Dispersion of a Passive Scalar Within and Above an Urban Street Network
- 333 Downloads
The transport of a passive scalar from a continuous point-source release in an urban street network is studied using direct numerical simulation (DNS). Dispersion through the network is characterized by evaluating horizontal fluxes of scalar within and above the urban canopy and vertical exchange fluxes through the canopy top. The relative magnitude and balance of these fluxes are used to distinguish three different regions relative to the source location: a near-field region, a transition region and a far-field region. The partitioning of each of these fluxes into mean and turbulent parts is computed. It is shown that within the canopy the horizontal turbulent flux in the street network is small, whereas above the canopy it comprises a significant fraction of the total flux. Vertical fluxes through the canopy top are predominantly turbulent. The mean and turbulent fluxes are respectively parametrized in terms of an advection velocity and a detrainment velocity and the parametrization incorporated into a simple box-network model. The model treats the coupled dispersion problem within and above the street network in a unified way and predictions of mean concentrations compare well with the DNS data. This demonstrates the usefulness of the box-network approach for process studies and interpretation of results from more detailed numerical simulations.
KeywordsDispersion model Street network Urban dispersion
Elisa V. Goulart gratefully acknowledges funding from National Council for Scientific and Technological Development (CNPq) and Espirito Santo Research Foundation (FAPES), Brazil. Omduth Coceal gratefully acknowledges funding from the Natural Environment Research Council (NERC) through their National Centre for Atmospheric Science (NCAS) under Grant No. R8/H12/83/002 and from the Engineering and Physical Sciences Research Council (EPSRC Contract No. EP/K040707/1).
- Goulart E (2012) Flow and dispersion in urban areas. PhD thesis, University of ReadingGoogle Scholar
- Hilderman T, Chong R, Kiel D (2007) A laboratory study of momentum and passive scalar transport and diffusion within and above a model urban canopy. Final report Contract Report DRDC Suffield CR 2008-025Google Scholar
- Pasquill F (1962) Atmospheric diffusion. The dispersion of windbourne material from industrial and other sources. D. van Nostrand Company Ltd, New YorkGoogle Scholar
- Soulhac L (2000) Modelisation de la dispersion atmospheric a l’interieur de la canopee urbaine. PhD thesis, Ecole Centrale de LyonGoogle Scholar
- Soulhac L, Salizzoni P, Mejean P, Didier D, Rios I (2012) The model SIRANE for atmospheric urban pollutant dispersion: part II, validation of the model on a real case study. Atmos Environ 320–337:49Google Scholar