Translated Titles

PIV Measurements of the Flow field Downstream of a Grade Control Structure – Preliminary Investigation of Bursting Phenomenon



Key Words

猝發現象 ; 固床工 ; 沖刷坑 ; 紊流特性 ; PIV ; 沖刷機率 ; bursting ; groundsill ; scour ; PIV ; exceeding probability ; turbulent characteristics



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Chinese Abstract

台灣地理環境因位於板塊交界處,地形陡峭且地質脆弱,河川短且水流湍急,使河道泥砂沖刷甚鉅,常危急固床工結構物之穩定。固床工下游經常產生局部沖刷形成沖刷坑,且常伴隨著猝發現象的發生,使底床泥砂擾動加劇,甚至造成固床工基礎破壞。因此,本研究針對固床工下游側沖刷坑之流場與猝發現象進行初步探討。 本研究使用質點影像測速儀( PIV )於動床渠道試驗進行沖刷坑水理特性量測試驗。主要係分析發生猝發現象時,於不同時程( T= 15 mins 、1 hr、5 hrs)、單寬流量( q= 0.0167、0.0283、0.0475 m2/s )及坡度( S= 1 %、1.5 % )沖刷坑內之紊流特性參數之變化,並計算沖刷坑內全剖面之沖刷機率。此外,並配合網路攝影機( CCD )觀測猝發現象發生頻率對沖刷深度之影響。 試驗結果顯示,水流以潛沒式射流( submergrd jet )進入沖刷坑,於沖刷坑內發生猝發現象。不同流量及坡度下,固床工下游側沖刷,會隨時間漸趨穩定,且在 1 hr後猝發發生頻率明顯降低。猝發現象影響高速水舌區及水躍區甚大。水躍區短暫位移較上游處,使高速水舌流速變緩。此外,受水舌擺盪發生猝發現象,碰觸底床泥砂,加速水流縱向流速,使沖刷坑內無法觀測到迴流區之存在。全剖面底床超越機率分析顯示,當發生猝發現象時,於沖刷坑最低點之沖刷機率因受水舌擺盪影響,隨時間增加而降低,而隨流量及坡度之改變則較無明顯之趨勢。就沖刷機率而言,發生猝發現象皆較未發生猝發現象之沖刷機率大。

English Abstract

Taiwan's geographical environment is located at the junction of the plates, with steep terrain and fragile geology. The rivers are usually short with rapid flows and heavy sediment load, wich often endangers the grade-control structure stability. The local scour and bursting phenomenon often occur in the scouring hole downstreamoften of a grade control structure. The local sour hole downstream of the downstream of the groundsill may grow with time, even causing the collapse of the structure. In order to increase our understanding of the temporal and spatial variations of the scour hole downstream of a grade-control structure, this study uses a PIV system to measure the turbulence characteristic parameters and to calculate the exceeding probabilities for the movable bed tests in a re-circulating flume. The independent variables include the experimental duration( T= 15 mins, 1 hr, 5 hrs ), unit discharge( q= 0.0167, 0.0283, 0.0475 m2/s ), and channel bed slope( 1 % and 1.5 % ). In addition, this study uses CCD camera to observe the effect of bursting fregueney on the scour depth. According to the experimental results, the flow enters the scour hole with a sudmerged jet, cauing the bursting phenomenon. Under different flow and slope cinditions. The scouring downstream of the groundsills is usually stabilized gradually over time, and the bursting frequency reduced dramatically after 1 hr. The bursting has huge influence in the areas of the high-speed flow nappe and the hydraulic jump. The hydraulic jump area may momently move upstream, which slows down the velocity in the high-speed nappe. In addition, due to the swinging of the high-speed flow nappe, the nappe sometimes touches the bed, accelerating the near bed flow velocity, and the re-circulation zone can no longer be observed in the scour hole. According to the experimental results, the exceeding probability for scouring at the lowers point of the scour hole decreases with time. However, no obvious trend was observed for the unit discharge and slope variations. As for the exceeding probability, it is much higher for the case with bursting than that without bursting phenomenon.

Topic Category 工學院 > 土木工程學系所
工程學 > 土木與建築工程
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