4.1. Wind Field Parameter Analysis
The occurrence of downbursts has a strong random characteristic. The speed parameters and shape parameters of the downbursts that occur each time are different.
Downburst wind loads acting on the transmission tower mainly change with the different downburst’s outflow parameters and the cylindrical coordinate position (r, α) of the downburst center relative to the center of the transmission tower.
Figure 3a shows the transmission tower and the downburst location. The parameters affecting the wind load of the transmission tower mainly included the following: the radial distance (r) of the transmission tower from the storm center and the wind direction angle (α), and the jet diameter of the downburst (D
jet).
In this part, the downburst wind field is steady. The radial distance r = 1.0 Djet, the wind direction angle α = 0° and the jet diameter of the downburst Djet = 800 m. The r and Djet were changed independently, while keeping the other factors unchanged.
Figure 4 shows the curve of the transmission tower base’s bending moment and the main material’s axial force with the radial distance. The main material’s force is the maximum axial force of the entire tower which was at bottom of tower. With the increase of the radial distance, the base bending moment and main material axial force increased firstly and then decreased in the range of r is 0.5–2.0 D
jet. The maximum value is about two to four times the minimum. With an increase in tower height, the influence of the radial distance on the bending moment and the axial force value became increasingly greater and greater. For towers 1 and 2, the force reaches the peak at r of 1.0 Djet, while the forces of towers 3, 4 and 5 reach an extreme value at r of 0.9 D
jet.
Figure 5 compares the change law of base bending moment and main material axial force with jet diameter for different tower heights. When the height of the transmission tower was small, the jet diameter had a small influence on the internal force of the transmission tower caused by wind load. In the range of D
jet = 600–2200 m, the maximum base bending moment was about 1.25 times the minimum. As the height of the tower increased, the load on the transmission tower was increasingly affected. The maximum base bending moment of tower 5 was about 1.43 times the minimum. This reflected the fact that within a certain range, the greater the height of the transmission tower, the more sensitive it was to the parameters of the downburst wind field. Within a certain range, with the increase of D
jet, the base bending moment of the transmission tower also increased until the peak was reached.
It is found that all towers reach their peaks at jet diameters of 900, 1100, 1300, 1400, and 1600 m, respectively. After that, if the jet diameter continues to increase, the base bending moment and main material axial force of the transmission tower gradually decrease.
4.3. Comparison of the Effects of Unsteady and Steady Downburst
Affected by the boundary layer wind, the actual downburst is mobile. The movement of the storm will change the wind field and eventually affect the wind load on structures [
23,
24]. Therefore, to study the impact of the movement effect of the downburst, the static downburst wind field were replaced with moving downburst wind field for numerical calculations.
About the unsteady downburst, the touch down location d
0 = 3600 m, the wind direction angle α = 0°, and the moving velocity of the unsteady downburst v = 12 m/s.
Figure 7 shows the wind speed time series of unsteady downburst with D
jet = 1000 m.
As moving downburst currents affected the transmission tower, the downburst center from the initial position continuously approached the transmission tower and then moved away.
Figure 8a shows the time history of the wind field and the normalized time history of the internal force of the transmission tower. The data was processed by the moving average method, using 100 and 200 points averages in
Figure 8a. F
N is the main material’s axial force of transmission towers. F
N gradually increased as the storm center approached. When r/D
jet approached 1, F
N reached its maximum value and then gradually decreased. When r/D
jet approached 4.5, there was a second small peak, but it was not as large as the former peak.
Figure 8b shows that the pulsating wind had a very large effect on the response. The pulsating wind also had two crests, and the position of the crests was similar to that shown in
Figure 8a.
Figure 9 shows the change curves of the main material axial force, base shear force, and tower top displacement of the transmission tower under moving downbursts with different jet diameters.
Figure 9 shows that the responses of towers 1, 3, and 4 gradually decreased when the D
jet was from 600 m to 1400 m. The response of tower 3 changed significantly and was reduced by about 25%. As the D
jet continued to rise, the responses of towers 1, 3, and 4 continued to increase and reached extreme values when the D
jet were 1800 m, 1900 m, and 1900 m, respectively. Among them, the extreme value of tower 3 was similar to its value when D
jet was 700 m, and the extreme values of tower 1 and tower 4 were both the maximum values in this process.
Figure 9a shows that the maximum top displacement of tower 1 and tower 3 was increased by 50% compared with the minimum. In
Figure 9b, the maximum value of the main material axial force of tower 1 was nearly doubled compared with the minimum value. The response of the above three towers gradually decreased with an increase in the jet diameter after reaching the extreme value. Tower 5 and tower 2 gradually increased with an increase in D
jet at the beginning. Tower 5 had a smaller extreme point when D
jet was 1100 m, and then it reached the maximum value at D
jet = 1700. Tower 2 had an extreme point when D
jet was about 1200, and then decreased. Unlike other power transmission towers, the response of tower 2 continued to go up with an increase in D
jet after the jet diameter reached 2200 m.
Compared with the response of the transmission tower under the static downburst wind field, the displacement and axial force value under the moving downburst were significantly increased. This conformed to the law of moving wind field, and the moving speed of downburst had a significant amplifying effect on the peak value. Though tower 2 had different response comparing with the others, the overall trend was still consistent with the laws that the higher the tower, the greater the response.
In general, this response was different from the regularity of static downburst. Under the moving downburst, the response of the transmission tower did not change significantly with Djet and it had two extreme points. Under static wind field, there was only one extreme point in response to the change of the jet diameter. Additionally, the moving effect of the wind field increased the Djet, which corresponded to the extreme value of the transmission tower.