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.

[1]  S. Dalziel.  Decay of rotating turbulence: some particle tracking experiments , 1992 .

[2]  Ruwim Berkowicz,et al.  OSPM - A Parameterised Street Pollution Model , 2000 .

[3]  Mathias W. Rotach,et al.  On the influence of the urban roughness sublayer on turbulence and dispersion , 1999 .

[4]  H. Schlichting.  Boundary Layer Theory , 1955 .

[5]  Stuart B. Dalziel,et al.  Rayleigh-Taylor instability: experiments with image analysis , 1993 .

[6]  O. Burggraf.  Analytical and numerical studies of the structure of steady separated flows , 1966, Journal of Fluid Mechanics.

[7]  H. Görtler,et al.  Berechnung von Aufgaben der freien Turbulenz auf Grund eines neuen Näherungsansatzes . , 1942 .

[8]  Mathias W. Rotach,et al.  A wind tunnel study of organised and turbulent air motions in urban street canyons , 2001 .

[9]  Lionel Soulhac.  Modélisation de la dispersion atmosphérique à l'intérieur de la canopée urbaine , 2000 .

[10]  Ari Karppinen,et al.  A measurement campaign in a street canyon in Helsinki and comparison of results with predictions of the OSPM model , 2001 .

[11]  A. M. Dhanak,et al.  Momentum Transfer in Turbulent Separated Flow Past a Rectangular Cavity , 1966 .

[12]  Sandrine Anquetin,et al.  Pollutant dispersion and thermal effects in urban street canyons , 1996 .

[13]  M. Nallasamy,et al.  On cavity flow at high Reynolds numbers , 1977, Journal of Fluid Mechanics.

[14]  M. Rotach.  Profiles of turbulence statistics in and above an urban street canyon , 1995 .

[15]  F. T. Depaul,et al.  Measurements of wind velocities in a street canyon , 1986 .

[16]  G. Batchelor,et al.  On steady laminar flow with closed streamlines at large Reynolds number , 1956, Journal of Fluid Mechanics.

[17]  Walter F. Dabberdt,et al.  Kinematics and dispersion characteristics of flows in asymmetric street canyons , 1988 .

[18]  R. Schiestel,et al.  Prevision numérique de la convection forcée turbulente dans une cavité bidimensionnelle entrainée , 1986 .

[19]  R. G. Harrison,et al.  Modified street canyon flow , 1998 .