深街道峡谷内高架桥对污染物传播特性影响的模拟研究

Numerical Simulation on the Effect of Viaduct Settings on the Air Flow and Pollutants Dispersion in the Deep Street Canyon

城市化进程导致在城市中出现通风条件较差的深街谷,建设于深街谷内的高架桥会加重周边街谷内空气污染.用计算流体力学模拟方法(CFD)探索在不同环境风速下的深街谷中,高架桥的高度和宽度对街谷内气流组织与污染物扩散的影响.结果表明:高架宽度小于0.8倍街谷宽度时的高架桥不会抑制桥下空间的流动;桥宽增加会改变桥下空间的涡旋结构和涡旋方向,近地面流动方向由之前的从右至左流动变为从左至右流动,因而桥下空间污染分布也发生明显改变;高架桥宽度的增加导致两侧低层住户受到较大影响,对背风面住户的影响更为明显;但高架宽度为0.5倍街谷宽度的高架桥能对迎风面中层住户造成影响;增加高架桥的高度,其下方污染物浓度增加;当高架桥位于街谷冠层时,下部空间的污染物浓度急剧增加;冠层处及涡旋交界面高架桥对两侧住户产生较大影响,而其他高度高架桥对两侧住户影响不大;随着环境风速的增加,高架桥对近地面源污染物扩散的阻碍作用逐渐减弱.研究显示,深街谷中增加高架桥的宽度、高度都会导致街谷内空气质量的恶化,而高架桥会阻碍因环境风速增加对街谷内空气质量的改善.

The progress of urbanization definitely leads to the occurrence of deep street canyon with poor ventilation condition,and the viaduct erected in deep street canyon absolutely further contributes to air pollution,thus it is imperative for the researchers to attach great importance to the effect of viaduct settings on the air flow and pollutants dispersion in the deep street canyon.Computational Fluid Dynamic(CFD)simulation was employed to investigate the influence of the height and width of viaduct on the air flow and pollutants dispersion in the deep street canyon under different ambient wind speed.The result shows that when the width of viaduct is less than B=0.8D(B is the width of the canyon,D is the width of the viaduct),the viaduct could not depress the flow of air under the viaduct,which is different from the case of shallow street canyon;with the increase of the width of viaduct,the viaduct alter the structure and rotate direction of lower vortex under the viaduct,thus the air near the ground flow from the left side to the right side instead of flowing from right side to left side,and the pollutant distribution under the viaduct is change;the low-rise residents on both sides are highly affected,when the width of viaduct increases;the pollutant concentration beneath the viaduct increases with the height of viaduct;when the viaduct is set in the canopy of street canyon,the pollutant concentration within the canyon dramatically increases;when the viaduct is set in the interface of these two overlapped vortexes,the viaduct can depress the escape of pollutants into the upper vortex,which is different from the case of shallow street canyon;the viaduct located in the canopy and interface can affect the residents living on the both side,but the viaducts of other height have a little effect on these inhabitants;with the increase of ambient wind speed,the depression of viaduct on the pollutant dispersion diminishes gradually.The study shows that the increase of width and height of viaduct can lead to the deterioration of air quality within the deep street canyon,and the presence of viaduct can hamper the improvement of outdoor air quality induced by the increase of ambient wind speed.