Study On Time-space Characteristics And Superimposed Models Of Disturbance Factors Of Wind-thermal Environment At Street Canyon Scale

In recent years,with the acceleration of urbanization,urban population is increasing year by year.Urban air environment quality directly affects the safety,health and comfort of most people.Urban street canyon is an area where indoor environment,outdoor environment and natural environment interact,and its internal wind and thermal environment has attracted more and more attention.The wind and thermal environment in the street canyon is not only directly related to the energy consumption of surrounding buildings and the comfort level of personnel,but also affects the urban heat island effect and the diffusion of pollutants.Therefore,It is very important to systematically study the formation mechanism and influencing factors of the wind and heat environment in the street canyon.The wind and thermal environment of street canyon is formed under the joint action of internal and external disturbance factors.External disturbance factors can be divided into "wind" and "heat" according to the source of the driving force.The wind disturbance mainly refers to the air flow outside the street valley space,and the heat disturbance mainly refers to the heat outside the street canyon air field.These two extrinsic disturbances are formed under the combined effects of atmospheric environment,mesoscale characteristics of cities and human activities,and have complex time variability.In order to explain the spatio-temporal variation characteristics of external disturbance factors and their influence on the wind-heat environment of street canyon,this study selected an east-west street canyon in Weishui Campus of Chang ’an University in Xi’an City as the object to carry out a long period continuous field measurement of the wind-heat environment of street canyon in winter and summer.The time-space characteristics of natural wind and surface temperature in the street canyon were explored through statistical analysis of the parameters of the natural wind flowing across the street canyon,the wall of the building facing the street and the surface temperature of the street canyon.The results show that the street valley flow natural wind has the characteristics of random pulsation.The high frequency range of the fluctuation is between positive and negative average wind speed,and the fluctuation range of the pulsation wind varies with the change of average wind speed.The greater the average wind speed,the more intense the pulsation.There are two typical forms of hourly change of natural wind: gradual change and step change.The wall temperature of buildings facing the street and the ground temperature of street canyons are significantly affected by meteorological conditions.The same weather in different seasons and different weather in the same season show different changing rules.Under sunny conditions in summer,the daily peak value of surface temperature is higher than that of the north wall than that of the south wall.In winter,the daily peak temperature of the north wall is higher than that of the ground than that of the south wall.Compared with sunny weather conditions,the daily peak values of three surface temperatures are closer under cloudy weather conditions.Through the statistical analysis of the diurnal variation of surface temperature under different weather conditions in different seasons,it is found that under the weather conditions in summer,the variation of surface temperature with time in the daytime conforms to the law of sine function,and the variation of temperature with time in the south wall and the north wall conforms to the law of first and second function,respectively.Under sunny weather conditions in winter,the variation of three surface temperatures with time in daytime conforms to the law of sine function.The variation of three surface temperatures with time in winter cold wave meteorological daily period conforms to the rule of primary function.Based on the analysis results of natural wind and street canyon surface temperature,the unsteady variation characteristics of street valley wind thermal environment under different superimposed modes of natural wind and surface temperature were calculated and analyzed by CFD numerical simulation technology.The results show that the surface temperature has an important effect on the formation and development of the flow field in the street canyon.The comprehensive thermal buoyancy generated by the surface temperature is the same as the direction of the circulation in the street canyon.The circulation is strengthened and the wind speed near the surface is higher.When the comprehensive thermal buoyancy generated by the surface temperature is in the opposite direction to the circulation in the street canyon,the circulation is inhibited,and the complex multi-vortex structure may be formed in the street canyon.