Numerical simulation of atmospheric pollutant dispersion in an urban street canyon: comparison between RANS and LES

Prediction accuracy of pollutant dispersion within an urban street canyon of width to height ratio W/H=1 is examined using two steady-state Reynolds-averaged Navier–Stokes (RANS) turbulence closure models, the standard k–ε and Reynolds Stress Model (RSM), and Large Eddy Simulation (LES) coupled with the advection–diffusion method for species transport. The numerical results, which include the statistical properties of pollutant dispersion, e.g. mean concentration distributions, time-evolution and three-dimensional spreads of the pollutant, are then compared to wind-tunnel (WT) measurements. The accuracy and computational cost of both numerical approaches are evaluated. The time-evolution of the pollutant concentration (for LES only) and the mean (time-averaged) values are presented. It is observed that amongst the two RANS models, RSM performed better than standard k–ε except at the centerline of the canyon walls. However, LES, although computationally more expensive, did better than RANS in predicting the concentration distribution because it was able to capture the unsteady and intermittent fluctuations of the flow field, and hence resolve the transient mixing process within the street canyon.