Experimental study on the flow structure of side by side longitudinal vorticies and their stability

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Flow structure of longitudinal vortices is one of the most fundamental research objects on large scall organized vortices in turbulent open channel shear flow from the view point of mass and momentum transport. But, the earnest regular investigation on this problem may be still insufficient, though the prototype surface flow pattern of side by side longitudinal vortices was shown about 30 years ago by an aerial photo-graph of flood flow.In this study, therefore, the flow structure was investigated by time-mean velocity distribution measurements and instantaneous visualizations, in an experimental side by side longitudinal flow, which was stimulated by air bubble ejection from the bed.From the experiments, the stable condition was proved to exsist when the lateral interval of longitudinal voritices was the distance of twice the depth, as was citel before. As for instantaneous flow structure of stable side by side longidunal vortices, locations of each boil at water surface are lined at the interval of twice the depth longitudinally, and also laterally, and furthermore at the same time alternatively. Each boil has mild slope of upward flow, and develops intermittently. On the contary the slope of downward flow after the boil is steep, and the flow is rather steady. The most of the downward flow is returned to the original longitudinal vortex at the bottom, but party shifted to the neighboring vortex.Above mentioned flow structure of side by side longitudinal vortices was shown probably for the first time, and such flow characteristics of the vortices as stability, influence of hydraulic condition, for example, are investigated in this study.