A unified theory for the estimate of ventilation and pollutant dispersion over hypothetical urban areas

Abstract

Urban roughness is a major factor governing the flows and pollutant transport in the urban atmospheric boundary layer (UABL) over buildings. Whereas, our understanding of its effects on ventilation and pollutant dispersion in/over urban areas is rather limited. In this paper, the ventilation of and pollutant dispersion over idealized street canyons flanked by eight types of building models is examined using computational fluid dynamics (CFD). As an initial attempt, the building models are of the same height so a roof level is well defined across the entire hypothetical urban area. The aerodynamic resistance, ventilation and pollutant dispersion are measured by the friction factor f, characteristic roof-level velocity scale and (vertical) dispersion coefficient , respectively. CFD results show that the ventilation and dispersion coefficient are largely governed by turbulence transport (over 60%). Moreover, with a consistent support from both analytical solution and CFD results, the turbulent components of and are linear functions of the square root ( ) and fourth root ( ) of friction factor regardless of the building geometry and UABL thickness. In view of the dominated transport processes by turbulence, it is proposed that the friction factor can serve as an estimate to the minimum ventilation rate and pollutant dispersion of urban areas.