Study On Wind Field Characteristics Of Urban Boundary Layer Based On Layered Structure

With the development of urbanization,the complex underlying surface structure in typical urban areas has replaced the original open and flat underlying surface,which significantly affects the motion law of turbulent flow field in urban atmospheric boundary layer and forms a special layered structure.The layered structure of Urban Boundary Layer（UBL）will have many unknown effects on the characteristics of urban wind field,which brings new security and economic challenges to the wind resistant design of wind-sensitive structures.Based on the basic theory and analysis method of atmospheric turbulence,the complex turbulent layered structure of UBL was determined by using the long-term wind speed data obtained from Beijing Meteorological Tower.On this basis,the aerodynamic parameters and fluctuating characteristics of strong wind of UBL were analyzed,and the statistical model of urban measured wind field under the layered structure was established.The main work completed is as follows:（1）The characteristics of mean wind speed and direction of long-term wind speed data in the central urban area of Beijing from 2013 to 2017 were analyzed,and the climate characteristics and the overall distribution characteristics of wind speed samples in the area were described;Based on the analysis of the temporal and spatial variation of temperature profiles in different seasons and the distribution of the overall atmospheric stability,the specific vertical layered structure of UBL in Beijing area has been divided.It has been pointed out that 8m and 16m belong to the Urban Canopy Layer（UCL）;the range below 80m belongs to the Roughness Sublayer（RSL）;the height of 140m is in the Inertial Sublayer（ISL）;and the heights of 200m and 280m are in the Mixed Layer（ML）.（2）Based on Monin-Obukhov similarity theory,the near-neutral strong wind data are selected,and four algorithms are used to fit the wind speed profiles of different structure sub layer combinations,and the values of roughness lengthz₀ and zero-plane displacement z_d are estimated.The results show that the layered structure of UBL has a significant influence on the determination of parameter values,and the wind field profile laws of the RSL,ISL and ML should be considered at the same time.（3）The direct fitting method and Lw method are used to discuss the influence of wind speed and direction on the analysis results.The results show that the parameter values change obviously with the relief of terrain,and decrease with the increase of wind speed,but the range of calculation results is relatively consistent.Then,above terrain parameters were substituted into different wind speed profile models to fit the measured wind profile,which shows that the exponential law model is the most suitable,and its profile index is close to the D-type geomorphology.（4）Based on the layered structure of UBL,the fluctuating turbulence characteristics of near-neutral strong wind field in different sublayers were discussed,including the variation of turbulence intensity,gust factor,power spectral density function of fluctuating wind speed and integral scale.The statistical analysis results were compared with the standard value of structural wind load,and the influence mechanism of layered structure on the fluctuating characteristics of wind field was described in detail.