Experimental and numerical study on the aerodynamic characteristics of steel tubular transmission tower bodies under skew winds

https://doi.org/10.1016/j.jweia.2021.104678Get rights and content

Highlights

  • The effect of wind velocity and turbulence intensity on the aerodynamic coefficient was investigated by wind tunnel test.

  • The variation of the drag coefficient with solidity ratio and tilt angle was obtained by CFD numerical simulation.

  • The modified calculation formulas for skewed wind load factor and tilted wind load factor were given.

  • The coupling influence of yaw and tilt angle on the wind load characteristics was given in the combined wind load factor.

Abstract

A study of the aerodynamic characteristics of circular steel tubular lattice tower bodies in an ultrahigh-voltage direct-current transmission tower under skew winds is presented. The objective of the study, which was based on experimental and numerical methods, was to investigate the aerodynamic coefficients of tower bodies under both yaw and tilt angles. Scaled rigid models of tower bodies with a scale ratio of 1:30 were tested, and the influence of the wind velocity and turbulence intensity on the aerodynamic coefficient was examined. Furthermore, a modified formula for the skewed wind load factor is proposed and compared with different standards. The aerodynamic coefficients of tower bodies with different solidity ratios and tilt angles were obtained via numerical simulation. Additionally, a recommended calculation formula of tilted wind load factor is proposed. The results show that the drag coefficients calculated by standards for different yaw angles were significantly underestimated and that the wind direction corresponding to the predicted maximum drag coefficient was inconsistent with observed wind direction. The formulas proposed in this study accurately describe the characteristics of wind load factor under different yaw and tilt angles. In particular, the influence of the tilt angle on the drag coefficient cannot be ignored.

Introduction

With the rapid development of the economy and the increasing demand for electricity, the design and maintenance of ultrahigh-voltage direct-current (UHVDC) transmission line systems has assumed considerable importance worldwide. In China, with the gradual reduction of the urban-rural divide, the focus of transmission tower construction is gradually shifting from developed cities to remote mountainous areas where infrastructure construction is relatively poor. It can be predicted that transmission towers will increasingly be constructed on steep mountainous terrain. For realizing a tower that could stand upright in high-slope mountain sections with minor environmental impact, Gan et al. (2020) designed the tower legs to have unequal heights and introduced additional transition sections in the tower. Apart from the disadvantage posed by the unevenness of the terrain, the effect of the skew wind, directed parallel to the mountain slope, should be considered in the design of transmission towers. Lou et al. (2020) and Shen et al. (2020) stated that climbing airflow in mountainous areas generates an updraft, and vertical wind speeds cannot be ignored. In the design of transmission towers for mountainous areas, the influence of the skew wind load, including the vertical wind component, cannot be ignored.

The effect of the skew wind should be considered not only in mountainous terrain affected by the boundary layer wind, but also in flat terrain that witnesses strong wind events such as tornadoes or downbursts. In East Asia, extremely strong wind events usually cause significant damage to buildings and structures. For instance, the EF4 tornado that occurred in 2016 in Jiangsu Province, China, damaged hundreds of ultrahigh-voltage transmission towers (Yang et al., 2018). In studies of strong winds, researchers have used various research methods, such as field measurements, wind tunnel tests, and numerical simulations, to investigate the characteristics of the wind fields, the wind-induced vibration response of transmission line systems, and the collapse damage (Mara and Hong, 2013; El Damatty and Elawady, 2018; El Damatty and Hamada, 2016; Abd-Elaal et al., 2018). There is increasing consensus among scholars that vertical winds cannot be ignored when considering strong wind action.

In investigation of the effect of the incident wind direction on a tower, Mara and Hong (2013) defined wind with a direction between the longitudinal and transverse directions as oblique wind, which is the result of ignoring the vertical component of the wind. In many studies on the vibration response and aerodynamic coefficient of transmission towers, the oblique wind load has been considered as a horizontal load (Deng et al., 2016; Yang et al., 2015). However, it is evident from the above analysis that the vertical component of the oblique wind should be considered. Therefore, the definition of oblique wind should be extended to skew wind with a direction between the transverse, longitudinal, and vertical directions. In other words, both the yaw angle θ and tilt angle β should be considered for the skew wind.

Aerodynamic coefficients have been used to determine the wind load acting on a lattice structure and have been obtained by wind tunnel tests and numerical simulations. Yang et al. (2016) combined the results from a high-frequency force balance (HFFB) wind tunnel test and a computational fluid dynamics (CFD) numerical simulation to determine the global drag coefficient, skewed wind load factor, and shielding factor of an angled steel triangular transmission tower for different yaw angles, but without considering the tilt angle. Zhou et al. (2019) obtained aerodynamic coefficients of circular steel tubular lattice towers at different yaw and tilt angles by increasing the number of micro force balances and simultaneously measuring the force at four main members. For a tilt angle of 0°, the experimental value of the skewed wind load factor peaked at the yaw angle of 30° or 60°, and it was 10% higher than the value calculated using codes. By performing wind tunnel tests with rigid models, Bayer (1986) found that the most unfavorable yaw angle specified in the applicable design codes was 45°.

The aerodynamic coefficients are thought to be related to the solidity ratio. Yang et al. (2016) added triangular auxiliary segments to a transmission tower body to set the solidity ratio to values of 0.2, 0.3, and 0.4 for analyzing the variation of the aerodynamic coefficient with different solidity. Zhou et al. (2021) combined the methods of adding the triangle frames and increasing the members’ sizes to increase the solidity ratio of tubular-angle steel tower body models from 0.14 to 0.24 and 0.34 respectively, thus analyzing the influence of the solidity ratio on the mean wind loads on equilateral triangular tower under skew winds. However, it is yet to be ascertained whether the different methods used for changing the solidity ratio in respective studies would affect the study results.

In present codes, the skewed wind load factor Kθ is usually used to evaluate the effect of different yaw angles θ. The method of calculating the skewed wind load factor for a tower in the presence of angular winds with only considering the yaw angle is different in different national codes. Expressions of the following form have been adopted in IEC 60826 (2017), EN 50341 (2012), American Society of Civil Engineers(ASCE), 1991, BS 8100 (1986), and AS/NZS 7000 (2016):Kθ=1+K1K2sin22θ

While DL/T 5154 (2012) does not provide any specific equation, it presents the horizontal and vertical wind load distribution coefficients for four wind angles (0°, 30°, 45°, and 90°), and the skewed wind load factors for these wind angles can be obtained after conversion from the wind load distribution coefficients. JEC-TR-00007 (2015) tabulates the wind load distribution coefficients of a tower in the transverse and longitudinal directions for different wind angles at intervals of 5°, and the skewed wind load factor can be obtained from the wind load distribution coefficients. Although the skewed wind load factor can be calculated using the aforementioned national codes, its accuracy needs to be improved. It is noteworthy that no code can calculate wind loads on transmission towers at nonzero tilt angle.

In this study, a ±800 kV UHVDC steel tubular transmission tower was chosen as a representative tower to develop a segmental model of tower bodies with a scaling ratio of 1:30. A series of wind tunnel tests were performed under skew winds, with different tower bodies (X-, V-, and K-type) and wind fields (10, 15, and 20 m/s for a uniform flow field and 15 m/s for 8% and 18% turbulence fields). The aerodynamic coefficients at different yaw angles θ were obtained via a HFFB wind tunnel test, and the variation tendency of the aerodynamic coefficient of the circular steel tubular lattice tower body was summarized to obtain the most unfavorable yaw angle. A fitting formula was derived for the skewed wind load factor and compared with the standard formula. The detailed design and an analysis of the experimental results are presented in Section 2. Next, CFD models were constructed with the Ansys Fluent software and verified using experimental data. The steady-state calculation was used to study the effects of parameters such as the solidity ratio and tilt angle on the drag coefficient and tilted wind load factor. The solidity ratio was changed by modifying the size of the diagonal members or by adding auxiliary members. The tilted wind load factor similar to the concept of the skewed wind load factor is proposed, and the formula for calculating the tilted wind load factor is derived by combining the effects of the solidity ratio. The numerical simulation procedure and an analysis of the simulation results are presented in Section 3. Finally, the results of this study are summarized in Section 4.

Section snippets

Test model

A ±800 kV UHVDC steel tubular transmission tower with a nominal height of 230 m was chosen as the test model. It is shown in Fig. 1, where the red dashed square shows the location of the wind tunnel test segment. The tower body of any transmission tower can be represented as a combination of three basic shapes, namely, X-, V-, and K-type sections (Wilcox, 1988). The geometric dimensions of the test models in Fig. 1 are shown in Fig. 2. The geometric scaling ratio is 1:30. Since the aeroelastic

Numerical simulation model

The X-type tower test section in Fig. 2 was selected as the main object in numerical simulation, and it was simulated using the Ansys Fluent software. The geometric dimensions of the selected object were identical to those of the experimental model.

Conclusion

In this study, a series of experiments and numerical simulations involving a circular steel tubular lattice tower body model was conducted. The aerodynamic characteristics, including drag and lift coefficients, of the tower body models with a scale ratio of 1:30 were obtained in wind tunnel tests, and the influence of parameters, such as the wind velocity, turbulence intensity, and yaw angle, were quantitatively analyzed. Experimental values of skewed wind load factors were compared with the

CRediT authorship contribution statement

Dekai Zhang: Methodology, Formal analysis, Software, Investigation, Writing – original draft. Xueqi Song: Validation, Data curation, Visualization. Hongzhou Deng: Conceptualization, Supervision, Project administration. Xiaoyi Hu: Resources, Funding acquisition. Xing Ma: Writing – review & editing, Supervision.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

The study described in this paper was fully supported by the National Natural Science Foundation of China (Project No. 51578421), and the support is gratefully acknowledged.

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