Ventilation Paths Excavation And Design Strategy Of Coastal Mountainous City Based On Frontal Area

In China,the cities are in the process of transforming from high-speed urbanization to high-quality development.In the coming decades,we will also face the double pressure of global warming and aging population.Especially,people in cold regions are not experienced enough in coping with high temperature,and may be more seriously affected by its adverse effects.How to take effective planning control and design countermeasures to mitigate and actively adapt to the adverse effects of climate change has attracted more and more attention from the researchers at home and abroad.Urban ventilation path as an important means of urban planning and design,is a new perspective of scientific quantitative urban planning and management.It can direct clean and low-temperature airflow on the outskirts or on the sea surface to the hot and sultry urban center,thus effectively relieving heat island effect and air pollution,and improving the urban air quality and human health.Currently,most of the research and practice on ventilation path mainly focus on the overall urban planning and macro level,while the study on the block scale and urban design stage,which are more important for pedestrian comfort,is relatively lacking.The situation is hard for planners and architects to use the application of ventilation path in the design of city space form or the street form.The architectural morphology research method selected in this paper based on the FAI(Frontal Area Index)has the advantages of simple calculation,good evaluation effect on pedestrian wind environment,and effective connection with urban planning and design.The purpose of this study is to use FAI to explore ventilation path at the city block scale,evaluating its effect on the improvement of wind environment,and propose corresponding planning and mitigation strategies.In addition,in order to make the research content,research methods and strategies possess representative and feasibility,the research area is seted in a typical coastal mountain city-Xinghai bay area of Dalian,a typical high-density block of 7 km×7 km.The excavation of ventilation path is faced with great challenges:First,how to accurately describe the coupling effects of atmospheric circulation and local thermodynamic circulation(solar wind,the wind valley)on wind environment of the peninsula city;Second,how to accurately describe the impact of mountain on urban wind environment in rugged terrain;Third,how to establish a set of tools that can quickly calculate mass urban information and architectural morphology parameters.Firstly,in view of the above problems,GIS(Geographic Information System)was used to conduct unified processing of the basic urban data(land and sea distribution,terrain,architecture,meteorological observation,etc.),then detailed analysis was made on the background climate conditions,thermal circulation and topographic features of peninsula cities.The results show that the study area is mainly affected by the south sea breeze and the north one,and the massifs mainly as mechanical resistance.The predominant wind direction is north,followed by south.The most frequent wind direction is south in spring and summer and north in autumn and winter.In summer and autumn,the night wind is relatively weak(<3.3m/s),which may form valley wind.Secondly,a new model of FAI of mountain city is proposed to fully and accurately reflect the blocking effect of mountain on urban wind environment.Furthermore,the reduction coefficient of FAI of mountain is proposed to reflect the difference of drag effect between mountain form and building.The calculation method of FAI is improved from two aspects.First,according to the characteristics of coastal mountainous cities,the sea level is taken as the reference surface,and the mountain and buildings are taken as a unified obstacle for FAI calculation.Secondly,CFD(Computational Fluid Dynamics)was used to simulate the different influences of mountain and building morphology over the leeside.According to the CFD results,a new method for calculating the reduction coefficient of mountain FAI was proposed.Thirdly,a set of script tools based on Python-GIS was developed to realize massive urban information processing and rapid calculation of architectural morphology parameters.In the100m×100m grid,FAI maps of south wind and north wind in the study area were calculated.The LCP(Least Cost Path)is used to calculate the ventilation path of south wind and north wind.There are mainly four south ventilation path and four north ventilation path.Considering the influence of thermal circulation on the dominant wind direction and ventilation path,the wind direction of thermal circulation such as valley wind and sea breeze was further analyzed,and the distribution of ventilation path was calculated.Fourthly,in order to verify the calculation results of ventilation path based on building morphology,CFD model and field measurement were used to compare the improvement effect of ventilation path.CFD simulation results show that the average wind speed of the ventilation path is 43%(south)and 18%(north)higher than that of perimeter zone.Field measurement were carried out on south wind and north wind respectively to further verify the wind speed improvement effect of the ventilation path.The field measurement results show that the average wind speed of the ventilation path is 100%(south)and 112%(north)higher than that of perimeter zone.These verification results show that the ventilation path excavated by FAI and LCP have sufficient credibility and consistency compared with the actual situation,which is helpful for planners and architects to excavate and evaluate the ventilation path.Finally,in order to connect the excavation results with the urban design,the planning and mitigation strategies of ventilation path layout,architectural form and landscape design are proposed.Through the superposition of ventilation paths,the control guidelines of ventilation path control scope,control width,street layout and other construction activities are carried out,and the corresponding architectural design strategies are put forward.Based on the superposition of the measured urban temperature field and the distribution of ventilation path,the thermal environment of the ventilation path and the urban built-up area is quantitatively evaluated,and the greening mitigation strategy is proposed.So as to improve the overall urban wind environment quality,improve the summer high temperature and winter haze,reduce the heat island effect,improve the environmental comfort.Through the study of wind environment of typical high-density urban blocks,this paper statistically analyzes and calculates the data of climate characteristics and building forms,and explores urban ventilation paths,which can be used as the quantitative basis for urban construction and development.The improved FAI model adopted in this paper,the new method of incorporating mountain FAI reduction coefficient and the analysis of climate and topographic features of coastal mountain cities,are of important reference value for the excavation of ventilation path in the areas mainly affected by land and sea breeze and complicated terrain,and provide strong planning guidance for the further construction of ecological livable cities.