CFD analysis of scalar transport phenomena within two real street canyons in the city of Bologna

The presence of variable heat fluxes between building walls and air within a street canyon, due to the non-uniform sun-light radiation at the street level, poses a key challenge for modeling the heat exchanges with local three-dimensional numerical approaches. In absence of trees, regions with temperature distribution much higher than air temperature, and therefore significant temperature gradients, might arise, while the trees shadowing effects on the walls mitigate these temperature peaks. Heat exchange processes between building walls and external air within the canyon is an important topic in air quality modeling, considering that the thermal structure largely affects the dynamics and pollutant concentration. The influence of trees is analyzed by considering different shadowing characteristics and crown shapes. The study is approached via the computational fluid dynamics finite-volume method, with large eddy simulations (LES) to model turbulence within the canyon and Boussinesq approach to model heat transfer [1]. Consequences on pollutant concentrations are analyzed. The thermal boundary conditions on the building surfaces are set by analyses on the basis of previous experimental results obtained from infrared camera during a UHI experiment [2], for the latitude and longitude of the case study, the orientation, the time of the day. A map with the maximum temperatures reached within the urban area analyzed as a function of the time instant is obtained. We propose a new formulation for the exchanges as a way to link thermal features in street canyons to pollutant concentrations [3]. Model simulations have been validated by previous studies done under the EU funded project iSCAPE (www.iscapeproject.eu). Temperature measured by distributed sensors, surface temperature using infra-red camera, together with measurements of particulate matter, ozone and traffic-related compounds concentrations will be used for the validation of the model. The model will give a tool to design and optimize the key characteristics within the city district (vegetation, geometry of buildings, etc.) in order to minimize the effects of pollutant concentration hotspots within an urban heat island.