Pollutant dispersion in urban street canopies

The effects of building configurations on pollutant dispersion around stree t canopies were studied numerically. The dispersion of pollutants emitted f rom ground sources was simulated by continuously discharging large number o f particles into the computation domain. The mean wind velocities at each t ime-step were firstly computed by solving the time-dependent incompressible Navier-Stokes equations, while the fluctuated velocities were determined u sing a statistical procedure. The trajectories of the discharged particles were obtained from a Lagrangian particle model. Three categories of numeric al simulation were conducted to study the effect of different canopy geomet ries on the pollutant dispersion. The computed wind field data were consist ent with the wind field characteristics described in the previous wind tunn el studies. A counter-clockwise vortex was found resulting in high pollutan t concentration at the windward side of the downstream building of the stre et canopy and low pollutant concentration at the leeward side of the upstre am building. The increase in height of the urban roughness buildings would facilitate the pollutant dispersion in urban street canopy under certain bu ilding configurations. Two or more vortices stacked vertically in a street canopy were found when height of the upstream and downstream buildings of a street canopy was increased, preventing pollutants from escaping out of th e canopy. (C) 2001 Elsevier Science Ltd. All rights reserved.