On the mechanism of air pollutant re-entrainment in two-dimensional idealized street canyons

Abstract

The two-dimensional (2D) idealized street canyon, which is the generic unit of a city, is the platform for our fundamental understanding of ventilation and pollutant removal at the neighborhood scale. The building-height-to-street-width aspect ratio h/b is the key geometric parameters affecting the flow structures in a street canyon. In this study, a series of computational fluid dynamics (CFD) sensitivity tests were performed to examine how the air pollutant concentration in a street canyon is related to the aspect ratio. The Reynolds-averaged Navier–Stokes (RANS) equations and the Renormalization Group (RNG) k−ε turbulence model were used in the mathematical model. The spatial behaviors of air pollutant transport from the facades, streets, and roofs to the shear layer were depicted by the local pollutant exchange rate ω. Besides, the bulk quantities, air exchange rate ACH, pollutant exchange rate PCH, and volume average pollutant concentration Θ, were used to elucidate the ventilation and pollutant removal mechanisms of the street canyon. The aspect ratios tested were in the range 0.067 ≤ h/b ≤ 2.5 that covered the isolated roughness, wake interference and skimming flow regimes in 2D street canyons. A local maximum Θ was determined in 0.2 ≤ h/b ≤ 0.5 that is different from the monotonic ACH or PCH. The CFD results showed that the mildly elevated air pollutant concentration is not caused by poor pollutant removal but the pollutant re-entrainment from the shear layer aloft back into the street canyon. It is thus suggested that ACH, PCH, and Θ should be used as complementary indicators.