On the flows over two-dimensional idealized street canyons with height variation

Abstract

Urban street canyon is the basic component of modern dense cities, the ventilation of urban street canyon could thus determine the air quality of the cities. Although realistic street canyons are usually three-dimensional (3D), the simpler two-dimensional (2D) idealised street canyons are useful for studying the fundamental mechanism. Therefore, hypothetical 2D idealized street canyons are used in the current study to examine the mechanism for flows, turbulence and pollutant dispersion over urban roughness. A series of LES models over 2D idealized street canyons with building height variation were performed. Buildings with two different heights are placed alternatively in the computational domains. Building-height-to-street-width (aspect, AR) ratio is defined as the height of the higher building divided by the street width. Building height variability (BHV), on the other hand, is defined as the difference in building heights divided by the height of the higher building. Three ARs (1, 0.5, 0.25) and three BHVs (0.2, 0.4 and 0.6) are considered in this study. Previous studies of 2D idealized street canyons with a uniform building height have shown that the air exchange rate (ACH) of the street canyons increased with increasing aerodynamic roughness. Preliminary results show that the relationship between the ACH and the aerodynamic resistance (friction factor) is persisted in the current LES. Results of the current LES also demonstrate that, apart from the width of street canyon, BHV affects the aerodynamic resistance for a given AR. In the range of 1/4 ≤ ARs ≤ 1, BHV increases the friction factor of the urban roughness to the level of a much wider street canyon with a uniform building height. Because of the large increase in friction factors, the ACHs are also greatly increased in the LES with BHV at the same building density. A significant difference in pollutant dispersion over the street canyons is also observed with BHV. In general, models with a higher BHV, which also have higher aerodynamic roughness, have a larger pollutant dispersion coefficient. Conclusively, introducing BHV into urban street canyon has the potential to improve the air quality in the city.