NUMERICAL STUDY OF AIR POLLUTANTS DISPERSION IN AN URBAN STREET CANYON

Abstract

The alarming increase of hazardous pollutants in South Asian cities such as Kathmandu, Delhi, Mumbai, Dhaka, etc risks the life of every individual there. The situation worsens especially, in the winter rising the air quality index to life-risking situations. A proper scientific study and modeling of the pollutants are necessary in order to manage the pollution level properly. A major source in the production of such harmful pollutants is vehicles and industries. A Computational Fluid Dynamics (CFD) approach is proposed to model the pollutants emitted by vehicles using different Reynolds Averaged Navier Stokes (RANS) turbulence models in the street canyon setup. The primary aim of the study is to understand the effect of turbulence on the transport of gaseous pollutants in the street canyon. This is accomplished by developing a turbulent steady-state solver for the passive transport of pollutants. RNG k − ε model performed best among other turbulence models with mean FB -0.105, NMSE 0.045, FAC2 0.915 and R 0.93 which is validated with the experimental data. An increase in dispersion of pollutants by 51.4% for AR=0.2 and by 42.33% for AR=0.5 was observed in comparison to AR=1. Street canyons with a low aspect ratio of 0.2 effectively assisted the escape of pollutants. Higher aspect ratios result in flow fields that are more constrained and stagnant, which restricts the dispersion of pollutants. Further, the balconied building lowered the concentration of gaseous pollutants in specific regions only and reduced the exchange concentration flux of pollutants with atmospheric air above the street canyon. Higher wind speeds allowed the concentration to disperse more effectively in Baghbazar’s street canyon, resulting in lower concentrations of pollutants