Experimental investigation of the impact of tree-like obstacles on ventilation in street canyons under isothermal and different levels of non-isothermal conditions

Urban heat island (UHI) is a well-known phenomenon¹ that affects thermal comfort of city dwellers, building energy consumption, and city sustainability in general. The planting of urban trees has been identified as one of the most effective methods for mitigating UHI effect and improving urban microclimate. The flow characteristics of an idealized symmetric urban street canyon containing trees of varying sizes is being investigated in this study. Particle Image Velocimetry (PIV) measurements in a water channel at the Environmental Fluid Mechanics Laboratory of the University of Cyprus, were used, to investigate the combination of buoyant and inertial forces in the canyon. A non-dimensional analysis of the measurements² was conducted to quantify the competition between thermally driven and inertially driven flow in the presence of varying sizes of tree obstacles. This study aims to provide insights into the urban microclimate by examining the flow pattern, turbulence characteristics, and air exchanges between the canyon and the boundaries above. Results show that trees have a significant impact on the formation and strength of vortical flow within the canyon. Smaller trees stimulate the formation of stronger vortices, while larger trees lead to the elimination of the vortex and the existence of recirculation cells. The location of the heated surface also affects the vortex structure and elevation within the canyon. In addition, trees can reduce normalized vertical velocities, which can be restrictive to the “breathability”^3,4 of the canyon. The volumetric ventilation rate ⁵ through the roof-level opening that refers to the time flow travels one circuit inside the canyon, is of high interest in the context of pollution escape/removal from the canyon, as well as the removal of other scalars (e.g., relating to heat, thermal comfort, and ¨breathability¨ of the urban canyon). This study found that in the case of a leeward heated surface, air removal from the canyon can change to air entrainment with the increase in tree size, whereas in the case of a windward heated surface, air removal dominates regardless of tree size. The study encompasses an examination of the vertical profiles of Reynolds Stresses (RS) and the extent of Roughness SubLayer (RSL). Our findings demonstrate that the range of layers extension, spans from 1.10 H (minimum) to 1.60 H (maximum), where H represents the height of the building. Moreover, a substantial increase in the RSL extent was observed in windward heated surfaces scenarios in comparison to leeward heated surfaces. In conclusion, our study sheds light on the impact of trees on flow characteristics and air exchange in urban street canyons. The findings of this study can inform urban areas' design and improve their overall sustainability.