Age of air and air exchange efficiency in idealized city models

Wind can provide relevantly clean external (rural) air into urban street network, i.e. city ventilation. The local mean age of air denotes the time it takes for the external air to reach a location after entering the urban canopy layer. The air exchange efficiency denotes the efficiency of flushing the street network with external air. However, difficulties exist in calculating the local mean age of air in a city due to open boundaries. The traditional experimental homogeneous emission method is adapted here in a CFD method to predict the urban local age of air and analyze the air exchange efficiency for city ventilation. Three simple city models are considered, including a round city model, a square city model and a long rectangular city with one main street parallel to the approaching wind or with two crossing streets. The difference in the city shape results in significant difference in the local mean age of air. In the round city of one narrow street, two inflows through street openings converge close to the city centre and exits through the street roof, so the air close to the city centre is relatively old and the air exchange efficiency is low (30%). For a round city with two crossing streets, a slightly non-parallel wind to the main street generates younger air and the higher air exchange efficiency in the city.

[1]  Yuanhui Zhang,et al.  Experimental study of ventilation effectiveness and air velocity distribution in an aircraft cabin mockup , 2008 .

[2]  Stephen E Belcher,et al.  Mixing and transport in urban areas , 2005, Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences.

[3]  Zhen Huang,et al.  Impact of building configuration on air quality in street canyon , 2005 .

[4]  T. Jongen,et al.  Extension of the Age-of-Fluid Method to Unsteady and Closed-Flow Systems , 2004 .

[5]  B. Givoni,et al.  Urban design factors influencing heat island intensity in high-rise high-density environments of Hong Kong , 2007 .

[6]  Rex Britter,et al.  Simulations of pollutant dispersion within idealised urban-type geometries with CFD and integral models , 2007 .

[7]  Mats Sandberg,et al.  Numerical and experimental studies of wind environment in an urban morphology , 2005 .

[8]  Dennis Y.C. Leung,et al.  On the prediction of air and pollutant exchange rates in street canyons of different aspect ratios using large-eddy simulation , 2005 .

[9]  Sture Holmberg,et al.  On the Assessment of ventilation Performance with the Aid of Numerical Simulations , 1997 .

[10]  Mats Sandberg,et al.  Effect of urban morphology on wind condition in idealized city models , 2009 .

[11]  Shinsuke Kato,et al.  Towards the application of indoor ventilation efficiency indices to evaluate the air quality of urban areas , 2008 .

[12]  M. Sandberg.  What is ventilation efficiency , 1981 .

[13]  D. Carruthers,et al.  Comparisons between FLUENT and ADMS for atmospheric dispersion modelling , 2004 .

[14]  Mattheos Santamouris,et al.  Experimental performance investigation of natural, mechanical and hybrid ventilation in urban environment , 2008 .

[15]  K. F. Fong,et al.  CFD study on effect of the air supply location on the performance of the displacement ventilation system , 2005 .

[16]  R. Britter,et al.  FLOW AND DISPERSION IN URBAN AREAS , 2003 .

[17]  J. Fenger,et al.  Urban air quality , 1999 .

[18]  Mukesh Khare,et al.  Wind tunnel simulation studies on dispersion at urban street canyons and intersections—a review , 2005 .

[19]  M. Sandberg,et al.  Building Ventilation: Theory and Measurement , 1996 .