Abstract
The propagation of a pollutant emitted from localized sources both within and above a regular street network is studied by analyzing data from direct numerical simulations of passive scalar dispersion. Two wind directions are considered, corresponding to aligned and oblique flow with respect to the street axes. Particular attention is paid to the role of entrainment of the scalar into the urban canopy from an elevated source and re-entrainment of material originally released further upstream from a ground source. The variation of concentration differences and vertical fluxes between the streets and the air above as a function of distance reveals important differences between the rate of lateral and vertical mixing for the two sources. Detrainment and entrainment need a longer fetch to equilibrate for the elevated source than for the ground source. There are large differences between the advection and detrainment velocities for the aligned and oblique cases, so that a change in wind direction could affect ventilation efficiency considerably. Time scales associated with different dispersion processes are computed and the time of first appearance of the scalar from the onset of release in different streets is mapped. It is shown that re-entrainment can provide a shortcut dispersion pathway for reaching certain parts of the network. This is particularly striking in the case of oblique flow, when material can be transferred by entrainment up to twice as rapidly as by advection. Taken together, these results highlight the overall message that vertical exchange is a two-way process and that entrainment needs to be considered in the context of emergency response as well as urban ventilation.
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Acknowledgements
Fernanda Cezana, Elisa V. Goulart and Neyval Costa Reis Jnior gratefully acknowledge funding from National Council for Scientific and Technological Development (CNPq), Capes (Coordination of Superior Level Staff Improvement) - Print Institutional Internationalization Program (88881.311735/2018-01.) 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 for the DIPLOS project (EPSRC Contract No. EP/K040707/1). OC also thanks the Federal University of Espirito Santo (UFES) for funding a two-week visit through their Capes_Print Institutional Internationalization Program.
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Cezana, F.C., Goulart, E.V., Reis, N.C. et al. Analysis of Pollutant Entrainment from Localized Sources in a Street Network. Boundary-Layer Meteorol 179, 241–258 (2021). https://doi.org/10.1007/s10546-020-00598-7
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DOI: https://doi.org/10.1007/s10546-020-00598-7