Research On Downburst Outflow Based On Rough Wall Jet And The Response Of Transmission Line

Downbursts are formed by a strong subsidence airflow violently hitting the ground during thunderstorm weather and propagating through the ground which have the characteristics of local,sudden and short-term.The downbursts are extremely destructive,especially the wind-induced response of large-span flexible structures such as transmission lines in this wind field needs to be focused on.Since the horizontal outflow field of downbursts which is similar with wall jet occupies a wider area,the probability of destructing structure is greater,so studying the wind-induced response of the transmission line in the wall jet wind field can evaluate the security of structure in the downbursts field effectively.In the current study of wall jets,water flow tests with less consideration of wall roughness and mostly with low Reynolds numbers cannot be effectively applied to the actual design of civil engineering.Most of the study on the wind-induced response of transmission lines is considering a stable and uniform wind field,but the study on the response of transmission lines in unsteady wind fields such as downbursts is not enough.So,this paper mainly made the following content:1)By adding a wall jet device to the conventional wind tunnel,the simulation of the wall jet wind field is realized.Considering the effect of wall roughness by paving sandpapers,three outflow speeds are set to study the effect of wall roughness on the characteristics of wall jet wind field under different Reynolds numbers.The effects of roughness are analyzed in terms of vertical average wind profile,turbulence,Reynolds stress,expansion rate,maximum velocity attenuation,and wall friction coefficient.The results show that the presence of rough walls will reduce the maximum wind speed and increase the height of the maximum wind speed in the same place,and change the selfsimilar curve of the vertical average wind profile.the roughness will obviously increase the turbulence near the wall surface,but as the vertical height increases,its effect becomes weaker.The expansion rate and maximum velocity attenuation of rough walls will be significantly greater than those of smooth walls.Roughness will increase the friction coefficient of the wall and the growth rate will increase with the increase of the downstream direction distance.2)The wind-induced response of the transmission line in the downbursts is divided into time-varying average response and dynamic response.Based on the quasi-steady assumption,the method of coordinate transformation is used to derive the calculation formulas of the time-varying average response and the mean square error of pulsation response when the transmission line has different height differences.The vertical profile of the average wind speed is simulated according to the formulas fitted by the test data,and the pulsating wind speed is obtained by superimposing the harmonics through the spatial coherence function and the Kaimal bilateral spectrum given in the Davenport exponential model.The time-history models of time-varying average wind speed and pulsating wind speed can be obtained by the time empirical model which is proposed by Homles.Using the time-history models to calculate the time-varying average response and the mean square error of pulsating response of the transmission line,and analyze the influence of height difference and wall roughness on the response of transmission line.3)Using ANSYS software to analyze the influence of structural geometric nonlinearity and aerodynamic damping on the longitudinal dynamic tension response of transmission lines in the downbursts.It shows that for large-span flexible structures like transmission lines,the effects of structural nonlinearity and aerodynamic damping cannot be ignored.Compared with the frequency domain calculation results in Chapter 3,it is found that the calculation results of time domain and frequency domain are in good coincide.And,considering the influence of height difference of transmission line and wind direction angle,it is found that these two factors have different impact on the structure response of transmission line when the height of the transmission lines changes...