The Turbulent Flow within a Wind-Aligned Street Canyon with Trees: an Experimental Study

Abstract Most studies on the aerodynamic impact of trees on street canyon ventilation focus on external winds perpendicular to the street axis, as these are crucial for dispersing ground-level pollutants like vehicle emissions. Other wind directions, despite their importance for urban pollutant dispersion, receive less attention. Non-perpendicular winds introduce an advective component along the canyon, which is strongly influenced by tree presence and density, significantly shaping pollutant spread across downwind streets. To address this gap, we conducted velocity measurements in a reduced-scale street canyon aligned with the external flow, varying the canyon’s height-to-width ratio and tree density. The aerodynamic effects of trees were simulated by placing plastic tree miniatures along the canyon’s lateral walls. Our results show that trees within a square canyon reduce the mean velocity by 80% and decrease velocity fluctuations by 15% compared to an empty canyon. At the rooftop, the interaction between the external flow and tree crowns leads to a 30% increase in velocity fluctuations. As the aspect ratio decreases (i.e., for wider canyons), a vegetation-free corridor between the rows of trees results in a 25% decrease in mean longitudinal velocity and a 30% increase in velocity fluctuations compared to the empty large canyon. This induces a street ventilation more efficient with respect to the vegetated square canyon, especially when pollutants are emitted at pedestrian level. This study provides valuable experimental data for evaluating the spatially-averaged mean longitudinal velocity in urban canyons, which can be incorporated into analytical and numerical models for better urban air quality management.