Impact of trees with varying size on street canyon flow under isothermal and non-isothermal conditions using water channel PIV measurements

Abstract

This study examines the influence of trees on urban microclimate, specifically focusing on the breathability and turbulence within urban canyons. Utilizing a simplified two-dimensional street canyon model with tree-like elements, we investigate air flow interactions influenced by both buoyancy-driven and inertial forces, using Particle Image Velocimetry (PIV) measurements. The study considers trees of varying heights ($h_t$), relative to the urban street canyon height ($H$), revealing that smaller trees ($h_t/H=1/4$) allow the establishment of stronger vortices on top of the trees, while larger trees ($h_t/H=1$) disrupt vortex formation, leading to recirculation cells. The study also explores the impact of the heated wall surface position (i.e., leeward or windward), on vortex formation and breathability at the rooftop level. Additionally, the critical value of the buoyancy parameter $\left(B\right)$, defined as the ratio of buoyancy to inertial forces, determines whether the flow field is shear- or buoyancy-dominant. These findings highlight the importance of trees placement and heated surfaces positioning in urban canyon modeling to enhance urban microclimate and pollutant dispersion. The results provide essential data for developing simplified models of tree impacts in urban areas, crucial for integrating with larger-scale weather and climate models.