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

城市化和道路运输的增加是造成城市环境空气污染和相关健康影响的主要因素。在机动车密度高的地方，由于建筑物的存在，污染物的扩散减少了。其直接后果是，空气污染水平经常超过国家空气质量限制值。此外，交通引起的湍流被认为是污染物扩散的关键因素。然而，当交通引起的湍流被认为是污染物扩散的主要过程时，扩散模型表现出较差的性能。拟议的项目旨在更好地了解污染物在城市环境中扩散所涉及的主要过程。因此，该项目的主要目标是通过高分辨率湍流分辨率大涡模拟来评估车辆引起的湍流和废气对城市街道峡谷中风流和污染物运输的影响。将复杂几何形状的明确移动车辆纳入其中的新方法将在现有风洞数据的基础上进行测试和验证。在第二步中，将对经典的街道峡谷设置进行一套全面的模拟，以评估车辆引起的湍流对气流的影响，以及对个别汽车排放的废气中污染物的扩散。模拟将集中在理想的设置上，但包括峡谷内植被的影响，如路边的树木和树篱，差热的街道和建筑墙壁，以及它们的综合影响，以评估车辆引起的影响的重要性。除了一般的模拟外，还会进行参数研究，例如考虑到平均风向和风速，以及不同的交通密度和车队组成。最后，将根据拟议项目中创建的大量数据集对现有的车辆诱导效应参数化进行审查，并将在第一个案例研究中测试适用于大涡模拟模型的新参数化。

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.