Spatial flow pattern, multi-dimensional vortices, and junction momentum exchange in a partially covered submerged canopy flume

Abstract

This study experimentally investigated the influence of a model-submerged canopy on three-dimensional hydrodynamic structures in a partially obstructed flume. Canopy density, water discharge, and water depth were varied to achieve generalized conclusions. With tracer experiments, multi-dimensional (vertical and horizontal) large-scale coherent vortices arising from both the top and lateral edges of the canopy were visualized. The results of the mean flow and turbulence field show that the hydrodynamic characteristics are highly three-dimensional, impacted by canopy density and submergence ratio. Typical shear layers are confirmed by scaled velocity distribution with local characteristic velocity and length scales in both vertical and transverse directions along the canopy edges except for the near-bed region where horizontal vortices are inhibited. Along the canopy edges, the average effect of the vertical coherent vortices overall outweighs that of the horizontal coherent vortices for dense canopies, the ratio of which can be regressed more precisely with the water-related Reynolds number. The vertical profiles of the longitudinal velocity in the near-junction region are characterized by near-bed velocity deflection, strengthened by the increase in canopy density. The negative velocity gradients are found to be more closely related to the drag length-related Reynolds number. An in-depth analysis of secondary flows and near-junction momentum budget has been conducted to show the mass and momentum exchange processes. The results show that multidimensional coherent vortices and turbulence-induced secondary flows play different roles in mass and momentum exchange, accounting for near-bed velocity deflections in the junction region.