Effects of Thermal Stratifications on the Flow and Pollutant Dispersion in Urban Street Canyons: a Large-Eddy Simulation Approach

Large-eddy simulations (LESs) for two-dimensional (2D) idealized street canyons are developed to examine how unstable, neutral, and stable thermal stratifications affect the characteristics of flow and pollutant dispersion in urban street canyons. An incompressible flow model with the Boussinesq approximation is employed and a street canyon of unity aspect ratio is considered. Street-level area sources of uniform heat and pollutants are used to model the pollutant dispersion in daytime and nighttime conditions.

In the stabilities studied, the flow patterns inside the street canyon are dominated by a large recirculation. In the unstable stratification with ground heating, the mean wind speed is enhanced but the streamlines exhibit a pattern similar to its neutral counterpart. In the stable stratification, the overall mean wind speed inside street canyon is decreased, moreover, a larger secondary recirculation is developed at the ground-level leeward corner that traps a stable layer of cold air at the street level.

In the unstable stratification, the turbulence kinetic energy (TKE) in the street canyon is increased at large compared with the neutral stratification. Unexpectedly, an even higher level of TKE is observed on the windward side of the street canyon in stable stratification. This change is believed to be caused by the decreased mean wind speed which slows down the turbulent transport from the roof-level windward corner to other parts of the street canyon.

In the neutral stratification, the pollutant is well-mixed inside the street canyon but its removal to the urban canopy layer aloft is rather poor. In the unstable stratification, the pollutant removal is enhanced mainly due to the warm updraft so the average pollutant concentration in the street canyon is significantly lowered down. In the stable stratification, the pollutant is not mixed well but a shallow layer of pollutant is accumulated at the street level demonstrating a large variation of pollutant concentrations in the street canyon.