| Summary: | 博士 === 國立成功大學 === 水利及海洋工程學系 === 103 === This dissertation presents an investigation on the dynamic response of density-stratified fluid within a trench under water waves. For each topics of interest, numerical simulations are supported by carefully conducted experiments. In laboratory experiments, the free surface wave and interfacial motion are captured using a CCD camera and image processing. The dense fluid transport and flow field are measured by laser-induced fluorescence (LIF) and particle image velocimetry (PIV) techniques, respectively. The three-dimensional numerical model Truchas is employed to trace the interfacial motion, dense fluid transport using the volume of fluid method. Comparisons between measurements and numerical results are performed for the free surface elevation, interfacial motion, the dense fluid transport and flow field.
Regular waves over density-stratified fluid in a submarine trench is first investigated. Both the numerical and experimental results show two types of interfacial motion, namely, partial standing wave patterns and travelling wave patterns. The numerical model is then employed to study the mechanisms of the different modes of interfacial motions (partial standing/traveling waves) and their corresponding amplification factors (external/internal modes). It is shown that the partial standing wave patterns are easily generated when the motion of the surface waves is 180 degrees out of phase at the two sides of the trench. However, the existence of partial standing wave patterns does not mean partial standing internal waves occur. The partial standing internal waves are triggered as internal wave wavelengths reach resonant condition. Furthermore, the ratio of the interface wave height to the surface wave height decreases with wave nonlinearity, suggesting that the nonlinear effect may significantly change the interfacial wave motion. It is found that the excited pairs of counter-rotating vortices around the interfacial wave can induce a large velocity in the lower layer for the internal mode, indicating that bottom erosion can be enhanced in this manner.
Next, solitary wave over density-stratified fluid in a submarine trench are investigated. A new experiment was conducted to measure the dense fluid transport and flow field. The dense fluid transport and flow field characteristics are discussed in the experiments. The height of the transported dense fluid over the left trench wall decreases as the dense fluid density increases, wave nonlinearity decreases, or trench width decreases. Furthermore, the dense fluid can be transported toward the upstream of the trench when a denser fluid or lower wave nonlinearity is considered. Some dense fluid can be trapped inside the trench due to the effect of lee side trench wall under the high wave nonlinearity and narrower trench width. Comparisons between measurements and numerical results are performed for the free surface elevation and the dense fluid transport and flow field. Good agreements are obtained. The calibrated model is then used to examine the transport processes of the various dense fluids and quantitative analyses also demonstrate that either density of dense fluid or wave nonlinearity significantly affects.
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