Influence of building density and roof shape on the wind and dispersion characteristics in an urban area: A numerical study

The Computational Fluid Dynamics code CFX-TASCflow is used for simulating t he wind flow and pollutant concentration patterns in two-dimensional wind-t unnel models of an urban area. Several two-dimensional multiple street cany on configurations are studied corresponding to different areal densities an d roof shapes. A line source of a tracer gas is placed at the bottom of one street canyon for modelling street-level traffic emissions. The flow field s resulting from the simulations correspond to the patterns observed in str eet canyons. In particular and in good agreement with observations, a dual vortex system is predicted for a deep flat-roof street canyon configuration , while an even more complex vortex system is evidenced in the case of slan ted-roof square street canyons. In agreement with measurement data, high po llutant concentration levels are predicted either on the leeward or the win dward side of the street canyon, depending on the geometrical details of th e surrounding buildings.