Quantitative city ventilation evaluation for urban canopy under heat island circulation without geostrophic winds: Multi-scale CFD model and parametric investigations

Abstract

Urban breathability is fundamentally important for improving urban air environment. Nowadays, air pollution and extreme heat waves heavily threaten modern cities and residents. In this paper, a methodology combined with an idealized High-Reynolds-Number Porous-Media city model and a multi-scale Computational Fluid Dynamics (CFD) method will be firstly proposed to quantify the urban ventilation breathability under no geostrophic winds. Breathability of city was then evaluated in terms of air change rate per hour (ACH) and age of air. The contribution of five factors to urban heat island and air quality is analyzed qualitatively and quantitatively. Numerical results further demonstrate that, 1) For heat island intensity, the dominant importance of influencing factors is: heat flux > temperature lap rate > urban scale > building height > urban density. Urban heat flux and temperature lapse rate have great influences on the urban heat island intensity and air quality. Comparing with neutral atmosphere, ACH in urban canopy decreased by more than a half under inversion condition, whereas ACH in pedestrian layer decreased by nearly 100%. 2) For air quality, the importance of the influencing factors could be listed as, building height > heat flux > temperature lapse rate > urban scale > urban density. Building height has the greatest impact on air quality. With the increase of urban scale, average age of air for a city decreases first and then increases. There exists an optimal city size to minimize the age of urban air. Urban building density (regardless of heat emission) on the buoyancy driven flow was very weak. Present model and research results could have theoretical significance for the council and urban planners to alleviate urban heat island and urban air pollution through reasonable planning of urban building layouts.