Aerodynamic characterization of model vegetation by wind tunnel experiments

Abstract

A better prediction of turbulent airflow around porous vegetation is required for urban environment studies as vegetation is being increasingly utilized to mitigate Urban Heat Islands (UHI). Trees in urban areas impact ventilation by disturbing the flow and lead to cooling by evapotranspiration. Furthermore, trees have been shown to play a role in pollutant removal. A first step in properly accounting for the impact of vegetation on UHI is to accurately determine the heat and mass exchange between vegetation and the environment. For such determination, an accurate model of the turbulent flow field around vegetation and an improved parameterisation of the turbulent momentum deficit using drag coefficient must be obtained. The aim of this paper is to investigate the drag profile and turbulent flow field of flexible and inflexible model trees. The drag coefficients of model trees are measured using a force balance and the turbulent flow fields are measured using a stereo-PIV setup. This paper provides a mean to better predict the turbulent airflow within and around porous vegetation by studying the relation of drag coefficients with turbulent flow fields for model trees. The drag coefficients of inflexible model trees are found to be nearly independent of wind speed whereas, for the flexible model with leaves and branches that streamline to the flow field, the drag coefficients decrease with increasing wind speed. These finding agree with the literature. The normalized mean velocity is related to the drag coefficient, where velocity deficit increases with the drag. Investigating the mean Reynolds stress component does not yield a definite correlation with drag coefficient.