Abstract
Previous researchers calculated air change rate per hour (ACH) in the urban canopy layers (UCL) by integrating the normal component of air mean velocity (convection) and fluctuation velocity (turbulent diffusions) across UCL boundaries. However they are usually greater than the actual ACH induced by flow rates flushing UCL and never returning again. As a novelty, this paper aims to verify the exponential concentration decay history occurring in UCL models and applies the concentration decay method to assess the actual UCL ACH and predict the urban age of air at various points. Computational fluid dynamic (CFD) simulations with the standard k-ε models are successfully validated by wind tunnel data. The typical street-scale UCL models are studied under neutral atmospheric conditions. Larger urban size attains smaller ACH. For square overall urban form (Lx = Ly = 390 m), the parallel wind (θ = 0°) attains greater ACH than non-parallel wind (θ = 15°, 30°, 45°), but it experiences smaller ACH than the rectangular urban form (Lx = 570 m, Ly = 270 m) under most wind directions (θ = 30° to 90°). Open space increases ACH more effectively under oblique wind (θ = 15°, 30°, 45°) than parallel wind. Although further investigations are still required, this paper provides an effective approach to quantify the actual ACH in urban-like geometries.
Highlights
The increase in number of vehicles in cities and the ongoing urbanization worldwide are causing more concerns about urban air pollution
Assessing application standard model withimplies presentagrid arrangement is acceptable for the purposes of acceptance requires considering relevant performance metrics rather thanbetween one specific index, so our research
Street-scale (~100 m), medium-dense, urban-like geometries are studied under neutral atmospheric conditions
Summary
The increase in number of vehicles in cities and the ongoing urbanization worldwide are causing more concerns about urban air pollution. The urban canopy layer (UCL) is defined as the outdoor air volume below the rooftops of buildings [1]. Raising UCL ventilation capacity by the surrounding atmosphere with relatively clean air has been regarded as one effective approach to diluting environmental pollutants [2,3,4,5,6,7,8] and improving the urban thermal environment in the hot summer [9,10] as well as reducing human exposure to outdoor pollutants [11,12,13]. For two-dimensional street canyons [14,15,16,17,18,19], street aspect ratio (building height/street width, H/W) is the first key parameter to affect the flow regimes and UCL ventilation. Four flow regimes have been classified depending on different aspect ratios (H/W) [14,15,16], i.e., the isolated roughness flow regime (IRF, in which the aspect ratio is less than 0.1 to 0.125), the wake interference flow regime (WIF, with an aspect ratio of 0.1 to 0.67), the skimming flow regime (SF, with an aspect ratio of 0.67 to 1.67), and the multi-vortex regime in Atmosphere 2017, 8, 169; doi:10.3390/atmos8090169 www.mdpi.com/journal/atmosphere
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