The Effect of Trees on Temperature Hot Spots within a Urban Heat Island: CFD Analysis of the Bologna Case Study

The presence of variable heat fluxes on the building walls, due to the non-uniform sun-light radiation at the street level, poses a key challenge for modeling the heat exchanges within the streets with local three-dimensional numerical approaches. In absence of trees, regions of concentrated high power (hotspots), and therefore significant temperature gradients might arise, while the non-uniform shadowing effects on the walls mitigate these temperature peaks. Heat exchange processes between building walls and external air within the streets is an important topic in air quality modeling, considering that the thermal structure largely affect the dynamics and pollutant concentration. Of interest in this study is the CFD modeling of radiative processes of building walls in terms of emissivity (i.e. is the ratio of the thermal radiation from a surface to the radiation from an ideal black surface at the same temperature), shape factors (i.e. the proportion of the radiation which leaves the surface of a building and strikes the surface of another building) and their relation to the heat exchanges between the building walls, the road surfaces and air between them. The influence of trees is analyzed by considering different shadowing characteristics and crown shapes. The study is approached via the computational fluid dynamics code OpenFOAM, with large eddy simulations (LES) to model turbulence within the canyon and Boussinesq approach to model heat transfer [1]. Consequences on pollutant concentrations are also analysed. The thermal boundary conditions on the building surfaces are set by correlations between the emissivity of a surface and the heat flux emitted by this surface, under certain weather conditions, 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 radiative building walls characteristics is obtained. We propose a new formulation for the exchanges as a way to link thermal features in street canyons to pollutant concentrations [2]. Model simulations are validated using data being acquired during the extensive 2017 summer campaign in Bologna city center, under the EU funded project iSCAPE (www.iscapeproject.eu). Acquiring knowledge about the interactions between urban climate, air pollution and climate change in European cities is at the core of iSCAPE project. 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 vegetation within the city district in order to minimize the effects of temperature hotspots within a urban heat island.

Acknowledgements

The iSCAPE (Improving Smart Control of Air Pollution in Europe) project is funded through the European Community’s H2020 Programme under the Grant Agreement No. 689954.

References

[1]. J. Allegrini, V. Dorer, J. Cameliet, Coupled CFD, radiation and building energy model for studying heat flux in an urban environment with generic building configurations, Sustainable Cities and Society, 2015, 19, 385-394.

[2]. F. Santese, S. Di Sabatino, E. Solazzo, R. Britter, Modelling urban heat island in the context of a Mediterranean city, Developments in Environmental Science, Volume 6, 2007, pp 55-63.