Redai dili (Aug 2023)
Construction of Urban Ventilation Corridors Based on the Spatial Enclosure Index: A Case Study of the Hengyang County
Abstract
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.
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