Three-Dimensional Organized Vortices above Flexible Water Plants

Abstract

Open channel flow over flexible bottom vegetation is studied experimentally. Two-component laser-Doppler velocimetry is used to measure the longitudinal and vertical velocity components of the flow field. It is observed that the temporally averaged velocity profile has an inflection below the top of the vegetation layer. The turbulence intensity and the Reynolds stress are largest near the top of the layer. An advanced technique using particle image velocimetry is developed to measure the instantaneous two-dimensional and three-dimensional flow field. The measurements reveal that organized vortices are generated intermittently above the vegetation. A side view of the vortex shows an elliptical shape, and the part of the three-dimensional structure above the vegetation layer consists of a pair of vortices inclined downward toward the front. The period of the vortex generation is explained in terms of linear stability analysis of flows with an inflectional velocity profile. The wavy motion of flexible vegetation is induced by the movement of such organized vortices.