The physics in natural ventilation of cities and buildings

Abstract

Sixty percent of the world’s population – i.e. 4.3 billion people – live in Asia in just 30% of the world’s landmass. Asian cities in the so-called valeriepieris circle are mostly taller, denser, deeper and larger than those in the West, and the effects of building drag and urban heat island circulation are also stronger. The physics of urban climate in these large cities is complex, as a combined result of local circulation and synoptic winds modified by the mountainous topography and land/sea breeze, if any. Urban air pollution and warming in the rapidly urbanizing Asia have been a great challenge to mankind. Natural ventilation of a city refers to the penetration and distribution of rural air into an urban canopy layer. The weakened city ventilation has become one major reason for worsening urban warming and air pollution in cities. Two distinct situations need to be considered, i.e. when the synoptic wind is strong; and when the synoptic wind is weak respectively. For the former, designers are interested to manage city ventilation for removal of the urban heat, moisture and pollutant, or retain of urban heat and moisture. The latter become mostly the conditions for the worst urban extreme heat or haze scenarios to occur. Natural ventilation of a building refers to the introduction of outdoor air into a building by natural forces such as wind and buoyancy. High-rise buildings present an interesting challenge as the top of the building may be in the urban roughness layer or even beyond the atmosphere boundary layer. I shall attempt to review the status of our understanding of the physics in city ventilation under both strong and weak wind conditions, and discuss our recent findings on urban extreme high temperature event in June 2016 in Kowloon, the spread of SARS CoV virus when there is inversion, and the urban domes and their merging using simple theoretical estimates, water tank models, city scale CFD, and meso-scale WRF. Our work aims to establish the need and an approach for designing city climate and environment as for buildings, for example, designing building density and height in a city for better urban climate, and between-city distance needed to avoid regional haze formation.