北京市潜在风道的数值模拟与综合识别

过去从局地尺度和微尺度优化规划和建筑设计的角度,城市规划与建筑学科的研究者提出了构建城市区域通风廊道的思想。但对目前的特大城市和城市群而言,无论城市热岛还是污染物输送都可能涉及更大尺度范围的区域影响。结合自然地理资源条件,对城市外围待发展区域的风道识别和规划,可能更具有现实意义。本文基于空气动力学粗糙长度计算通风指数,从动力学角度初步识别出北京不同区域的通风潜力：① 利用数值模拟输出的1月和7月平均水平风场发现,在背景风较强的冬季,水平风速的分布与下垫面的粗糙度保持高度一致,在北京城区东北方向存在一条明显的风道,在通过城区时受城市下垫面的拖曳影响出现显著的风速下降,在城市下风方向风速又有所回升;② 与热力分析对比发现,夏季城市外的低温区域与盛行风向相悖,偏南方向上的补偿空间面积比冬季小且与作用空间的温差也小,流向城市的可利用风资源匮乏;③ 基于近地面温度和粗糙长度加权计算后得到通风指数,冬季为0~0.25,夏季为0~0.60,数值越小通风能力越强,受季节热力差异影响,冬季通风能力显著优于夏季。④ 进一步结合数值模拟的风速分布,将通风评价结果划分为4个等级,从北京市全域尺度分冬、夏两季识别了北京市的潜在风道,冬季贯穿南北的风道全长约200 km,从城市外围引入风资源,可有效提高城市自净能力,而在背景风较弱的夏季,风道贯通性较差,气流疏导能力弱,亟待区域联动优化城市群发展规划。

Numerical Simulation and Comprehensive Identification of Potential Wind Corridors in Beijing

From the perspective of local and micro scales, construction of ventilation corridor in urban area was proposed by optimized planning and architectural design in previous studies. However, for contemporary megacities and urban agglomerations, both urban heat island and pollutant transport may involve a larger scale of regional impact. It may have more practical significance to integrate regional natural geographical resources to identify and plan ventilation corridors in urban peripheral areas to be developed. Taking Beijing as an example, potential ventilating areas is preliminarily identified by drawing the map of aerodynamic roughness length in this study. First, monthly mean wind fields from numerical simulation outputs of January and July demonstrate that, in winter with strong background wind, the distribution of horizontal wind speed is highly consistent with the distribution of underlying surface roughness. There is an obvious wind path in the northeast of urban Beijing. Wind speed drop significantly when passing through the urban area due to the high surface roughness, and partially recoveries in the urban downstream. Second, thermodynamic analysis suggests that the low temperature area outside the city in summer is contrary to prevailing wind direction; both the size of the compensation space and thermal gradient between the compensation and the action space are smaller than that in winter, hence the available wind resources to refresh the city are scarce. Third, a dimensionless ventilation index is estimated by integrating the weighted surface temperature and roughness length. In Beijing, the index is in between 0 and 0.25 in winter, and is in between 0 and 0.60 in summer. The smaller the value is, the stronger the ventilation capacity. The ventilation capacity in winter is significantly better than that in summer due to the effect of seasonal thermodynamic difference. Fourth, the simulated velocity is further combined to divide the ventilation performances into 4 grades, and the potential ventilation corridors in Beijing are identified in winter and summer, respectively. In winter, the length of the ventilation corridor running through Beijing is about 200 km; it introduces wind resources from the periphery of the city to improve the self-purification capacity of the city. In summer when the background wind is weak, the connectivity of the wind path is poor and the ventilating capacity is weak. Optimizing urban agglomeration planning will be hence practical helpful.