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Flow Characteristics for Rapid Channel Flow with Rough Bed
本實驗係採用光纖雷射杜卜勒測速儀（Fiber-optic Laser Doppler Velocimetry，簡稱FLDV）量測底床鋪設8mm玻璃顆粒之陡坡粗糙底床明渠流場。實驗寬深比介於4.63與11.36之間；坡度變化分別為0.3%、1%、2%與3 %；福祿數則介於0.632與1.99；雷諾數涵蓋6000與41000之範圍。實驗之目的主要在探討寬、窄流場於陡坡粗糙底床之水力特性與光滑渠流之異同，分為二維與三維討論之，大部份實驗屬於超臨界流況。
本文主要分析項目包括：平均速度剖面、紊流強度及雷諾應力分佈等水力特性，並分別與Nezu & Rodi（1986）、Tominaga & Nezu（1992）、楊(1998)及劉（1995）等之結果比較。在平均速度分佈方面，乃與壁定理、冪次定理等流速分佈定理比較。紊流強度分佈及雷諾應力分析，則針對垂線最大流速發生於液面上與液面下之流速剖面進行探討，以研析流場之水力特性與機制。
實驗結果顯示，本研究試驗範圍之平均速度剖面與冪次定理符合良好，而壁定理則因粗糙底床影響，於底床產生一均勻流速分佈區，不似光滑流場呈線性變化。另外Nezu（1986）之光滑底床主流向紊流強度模式曲線，已不適用於本流場，而其垂直向紊流強度與楊（1998）之模式和本研究之實測值大致上符合良好。在雷諾應力方面，最大流速發生於液面上（α≧1 ）之結果與楊（1998）雷同；最大流速發生於液面下（α<1 ）時，因受二次流影響，雷諾應力之峰值則有回升之現象。|
ABSTRACT The Fiber-optic Laser Doppler Velocimetry (FLDV) was adopted in this experimental study to investigate the characteristics of the steep open channel flow over a rough boundary, which were formed by gluing a layer of uniform 8 mm glass particles on the channel bottom. In this study, the aspect ratio varied from 4.63 to 11.36, the Reynolds number varied from 6000 to 41000, and the channel bed slope of 0.3%, 1%, 2% and 3% were selected. The experiments aimed at searching into the dissimilarities of the characteristics of the steep open channel flows over rough and smooth boundaries wide different channel widths. The Froude number ranged from 0.632 to 1.99 and most of the experiments belonged to supercritical flow. The major items of this research were the mean velocity profiles, the distributions of the distributions of the turbulence intensities and the Reynolds stress for the steep open channel flow. In addition, the experimental data were compared with the results of Nezu and Rodi (1986), Tominaga and Nezu (1992),Yang(1998), and Liu(1995). The measured velocity profiles were compared with the law of the wall, and the power law. The measured distributions of the turbulence intensities and the Reynolds stress were analyzed for both the conditions with the maximum point velocity occurred above and below the water surface. The measured mean velocity profiles compared well with the power law. For the law of the wall, the velocity distributions were fairly uniform near the rough bed, which was different from the linear variation for the smooth bed. Besides, Nezu''s(1986) longitudinal turbulence intensity model, which was developed for the open channel flow with smooth bed, was not suitable for the current study with rough bed. Both Nezu''s(1986) and Yang''s(1988) vertical turbulence intensity model were consisted with the data measured in this study. For the flow with maximum point velocity located above the water surface (α≧1 ), the measured distributions of the Reynolds stress for the rough bed were similar to those measured by Yang(1998) for the smooth bed. For the flow with maximum point velocity located the water surface (α<1 ),the measured Reynolds stress values tended to increase with the increase of the secondary flow effect.
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