Turbulent Structure and Bursting Phenomena over Roughners Discontinuity

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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.