Car exhaust dispersion in a street canyon. Numerical critique of a wind tunnel experiment

Due to increasing car traffic in cities, problems related to car induced air pollution in street canyons have become important. Physical modeling in wind tunnels or numerical codes may be used for dispersion simulation when investigating air quality. Rafailidis [in: Annalen der Meteorologie] carried out an extensive set of test runs recently in the BLASIUS wind tunnel at the Meteorological Institute of the University of Hamburg, Germany. In the present study the wind tunnel experiments were simulated numerically using the CFD-code Fluent(R). In a first approach, the idealized two-dimensional case was calculated. Several test runs have been carried out to study the effect of emission rate and source design on flow structures and dispersion in the street canyon. It could be shown that alternative emission conditions and the source design might affect the concentration field within a modeled street canyon. A second set of calculations for a simplified three-dimensional simulation of the street canyon setup was performed to investigate the presence of secondary flow patterns found during wind tunnel tests. The lateral flow structure within the street canyon observed during wind tunnel measurements was simulated, and the effect of changing boundary conditions on the secondary flow structure was studied. In the paper the advantages of CFD simulations for planning wind tunnel dispersion tests are discussed.