Airflow Mitigation and Pollutant Purification in an Idealized Urban Street Canyon with Wind Driven Natural Ventilation: Cooperating and Opposing Effects of Roadside Tree Plantings and Non-uniform Building Heights

A refined CFD research was reported, concerning on the individual characteristics of different roadside tree planting and street canyon with non-uniformity of buildings. In this work, airflow field and pollutant dispersion were simulated by the use of a full three-dimensional simulations. Ventilation air exchange rate (ACH) and net escape velocity (NEV) were, respectively applied to assess ventilation exchange and pollutant dispersion in the street canyons. Numerical results show that the airflow patterns within the street canyon varies significantly with canopy leaf area density (LAD) and trunk height (Htb), whereas the distribution of pollutants at pedestrian level in the street canyon is mainly determined by the symmetrical inverse vortex structure of the street canyon. Promotions of LAD and Htb of the road trees simultaneously reduce the wind velocity, ACH and NEV of the street canyon, deteriorating the ventilation exchange conditions as well as the pollutant dispersion performance. As the value of the Htb becomes larger, NEV of the step-up canyon is approximately approaching to that of the step-down canyon. In addition, as the value of standard deviation (σH) of the street canyon increases, the vortex structure of the canyon disappears and promotes the diffusion of pollutants inside canyons. As σH further increases, ACH of the step-down canyon also boosts, indicating better ventilation performance of street canyon, whereas the step-up canyon only tops the best ventilation performance at the value of σH= 33.3%. The best dispersion of pollutants is observed at σH= 16.7% in the step-down canyon while in the step-up canyon the dispersion of pollutants becomes better continuously with increasing σH. In particular, as σH= 66.7% was maintained, the dispersion of pollutants is better in the step-up canyon compared to the street canyon without trees. At the same time, the difference between the NEV of the step-down canyon and the step-up canyon increases as σH increases, which indicates that the difference in pollutant dispersion performance between adjacent street canyons becomes greater. This research provides a theoretical reference for urban design and vegetation planning in urban areas to improve the habitat.