Study On The Influence Of Topography On The Characteristics Of Near Surface Wind Field

Understanding the wind field under undulating terrain is of great significance for weather forecast,wind power generation,air pollution prediction and power transmission.In this paper,the wind tower measurement and CFD numerical simulation are used to analyze the near surface three-dimensional wind field on Nan’ao Island,Shantou City,Guangdong Province,and reveal the wind characteristics under the real terrain.The main contents and conclusions are as follows:Firstly,the 100-meter wind pressure field generated by 16 wind directions in the neutral atmospheric junction at 10～500 m of Nan’ao Island is simulated;Using the gradient observation data of 5 wind towers,a set of technical process for verifying and evaluating the CFD results of the actual terrain is proposed.The evaluation results show that in each observation level,the CFD simulated wind speed is only 0.3～0.5 m?s^-1 greater to that of wind tower.Further,the bias of numerical simulation is analyzed by four factors.The analysis results show that for the observation points with different altitude,slope position and slope angle,the numerical calculation shows good results.However,the CFD simulation does not take thermal effect into account,so there are some errors in the coastal area.Therefore,the numerical calculation grid points within 1.2km from the coastline are eliminated in this paper.Then,by calculating the terrain parameters of slope angle and wind speed ratio,the influence of real undulating terrain with different grid spacing and slope angle on wind characteristics is explored.When going uphill,the maximum wind field acceleration near the ground can reach 45%;When going downhill,the maximum wind field deceleration can reach 67%.When the slope angle is small（|α|<7°）,the downhill can produce 15%wind field acceleration effect.In addition,the acceleration and deceleration effect is significantly affected by grid spacing.Larger grid makes bigger effect,but the 1.6 km grid spacing will smooth of the effect.At the same time,the"critical slope angle"also decreases with the grid spacing.Under the fine grid spacing of 100 m,the critical slope angles of uphill and downhill are 27°and-25°respectively.When the terrain slope angle is less than the critical slope angle,the larger the slope angle is,the more significant the effect of the wind field is;When the slope angle is greater than the critical slope angle,the acceleration and deceleration effect of the wind field decreases with the increase of the slope angle.Finally,the comparison between the real terrain wind characteristics and the research results is carried out.By comparing the wind characteristics of ideal terrain,it is found that the current wind speed ratio obtained from simplified terrain simulation is not suitable for the case of thin grid spacing（within 800m）,and its calculation result is about 5%～20%smaller than that of real terrain.By comparing the wind speed ratio criteria formulated in the building structure load codes of various countries,it is found that the Chinese code can provide a sound prediction for the surface 10m wind,but its result for high-level wind field is not reliable.Because the distribution of wind speed in the vertical direction is more in line with the index rate formulated by the American and Canadian specifications than the linear decline considered in the Chinese specifications.And the vertical range of the wind field affected by terrain is much larger than that considered in current specifications.To sum up,the wind characteristics obtained from the CFD simulation of Nan’ao Island is closer to the actual situation.This paper provides important data support for improving the structural wind resistance design specification under complex mountainous conditions.It has important practical significance in wind disaster early warning and risk assessment.