A Wind Tunnel Study of Flows over Idealized Vegetation Canopy with Roughness Sublayer Correction

Abstract

Dynamics in the roughness (RSLs) and inertial (ISLs) sublayers in the turbulent boundary layers (TBLs) over idealised vegetation canopies are investigated analytically and experimentally. In this paper, we derive an analytical solution to the mean velocity profile, which is a continuous function applicable to both RSL and ISL, over vegetation canopies in isothermal conditions. Afterwards, a modified mixing-length model for RSL/ISL transport is developed that elucidates how surface roughness and vegetation density affect the turbulence motions. A series of wind tunnel experiments are conducted to measure the vertical profiles of mean and fluctuating velocities, together with momentum fluxes over various configurations of vegetation canopy using hot-wire anemometry (HWA). The analytical solution agrees well with the wind tunnel result that improves the estimate to mean velocity profile over vegetation canopies and TBL dynamics as well. The thicknesses of RSL and ISL are calculated by monitoring the convergence/divergence between the temporally averaged and spatio-temporally averaged profiles of momentum flux. It is found that the height of RSL/ISL interface is a function of vegetation type and density. Examining the direct, physical influence of vegetation elements on near-surface RSL flows reveals that the TBL flows over vegetation canopies exhibit turbulence motions of two different length scales which are functions of the RSL and ISL structure. Conclusively, given a TBL, the denser the vegetation, the higher is the RSL intruding upward that would thinner the ISL. Therefore, the conventional ISL log-law approximation to TBL flows over urban surfaces should be applied with caution.