title: A theory of ventilation estimate over hypothetical urban areas.
abstract: Urban roughness is a major factor governing the flows and scalar transport in the atmospheric boundary layer (ABL) but our understanding is rather limited. The ventilation and pollutant removal of hypothetical urban areas consisting of various types of street canyons are examined using computational fluid dynamics (CFD). The aerodynamic resistance, ventilation efficiency, and pollutant removal are measured by the friction factor f, air exchange rate (ACH), and pollutant exchange rate (PCH), respectively. Two source configurations of passive tracer, ground-level-only (Tracer 0) and all-solid-boundary (Tracer 1) are employed to contrast their transport behavior. It is found that the ventilation and pollutant removal are largely attributed to their turbulent components (over 60%). Moreover, with a consistent support from analytical solution and CFD results, the turbulent ACH is a linear function of the square root of the friction factor (ACH'∝f(1/2)) regardless of building geometry. Tracer 0 and Tracer 1 exhibit diversified removal behavior as functions of friction factor so analytical parameterizations have not yet been developed. In view of the large portion of aged air removal by turbulence, it is proposed that the aerodynamic resistance can serve as an estimate to the minimum ventilation efficiency of urban areas.
authors.affiliation: Department of Mechanical Engineering, The University of Hong Kong, Hong Kong, China. Electronic address: liuchunho@graduate.hku.hk.
authors.forename: Chun-Ho
authors.initials: CH
authors.lastname: Liu
authors.affiliation: Department of Mechanical Engineering, The University of Hong Kong, Hong Kong, China.
authors.forename: Chi-To
authors.initials: CT
authors.lastname: Ng
authors.affiliation: Department of Mechanical Engineering, The University of Hong Kong, Hong Kong, China.
authors.forename: Colman C C
authors.initials: CCC
authors.lastname: Wong
mesh_terms: D000388:Air / Q000592:standards*; D000392:Air Movements; D000393:Air Pollutants / Q000032:analysis*; D001108:Architecture; D002947:Cities; D008962:Models, Theoretical
chemical_list: D000393:Air Pollutants
keywords: Air quality; City ventilation; Hypothetical urban areas; Pollutant removal; Reynolds-averaged Navier–Stokes (RANS) k-ϵ turbulence model