| Summary: | 碩士 === 國立中興大學 === 土木工程學系 === 92 === Secondary currents not only transport heat, energy, mass, and pollution, but also cause the structures of downstream river by interaction with river bed. Sand stripe, which includes trough and ridge, is a 3-D complex structure that may exist in a river. A trough is formed by the downward flow of the secondary current, and the ridge is caused by the upward flow.
In this study, experimental investigations are performed in a straight open channel with rough bed (two rough stripes) and fully developed turbulent flow to increase our understanding of the flow structures. The channel width and slope are 25 cm and 0.1%, respectively, and the aspect ratios are 2 and 4. The results are compared with Hong’s (2003) smooth bed experiments.
In contrast to the smooth bed experiments, the major difference occurs around the ridge. The bulge structures have significant effect on the transverse flow. The relative roughness (roughness height over flow depth) influences the transverse distribution of the boundary shear stress, which can be observed by analyzing the plot of iso-Reynolds’ stress curves.
Secondary currents in a straight open channel are induced by the isotropic property of turbulent flow, and frequently occur near the corners or channel boundaries. Although the velocity of the secondary currents is usually very small (less than about 5%) as compared to the maximum velocity in the longitudinal direction, it has significant effect on the 3-D structures of the velocity, temperature and concentration of material in the flow.
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