The influence of buoyancy on flow and pollutant dispersion in street canyons

In this paper, the effect of buoyancy on flow and pollutant dispersion within street canyons is studied by means of computational fluid dynamics simulations. We consider a neutral boundary layer approaching a 3D street canyon assuming a wind direction perpendicular to the street canyon. The Boussinesq hypothesis for incompressible fluids is chosen for modelling buoyancy. We distinguish three cases: leeward, ground and windward wall heating. Thermal effects on both the flow and dispersion are investigated for several Richardson numbers. The analysis focuses on the influence of street canyon geometry on flow and temperature distribution, by considering different aspect ratios W/H canyon between 0.5 and 2, where W is the width and H the height of the street canyon. Three-dimensional effects are observed, depending on L/H, where L is the length of the canyon. Three dimensional effects become negligible for aspect ratio L/H larger than 20. Results obtained for the case with a large Richardson number show that dispersion patterns in a street canyon differ substantially the isothermal case. In case with windward heating large concentration values are found close to the windward wall. Our findings can be of interest for many urban environment applications in which natural ventilation and thermal comfort are being of concern.