During sunny days with periods of low synoptic wind, buoyancy forces can play a critical role on the air flow, and thus on the dispersion of pollutants in the built urban environments. Earlier studies provide evidence that when a surface inside an urban street canyon is at a higher temperature than that of local ambient air, buoyancy forces can modify the mechanically-induced circulation within the canyons (i.e., gaps between buildings). The aspect ratio of the urban canyon is a critical factor in the manifestation of the buoyancy parameter. In this paper, computational fluid dynamics simulations are performed on urban street canyons with six different aspect ratios, focusing on the special case where the leeward wall is at a greater temperature than local ambient air. A non-dimensional measure of the influence of buoyancy is used to predict demarcations between the flow regimes. Simulations are performed under a range of buoyancy conditions, including beyond those of previous studies. Observations from a field experiment and a wind tunnel experiment are used to validate the results.
Buoyancy dominant flow Buoyancy parameter CFD simulations Urban street canyons
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This research was supported by the Singapore National Research Foundation through the Singapore-MIT Alliance for Research and Technology’s Center for Environmental Sensing and Modeling (CENSAM). During the preparation of this paper, H.J.S. Fernando was supported by National Science Foundation (CMG; Grant # 0934592). The authors would like to thank the two reviewers for their useful comments and suggestions.
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