Large-eddy simulation study on the effect of vehicle-induced turbulence and exhaust fumes on wind flow and pollutant dispersion in urban street canyons

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Increased urbanization and road transport are major factors contributing to air pollution and associated health effects in urban environments. On local scale, where motor vehicle density is high, pollutant dispersion is reduced due the presence of buildings. As a direct consequence, air pollution levels often exceed national air quality limit values. Also, traffic-induced turbulence is identified to be a key factor for the dispersion of pollutants. However, dispersion models show poor performance under conditions, where traffic-induced turbulence is considered to be a major process for the dispersion of pollutants. The proposed project aims at acquiring a better knowledge about the principal processes involved when it comes to the dispersion of pollutants in urban environments. The main objective of the proposed project is thus the evaluation of the effect of vehicle-induced turbulence and exhaust fumes on the wind flow and transport of pollutants in urban street canyons by means of high-resolution turbulence-resolving large-eddy simulations. Novel methods to incorporate explicitly moving vehicles of complex geometry will be tested and validated in a first step against existing wind tunnel data. In a second step, a comprehensive set of simulations will be conducted for the classic street canyon setup to evaluate the effect of vehicle-induced turbulence on the wind flow and on the dispersion of pollutants releases from exhausts of the individual cars. The simulations will focus on idealized setups, but include the effects of in-canyon vegetation such as roadside trees and hedges, differentially-heated steets and building walls, as well their combined effect to evaluate the importants of vehicle-induced effects. This general set of simulations will be complemented by parameter studies where, e.g. the mean wind direction and speed as well as different traffic densities and fleet compositions are taking into account. Finally, existing parameterizations for vehicle-induced effects will be reviewed in the light of the extensive data set created in the proposed project and a new parameterization that is suited for large-eddy simulation models will be tested in a first case study.