Influence of shrub hedge on CO diffusion in elevated bridge street canyons
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摘要:
为探究灌木篱墙布局对高架桥街道峡谷内部流场及污染物扩散的影响，将灌木篱墙假设为多孔介质，采用标准k-$ \varepsilon $模型结合组分运输方程对布置中央灌木篱墙和两侧灌木篱墙的高架桥街谷的底部和桥面道路机动车排放的CO扩散过程进行模拟，并与风洞试验结果比较。结果表明：数值模拟方法可靠，且与标准街谷不同，高架街谷中的灌木篱墙不仅不会阻碍流场运动，还能加强底部受阻风场与上部风场的对流，有利于底部道路排放污染物的扩散，能够有效降低街谷的整体污染物浓度，改善污染物在背风侧、桥体上方和底部集聚的问题。两侧均设置篱墙时风场的上下对流更强，背风面壁面处CO质量分数降低60%，增设灌木篱墙可作为改善其内部空气污染程度的有效措施。
Abstract:To investigate the influence of shrub hedge layout on the flow field and pollutant dispersion in elevated bridge street canyons, the shrub hedges were assumed to be porous media, and the standard k-ε model was used in combination with the species transport equation in the simulation. The diffusion process of CO emitted from motor vehicles on the bottom and bridge deck of the elevated bridge street canyons with central two-side shrub hedges was simulated separately. The simulation results were compared and validated with wind tunnel test ones. The results indicate that this numerical simulation method is reliable, and unlike standard street canyons, the shrub hedges in elevated bridge street canyons not only do not hinder the movement of the flow field, but also enhance the convection between the blocked wind field at the bottom and top. This is beneficial for the dispersion of pollutants emitted from the bottom of the road, and can effectively reduce the overall pollutant concentration in the street canyon and improve the issues of pollutant accumulation on the leeward side, above the bridge deck, and at the bottom. When hedges are set on both sides, the convection between the upper and lower wind fields is stronger, and the CO concentration at the leeward wall is reduced by 60%. Therefore, adding shrub hedges can be an effective measure to reduce the level of air pollution in elevated bridge street canyons.
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Key words:
- elevated bridge /
- shrub hedge /
- street canyon /
- pollutant diffusion /
- numerical simulation
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图 3 数值模拟与风洞试验CO无量纲浓度对比
Figure 3. Comparison of dimensionless concentrations between numerical simulation and wind tunnel experiment for CO
图 4 不同布局的街谷内部风速云图
Figure 4. Wind speed contour maps inside street canyons with different layouts
图 5 不同布局的街谷内部流线
Figure 5. Streamline maps inside street canyons with different layouts
图 6 不同布局的街谷内部CO质量分数分布云图
Figure 6. Cloud maps of CO mass fraction distribution inside street canyons with different layouts
图 7 不同布局和污染源位置的街谷内部CO质量分数分布云图
Figure 7. Cloud maps of CO mass fraction distribution inside street canyons with different layouts and pollution source locations
图 8 背风侧建筑物壁面处CO质量分数垂直分布
Figure 8. Vertical distribution of CO mass fraction at leeward building facades
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