Modelling of vehicle-induced turbulence in air pollution studies for streets

Vehicle-induced turbulence can be an important factor of pollutant dispersi on in urban areas, especially under conditions of low wind speeds which are typical for street canyons. An experimental concept (Plate, 1982) for mode lling the effects of vehicle-induced turbulence was applied in the present study. The movement of vehicles was simulated in a boundary-layer wind-tunn el by small metal plates mounted on two belts moving along a modelled stree t canyon. The scaling factor was based on the ratio of turbulence productio n by cars to that by wind flow. The traffic was represented by the velocity , density, frontal area and drag coefficients of the vehicles. The velocity and traffic density were varied, and the influence of the vehicle-induced turbulence on concentration patterns at the canyon walls was studied. It wa s found that concentration decreases with an increasing ratio of vehicle to wind-velocity and with an increase of traffic density. A dimensionless com bination of vehicle to wind-velocity ratio and density factor was proved to be a universal parameter describing the dependence of the concentration on vehicle-induced turbulence. The wind-tunnel measurements were compared wit h predictions by the numerical Operational Street Pollution Model (OSPM; He rtel and Berkowicz, 1989a). Differences between the wind-tunnel and numeric al results regarding effects of vehicle-induced turbulence are discussed. T he comparison revealed general agreement between wind-tunnel and numerical data. Turbulence and concentration measurements in a street canyon in Copen hagen have been additionally employed for analysis of the model results.