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Flow Characteristics in Skimming Flow over a Vertical Drop Pool
|關鍵字:||Vertical drop pool|
Turbulent energy budget
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|摘要:||本文針對垂直式跌水消能池之滑流流場特性進行實驗研究與探討。文中應用質點軌跡可視化法與質點影像測速儀(簡稱PIV)量測系統，對於滑流流場現象進行定性觀察、以及速度場與紊流特性進行定量量測。本文中探討在不同的來流流量下，在固定跌水消能池之長度(L)，改變尾檻高度(h)與跌水池高度(H)之高度比(h/H)分別為0、0.12、0.43、0.71 (dropping flow case)以及1.0 (cavity flow case)時之滑流流場特性。於流場可視化觀察中發現，當水流通過跌水消能池時，大部分的水流會形成表面滑動射流之滑流型態而順勢越過尾檻向下游流動，而部分水流會在撞擊尾檻前緣角隅後，在消能池中形成迴流區，產生一個具旋轉性的大尺度渦流。在滑流與此大尺度渦流間會形成具明顯速度梯度之剪力層。因此，本文中分別針對全域之整體性流場(full-field point of view)特性，以及對此剪力層與消能池底版附近局部區域(local point of view)之速度場與紊流特性進行分析與探討。
The characteristics of non-ventilation skimming flow over a vertical drop pool were investigated experimentally, using flow visualization technique and high speed particle image velocimetry (PIV). Five series of experiments having different end sill ratios [h/H = 0, 0.12, 0.43, 0.71 (referred as dropping flow case); and 1.0 (referred as cavity flow case), where h is the end sill height and H is the drop height] with various approaching flow discharges were performed to measure the detailed quantitative velocity fields. The mean velocity, vorticity, turbulence intensity, Reynolds shear stress and turbulent energy budget terms were obtained by ensemble averaging the repeated measurements. Flow characteristics were discussed from the full-field point of view, and from the local point of view which is focused in the shear layer region between sliding jet and pool as well as focused at the bottom boundary in pool region. In addition, energy loss was examined for non-ventilation skimming flow and compared with those for the ventilation dropping flow given by Rajaratnam and Chamani (1995). In the shear layer between sliding jet and pool, it is found that the growth of the shear layer in the downward direction as the jet slides down the pool represents the momentum exchange. Analyzing the distribution of measured velocity, the similarity profile of the mean velocity at different cross-sections along the shear layer was obtained. The proposed characteristic scales provided unique similarity profiles having promising regression coefficient. The selection of the characteristic scales is also discussed. In the magnitude of relative Reynolds number ranging from O to O, the flow characteristics of mean velocity field should be Reynolds number independent in the shear layer region. Further, the spatial variations of mean velocity profiles, turbulence intensities, in-plane turbulent kinetic energy and Reynolds shear stress were also elucidated in detail. The imperative observation is that the Reynolds shear stress dominates the major part along the shear layer as compared to the viscous shear stress. The study also provides an insight into the energy budget balances at different cross-sections in the shear layer. Near the bottom boundary in pool region, similarity profile for the mean horizontal velocity was obtained, which showed the similar trend of velocity distribution of the plane turbulent wall jet. The distributions of turbulence intensity, Reynolds shear stress, and turbulent energy budget terms were also discussed for different X-sections on the bottom boundary in the pool. Furthermore, the study also discusses the energy budget balances at different X-sections on the bottom boundary in the pool.
|Appears in Collections:||土木工程學系所|
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