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Impacts of Urban Layouts and Open Space on Urban Ventilation Evaluated by Concentration Decay Method

1
School of Atmospheric Sciences, Sun Yat-Sen University, Guangzhou 510275, China
2
Laboratory of Ventilation and Air Quality, University of Gävle, SE-80176 Gävle, Sweden
3
Department of Mechanical Engineering, The University of Hong Kong, PokFuLam Road, Hong Kong, China
*
Author to whom correspondence should be addressed.
Atmosphere 2017, 8(9), 169; https://doi.org/10.3390/atmos8090169
Received: 20 August 2017 / Revised: 6 September 2017 / Accepted: 6 September 2017 / Published: 11 September 2017
(This article belongs to the Special Issue Recent Advances in Urban Ventilation Assessment and Flow Modelling)
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. View Full-Text
Keywords: small open space; air change rate per hour (ACH); concentration decay method; urban age of air; computational fluid dynamic (CFD) simulation small open space; air change rate per hour (ACH); concentration decay method; urban age of air; computational fluid dynamic (CFD) simulation
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Wang, Q.; Sandberg, M.; Lin, Y.; Yin, S.; Hang, J. Impacts of Urban Layouts and Open Space on Urban Ventilation Evaluated by Concentration Decay Method. Atmosphere 2017, 8, 169.

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