Modelling the Impact of Trees on Vehicular Emissions in the Urban Environment Using Computational Fluid Dynamics

This thesis focuses on the simulation of the impact of trees on vehicular emissions in the urban environment, using CFD (Computational Fluid Dynamics) simulations of air-pollutant concentrations performed under the OpenFOAM software platform (k-ϵ model). Special attention was paid to the evaluation of the CFD model, by assessing the model results against wind tunnel and tracer experiments, as well as against a road side monitoring station. An overall accuracy of 30 to 40% on simulated concentrations was found, which is comparable to the results of previous studies. Most of the statistical parameters were also found to lie within an acceptable range. CFD models in the literature typically use idealised buildings to model wind flow and pollution dispersion. However, the methodology used in this thesis uses actual LIDAR data of buildings and trees to reconstruct a 3D representation of the different modelled scenes, such as Leicester City centre (2 x 2 km area). The modelled areas were on a scale larger than those usually used in other CFD studies. Furthermore, a special focus of this thesis details the interaction between trees and wind flow dynamics. In addition to the study of the aerodynamic effects of trees, the reduction of air pollution by deposition was investigated, which is something that has not yet been modelled at this scale. A final focus of this thesis was the ranking of current and prospective NO2 mitigation strategies including trees, building facades coated with photocatalytic paint and solid barriers. Trees were shown to be the most beneficial strategy when combining both economic and environmental effectiveness. With an attractive cost compared to other solutions, tree-planting policies could offer benefits to urban planning when funding is limited.