Numerical Modeling And Analysis Of Urban Wind Environment In Xianyang City

The city's well-ventilated environment can effectively reduce the city's haze and heat island effects.In recent years,due to the rapid development of social economy and the shortage of urban land,dense high-rise buildings gradually become the main features of the city,the resulting urban air environment caused a high degree of social concern.This subject was proposed by Xianyang City Planning and Design Institute.In the urban construction planning,the urban ventilation environment as an important part of the development of the program.Through the numerical simulation of the urban current situation and the preliminary planning of the urban ventilation environment to comprehensive and clear understanding of the current situation of urban ventilation and what level may be achieved under planning conditions,which is helpful for urban planning.In view of this situation,the study of ventilation in a single building or small scale area cannot reflect the overall situation of urban ventilation environment,and it is necessary to establish a large scale model of the city to study the whole wind environment of the city.The key problem needs to be addressed of the research task was establishing geometric model of the city panorama under the condition of incomplete urban construction information and establishing numerical calculation model of the large scale urban wind field with the original size and basically the original appearance.As the current domestic and foreign basic did not do such a scale of the wind field simulation,so this is the innovation of the subject.In this paper,we established a numerical model of the large scale wind field in the city based on the urban ventilation problem in Xianyang City.On this basis,the characteristics of velocity distribution,pressure distribution and temperature distribution of urban wind field were studied.This paper first establishes the full-size geometric model of Xianyang urban area with the help of Google earth satellite map and field investigation and according to certain integration principle.Based on the basic flow field characteristics of the urban ventilation,the RNG k-? turbulence model was selected and a large-scale numerical model of the wind field was established.The boundary of the velocity entrance of the atmospheric boundary layer was determined and the method of thermal response test of rock and soil was used to determine the boundary of 20 m constant temperature layer and the thermodynamic parameters of rock and soil.Based on the numerical model established in Xianyang City,the urban wind field was calculated and analyzed.The main contents of this paper are as follows: the velocity vector field is analyzed from the horizontal and vertical directions,respectively.The wind speed of each city district was compared and analyzed.The pressure distribution,temperature distribution and ventilation rate of the each area were analyzed.The results show that: a,at 2.4 m/s wind speed,the urban ventilation in the cold and hot conditions is controlled by the natural winds of the dominant wind direction,and the average flow rate in the hot conditions is slightly larger than that in the cold condition,the relative difference is 8.97%.At 0.5 m/s wind speed,the cold and hot conditions of the city-intensive areas of the wind speed is small,do not see the dominant wind direction,in the upper reaches of the city have a clear upward airflow,and the city has a wind around;For the hot conditions,the horizontal direction of the incoming air flow is much larger than the horizontal outflow in each partition.And the hot conditions of the region's average flow rate is much larger than the cold conditions,the relative difference is 34.33%.Which shows that the impact of hot air on the wind field is relatively larger with small wind speed.b,The influence of building wake is very long,at 90 m high,there are still before and after the superposition of the wake,that is,The flow around buildings of upper wind side will affect buildings downwind side.c,when the natural wind into the city,the city entrance and the road that in the same direction as the flow has a decisive role in ventilation.Xianyang City,the main line of urban ventilation is the east-west biased south direction of the road.d,the ventilation rate of each case comparison: for 2.4 m/s wind speed,cold and hot conditions,the same area of the two conditions of ventilation rate is very close and the relative difference in the overall average of the city mean value per unit area flow rate is 7.9%.For the 0.5m/s wind speed,cold and hot conditions,it is clear that the ventilation rate in the cold condition is much smaller than the thermal condition and the relative difference in the overall average of the city mean value per unit area flow rate is 56%.e,the temperature field analysis: the volume of 200 m height within the space of the average temperature,the maximum temperature of 35.498°C at 2.4 m/s wind speed hot conditions,high temperature area in the city's downwind side of the building-intensive area;0.5 m/s wind speed of the hot conditions,the maximum temperature reached 35.858°C,high temperature area in the middle of the city.In this paper,using the established numerical calculation model,the calculation of urban wind field was carried out for Xianyang city planning scheme.Get the following conclusions: the planning layout of the city streets opened up the north-south and east-west air-duct which improve the city's ventilation environment.Overall,in the dominant wind direction,north-south direction of the ventilation is better than the east-west direction;the urban area of the western part of the new town area is much better than the eastern part of the old city.This study has provided some help for the development of urban planning scheme in Xianyang City.At the same time,it has accumulated some experience in the large scale numerical modeling of urban ventilation environment.The research results also have some guiding significance to how to improve the urban ventilation environment.