Characteristics of Free Overfall-Impacted Supercritical Flows and Its Windows-Based Application

Abstract

水利工程設施常設置跌流工等水工結構物，以達消減水流能量之目的。然而跌流時水流沖擊力與剩餘能量之作用下，易導致跌流工損毀，失去原先較佳消能效果，故跌流處之沖擊力與能量損失間相互關係甚是重要。據臺灣965座防砂壩資料統計，其上游因淤砂導致渠床坡度 小於6 %者占全部防砂壩之八成二。當水流於上游渠道非水平時跌流，流況常呈超臨界流，其沖擊水力之機制為何，實應進一步探究之。 本研究推導沖擊力與能量損失之理論公式，並採定量清水流進行渠槽試驗以驗證之。試驗係改變不同渠床坡度 (=0 %、2 %、4 %及6 %)與堰壩高度 (=0.15 m、0.20 m、0.25 m及0.30 m)，獲致一系列超臨界自由跌流之單寬沖擊力 與能量損失 數據，藉由迴歸分析得其經驗式，以期提供未來水工結構物設計強度時作參考之用。 又因科技日新月異，電腦之強大功能可有效輔助工程師設計水工結構物。本研究將試驗迴歸之經驗式，並結合前人研究結果，運用Visual Basic程式語言撰寫一視窗化軟體。經由本研發之視窗化模式演算後，得自由跌水流作用下之上游流況、水流特性及力與能量等數據，且呈列於圖表中，可使工程師於水工結構物之設計上更為便捷。

For energy dissipation, the cross-river structures have been widely used in both natural and artificial channels. But the structures could be damaged by the impact forces of the free overfall flows, causing the reduction of their energy dissipation efficiencies. For this reason, investigation of the relationship between the impact force and energy dissipation is very important. According to the statistics of 965 check dams in Taiwan, 82 percent of the check dams has average slope less than 6 percent due to the sediment deposition. It is worth to study the characteristics of free overfall-impacted flows and their relationship with the upstream channel bed slopes. This study is proposing to investigate the flows passing through steep sloping beds with various upstream bed slope S (0, 2, 4, and 6 %) and drops height H (0.15, 0.20, 0.25, and 0.30 m), and to identify the theory of the unit width impact force Fd and the energy dissipation ΔE. A series of experiments were carried out under the clear water, steady flow conditions to clarify the effect of water impact, and to calculate the unit width impact force Fd and the energy dissipation ΔE. A linear regressive equation between upstream bed slopes S, unit width impact force Fd and the energy dissipation ΔE was derived, which can be used to design the strengths of the hydraulic structures. Due to the rapid progress of computer technology, engineer now is able to use computers to design the hydraulic structures with remarkable computing power. This study combines the data collected in this study and the previous research results to develop a windows-based design model. It could show the upstream flow conditions, hydraulic characteristics, impact force and energy dissipation on a chart based on the calculation of the model. Therefore, designing hydraulic structures could be much faster than in the past.