solitary wave using high-speed particle image velocimetry (PIV). Image processing techniques and recursive supper reduction were utilized in the study to improve the time and space resolutions of PIV system. Particle trajectory photography was also used for flow visualization to observe flow fields near the bottom boundary.
The characteristics of bottom boundary layer flow induced by a solitary wave are discussed in the following items: (1)The pressure gradient decreases after the passing of solitary wave and also results in the reverse velocity of the boundary layer flow. The occurring time of reverse flow becomes earlier and the location of the separation point gets close to the center of the area with respect to the ratio of wave height to water depth. (2)The velocity distribution of boundary layer can be classified into two parts for similarity analysis. Before the passing of wave crest of the solitary wave, a regression curve which consists of two hyperbolic tangent functions is used to fit the velocity profiles. Corresponding characteristic values were calculated by this fitting curve. These characteristic values were used to conduct similarity analysis. Boundary layer, displacement layer, momentum layer and energy layer were tried to be various length scales. Finally a similarity curve could be obtained and it is also found that displacement layer thickness, momentum layer thickness and energy layer thickness are the multiple of boundary layer thickness. A regression curve combining both hyperbolic tangent function and cosine function was used to fit the velocity profiles after the passing of wave crest of the solitary wave. The height of corresponding of minimum velocity was tried to be used as a length scale. The result is not good. Besides, the thickness of maximum reverse flow could also be used as the length scale for each velocity profiles. This study tried to selecting the subtraction of the potential velocity and the minimum velocity as velocity scale, and to use the maximum reverse flow layer thickness and half-velocity-defectas different length scales, using the maximum reverse flow layer thickness of physical significance as length scale, a similarity profile can be obtained. (3)Based on the similarity profile, the viscous bottom shear stress can be calculated. It is found that the bottom shear stress increases with the ratio of wave height to water depth. The maximum viscous bottom shear stress occurs for T = - 2 ~ 0.
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