Turbulent Structure and Bursting Phenomena over Roughners Discontinuity

试验研究明渠水流中流速分布、床面切应力和紊流特性对床面粗糙度突变的响应，并利用精细紊流模型预测这种响应函数，是水利工程和流体力学中最重要的课题之一。近年来，利用热膜风速仪和激光多普勒风速仪（LDA）研究了明渠水流中粗糙度间断处的湍流结构。例如，根津（1991）通过使用LDA系统，对从光滑到粗糙床和从粗糙到光滑床的砂粗糙度不连续性进行了精确的速度测量。他们研究了平均流速和河床剪切应力的流向变化。特别重要的是明渠水流的第一个观察，即与平滑到粗糙的河床变化相关的过冲特性，以及与粗糙到平滑的河床变化相关的下冲特性，发生在粗糙度不连续的下游。本文还用一个双通道湍流测量系统对明渠流中粗糙度不连续性进行了精确的湍流测量，为了揭示这种显著的床面切应力和湍流特性的超调特性，本文采用了一种新的LDA系统。粗糙度条件的变化范围比根津（1991）的更广。然后利用k-ε模型进行了数值计算，预测了速度和湍流廓线的变化特性。

It is one of the most important topics in hydraulic engineering as well as in fluid mechanics to investigate experimentally the response of velocity distributions, bed shear stress and turbulence characteristics to abrupt changes of bed roughness in open-channel flows and also to predict such response functions by the use of a refined turbulence modeling.In contrast with boundary layers, researches on turbulent structures over roughness discontinuity in open-channel flows have recently been investigated by making use of hot-film anemometers and laser Doppler anemometer(LDA). For example, Nezu(1991) have conducted accurate velocity measurements over sand roughness discontinuity from smooth to rough beds and conversely rough to smooth beds, by the use of an LDA system. They examined the streamwise variations of mean velocity and bed shear stress. Of particular importance is the first observation of open-channel flows that the overshooting property associated with the smooth to rough bed change, and the downshooting property associated with the rough to smooth bed change occur immediately downstream of the roughness discontinuity.In the present study, accurate turbulence measurements over roughness discontinuity in open-channel flows have been furthermore carried out with a two-component LDA system in order to reveal such a notable overshooting property of bed shear stress and turbulence characteristics. The roughness conditions were varied more widely than those of Nezu(1991). Some numerical calculations were made then to predict the variation properties of velocity and turbulence profiles by making use of k-epsilon model.