The aerodynamic role of trees on city breathability in urban-like geometries

Recent literature has demonstrated that local vegetation affects the flow pattern, and turbulent and mass exchange in urban street canyons, which in turn alters the pollutant concentration distribution. It has been identified that reduction in pollutant concentration through leave deposition is counterbalanced by blocking effects of trees resulting in larger street-level concentrations. On the other hand urban vegetation plays a significant role in mitigating heat island effects particularly in hot climate cities, among other benefits. It is therefore imperative to quantify the overall role of trees on city breathability and pedestrian comfort which is still an open question. The present study takes into consideration the combined effects of building arrangement and vegetation on city breathability using indoor ventilation concepts combined with Computational Fluid Dynamics (CFD) numerical modelling. Reynolds-Averaged Navier-Stokes (RANS) turbulence modelling and Large Eddy Simulation (LES) are employed for the CFD investigation. By simulating airflow and pollutant dispersion within street canyons in the absence and presence of trees, and validating against extensive wind tunnel experimental measurements, it is observed that LES performs better than RANS. This is because LES is able to capture and resolve the unsteady fluctuations of the flow and concentration fields thus accounting for the turbulent mixing process within the street canyons, although time-averaged quantities are compared ultimately. The CFD simulations are used to assess pollution dilution and city breathability within tree-lined urban street canyons. It is shown that the presences of trees reduces the overall in-canyon air circulation and ventilation, resulting in less pollutants being transported out and away from the canyons, in comparison to tree-free streets. The technique used in the present paper provides a range of information regarding the overall breathability of a given city and/or neighbourhood with tree planting and situations where there is likelihood that allowable mean concentrations could be exceeded. Therefore, this study aims to provide some guidelines for the planning and re-development of urban areas taking into account the most suitable tree type/shape and arrangement for a particular local meteorology and city morphometry in order to improve local air quality and microclimate.