Numerical and experimental modelling of pollutant dispersion in a street canyon

Abstract

The pollutant dispersion in a two-dimensional street canyon is studied in this project. The principal parameter investigated is the height of the downstream building. The pollutant source is situated in the middle of the street. The investigation is performed in two ways. Experiments have been carried out in the L-2B wind tunnel at von Karman Institute and numerical simulations have been done with the CFD software Fluent 5.2. The concentration measurements have been performed by means of light scattering technique and the velocity field has been measured with particle image velocimetry. In the numerical simulations, a preliminary study about the backward-facing step has been performed in order to select the best turbulence model in Fluent for these complex flows characterized by separation, stagnation, recirculation, reattachment, etc. The best model appeared to be the realizable k– ε model with the two-layer zonal approach to the wall, which predicts the reattachment length after the step with <1% error in comparison with the value obtained from direct numerical simulation by Le, Moin and Kim (Direct numerical simulation of turbulent flow over a backward-facing step, Report No. TF-58, 1996). This model has been applied in the street canyon simulations. Comparison with the experimental results has been made. Besides the height of the downstream building, the influence of a third building situated upstream of the street canyon in the flow and dispersion inside the street has been investigated.