Abstract
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.
Highlights
Global air pollution has been a subject of study for several decades
Another problem associated with city level pollution is the urban heat island (UHI) effect, which causes an increase in urban air temperatures as opposed to the surrounding rural region [7,8,9]
Additional “urban buoyancy flow” studies focusing on the combined effect of “building wall” and “mountain slope” are necessary
Summary
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 sealevels by several centimetres [1]. One of the key issues with urban megacities like Hong Kong is their geographic location and complex topography These cities are surrounded by mountains and skyscrapers, where wind penetration is greatly reduced causing accumulation of pollutants [4]. During low wind speeds the buoyancy driven flows caused by heating and cooling of surfaces (mountain slopes and building walls) can greatly help disperse these pollutants [5, 6] Another problem associated with city level pollution is the urban heat island (UHI) effect, which causes an increase in urban air temperatures as opposed to the surrounding rural region [7,8,9]. Recent studies indicate that buoyancy driven flows (along mountains and building walls) can help reduce urban air pollution levels and improve the thermal environment in a city. Future developments that are likely to occur (Section 7), followed by conclusions from this review (Section 8), form the concluding parts of this paper
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