This paper reviews some recent studies (after 2000) pertaining to buoyancy driven flows in nature and thier use in reducing air pollution levels in a city (city ventilation). Natural convection flows occur due to the heating and cooling of various urban surfaces (e.g., mountain slopes), leading to upslope and downslope flows. Such flows can have a significant effect on city ventilation which has been the subject of study in the recent times due to increased pollution levels in a city. A major portion of the research reviewed here consists of natural convection flows occurring along mountain slopes, with a few studies devoted to flows along building walls. The studies discussed here primarily include field measurements and computational fluid dynamics (CFD) models. This review shows that for densely populated cities with high pollution levels, natural convection flows (mountain slope or building walls) can significantly aid the dispersion of pollutants. Additional studies in this area using CFD and water channel measurements can explain the physical processes involved in such flows and help improve CFD modelling. Future research should focus on a complete understanding of the mechanisms of buoyancy flows in nature and developing design guidelines for better planning of cities. 1. Introduction Global air pollution has been a subject of study for several decades. For instance, global warming has caused the melting of ice in the west Antarctic region, thereby increasing the sea-levels by several centimetres [1]. However, pollution levels within a city may increase only due to the surrounding topography and local environmental conditions. For example, the London smog in 1952 consisted of large amounts of dusty fumes from nearby factories causing poor visibility and resulting in the deaths of several people during the time [2]. Lack of available land and increased population have led to the construction of high density and high rise buildings in many cities like New York and Hong Kong, thereby increasing city pollution. This situation has been aptly described by Fernando [3] as “The future of humankind is sure to evolve in large urban areas that proffer the highest quality of life. Accelerated growth, however, has overstressed some urban environments, thus calling for sound planning tools for sustainable growth. In this context, of cardinal importance is the prediction of the urban atmosphere (i.e., key meteorological variables and turbulence) and air-quality indicators (e.g., pollutant levels, comfort index).” One of the key issues with urban megacities like
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