Coherent Structure over Rough Bed and Movable Bed in Open Channel Flow.

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It is very important in llydraulic Engineering as well as in Fluid Mechanics to reveal the response of velocity profiles and bed shear stress to abrupt change of bed roughness in open channel flows. In the present study, accurate velocity measurements over abruptly changed k-type roughness elements were conducted in open channel flows by making use of two-component LDA. An inner boundary layer develops from the boundary point (x=O) between smooth and rough beds. The velocity and turbulence distributions in the inner boundary layer rapidly respond to abruptly changed roughness. Of particular significance is that the bed shear stress overshoots immediately downstream of the abrupt roughness change. This finding is very important to construct bed protection works in rivers. It was also strongly suggested that the burst period was governed by outer variables rather than by inner variables.