Investigation of the flow characteristics with one-line emergent canopy patches in open channel

Abstract

Experiments are performed on channels with emergent one-line porous canopy patches distributed along the channel sidewalls in which flow dynamics and turbulent structures are analyzed. The evolution of the transverse distribution of the mean streamwise velocity and Reynolds stress profiles in specific transverse sections with downstream distance suggests that a fully equilibrated zone is reached beyond an adjustment distance, and the adjustment distance needed for vegetation with high solid volume fractions is short. The flow field can be divided into three main different regions based on the transverse distribution of the mean streamwise velocity and Reynolds stress in the equilibrium zone: a boundary-affected region, an analog mixing layer, and a uniform region in a non-vegetated area. The analog mixing layer exhibits a two-layer structure governed by local coherent vortices. The dominant frequencies of the vortices at the interface are determined through spectral analysis and proven to correspond to a patch scale. The development of the shear layer, the analysis of the frequencies and scales of the vortices, and the experiment on flow visualization indicate that von-Karman vortices continue to form downstream despite the interference of the sidewall and the special one-line distribution pattern of patches. Quadrant analysis is adopted to illustrate the lateral momentum exchange caused by different motions. Results show that ejections and sweeps are the main contributions to Reynolds stress in the analog mixing layer. Different signs of Reynolds stress in the boundary-affected region and the analog mixing layer can also be explained through quadrant analysis.