报 告 人：林呈 教授 台湾中兴大学

时    间：2014年11月13日（周四）上午9:30

地    点：严恺馆502

主办单位：港口海岸与近海工程学院

报告简介：

    As pointed out by Russell (1838), solitary wave propagates with very stable motion over a considerable distance along a constant water-depth channel. This wave travels steadily neither steepening its wave height nor widening its effective wave length. Solitary waves are often investigated due to their simple and permanent wave form. In addition, the investigation of shoaling solitary waves can help in simulating the run-up and run-down motions as well as the shoreward inundation of a tsunami. However, the characteristics of unsteady hydraulic jump (Sumer et al., 2011) during the run-down motion of a shoaling solitary wave is still very rudimentary; and the systematical measurements has not yet been made up to now. Herein, the vortical structures and velocity fields in the unsteady hydraulic jumps during the run-down motion of shoaling solitary waves propagating over three different slopes are investigated experimentally.

    The experiments were conducted in a glass-walled and glass-bottomed wave flume with dimensions of 14.0 m long, 0.25 m wide and 0.5 m deep. The wave flume was equipped with a piston-type wave maker at one of its end, which was triggered by a servo motor. The wave maker can generate ideal solitary wave forms. Two capacitance type wave gauges were employed to measure the water surface elevation. Three sloping bottoms all made of acrylic and having slopes of 1/10, 1/5 and 1/3, were used for the experiments. Two flow visualization techniques, including thin-layered fluorescent dye and neutrally suspending Titanium Dioxide particles, were both used to examine: the occurrence of unsteady hydraulic jump. A laser light sheet was used to illuminate the 2-D motion of the tracers used on a vertical plane along the longitudinal direction of the flume. A high-speed camera was used to capture images with maximum framing rate of 1200 Hz and maximum image resolution of 1,024 × 1,024 pixel. A HSPIV system was also used to measure the two-dimensional velocity fields of hydraulic jumps and the underlying vortex structures when flows retreated from different slopes.

    In the present study, the temporal variations of visualized serial images and (phase-averaged) velocity fields of the hydraulic jumps are illustrated, respectively. Main targets are focused on the effect of the slope on formation of the separated shear layers from the sloping bottoms and on the temporal and spatial variations of velocity fields in the vortical structures underlying the unsteady hydraulic jumps. The occurrence condition for the unsteady hydraulic jumps is elucidated by examining the variation of time-dependent Froude numbers in the vicinity of the flow separation points for different slopes.