| Summary: | 碩士 === 國立成功大學 === 水利及海洋工程學系碩博士班 === 101 === Over the past years, researchers have done lots of research on sand drift, sand erosion and overtopping in different kinds of sand dunes. They submitted erosion mechanisms, sand drift equations and the results of simulation, and finally conclude that the dune with berm has the best protection against wave erosion. But they didn’t conduct any investigation about the internal flow field and vortex of this kind of sand dune. So this paper is to present the characteristic of flow field about the forward and backward of this type of sand dune, and show the temporal and spatial distribution of flow filed and vortices.
The experiment is conducted in a medium-sized glass tank at Tainan Hydraulics Laboratory. The sand dune model is made of acrylic material to understand the change of hydrodynamic mechanism when the water wave overtops the sand dune. Because the top layer the sand dune is too thin, it can only take pictures from the forward and backward of the sand dune. Yet, this experiment tries to get the clearest particles within the maximum range of our target. And this experiment choses a mature wave with a complete wave period to get substantial analysis. This experiment uses an optical gate to control the camera and wave gauges synchronously to get the image data. After getting the image data, we analyze the data by using the canny method to detect water surface line to get the internal flow field we are interested in. Besides, this experiment repeated 20 tests in every wave period test and shooting area to get average velocity with ensemble average method. The variation of surface elevation is less than 0.3% in every test.
This research shows the temporal and spatial distribution of flow filed and vortices in front slope and back slope of BD model in five kinds of period with 5 cm deep sea wave high. All kinds of waves of front slope of the BD model runs up with lots of bubbles, and the flow field is chaotic so as to set apart while moving, which results in stagnation in the model bed. The short wave of the rear part of the BD model produces violent current and vortex in the surface of the liquid. As to long wave, besides the dramatic change in the speed and vortex of the surface liquid, there is obvious and deeper overwash in the bottom part, which leads to diverse direction of vortex. To sum up, this report presents the changes and calculation of flow field and vortex of wave in various periods.
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