Flow and pollution concentration large-Eddy simulation and transition conditions for different street canyons and wind speeds: Environmental pollution reduction approach

• Published in: Urban climate 2021-01, Vol.35, p.100731, Article 100731
• Main Authors: Niroobakhsh, Ali, Hassanzadeh, Smaeyl, Hoseinibalam, Fahimeh
• Format: Article
• Language: English
• Subjects: Aspect ratio; Large-Eddy simulation; Pollutants concentration; Street canyon; Transition condition
• ISSN: 2212-0955; 2212-0955
• DOI: 10.1016/j.uclim.2020.100731

Effects of different ambient wind speeds and aspect ratios on turbulence and pollutant concentration in and above street canyons are numerically investigated and compared using parallelized LES model (PALM) and ((Transition Conditions)) are introduced for the first time. The results show that, increasing of ambient wind speed does not change the shape of streamlines but increases the magnitude of time- and area-averaged turbulent kinetic energy with the rate of square of speed ratios resulting in better ventilation of pollutants. As aspect ratio increases, the magnitude of turbulent kinetic energy and turbulent fluxes especially in lower depths of the canyon decrease sharply causing decreased pollution transport to higher levels. An applied case is between 2 and 4 aspect ratios in which there are two vortices along the depths of the canyon. As the width of the canyon increases, the rate of the upper vortex penetration increases and eventually at aspect ratio 2 makes a transition from two to one vortex; we call such conditions as transition conditions. In transition conditions increasing of less than half a meter of street canyon width will drastically decrease the pollution concentration inside the canyon; this is useful in optimizing canyon dimensions for better ventilation. •Different ambient wind speeds have no effect on the structure of streamline fields (number of vortices)in the street canyon.•The increase of vortices number along the depth of the canyon has very negative effects on the discharge of pollutants.•In transition conditions, increasing of less than half a meter of the canyon width will drastically increase the ventilation.•Transition condition phenomenon can be used to optimize canyon dimensions in designing urban areas for better ventilation.