Study On Aerodynamic Characteristics Of Cantilevered Circular Cylinders And Transmission Towers Composed Of Circular Section Members

For towers composed of circular-section members,the aerodynamic coefficients are sensitive to the flow conditions.Therfore it is of great significance to esitimate the aerodynamic coefficients accurately.Researches on aerodynamic characteristics of a cantilevered cirlular cylinder,cantilevered circular cylinders in a tandem arrangement and transmission towers composed of circular-section members were carried out.The experiment results are useful for the design of transmission towers composed of circular-section members.A cantilevered cirlular cylinder with L/D=2 was manufactured and wind tunnel tests were carried out in a smooth flow and in a uniform turbulent flow both with Reynolds number ranging from 1.04 ×105 to 6.09 × 105.Analysis is focused on the influence of Reynolds numbers and turbulence intensity upon aerodynamic coefficients and the experimental results are compared with the research results by other scholars.The results show that there is difference between the pressure distributions in the rear side of the cantilevered cylinder and those in the rear side of 2D circular cylinder,which is reduced in the supercritical regime.The results in the smooth flow show that:1)in the subcritical regime,the drag coefficients for the cantilevered cirluar cylinder are smaller than those for a 2D circular cylinder;2)in the critical regime,the drop of drag coefficients grow sharper with increasing aspect ratios;3)in the supercritical regime,the drag coefficients decrease with increasing aspect ratios.Flow around two cantilevered circular cylinders in a tandem arrangement were also carried out in the smooth flow and in the uniform turbulent flow.The effects of the gap spacing between the two cylinders were focused on.In the smooth flow with subcritical Reynolds numbers,the upstream cylinder is influenced by the downwash and the drag coefficients are smaller than those of the upstream cylinder with infinite aspect ratio.The interference factors of the downstream cylinder increase monotonously with extending gap spacings and the values are smaller than 1 for all the gap spacings.In the smooth flow with supercritical Reynolds numbers,the measured values of drag coefficients for the tandem cylinders are smaller than those 2D tandem circular cylinders.In the flow with a high turbulence intensity or supercritical Reynolds numbers,the interference factors of the downstream cylinder vary almost the same with increasing gap spacings and come close to 1.0 at large gap spacings.Sectional test models composed of circular section members were constucted and wind tunnel tests based on high-frequency-force-balance technique were carried out in a smooth flow and a uniform turbulent flow.The overall.drag coefficients for transmission towers,shielding factors and drag coefficients for single frames were measured and analyzed.The results show that high intensity turbulence reduces the drag coefficients for single frames and increases the shielding factors,which results that the overall drag coefficients for transmission towers measured in the smooth flow and in the uniform turbulent flow are almost equal.The drag coefficients for single frames suggested by Chinese codes(DL/T 5254-2010)are smaller than those measured in this paper,particularly when the solidity ratio is small.It is suggested that Chinese codes should consider the effect of solidity ratios on drag coefficients for single frames and increase the values of drag coefficients for single frames.This paper also provides some suggestions on the shielding factors because the values of shielding factors suggested by Chinese codes are larger than those measured in this paper and also larger than the values suggested by British codes.Based on the high frequency force balance technique,power spectrums of along-wind loads on sectional test models with different solidity ratios and aspect ratios were analysed.In the same flow conditions,the normalized spectrums of along-wind loads are not sensitive to the solidity ratios and aspect ratios(b/a)for the reason that the fluctuating wind loads are caused by the velocity fluctuatons.The normalized power spectrums of along-wind loads are simulated by applying least square method and achieve perfect results.