Please use this identifier to cite or link to this item: http://hdl.handle.net/11455/90895
標題: 明渠流通過固床工下游粗糙底床沖刷坑之紊流流場分析
Investigation of turbulent channel flow over a rough scour hole downstream of a groundsill
作者: 王忠翔
Zhong-Xiang Wang
關鍵字: Particle image velocimetry
rough bed
scour hole
Turbulence intensity
Reynold stress
質點影像測速儀
粗糙底床
沖刷坑
紊流強度
雷諾應力
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摘要: 臺灣因地形陡峭且河短水急,再加上地質鬆軟,使河道沖刷劇烈,危及結構物之安全。固床工有助於護河床面泥沙之穩定,惟其亦伴隨著下游之局部沖刷,造成結構物破壞。近年來有關沖刷坑之研究多著重於平衡情況下最大沖刷深度之推估與流場探討,較缺乏未平衡沖刷坑流場之研究。然結構物破壞時未必發生於平衡沖刷,本研究主要目的,為深入瞭解平衡前後沖刷坑流場特性之異同、以供未來數值模擬與實務應用之參考。 本研究進行重複性動床剖面試驗,經無因次化後比對文獻中之沖刷坑剖面,選定未平衡與平衡代表性沖刷坑各一,製作定床沖刷坑模型。藉由PIV進行明渠流通過固床工下游粗糙底床沖刷坑之流場試驗,並以同坡度(S = 1%)不同流量(Q=0.003 ~ 0.0085 cms)條件進行流場特性與物理機制之探討。 實驗結果顯示,平衡沖刷坑來流流量與水躍大小正相關性,且其發生位置隨流量之增加而向下游移動;然逆流速度大小與範圍(重匯點上游)則隨來流流量之增加而減小。反之,未平衡沖刷坑逆流區大小與範圍則未明顯地受來流流量增減之影響。此外,平衡沖刷坑流場之無因次紊流強度、無因次雷諾應力皆大於未平衡沖刷坑。然而,平衡沖刷坑之無因次紊流強度、無因次雷諾應力其各垂線之(絕對值)峰值均發生於近液面處,對底床沖刷較無影響;反之,未平衡沖刷坑流場各垂線之峰值較接近底床,其對底床沖刷將有明顯之影響。
Due to the precipitous topography, rapid flow and fractal geology, severe scouring of the riverbed which may damage the hydraulic structures, frequently occurs in Taiwan. Although groundsill helps to stabilize the riverbed, it usually causes local scour downstream. In recent year, most researchers emphasize the estimation of the maximum scour depth and the characteristics of the flow field during the equilibrium condition. The main objective of this study is to clarify the similarities and differences of the flow characteristics between the equilibrium and non-equilibrium scour holes so as to provide a reference for numerical simulation and practical application in the future. In this study, typical equilibrium and non-equilibrium scour holes were selected based on the laboratory movable bed tests and the results in the literature. Two acrylic models of the scour holes were then placed into a 12 m long re-circulating flume for the fixed bed tests. The flow characteristics and physical mechanism were analysed for conditions with a fixed slope (1%) and various flow discharges(0.003~0.0085 cms). According to the experiment with the equilibrium scour hole, the size of the hydraulic jump has positive correlation with the flow discharge, and the location of the jump moves downstream with an increase of the flow discharge. However, the re-attachment point (size of reverse flow zone) decreases with an increase of the flow discharge For the non-equilibrium scour hole tests, howerer, the location of the re-attachment point are not significantly affected by the flow discharge. In addition, the dimensionless turbulence intensities and Reynolds stress for the equilibrium scour hole tests are higher than those for the non-equilibrium scour hole tests. the peak value (absolute) of the dimensionless Reynolds stress in a vertical usually occurs closer to the water surface for the equilibrium scour hole test as compared with that for the non-equilibrium case, indicating move severe bed scour may occur at the early stage of the scouring process (non-equilibrium) in a movable bed condition.
URI: http://hdl.handle.net/11455/90895
其他識別: U0005-0608201523462400
文章公開時間: 2017-08-25
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