Large-eddy simulation of flow and pollutant dispersion in a 3D urban street model located in an unstable boundary layer

Large-eddy simulation (LES) based on the standard Smagorinsky model was employed to simulate microscale pollutant dispersion in a three-dimensional urban street model in an unstable boundary layer. First, an inflow turbulence generation technique based on Kataoka's method was investigated under unstable conditions. Moreover, the method was validated by comparing with experimental data. An objective of this study was to assess the efficiency of LES for estimating thermal/pollutant dispersion in weak-wind regions. Results revealed that given appropriate inflow data, the calculated mean flow variables and their fluctuations agreed favorably with the experimental data under unstable conditions. The flow and pollutant dispersion inside a street canyon were investigated through LES. A primary recirculation was formed inside the street canyon. Consequently, the temperature and pollutant concentration were highly affected. Another objective of this study was to investigate the transport mechanism of pollutant between a 3D street canyon and outer space. Thus, analyses of the concentration fluxes and pollutant flow rate were conducted for the street canyon, the role of turbulence in pollutant transportation was determined. Results revealed that total pollutant inflow rate was higher than its outflow rate for the two side surfaces but opposite for top surface. Turbulence considerably contributed to pollutant transportation, especially to the pollutant inflow rate at side surfaces and pollutant outflow rate at top surface. For top surface, turbulence contribution for pollutant outflow rate reached 75%. Analysis of the air flow rate revealed that air exchange between street canyons and outer space is mainly through the side surfaces.