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Laboratory Experiments for Stochastic Characteristics of Rain-impacted Sheet Flows over a Smooth Boundary
|引用:||1.胡春宏、惠遇甲 (1995)，“明渠挾沙水流運動的力學和統計規 律”，科學出版社。 2.張福顯 (1996)，“雨點作用下之明渠流紊動特性研究”，中興大學土木工程學系碩士論文。 3.馬明明 (1995)，“台灣中部與台北地區降雨特性及沖蝕性關係之研究”，中興大學土木工程學系碩士論文。 4.劉家盛 (1995)，“降雨對粗糙底床淺水渠流之試驗研究”，中興大學土木工程學系碩士論文。 5.賴奕誠 (1994)，“雨點打擊對光滑底床淺水明渠場之影響”，中興大學土木工程學系碩士論文。 6.Ghosh, S. N. and Roy, N. (1970), “Boundary shear distribution in open channel flow,” J. Hydr. Div., ASCE, 96(4): 967-994. 7.Grass, A. J. (1971), “Structural features of turbulant flow over smooth and rough boundaries,” J. Fluid Mech., 50: 233-255. 8.Gessner, F.B. and Emery, A. F. (1981), “The numerical prediction of developing turbulent flow in rectangular ducts,” J. Fluids Engrg., ASME, 103: 445-455. 9.Knight, D. W. and Macdonald, J. A. (1979), “Hydraulic resistance in Artificial strip roughness,” J. Hydr. Div., ASCE, 105(6): 675-690. 10.Knight, D. W. (1981), “Boundary shear in smooth and rough channels.” J. Hydr. Div., ASCE, 107(4): 839-851. 11.Knight, D. W., Demetriou, J. D. and Hamed, M. E. (1984), “Boundary shear in smooth rectangular channels,” J. Hydr. Engrg., ASCE, 110: 405-422. 12.Mutchler, C. K. (1967), “Parameters for describing raindrop splash,” J. Soil and Water Conservation. 13.Nezu, I. (1977), “Turbulent structure in open channel flows,” Ph. D. Thesis presented to Kyoto University, Kyoto, Japan. 14.Nezu, I. and Rodi, W. (1986), “Open-channel flow measurements with a laser Doppler anemometer,” J. Hydr. Engrg., ASCE, 112(5): 335-355. 15.Nezu, I. and Nakagawa, H. (1993), “Turbulence in open-channel flows,” A. A. Balkema Publishers. 16.Pao, H. F. (1967), “Fluid dynamics,” Charles E. Merrill Books, Inc. Columbus, Ohio. 17.Prandtl, L. (1925), “Uber die ausgebildete turbulenz,” ZAMM, 5:136. 18.Shen, H. W. and Li, R. M. (1973), “Rainfall Effect on Sheet Flow over Smooth Surface,” J. Hydr. Div., ASCE, 99: 771-792. 19.Tennekes, H. and Lumley, J. L. (1972), “A first course in turbulence,” The Massachusetts Institute of Technology. 20.Tominaga, A. and Nezu, I. (1991), “Turbulent structure in compound oper-channel flows,” J. Hydr. Engrg., ASCE, 117(1): 21-41. 21.Webel, G. and Schatzmann, M. (1984), “Transverse mixing in open channel flow,” J. Hydr. Engrg., ASCE, 110(4): 423-435. 22.Wenzel, H. G., Jr., Yoon, Y. N. and Wang, R. C. T. (1969), “The effect of rainfall on sheet flow,” presented at the 50th Annual Meeting of AGU, Washington. D. C. 23.Yoon, Y. N. (1970), “The effect of rainfall on the mechanics of steady spatially varied sheet flow on a hydraulically smooth boundary,” Ph. D. Thesis. University of Illinois, Urbana-Champaign, IL.|
|摘要:||本研究採用光纖杜普勒測速儀( Fiber-optic Laser Doppler Velocimetry，簡稱FLDV )，針對二維、完全發展流在光滑底床進行流場之量測。實驗之量測水深介於4 mm與10 mm之間，同時考慮降雨及無降雨兩種情況，並延續以往之試驗，將量測坡度擴大為0.5 %及1.0 %，福祿數介於0.62~1.84之間，包含亞臨界流與超臨界流之流場，雷諾數則約介於900與6000之間，流場多屬過渡區及紊流邊界層。
本研究根據量測資料分析處理後，主要分析項目包括平均速度剖面、二維紊流強度、壁定理與速度欠損律之比較，並探討雷諾應力分佈及Darcy-Weisbach摩擦係數之變化。關於可貫穿臨界水深及Darcy-Weisbach摩擦係數於過渡段之無因次模式，分析時並加入張(1996)之低坡度( 0.1 %、0.3 % )資料，使其適用範圍提高。此外，更分區推導出雷諾應力之偏態模式與峰度模式，及降雨與無降雨之Darcy-Weisbach摩擦係數模式，擬供紊動流場分析之參考。
在探討降雨影響下，水流全深是否能被雨滴貫穿時，發現可貫穿臨界水深並非定值，而係坡度、降雨強度、雨滴粒徑與雷諾數之函數。本實驗其他重要結果如下：(1) 相同坡度、水深條件下，降雨之黏滯剪應力小於無降雨。就低坡度而言(即S = 0.1 %、0.3 % )，最大雷諾應力與黏滯剪應力之比值，降雨時遠大於無降雨。(2) 隨著坡度增大，雷諾應力之偏態分佈愈為集中，且降雨強度與雨滴粒徑之影響減小，並逐漸趨近於無降雨之情況。(3) 隨著坡度增大、水深減小、降雨強度與雨滴粒徑增大，其雷諾應力之峰度值愈大，表其分佈愈形高狹，並愈偏離常態分佈。(4) Darcy-Weisbach摩擦係數f，因降雨作用而增大，且隨雷諾數增大有逐漸減小之趨勢。當雷諾數大於1000時，降雨之影響則不甚明顯。|
The Fiber-optic Laser Doppler Velocimentiy ( FLDV ) technique was used to measure the characteristics of two-dimentional, fully developed open channel flow over a smooth bed with and without raindrop impact. This study widened the flow conditions of the earlier experimental studies. The flow depth ranged from 4 mm to 10 mm, and the bed slope ranged from 0.5 % to 1.0 %. The Froude number varied from 0.62 to 1.84, including both subcritical and superiical flows, and the Reynolds number varied from 900 to 6000. In general, the flows belong to transition region and turbulent boundary layer. Based on the analysis of the measured data, the mean velocity profiles, and the two-dimensional turbulence intensity distributions were compared with the law of the wall and the velocity defect law. The variations of the Reynolds stess and Darcy-Weisbach friction factor were also discussed. To increase the applicability of the models, Chang's (1996) data with lower bed slopes ( 0.1 % and 0.3 % ) were included in the development of the dimensionless penetratable critical flow depth ( flow depth above which the raindrops cannot penetrate through ) and the Darcy-Weisbach friction factor in the transitional region.With regard to the Reynolds stress, models were developed to predict the vertical distributions of the skewness and kurtosis of Reynolds stress for the rain-impacted flows. In addition, a model was also developed to predict the Darcy-Weisbach friction factor. It was found that the critical flow depth was not a constant. Instead, it was a function of slope, rainfall intensity, raindrop size and Reynolds number. Other important findings of the experiments were: (1) With the same slope and flow depth, the viscous shear stress for the condition with rainfall is smaller than that without rainfall. Under the low slope conditions ( S = 0.1 % and 0.3 % ), the ratios of the maximum Reynolds stress to the viscous shear stress for the condition with rainfall is larger than that without rainfall. (2) As the bed slope increases, the distribution of the skewness of the Reynolds stress tends to vary within a small range. The effects of the rainfall intensity and raindrop diameter also decreases, and gradually approaches the condition without rainfall. (3) With an increase of the bed slope ( decrease of flow depth ) and the increases of rainfall intensity and raindrop diameter, the kurtosis of the Reynolds stress increases and the distribution of the Reynolds stress deviates further away from the normal distribution. (4) The Darcy-Weisbach friction factor f increased with the raindrop impact, and decreased with an increase of the Reynolds number. The effect of rainfall was insignificant when the Reynolds number exceeds about 1000.
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