Please use this identifier to cite or link to this item: http://hdl.handle.net/11455/16117
標題: 跌水消能池之滑流流場特性探討
Flow Characteristics in Skimming Flow over a Vertical Drop Pool
作者: 林蔚榮
Lin, Wei-Jung
關鍵字: Vertical drop pool
跌水消能池
Skimming flow
PIV
Turbulence
Turbulent energy budget
滑流
紊流特性
能量收支
出版社: 土木工程學系所
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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。 為能充分瞭解此剪力層的特性,本研究乃針對沒有通氣情形(即滑動射流流與消能池迴流區間沒有夾氣袋之存在、亦無通氣管之連通大氣,以期能將剪力層之長度範圍由跌水處順水流方向往下游尾檻處延伸至最大情形)之滑流流場探討。對於跌水消能池之消能效益研究方面,發現在沒有通氣的滑流流場下,消能效益將大於前人研究中對於有通氣情形下之消能情形。此乃因迴流區的動量大多是靠滑動射流與迴流區間具高速度梯度之紊亂剪力層的動量交換而來。因此,剪力層往下游延伸的長度的增加,增進滑動射流與迴流區之摩擦損耗,進而增加消能效益。 於剪力層分析方面,發現此剪力層會隨著水流由跌水處往下游尾檻處逐漸增長。在選定適當的長度尺度與速度尺度,並對於剪力層之速度分布進行無因次化後可獲得一相似性剖面。文中亦針對特徵長度尺度、速度尺度與實驗參數的相關性進行探討。根據此剪力層區域中之雷諾數分析,可知在剪力層之速度剖面特性應不受雷諾數效應之影響。此外,本文探討在不同的剪力層斷面處之速度剖面、雷諾剪應力以及紊流能量收支平衡等分佈特性。 對於消能池底版附近區域分析方面,將位於底版不同水平位置斷面上之速度剖面進行分析。並由速度剖面中再選定適當的長度尺度與速度尺度,並進行無因次化後也可獲得一相似性剖面。此相似性剖面則顯現出類似壁射流之速度剖面特性。最後,本文亦對於在不同的底版位置處之雷諾剪應力、紊流特性以及紊流能量收支平衡等分佈特性進行探討。
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[103] to O[104], 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.
URI: http://hdl.handle.net/11455/16117
其他識別: U0005-2808200913000000
文章連結: http://www.airitilibrary.com/Publication/alDetailedMesh1?DocID=U0005-2808200913000000
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