Large-Eddy Simulation of Pollutant Dispersion over Urban Street Canyons: Local Turbulence and Local Reactions

Pollutant dispersion in urban areas is a complicated process with the interaction between rough ground and chemical reactions. While the upper part of the pollutant plume dispersion from a line/point source resembles the Gaussian shape, its lower part is far from the traditional analytical solution because of the bottom surface roughness.

In this study, large-eddy simulation (LES) is employed to calculate the pollutant dispersion with bisubstrate chemical reactions in and over an array of idealized street canyon models. The flow is driven by a constant pressure gradient that is perpendicular to the streets. Thirty-six identical street canyons of unity aspect ratio are used as the roughness elements to initiate the turbulent flows. Reactant A is a kind of vehicular pollutant which is emitted from the ground surface of the first street canyon. Another reactant B, which is doped in the inlet of the boundary layer over the street canyons, reacts with reactant A producing reactant C. The chemical reactions are irreversible whose reaction rates are assumed to be constant. The velocity, turbulence and concentration data are collected downstream over the street canyons.

Local time scales of turbulence and chemical reaction are examined at every grid point to study the mechanism of pollutant dispersion with chemistry over urban areas. Turbulence intensity at the boundary of the pollutant plume is found to be important for the pollutant dilution and removal.