基于空间封闭度的城市通风廊道构建——以衡阳县城为例

Construction of Urban Ventilation Corridors Based on the Spatial Enclosure Index:A Case Study of the Hengyang County

南方季风气候夏热冬冷的特殊性,让人们意识到城市在夏季应加强自然通风,冬季则需抵御寒冷的冬季风,而构建城市通风廊道不仅有利于缓解城市夏季的热岛效应、减轻大气污染,还能减少冬季风的侵袭。当前城市风环境研究一般选用数值模拟的方法,其可实现不同情境下的城市风环境模拟研究;但也存在短板,特别是在城市总体规划等大尺度的设计方案初期,受计算机性能限制需要花费大量的时间和精力。文章以湖南省衡阳县城为研究对象,提出通过规则建模的方法建立城市3D模型,并利用ArcGIS空间叠加构建城市空间封闭度综合评价模型。通过该方法计算出城市的潜在通风廊道,并与PHOENICS模拟的风速云图进行验证,结果基本吻合,验证了模型的可行性。研究发现:在设计初期阶段应考虑城市风环境变化,选择规则建模不仅有利于方案的快速建模,还能快捷构建规划方案的通风廊道;城市通风廊道的构建需要考虑多项城市要素,根据风道的不同作用空间在规划方案中构建了8条一级通风廊道和14条二级通风廊道,并比较PHOENICS的风速云图和综合评价结果,证明了由建筑、路网、水体、绿化系统所构建的廊道系统能较好地与通风廊道相重叠,并证明了风口、开敞空间、水域、道路和建筑的优化措施能有效改善规划方案的通风能力;对比单因子与多因子的评价结果,进一步论证了单一标准评价的局限性,而多因子评价更适合复杂的城市风环境;空间封闭度模型不仅考虑了地形高差,还考虑了建筑环境因子(天空开阔度、绝对粗糙度、迎风面积密度、平均高度、建筑密度),为城市通风廊道构建提供了一种更加便捷有效的新方法,有利于城市的绿色低碳建设。

The monsoon climate in southern China is characterized by hot summers and cold winters.These extreme conditions have caught the attention of researchers because of the importance of designing cities so as to strengthen natural ventilation in summer and reduce exposure to cold winds during winter.Not only do the design and construction of urban ventilation corridors lead to the mitigation of the urban heat island effect in summer,the corridors also contribute to reducing atmospheric pollution and the invasion of winter monsoons.This study is specifically concerned with an analytical method that can be used,especially during the planning and design of cities,to improve the urban wind environment.Currently,most urban wind environment studies generally adopt numerical simulations as the investigation method.To some extent,this approach can be helpful with urban wind environment analyses under different planning and design scenarios.Many existing studies indicate that this technique has the advantage of rendering urban wind environment predictions with reliable accuracy.Numerical wind environmental simulations—especially,Computational Fluid Dynamics(CFD)simulation models—have been used for urban planning and design optimization.However,these models have certain limitations about the planning and workflow that the numerical simulation techniques require.For instance,in the early stages of urban design schemes,such as the master planning stage,the workload and amount of time required to perform the necessary repetitive simulations become enormous(not only in terms of time and energy but also with regard to expertise).The thorough and exhaustive numerical simulation task sometimes becomes impossible because of the limitations of computer performance,particularly for large city-scale areas.In this study,Hengyang County,Hunan Province,was selected as the study area.A new method was proposed to reduce the workload,time,and computational equipment requirements for analyzing the wind environment of a large city-scale area during the early stages of urban planning and design.The proposed method starts with the construction of a 3D model of the city using rule-based modeling,followed by a comprehensive evaluation model implemented in ArcGIS to assess the urban spatial enclosure and its influence on urban ventilation corridors.We used the least-cost path to generate ventilation corridors under different wind directions.The measurement points in the ventilation corridors of the city were selected to validate the wind speed cloud map simulated by PHOENICS.The simulation results were basically consistent with the site-measurements,thus demonstrating the validity of the evaluation model proposed in this study.Several findings are derived from this study.First,changes in the urban wind environment should be considered at the early stage of urban design.Second,the choice of rule-based modeling is not only beneficial to the quick modeling of the design scheme but is also efficient for a quick modeling of ventilation corridors.Third,the construction of urban ventilation corridors needs to consider several urban elements.The 8 primary ventilation corridors and 14 secondary ventilation corridors are constructed in the planning scheme according to the different action spaces of the wind corridors.Comparing the wind speed map of PHOENICS and the comprehensive evaluation results,it is proved that the urban ventilation corridor system constructed by buildings,road networks,water,and greenery systems can be quite overlapped with the ventilation corridors,and it also proved that the optimization measures based on wind openings,open space,water,roads,and buildings can effectively improve the ventilation capacity of the planning scheme.The study further demonstrates the limitation of single-criteria evaluation compared to multi-criteria evaluation which is more suitable for the complex urban wind environment.In addition to terrain height differences,the urban spatial enclosure model reflects consideration for building environment factors,such as the sky view factor,absolute roughness,frontal area density,average height,and building density.Thus,we propose a more convenient and effective method for the design of urban ventilation corridors that can be beneficial for achieving green and low-carbon urban planning.