The University of Hong Kong, Department of Mechanical Engineering, Pokfulam, Hong Kong, China.
Sci Total Environ. 2012 Feb 1;416:362-73. doi: 10.1016/j.scitotenv.2011.12.016. Epub 2012 Jan 5.
This paper analyses the contribution of mean flow and turbulence to city breathability within urban canopy layers under the hypothesis that winds from rural/marine areas are sources of clean air (inhale effect) and main contributors to local-scale pollutant dilution (exhale effect). Using Computational Fluid Dynamics (CFD) simulations, several idealized long streets flanked by tall buildings are investigated for wind flow parallel to the street axis. Aspect ratios (building height/street width) ranging from 2 to 4 and street lengths ranging from neighborhood scales (1km in full scale) to city scales (10km in full scale) are analyzed. To assess the inhale effect, the age of air concept is applied to quantify the time taken by a parcel of rural/marine air to reach a reference location within the urban canopy layer. To simulate the exhale effect, removal of pollutants released from a ground level source is considered. Numerical results agree with wind tunnel observations showing that a bulk portion of rural/marine air enters the streets through windward entries, a smaller part of it leaves through street roofs and the remaining fraction blows through the street aiding pollutant dilution. Substantial differences between neighborhood-scale and city-scale configurations are found. For neighborhood-scale models, pollutant removal by rural/marine air is mainly associated to mean flow along the streets. Breathability improves in streets flanked by taller buildings since in this case more rural/marine air is captured inside canyons leading to stronger wind along the street. For city-scale models, pollutant removal due to turbulent fluctuations across street roofs competes with that due to mean flows along the street. Breathability improves in streets flanked by lower buildings in which less rural/marine air is driven out and pollutant removal by turbulent fluctuations is more effective. Based on these findings, suggestions for ventilation strategies for urban areas with tall buildings are provided.
本文假设从农村/海洋地区吹来的风是清洁空气的来源（吸入效应），并对局部尺度的污染物稀释做出主要贡献（呼出效应），分析了在城市冠层下，平均流和湍流对城市透气性的贡献。利用计算流体动力学（CFD）模拟，研究了几个理想的长街道，街道与建筑物平行，建筑物高度与街道宽度之比（建筑高度/街道宽度）从 2 到 4 不等，街道长度从街区尺度（全尺度 1km）到城市尺度（全尺度 10km）不等。为了评估吸入效应，应用空气龄概念来量化农村/海洋空气到达城市冠层内参考位置所需的时间。为了模拟呼出效应，考虑从地面源释放的污染物的去除。数值结果与风洞观测结果一致，表明大部分农村/海洋空气通过迎风入口进入街道，一小部分空气通过街道屋顶排出，其余部分空气通过街道吹入，有助于污染物稀释。发现街区尺度和城市尺度的配置之间存在很大差异。对于街区尺度模型，污染物的去除主要与街道上的平均流有关。在两侧建筑物较高的街道上，透气性会提高，因为在这种情况下，更多的农村/海洋空气被吸入峡谷，导致街道上的风速更强。对于城市尺度的模型，由于街道屋顶上的湍流波动引起的污染物去除与沿街道的平均流引起的去除竞争。在两侧建筑物较低的街道上，透气性会提高，因为在这种情况下，较少的农村/海洋空气被排出，而湍流波动引起的污染物去除更为有效。基于这些发现，为高层建筑城市的通风策略提供了建议。
