Large-eddy simulation of ventilation and pollutant removal in neutrally and unstably stratified street canyons

Abstract

Large-eddy simulation model was developed to study the ventilation and pollutant removal of urban street canyons in neutral and unstable stratifications. Street canyons of unity building-height-to-street-width ratio were considered. For the case with unstable stratification, the ground was heated up to a Richardson number Rb (= gh/Uh2(Θh-Θ0)/Θref) of -10, where g is the gravitational acceleration, h the building height, Uh the roof-level velocity scale, Θref the reference temperature, Θh the roof-level temperature and Θ0 the ground temperature. The gaseous pollutant was modeled as a passive scalar. Ground-level area sources with uniform pollutant concentrations were used to model traffic emission. In neutral stratification, skimming flow and poor pollutant removal are observed. A primary recirculation is developed in the street canyon core by the prevalent wind (Fig 1a). It occupies nearly all the space inside the street canyon leaving three small secondary recirculations at the ground-level leeward, ground-level windward and roof-level leeward corners. The pollutant emitted from the street is mostly trapped inside the street canyon hence elevated pollutant concentration is observed. Unstable stratification modifies the flow pattern significantly that enhances the pollutant removal. An enlarged secondary recirculation is observed at the ground-level windward corner (Fig 1b). It pushes the primary recirculation upward which eventually extends over the roof level of street canyon immersing into the shear layer aloft. The sizes of the two small recirculations on the leeward side shrink instead. The wind speed inside the street canyon increases that enhances the pollutant mixing. As a result, the overall pollutant concentration is lower compared with that in neutral stratification. In contrast to a roof-level thin layer of pollutant in neutral stratification, pollutant is carried upward by the convective updraft moving from the building roof level into the shear layer. Unstable stratification thus promotes pollutant dilution and removal in street canyons. Different locations of substantial turbulence kinetic energy (TKE) production were observed from the LES results of street canyons in neutral and unstable stratifications. In neutral stratification, TKE production is high at roof level because of the large velocity gradient there. In unstable stratification, because of the bottom heating, elevated TKE production locates near the windward facade. The large temperature gradient and the buoyancy force strengthen the recirculating wind at the ground-level windward corner. Therefore, the TKE production is peaked along the interface between the primary and secondary recirculations. Velocity and pollutant concentrations in a street canyon under: (a) neutral and (b) unstable stratifications.