本研究使用Flow-3D水理分析數值模式模擬水流流經複合寬頂堰所產生之流場。模擬所使用的複合寬頂堰包括矩形底矩形寬頂堰、三角形底矩形寬頂堰及矩形底梯形寬頂堰三種堰型，其尺寸大小和Jan et al.(2009)實驗研究所採用的尺寸相同。FLOW-3D模擬之相關邊界條件和Jan et al.(2009)實驗所採用的條件相同。模擬時上游採用固定水位條件。模擬結果顯示過堰流量與實驗流量非常相近，其誤差約在1%以內。水流經過堰會有水位跌降現象，模擬分析結果顯示跌降後水頭約為未跌降前水頭的40%。
模擬成果輸出流速、壓力場及流線之分布情形，顯示流速及壓力變化均受堰的大小及形狀影響。複合堰口形狀不同會影響斷面最高流速發生位置及影響水平流場及垂直流場的分布。本研究也分析複合堰之水舌射流特性及負壓面現象。

The characteristic of the water stream through a compound cross-sectional weir is studied in this paper by using FLOW-3D simulation software. The boundary conditions of the simulation are reproduced according to Jan et al. (2009), this included factors such as the size of the model and fixed upstream water level conditions. The type of compound cross-sectional weirs used in this study are compound rectangular-rectangular broad- crested weir, compound triangular-rectangular broad-crested weir, and compound rectangular-truncated broad-crested. The results obtained by the simulation are closed to the experimental value by Jan et al with an error under 1%. The analysis calculated a 40% differences before and after the water head drop.
The simulation indicated the flow velocity, distribution of pressure field and flow lines around the weir are highly dependent by the size and shape of the weir. The occurrence location of highest flowrate and the distribution in the horizontal and vertical flow field are affected by the shape of the weir. This study also analyzed the water tongue jets and suction surface phenomenon of the compound weir.
Key words: Broad-crested weir, compound cross-sectional weir flow, three-dimensional numerical simulation.

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