Evaluating different categories of turbulence models for calculating air pollutant dispersion in street canyons with generic and real urban layouts

Turbulence models are crucial for simulating urban pollutant dispersion by computational fluid dynamics (CFD) methods. This study aimed to evaluate the performance of steady-state and unsteady-state Reynolds-averaged Navier-Stokes simulation (SRANS/URANS) and large-eddy simulation (LES) in calculating air pollutant dispersion in street canyons with generic and real urban layouts. For each layout, wind tunnel experiments with measured wind speed and pollutant concentration were available as benchmarks. In addition, instantaneous concentration fields were analyzed to assess the transient models. The results showed that in both generic and real urban layouts, the RNG $k-\epsilon$ model and SST $k-\omega$ model provided similar results for time-averaged wind speed and concentration distributions in SRANS and URANS simulations. LES performed the best in calculating wind speed and pollutant dispersion. In a generic urban layout, URANS with the SST $k-\omega$ model captured large-scale fluctuations, while instantaneous results from URANS with the SST $k-\omega$ model did not change over time in a real urban layout. SRANS/URANS with the SST $k-\omega$ model can provide acceptable results for time-averaged pollutant concentration fields in a generic urban layout with simple building shapes and placements. However, for real urban layouts, the LES approach is the most accurate way to calculate air pollutant dispersion.