Study on Velocity Fields of Bottom Boundary Layer and Vortex Structures in the Run-down Flow of Shoaling Solitary Wave

• Main Authors: Sung-Chen Chang, 張松宸
• Other Authors: Chang Lin
• Format: Others
• Language: zh-TW
• Published: 2014
• Online Access: http://ndltd.ncl.edu.tw/handle/45698183184722757967

碩士 === 國立中興大學 === 土木工程學系所 === 102 === Laboratory experiments were conducted to study the bottom boundary layer flow of a solitary wave propagating over a 1:3 slope. High time-resolved PIV and flow visualization techniques were applied to investigate both the spatial and temporal variations of velocity filed. The results show that the profile of particle streamwise velocity can be divided into different type of stages during the whole process. Before the passage of wave crest, the particle velocity undergoes a positive acceleration, and the velocity profile is similar to the boundary layer flow over a flat plate with zero pressure gradient. The velocity starts to decelerate after the crest passing, and the velocity profile shows inflection point as the velocity decreases. An overshoot exhibited on the profile indicates the startup of flow rundown, and continues to decrease until it reaches a minimum magnitude. The overshoot gradually diffuses toward the outside of the boundary layer as the velocity profile transforms to a negative profile similar to the boundary layer flow over a flat plate. The velocity accelerates again in the bottom as the second run-up begins, with the scale smaller than the first run-up stage. The maximum velocity profile in the wave run-up stage is found to be affected by the solitary wave passing; on the other hand, the minimum (negative maximum) velocity profile in the flow rundown stage is affected by the gravity. This study use these two free stream velocities as the velocity scales, and the corresponding boundary layer thicknesses as the length scales to perform similarity analysis. The results show that even though the flow fields are driven by different force mechanism in the two stages, the velocity profiles exhibit great analogy and symmetry. During flow rundown, the flow field transforms from subcritical to supercritical flow condition; a hydraulic jump take place at the end of the process as the retreated down-wash meets the backside deeper water. Vorticial motion occurs within the separated shear layer close to the bottom boundary. Two large vortices with opposite direction are thus generated during the process of flow rundown. This study selects the maximum horizontal velocity as a velocity scale, the water depth as a length scale. Finally, three similarity curves could be obtained and all of the correlation coefficients are at least 0.995.