Study on velocity characteristics of undular bore propagating over horizontal bottom

• Main Authors: Yi-Po Shao, 邵亦柏
• Other Authors: Chang Lin
• Format: Others
• Language: zh-TW
• Published: 2018
• Online Access: http://ndltd.ncl.edu.tw/handle/7gjke3

碩士 === 國立中興大學 === 土木工程學系所 === 106 === The aim of this study is to investigate the flow characteristics of undular bore propagating over horizontal bottom experimentally. Special attention is paid to the evolution of the bottom boundary layer flow as the observation results are still lacking up to date. Preliminary tests are conducted to validate and ensure the exactness of the generated undular bore in this study. According to Shimmozono (2011), a weak bore is defined when h1/h0 ＜ 1.75. Therefore, two undular bores with the water-depth ratios of h1/h0 = 1.70 and 1.64 are tested in this study. Two wave gauges are deployed at different positions in the flume to measure the spatial variation of wave profiles simultaneously. A flow visualization technique (FVT) using particle trajectory photography method and high-speed particle image velocimetry (HSPIV) system for velocity measurement are conducted, together with the employment of high-speed digital camera and laser light sheet. The velocity fields are obtained by ensemble averaging the repeated measurements. To understand the characteristic of the free surface variation of the undular bore superposed by several leading waves, the celerity and wave period of each leading wave are calculated from the time series profiles of free surface elevation. The variation of the wave period shows that the leading waves are dipersive. In addition, a very close-up field of view (FOV) for HSPIV measurement is used to measure the velocity field of the boundary layer flow. Accelerations and pressure gradient are calculated by using Navier-Stokes equation. The velocity shifting in the internal flow which caused by the effect of the leading waves do not influence the growth of the boundary layer thickness. Further, flow reversal in the boundary layer occurs when the magnitude of adverse pressure gradient reaches the maximum value. Finally, several similarity profiles are proposed to describe the velocity distribution of the boundary layer flow.