Dispersion of a Passive Scalar Within and Above an Urban Street Network
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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).
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