Pollutant dynamics in urban canyons: wind-tunnel experiments and numerical simulations

The main goal of this study has been to advance the understanding of the effect of the presence of trees on the ventilation of pollutants in an urban geometry. This has been pursued by an experimental campaign conduced in the wind tunnel of the Laboratoire de Mécanique des Fluides et d’Acoustique at ECL. The urban geometry considered consists in a street canyon placed perpendicular to the wind direction within the wind tunnel. Three main configurations of trees were analysed, namely: no trees, two rows of scattered trees and two dense rows of trees. A linear source emitting a mixture of ethane and air has been used to simulate the pollutant. Concentration measurements were performed by means of a Flame Ionization Detector (FID). The study is enriched by the realisation of RANS numerical simulations using Ansys Fluent. The obtained results are in a good agreement with the experimental campaign conducted at ECL. However, due to the inherent constraints of the RANS approach, the numerical results have provided only a qualitative behaviour of the concentration field. To analyse the effect of the vegetation on the dispersion of pollutant, the study has been focused on two aspects: the analysis of the concentration field and the computation of the vertical exchange velocity with the surrounding environment. The study has brought interesting results on both these two aspects and has led to new questions and analysis to be done to better understand the dynamic of pollutant in urban environments.