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Flow mechanics aspects of city planning with respect to the reduction of vehicle emissions

Summary

Dispersion conditions inside street canyons were investigated in an atmospheric boundary - layer wind tunnel, within the framework of the research project "Flow mechanics aspects of city planning with respect to the reduction of vehicle emissions". The consideration of vehicle-induced turbulence presented an important aspect of the study. Series of experiments were designed, with the goal to derive general conclusions about the effect of vehicle-induced turbulence on dispersion of vehicle emissions in urban areas. A law of modeling for vehicle-induced turbulence in a wind tunnel, proposed by Plate in 1982, formed the background of the studies. It was confirmed by the first part of experiments during which a technique of simulating the vehicle motions by small metal plates mounted on belts moving along the street was used in an idealized street canyon with a perpendicular approach flow. Concentration measurements carried out at the canyon walls documented the effect of the vehicle motions on the dispersion patterns. It is necessary to distinguish between situations with two-way traffic and one-way traffic. The second case results in a significant transport of pollutants along the street. It can only be explained by the presence of a mean flow velocity caused by the vehicle motions. For perpendicular approach flow the concentration distribution at the building walls becomes asymmetric with an increase of concentration along the street in the direction of traffic flow. In the case of two-way traffic, the concentration distribution is similar to the reference case without traffic movement, and is approximately symmetric to the center of the buildings. The vehicle movement induces an increase of turbulence which in turn causes a reduction of concentrations. Results from studies with different ratios of vehicle velocity and density to wind velocity verify the similarity criterion derived from the law of modeling by Plate (1982). A comparison of the wind - tunnel data with calculations by numerical dispersion models which take into account the effects of traffic induced turbulence has shown a good agreement for the idealized street canyon situations with two - way traffic. Based upon these results, a study scheme was developed which takes into account the effects of vehicle-induced turbulence for modeling the dispersion conditions in the area around the Hauptstätter Straße in the city of Stuttgart. The entire measurement procedure is documented and the influence of the vehicle movements on the concentration distribution are presented. The results can be expressed as follows:

In comparison to the situation without traffic movement, the gradients of the horizontal concentration distribution are substantially smaller. The differences between the concentrations at the various measuring points, which for the case of non-moving vehicles are caused by flow between and around the buildings, are diminished. A reduction of the concentration levels in the Hauptstätter Straße is observed for higher ratios of vehicle to wind velocity. Finally a comparison of concentration distributions measured in the wind tunnel with predictions by the numerical dispersion models MISKAM and ABC is presented. Unfortunately a comparison with full-scale data was not possible during this project. It should be a goal for future studies to verify the wind - tunnel results against observations in the nature, especially for the situations with one - way traffic.

 


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