CFD simulations of wind flow and pollutant dispersion in a street canyon with traffic flow: Comparison between RANS and LES

The dispersion of traffic pollutants in street canyons takes place under the joint influence of the urban morphology and traffic-induced air motions. Computational fluid dynamics (CFD) are increasingly used to explore air quality problems in cities, however, numerical setup guidance for studies considering moving traffics remains missing. This study presents a systematic evaluation and comparison of steady Reynolds-averaged Navier-Stokes (RANS) and large-eddy simulation (LES) in reproducing the wind flow and pollutant dispersion in a street canyon under traffic flow using the quasi-steady method. The RANS simulations are performed with five turbulence models, and the standard k-epsilon (SKE) turbulence model is found to yield the best agreement with the wind-tunnel data. The LES simulation with the wall-adapting local eddy-viscosity subgrid-scale model outperforms all RANS models in simulating the mean velocity and mean pollutant concentration, and can accurately predict the turbulent velocity. The counter-gradient turbulent mass flux captured by LES reveals that the gradient-diffusion hypothesis used in steady RANS fails in the regions near moving traffics, which strongly affects the predicted mean concentration. The findings of this study can support future CFD studies on pollutant dispersion in urban environments.