Conditional statistics of Reynolds stress in open channel flows with modeled canopies of homogeneous and heterogeneous density

Abstract

The flow structures under the effects of heterogeneous canopies have been shown to be significantly different from those under the effects of homogeneous canopies. The purpose of this study is to investigate how the changes in density and density uniformity of the canopy affect the turbulent characteristics of the flow in a partially vegetated channel. A comparative experiment is conducted, including two cases of homogeneous canopy with different densities and one case of heterogeneous canopy consisting of alternating sparse and dense vegetation patches. While the lateral profiles of Reynolds stress, magnitudes of quadrant motions, and high-order moments of velocity fluctuations present a high similarity within the shear layer, variations in both the density and density uniformity of the canopy markedly affect the turbulence at the interface between the canopy and the main channel. The results show that canopy density heterogeneity enhances the momentum exchange at the interface and promotes the penetration of stress-driven flow into the sparse vegetation patch while inhibiting its penetration into the dense vegetation patch. An analogy can be drawn between the canopy flow with sufficient density and the turbulent rough-wall boundary layers based on the turbulent statistics within the shear layer. Furthermore, the effect of increased canopy density on the flow corresponds well to the effect of decreased wall roughness. By using the cumulant expansion method, the assumption of structural similarity present in wall-bounded flows is found to be applicable to the canopy flows considered in this study.