Mechanisms of ventilation in real street canyons in presence of vegetation

Street canyons are the unit elements of the urban morphology. Fluid dynamics within these elements has been widely studied especially in the context of air pollution associated to traffic emissions. Gases and particulate matter resulting from the combustion processes are the main components of these emissions, but also pneumatic abrasion, brake discs consumption and road dust resuspension contribute to air quality deterioration. In general terms, the presence of dense built up areas limits the efficacy of atmospheric winds to disperse pollutants, enhancing near ground concentrations. It is also known that the presence of trees in urban street canyons affects pollutants dispersion and exchange of momentum and scalars with the free atmosphere. Depending on wind direction, the establishment of canyon vortices, corner vortices at street entrance and intersections, intermittent flows within the canyon are drivers for near surface pollutant removal. The presence of trees alter the flow structure and may lead to a reduction of ventilation in streets. In real world applications, the actual quantification of tree influence depends on different and mutual interacting factors: synoptic meteorological conditions, wind stress at the top of the canyon, geometry of the street canyon and vegetation aerodynamic characteristics. Within the recently EU-funded project iSCAPE, the role of vegetation in street canyons has being analyzed in real street canyons in the city of Bologna (Italy). Specifically, during summer 2017, turbulent fluxes are being measured at different heights in two typical street canyons of similar aspect ratio and orientation with respect to the prevailing wind but with different vegetative elements i.e. one is almost free of vegetation and the other has trees at the edge of each traffic lane. Data interpretation is assisted by Computational Fluid Dynamics (CFD) modelling using large eddy simulations (LES). Validated simulations are used to formulate ventilation indices typical for Italian cities. Results are sought for extension to other cities in Europe and elsewhere that are characterized by low synoptic conditions and similar morphological structures