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

Abstract

<p>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 <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/geostrophic-wind" title="Learn more about geostrophic winds from ScienceDirect's AI-generated Topic Pages">geostrophic winds</a>. Breathability of city was then evaluated in terms of air change rate per hour (ACH) and age of air. <strong><em>The contribution of five factors to urban </em><a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/urban-heat-island-effect" title="Learn more about heat island from ScienceDirect's AI-generated Topic Pages">heat island</a><em> and air quality is analyzed qualitatively and quantitatively</em></strong>. 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 <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/lapse-rate" title="Learn more about lapse rate from ScienceDirect's AI-generated Topic Pages">lapse rate</a> have great influences on the urban heat island intensity and air quality. Comparing with <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/neutral-atmosphere" title="Learn more about neutral atmosphere from ScienceDirect's AI-generated Topic Pages">neutral atmosphere</a>, ACH in <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/urban-canopy" title="Learn more about urban canopy from ScienceDirect's AI-generated Topic Pages">urban canopy</a> 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 <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/lapse-rate" title="Learn more about lapse rate from ScienceDirect's AI-generated Topic Pages">lapse rate</a> > 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 <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/buoyancy-driven-flow" title="Learn more about buoyancy driven flow from ScienceDirect's AI-generated Topic Pages">buoyancy driven flow</a> was very weak. Present model and research results could have theoretical significance for the council and urban planners to alleviate urban heat island and <a href="https://www.sciencedirect.com/topics/earth-and-planetary-sciences/urban-air-pollution" title="Learn more about urban air pollution from ScienceDirect's AI-generated Topic Pages">urban air pollution</a> through reasonable planning of urban building layouts.<br></p>