Effects of different ambient wind speeds and aspect ratios on turbulence and pollutant concentration in and above street canyons are numerically investigated and compared using parallelized LES model (PALM) and ((Transition Conditions)) are introduced for the first time. The results show that, increasing of ambient wind speed does not change the shape of streamlines but increases the magnitude of time- and area-averaged turbulent kinetic energy with the rate of square of speed ratios resulting in better ventilation of pollutants. As aspect ratio increases, the magnitude of turbulent kinetic energy and turbulent fluxes especially in lower depths of the canyon decrease sharply causing decreased pollution transport to higher levels. An applied case is between 2 and 4 aspect ratios in which there are two vortices along the depths of the canyon. As the width of the canyon increases, the rate of the upper vortex penetration increases and eventually at aspect ratio 2 makes a transition from two to one vortex; we call such conditions as transition conditions. In transition conditions increasing of less than half a meter of street canyon width will drastically decrease the pollution concentration inside the canyon; this is useful in optimizing canyon dimensions for better ventilation.

数值研究了不同环境风速和长宽比对街道峡谷内外湍流和污染物浓度的影响，并首次使用并行LES模型（PALM）进行了比较，并首次引入了（（过渡条件））。结果表明，环境风速的增加并不会改变流线的形状，而是以速度比的平方比增加时间和面积平均湍动能的大小，从而使污染物更好地通风。随着长宽比的增加，湍流动能和湍流的大小（尤其是在峡谷的较低深度处）急剧下降，从而导致污染物向较高水平的迁移减少。一个应用案例的纵横比在2到4之间，沿着峡谷的深度有两个涡流。随着峡谷宽度的增加，上层涡旋的渗透率也随之增加，最终在纵横比为2时从两个涡旋过渡到一个涡旋。我们称这种条件为过渡条件。在过渡条件下，街道峡谷宽度增加不到半米，将大大降低峡谷内的污染浓度。这对于优化峡谷尺寸以获得更好的通风很有用。