Simulation Study on Impacts of Viaduct Height on Pollutant Dispersion in Street Canyons Using LES and RANS Models

To assess the impact of different heights of the viaduct on air pollutant dispersion and the prediction accuracy of pollutant concentration in urban street canyons, simulation results based on LES and RANS models are analyzed. The presence of a viaduct generated a poorly ventilated region underneath it, and RANS significantly underestimated the wind speed and grossly overestimated the pollutant concentration. LES gives better results for the flow pattern, distribution of turbulent kinetic energy and mean pollutant concentration. With a fluctuation of less than 15% of the pollutant concentration, both RANS and LES cases show that an increase in the viaduct height has a weak impact on the concentration of pollutants in most areas of the canyon except windward, and cases with a viaduct height of 0.75H had the lowest predicted pollutant concentration relative to other cases with a viaduct as a result of better ventilation. In addition, LES found a subregion of pollutant accumulation above the ground, but RANS did not.