局部沖刷對沖積河川影響之研究

Abstract

本論文主要以數值模擬來探討局部沖刷於橋墩附近的流況變化及其物理機制，使用了兩個不同的現場案例，分別為后豐大橋的橋樑沖刷以及大甲溪實施固床工保護的間隙沖刷。數值模擬應用於此問題一直以來都是相當困難的課題，因此，於本論文中藉由商業軟體Flow-3D做分析及討論，其分析內容同時包含了定床跟動床兩種數值模型設置，並藉由實驗結果輔助之。於橋樑沖刷部分，定床的模擬中，分析第一根橋柱前的水深比較，其數值結果實驗結果相當吻合；於動床模擬中，在實驗設置裡加入了超音波儀器，從模擬結果可以清楚發現其存在位置會影響到底床的沖刷深度以及整個流場的變化，因此移除了該裝置之後，試著去探討在橋墩前所產生的跌水現象所引起的沖刷所帶來的影響。由模擬結果發現，有跌水沖刷的部分，主要沖刷位置會集中在橋柱前方形成一個沖刷坑，同時底床被掏刷過程中，橋柱迎水面會受到強勁的水流的流速衝擊，橋柱靠近底床部分最大，往上遞減。然而沒有跌水沖刷的部分，主要沖刷位置落在第一根橋柱周圍，其因素歸因於向下射流以及馬蹄型渦流。再者，固床工的間隙沖刷模擬，當底床設置為固床時，可以發現不論在何種大小流量下都會在第三階部分產生一個明顯的Z方向流速，並於固床工編原產生明顯的渦流。由於此間隙沖刷案例的模擬，其物理問題設置屬不連續體排列，因此在數值模擬過程中，不易觀察其確切的物理現象，因此，此部分的數值模擬尚待開發。

In this thesis, the flow field and physical mechanism of local scour problem around the bridge piers is mainly used by numerical simulation. Two different field cases are studied: they are the bridge scour effects surrounding the Hou-Feng Bridge and the gap scour effects about the stepped concrete block grade control structure in Ta-Jia river, respectively. It is an extremely difficult problem by using the numerical programming to solve these problems. Hence we seek the help from the commercial software, Flow-3D, to analyze the problem. The problem setting includes the rigid-bed and mobile-bed simulation and the laboratory experiments are aided in the meantime. The numerical results of the rigid-bed simulation are compared with the laboratory results in terms of the fluid depth in front of the first pier. The results from numerical studies and laboratory experiments match very well. In the mobile-bed simulation, the ultrasound device is added in the experiment and the scour depth and the variation of the whole flow field are interfered with the sensor. Hence after the ultrasound device is removed, the influence of the overfall flow in the bridge scour simulation can be clearly classified. The mainly scour hole is concentrated in front of the first pier in the numerical simulation with the overfall flow. In the meantime, the upstream of the first pier is borne by the current flow and the maximum flow velocity is measured close the bottom of the first pier. If the overfall flow is not considered in the simulation, the mainly scour hole is concentrated around the first pier. The reasons are attributed to the down flow and the horseshoe vortex. As for the rigid-bed simulation of the gap scour, the third step of the stepped concrete block grade control structure always existed the obvious velocity on the Z-axis and the strong vortex near the edge of the stepped concrete block grade control structure in the numerical simulation no matter how strength of the flow. In the gap scour simulation, it is difficult to observe the accurate physical phenomenon during the numerical simulation in light of its physical problem setting belongs to the discontinuous arrangement. Hence the applied numerical simulation still requires further development.