Simulation and Comparison of Windbreak Structures on Highway

1st International Conference on Transportation Infrastructure and Materials (ICTIM 2016) ISBN: 978-1-60595-367-0

Lu Bai1

1Doctoral Student, Chang’an University, Middle Section of South Erhuan Street, Xi’an, China. [email protected]

ABSTRACT: High wind is among top highway hazard in Xinjiang, China, especially for freeways. Windbreak structures are installed along the roadway in windy-zone to ensure the driving safety. This study was conducted to collect in-situ data of the wind speed using different types of Windbreak structure. Simulations were carried out to simulate the wind field of highway using computational fluid dynamic (CFD) software. Types of Windbreak structure, wind speed, location of installation and types of vehicles were considered in the simulation. The optimum installation location of Windbreak structure was recommended.

INTRODUCTION

High wind is a primary climate hazard to roadway safety in Xinjiang, China. In recently years, accidents, transportation suspension and infrastructure damage caused by high wind have been reported, especially on freeway, where vehicles travel at a high speed and are vulnerable to overturn (Wang 2005, Liu 2006). In addition, data collected from 50 weather stations along highways from 1990s till now, shows that there is an increasing trend of annual maximum wind speed and days of high wind, which might be caused by global climate change. High wind hazardous on highway is drawn a rising attention. Windbreak structure is a barrier used to reduce the wind speed on the carriageway. In China, the most commonly used windbreak structures include: windbreak net, windbreak steel fence and windbreak wall (Ge and Jiang 2009, Zeng et al. 2012). The windbreak density is the ratio of the solid portion of the barrier to the total area of the barrier. Among these three types of windbreak structure, windbreak net has the least windbreak density and windbreak wall has the most windbreak density. As the windbreak density increase, the wind pressure acting on the structure increase, which requires the higher strength of the windbreak structure itself and also limit the height of the structure (Boselly and Doore 1993, Wang 2004, Li 2007, Li et al. 2012). The windbreak height is the primary factor inflicting the distance of protected area on the downwind side. Usually, the distance of protected area from the windbreak structure is two to five times of the windbreak height. In this study, the onsite wind fields were measured to evaluate the performance of three types of windbreak structure, including windbreak wall, windbreak fence and windbreak fence. Based on the measured field performance, the windbreak fence was recommended. Computational Fluid Dynamic (CFD) simulation was further carried out to determine the optimum windbreak fence installation location.

529 ONSITE WIND FIELD MEASUREMENT The onsite wind field measurement was carried out in the windy zone in Xingjiang, China. Three types of windbreak structures were included. The windbreak net and windbreak fence were installed on the Lian-Huo freeway (K3781+600-K3781+800). 15 wind speed stations were installed to capture the wind speed distribution in the area. Five sensors were installed on each test station at different height, i.e. 4m, 2m, 1m 0.5m and 0.15m. The windbreak wall was installed on Lan-Xin railway (DK1451+870- DK1451+960), 13 wind speed sensors were installed. Table 1 shows the maximum wind speed on the Lian-huo freeway, where the windbreak net was installed. The maximum speed 11.6m/s was obtained at distance of - 10m from the net at the height of 4m. After wind pass through the net, the speed reduced to 6m/s. As the distance increases to 65m from the net, the wind speed resumes to the speed before the wind past through the windbreak net. The average wind speed reduction is approximately 4.6m/s. The field wind speed distribution is shown in Fig. 1.

Table 1. Maximum wind speed on the Lian-Huo freeway K3781+600 with windbreak net (m/s).

Height Distance from the wind net (m) (m) -20 -10 -5 -2 2 9 15 20 35 51 58 65 70 75 80 4.00 10.4 11.6 9.4 10.2 6.0 9.5 7.0 6.9 8.6 8.1 10.2 11.6 8.2 9.2 11.4 2.00 10.0 10.2 8.3 9.5 4.9 7.9 6.7 5.9 6.2 5.6 9.0 10.0 6.9 8.5 10.0 1.00 9.2 8.9 7.8 9.0 4.7 6.0 6.0 4.8 5.8 6.6 8.5 9.0 6.0 7.6 9.3 0.50 8.1 8.3 7.1 8.5 4.0 5.8 5.4 4.6 5.4 5.7 7.6 8.9 5.8 7.1 8.9 0.15 6.5 7.0 5.6 7.2 3.2 5.0 4.6 3.0 4.3 5.1 6.7 8.2 4.8 5.7 7.5

度高 Height

Distance距离： m Figure 1. Counter map of maximum wind speed on the highway section with wind net.

Table 2 shows the measured wind speed on the Lian-Huo freeway K3781+700, where the windbreak fence was installed. The windbreak density of the fence is 70% (solid portion/area of entire fence). Before the wind reached the fence, the captured maximum

530 speed is 16.0m/s. As the wind went through the fence, speed reduced to 6m/s. As the distance increases to 70m, the wind speed resumed. The effective speed reduction is about 10m/s. The affected area of windbreak fence is proximately the same as the windbreak net. Fig. 2 shows the field wind speed distribution, the pattern is similar to the sited, where windbreak net is installed.

Table 2. Maximum wind speed on the Lian-Huo freeway K3781+700 with windbreak fence.

Height Distance from the wind net (m) (m) -20 -10 -5 -2 +2 +9 +15 +20 +35 +51 +58 +65 +70 +75 +80 4.00 19.6 17.0 14.3 16.0 6.0 12.0 10.9 14.1 17.3 18.9 17.7 12.0 15.1 19.3 15.7 2.00 15.7 16.4 13.9 15.0 5.0 11.0 9.5 13.5 16.9 18.4 17.4 11.6 15.0 17.2 13.6 1.00 12.7 13.8 12.2 14.7 4.7 10.9 8.8 13.0 14.7 10.9 11.7 11.3 12.3 13.4 13.3 0.50 12.4 12.0 10.6 12.2 4.3 10.5 7.9 12.5 12.0 9.8 10.9 11.2 11.8 11.3 13.1 0.15 11.1 11.1 9.2 9.2 3.3 8.8 7.5 12.2 11.4 7.8 9.8 11.0 9.9 9.7 11.3

Table 3 shows the measured wind speed on the Lian-Xin railway Dk1451+910, where the windbreak wall was installed. Because the rail track is much narrower than the freeway carriageway, the windbreak wall needs to covers much shorter distance than the windbreak net and fences. Therefore, the height of the windbreak wall is shorter. The maximum wind speed was captured at -20m from the wall, and the minimum speed is obtained after the wind pass through the wall at the distance of 2m. The wind speed reduction is 10m. Comparing three types of the windbreak structure, windbreak wall and fence have a similar wind speed reduction. The windbreak net has the least. The windbreak fence is easier to be installed and the construction time is much less than windbreak wall, which is usually casted on site and requires a curing time. Therefore, windbreak fence is recommended.

度高 Height

Dista距离：nce m Figure 2. Counter map of maximum wind speed on the highway section with windbreak fence.

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Table 3. Maximum wind speed on Lanxin Highway Dk1451+910 with windbreak wall.

Height Distance from the wind net (m) (m) -20.0 -10.0 -5.0 -2.0 -1.0 2.8 4.5 7.5 9.4 12.5 14.0 18.5 4.00 15.7 17.4 15.1 12.1 16.7 8.0 11.2 15.8 9.2 13.6 14.8 19.0 2.00 14.5 17.3 12.7 9.4 16.3 6.8 10.7 15.4 8.6 12.6 13.3 17.9 1.00 13.8 15.8 12.1 8.4 12.5 6.5 9.1 15.2 7.8 12.0 16.4 16.3 0.50 13.0 11.1 8.0 7.4 10.2 6.4 8.4 14.6 10.0 11.0 16.0 15.1 0.15 11.8 8.6 7.2 6.2 9.6 6.3 8.3 13.9 9.7 11.6 14.3 12.2

度高 Height

Distance距离： m Figure 3. Counter map of maximum wind speed on the highway section with windbreak wall.

OPTIMUM INSTALLATION LOCATION OF WINDBREAK FENCE

In order to determine the optimum installation location of windbreak fence, a CFD software is used to simulate the field wind speed distribution when windbreak fences are installed at different location. In the simulation, three types of cars were considered, passenger car, bus and truck. The wind speed disruptions around these three types of cars are shown in Fig. 4.

a) Passenger Car b) Bus

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c) Truck Figure 4. Wind field Vehicle cross of highway with different types of vehicle.

The purpose of using windbreak structure is to improve the driving safety under high wind. Therefore, the side force on a vehicle caused by wind is used to evaluate the effects of installed windbreak fence. As shown in Fig. 5, due to the smaller size of the passenger car, the side fore on it is much small than that on bus and truck. Even through the bus and truck have similar height and length, due the sealed box shape of the bus, its side force is higher than truck’s. As the distance of installation went far away from the freeway shoulder, the side force increase. If the fence is installed too close to the carriageway, it may affect driving and force driver to keep a distance from the side of the road. Passinger Car Bus truck 10000

8000

6000

4000 Side Force(N) Side

2000

0 -1 0 1 2 3 4 Distance from Shouder (m)

Figure 5. Comparison of Side Force on Different types of Vehicle with Different Windbreak Fence Installation Distance.

CONCLUSIONS In this study, the onsite wind field was measured to evaluate the performance of three types of windbreak structure, including windbreak wall, windbreak fence and windbreak fence. Based on the measured field performance, the windbreak fence was recommended and the optimum installation location is the on the shoulder according to the CFD simulation.

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