POLLUTANT DISPERSION AND THERMAL EFFECTS IN URBAN STREET CANYONS

A numerical model has been built to simulate the small-scale atmospher ic flows within the urban canopy, based on the lower atmosphere classi cal assumptions and the ''standard k-epsilon'' two-equation turbulence model. This model is used here to study the flows and vertical exchan ges of pollutants within the street and at the interface with the atmo spheric layer above the roofs, in the asymptotic case of infinitely lo ng street canyons. A thorough study of the influence of the street geo metrical aspect ratio leads to a refinement of the flow separation in three regimes, popularized by Oke (1988, Energy Bldg 11, 103-113), tak ing into account not only the influence of the buildings on the flow o f the surface layer just above the roofs but also the structure of the recirculating flow within the street. The time evolution of pollutant concentration within the street canyon and al the pedestrian level is analysed as a function of the geometry and pollutant doses are presen ted. The number and arrangement of vortex structures within the street canyon largely influence the vertical exchange rates. A preliminary s tudy shows that the differential heating of street surfaces can largel y influence the flow's capability to transport and exchange pollutants . In fact, differential heating can even shift the in-street flow stru cture from one regime to another, e.g., from a one-vortex flow to a fl ow with several contra-rotative vortices. (C) 1996 Elsevier Science Lt d