Modeling transverse momentum exchange in partially vegetated flow

Abstract

In this study, theoretical and experimental methods are applied to investigate streamwise velocity profiles and transverse momentum exchange in the quasi-equilibrium region in partially vegetated channels. A new eddy viscosity model is proposed to model the transverse momentum exchange in consideration of bottom turbulence, stem-scale turbulence, and turbulence induced by coherent structures. Results show that turbulence induced by coherent structures is the dominant contributor to the momentum exchange. The proposed eddy viscosity model can be well applied for different setups by varying only one proportionality constant (ξ), which is inversely proportional to the canopy drag (CDa). The lateral distribution of eddy viscosity is also well predicted and shows a peak just outside the canopy. By assuming the canopy as a rough wall and setting the zero-plane displacement within the canopy to the distance of penetration width, we find that the lateral profiles of the mean streamwise velocity in the outer shear layer follow logarithmic curves. A quantitative relationship between roughness length and canopy drag (CDa) is, thus, determined similar to that in turbulent rough-wall boundary layers.