Dispersion mechanisms in a street canyon

In this article, we investigate experimentally and analytically the dispersion mechanisms of a passive tracer in a two-dimensional model of a street canyon. The principal concern is the concentration transfer between the street and the external flow. In contrast to previous studies, the mass fluxes are not only inferred from mean concentration measurements but also directly measured thanks to a Particle Tracking Velocimetry technique. Visualizations of the evolution of the concentration field show the role of the shear layer at the top of the street canyon. Analytical transfer and dispersion models are derived, demonstrating the importance of external turbulence properties on the transfer. Those models are in excellent agreement with the measurements. The results presented in this article strongly suggest that the transfer in a street canyon does depend on the structure of the incoming turbulence, i.e. on the local stability conditions and on the upwind buildings. (C) 2003 Elsevier Science Ltd. All rights reserved.