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Windtunneltestonaerodynamiceffectofwindbarriersontrain-bridgesystem

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Special Topic: High-speed Railway Infrastructure (II) February 2015 Vol.58 No.2: 219–225

• Article • doi: 10.1007/s11431-014-5675-1

Wind tunnel test on aerodynamic effect of wind barriers on train-bridge system

GUO WeiWei1,2*, WANG YuJing1, XIA He1,2 & LU Shan1

1 School of Civil Engineering, Beijing Jiaotong University, Beijing 100044, China; 2 Beijing Key Laboratory of Structural Wind Engineering and Urban Wind Environment, Beijing 100044, China

Received June 4, 2014; accepted August 5, 2014; published online October 23, 2014

To investigate the aerodynamic effect of wind barriers on a high-speed train-bridge system, a sectional model test was con-ducted in a closed-circuit-type wind tunnel. Several different cases, including with and without barriers, with different barrier heights and porosity rates, and with different train arrangements on the bridge were taken into consideration; in addition, the aerodynamic coefficients of the train-bridge system were measured. It is found that the side force and rolling moment coeffi-cients of the vehicle are efficiently reduced by a single-side wind barrier, but for the bridge deck these values are increased. The height and porosity rate of the barrier are two important factors that influence the windbreak effect. Train arrangement on the bridge will considerably influence the aerodynamic properties of the train-bridge system. The side force and rolling mo-ment coefficients of the vehicle at the windward side are larger than at the leeward side.

wind barrier, train-bridge system, wind tunnel, aerodynamic coefficient, windbreak effect

Citation: Guo W W, Wang Y J, Xia H, et al. Wind tunnel test on aerodynamic effect of wind barriers on train-bridge system. Sci China Tech Sci, 2015, 58: 219225, doi: 10.1007/s11431-014-5675-1

1 Introduction

As modern trains become more high-speed and light-weight, they are more sensitive than ever to wind [1]. A train may become unsafe in strong wind conditions, especially when it runs on a bridge. To carry out safety analysis of a train run-ning on a bridge in high winds, the aerodynamic forces act-ing upon the train-bridge system should be determined [2–7].

Computational fluid dynamics (CFD) and wind tunnel tests are the usual approaches for obtaining the basic aero-dynamic forces of a structure [8–12]. Some challenging is-sues and uncertainties remain in CFD technique, which leads to appreciable differences between calculated and actual results [7]. Compared to CFD, a wind tunnel test is a

more reliable way to explore the aerodynamic or aeroelastic phenomena of a structure.

The high-speed railway line between Lanzhou and Xin-jiang in northwest China, which is still under construction, passes through a region that suffers more from windstorm damage than almost anywhere else in the world; the wind velocity often reaches 50–60 m/s. Because these strong winds are the main natural disaster that affects the running safety of high-speed trains, wind barriers are required for bridge designs in this region [13,14]. Wind barriers can re-duce the aerodynamic forces on the train, but they can also have negative influence on a bridge because they enlarge the windward area of the structure. Moreover, the barrier will exert a disturbance on the wind field and increase tur-bulence effects, both of which make the wind field around the bridge structure more complex [15,16]. We studied the aerodynamic effect of a single-side wind barrier on a cou-

220 Guo W W, et al. Sci China Tech Sci February (2015) Vol.58 No.2

pled train-bridge system by a section-model test that was conducted in a wind tunnel at Beijing Jiaotong University (China). Based on this test, which considered wind effects with and without a wind barrier, with barriers of different heights and porosity rates, and with different train arrange-ments on the bridge, we calculated the aerodynamic effect of the barrier on the train-bridge system. Some useful re-sults were obtained, which can provide fundamental data for barrier design at high-speed railway bridges and running safety analyses of trains on such bridges.

2 Experimental method

Dimensionless force coefficients of a bridge and a train can be obtained by a force test or a surface-pressure test on a rigid-section model in a wind tunnel. Such tests are usually conducted under smooth wind flow.

In a force test, the section model is mounted on one or two force balances for the measurement of wind forces. The force balance is usually fixed to a turntable so the attack angle of the wind can be adjusted. The force balance must be designed according to the mass of the model and the frequency range of interest. The force coefficients of the model can be calculated by the following equations:

2D D( ) ( ) 0.5 , C F U HL (1)

2L L( ) ( ) 0.5 , C F U BL (2)

2 2M ( ) ( ) 0.5 , C M U B L (3)

where CD, CL and CM are the drag, lift, and moment coef-ficients, respectively; FD, FL, and M indicate the measured drag, lift and moment forces, respectively; H, L, and B represent the height, length, and width of the section model (Figure 1), respectively; is the air density of the wind tunnel; U is the velocity of the wind flow; and is the attack angle of the wind flow.

In the pressure test, the surface-wind pressures on the structure are measured at pressure taps that use high-sensi- tivity silicon piezoresistive sensors and pneumatic connect-ors. The sensors are referenced to the static pressure of the test section, measured sufficiently far from the model. As shown in Figure 2, the drag, lift and moment forces on the ith tap can be expressed as the product of the pressure and the surface area:

Figure 1 Aerodynamic forces and moment on section model.

Dd cos d ,i i i iF p s (4)

Ld sin d , i i i iF p s (5)

d d , i i i iM p d s (6)

where pi is the measured surface pressure at the ith tap; dsi

is an elementary surface area near the ith tap; i denotes the angle between the normal line of dsi and the horizontal line; and di is the vertical distance from the centroid of the section to the normal line.

By summing all the forces that act on each of the pres-sure taps surrounding the section surface of the model, the overall force components can be obtained as

D D L L1 1 1

d , d , d ,

N N N

i i ii i i

F F F F M M (7)

where N is the number of pressure taps on the section surface.

The force coefficients of the model can be calculated by substituting eq. (7) into eqs. (1)–(3) and letting L=1.

3 Wind tunnel facilities, models and test cases

The wind tunnel lab at Beijing Jiaotong University is a closed-circuit type (Figure 3). The total length of the test section (from the nozzle outlet to the diffuser inlet) is ap-proximately 15.0 m. The test section is 3.0 m wide and 2.0 m high. Wind velocity through the test section can be regu-lated between 0 and 40 m/s by adjusting both the pitch blade angle and the speed of the fan (which powered by a 400 kW engine), where the fan is placed at the outlet of the test section. The maximum wind velocity used in this study was 15 m/s.

The structural models for the vehicle, deck, and wind barrier are made of acrylonitrile butadiene styrene (ABS) plastic boards. In this study, the model scale is 1:32. The scaled dimensions of the vehicle, deck, and a single-side barrier 3.5 m in height are shown in Figure 4.

Figure 2 Illustration of surface wind pressures and forces.

Guo W W, et al. Sci China Tech Sci February (2015) Vol.58 No.2 221

Figure 3 Wind tunnel lab at Beijing Jiaotong University.

Figure 4 Scaled model of train-bridge system with barrier (unit: mm).

The test was carried out for four cases (Table 1 and Fig-ures 5 and 6).

All of the cases were carried out in wind environments with uniform flow. The blockage ratio of the models to the cross-section of the test section was 4.4%–12.23%. Accord-ing to some research, a blockage ratio higher than 10% is acceptable in practical engineering [17].

Figure 7 depicts one of the schematic diagrams in case 2. Because the model can rotate around the deck’s longitudinal axis, the drag, lift, and moment are measured as functions of the wind attack angle . A wind barrier is installed at the windward side of the deck (Figure 7(a)). The train model is also set on the windward track on the deck, which is equipped with 5 groups of pressure taps on its surface; each group has 40 measuring points (Figure 7(b)). Because of interference from the tunnel walls, the pressure data of taps at both ends of the vehicle are dispersed from those at the middle (RD=33%–77%). Therefore, we used the average of the pressure data obtained from the three groups in the mid-dle part of the vehicle for analysis.

The deck model is placed on a dynamometric system. The aerodynamic forces and moments of the deck are measured with a 5-component (Fy, Fz, Mx, My, Mz) force balance at each end (Figure 8(a)).

Four rack-mount and intelligent-pressure scanner valves (Figure 8(b)) were used to measure the pressures on the vehicle surface (256 channels in total). Instantaneous and simultaneous pressure measurements were carried out at

Table 1 Four cases in the test

Case Train on deck Measurement condition

Case 1 no train Without and with a single-side wind barrier Case 2 a train Different train arrangements are compared Case 3 a train Different barrier heights are compared Case 4 a train Different barrier porosity rates are compared

Figure 5 Illustration of test cases. (a) Case 1: with and without barrier on deck; (b) case 2: with different train arrangements on deck; (c) case 3: with different barrier heights H; (d) case 4: with different barrier porosity rates

Figure 6 Illustration of barrier with different porosity rates .

each tap at 312.5 Hz for 80 s.

4 Analysis of results

4.1 Case 1

We measured the influence of a single-side wind barrier

222 Guo W W, et al. Sci China Tech Sci February (2015) Vol.58 No.2

Figure 7 Schematic diagram of wind tunnel test (unit: mm). (a) Connec-tions, supports, and layout of section model; (b) arrangement of one group of pressure taps on vehicle surface.

Figure 8 Experimental instruments. (a) TG351B-type force balance; (b) ESP-64HD pressure scanner.

with a height of 3.5 m and a porosity rate of 30% on the aerodynamic coefficients of the deck.

Figure 9 displays the tri-component coefficients of the deck with and without wind barrier for the wind attack an-gle ranging from 6° to 6°.

The lift force and moment coefficients of the deck de-crease due to the existence of the wind barrier, whereas the drag force coefficients are larger than those of the bare deck due to the increase of windward area. Table 2 shows the drag, lift, and moment coefficients of the deck at zero wind-attack angle.

To verify the measurements, the tri-component coeffi-cients of the deck at zero wind-attack angle between the CFD simulation and the wind tunnel test are compared in Table 3. The relative discrepancies (RD) between the two sets of results are also listed in Table 3. The relative

Figure 9 Tri-component coefficients of deck in case 1. (a) Drag force coefficients; (b) lift force coefficients; (c) moment coefficients.

Table 2 Tri-component coefficients of deck in Case 1

Case 1 CD CL CM

Without barrier 1.546 0.567 0.235 With barrier 3.021 0.074 0.180

Table 3 Comparisons of tri-component coefficients of deck between CFD simulation and wind tunnel test

° Wind velocity (m/s) Results CD CL CM

0 10 Simulated 1.426 0.614 0.228Measured 1.546 0.567 0.235RD (%) 7.76 8.29 2.98

discrepancy is defined as the computed result minus the measured result, divided by the measured result. The com-puted tri-component coefficients are all close to the meas-urements. The absolute relative discrepancies are less than 10%.

4.2 Case 2

We measured the wind forces and pressures on the train-

Guo W W, et al. Sci China Tech Sci February (2015) Vol.58 No.2 223

bridge system under different train arrangements on the deck, herein installed at the windward side as a wind barrier with a height of 3.5 m and a porosity rate of 30%.

Figures 10 and 11 show the results of the wind tunnel test on the tri-component coefficients of the vehicle and the deck for the wind attack angle ranging from 6° to 6°. The aerodynamic characteristics of the train-bridge system differ when the train is at the windward and the leeward track. The side force and rolling moment coefficients of the vehicle at the windward side are larger than at the leeward side because the wind velocity acting on the vehicle at the leeward side is smaller. Compared to Figure 9, the tri-component coefficients of the deck change considerably when the train is present.

The tri-component coefficients of the vehicle and the deck at zero wind-attack angle are listed in Table 4.

4.3 Case 3

We measured the wind forces and pressures on the train- bridge system under different barrier heights, herein kept constant as the barrier porosity rate and with the value of

Figure 10 Tri-component coefficients of vehicle in case 2. (a) Side force coefficients; (b) lift force coefficients; (c) rolling moment coefficients.

Figure 11 Tri-component coefficients of deck in case 2. (a) Drag force coefficients; (b) lift force coefficients; (c) moment coefficients.

Table 4 Tri-component coefficients of train-bridge system in case 2

Case 2 Deck Vehicle

CD CL CM CD CL CM

Windward 2.700 0.095 0.065 0.382 0.325 0.044

Leeward 2.822 0.106 0.076 0.094 0.122 0.017

30%.

Figure 12 shows the pressure distribution curves on the vehicle surface at different barrier heights (the measuring point positions are shown in Figure 7(b)). With increasing barrier height, the pressures on the windward side of the vehicle surface (points 1 to 11) efficiently decreased.

Figure 13 show comparisons of the tri-component coeffi-cients of the train-bridge system at zero wind attack angle by different barrier heights. The aerodynamic coefficients of the vehicle are sensitive to the barrier height. As the height increases from 0 to 3.5 m, both the side-force and roll-ing-moment coefficients gradually decrease. The former drops much more obviously than the latter but both indices slightly increase for the barrier higher than 3.5 m. Unlike

224 Guo W W, et al. Sci China Tech Sci February (2015) Vol.58 No.2

Figure 12 Pressure distribution on vehicle surface in case 3.

Figure 13 Tri-component coefficients of train-bridge system vs. barrier height H. (a) Vehicle; (b) deck.

the vehicle, the drag force and moment coefficients of the deck increase slowly with the barrier height. In short, when the height of barrier is 3.5 m, the side-force and roll-ing-moment coefficients of the vehicle are closest to zero, so that the vehicle is easier to resist overturning. Meanwhile, the aerodynamic coefficients for the deck are the most agreeable. Therefore, the optimal barrier height for this train-bridge system is 3.5 m.

The tri-component coefficients of the vehicle and the deck at zero wind attack angle in this case are listed in Ta-ble 5. The side-force and rolling-moment coefficients of the vehicle are efficiently reduced due to the existence of the wind barrier, but the barrier brings a negative effect to the deck. With a single-side wind barrier of a 3.5 m height and a 30% porosity rate, the drag force and moment coefficients for the vehicle are reduced by 71.4% and 78.7%, respec-tively; for the deck they are increased by 3.3% and 242.1%.

4.4 Case 4

We measured the wind forces and pressures on the train-

Table 5 Tri-component coefficients of train-bridge system in case 3

Case 3 Deck Vehicle

CD CL CM CD CL CM

H=0 m 2.614 0.253 0.019 1.335 0.480 0.207H=2.0 m 2.628 0.290 0.017 1.230 0.355 0.126H=3.5 m 2.700 0.095 0.065 0.382 0.325 0.044H=5.0 m 3.247 0.238 0.083 0.386 0.060 0.070

bridge system under different barrier porosity rates, herein kept constant as the barrier height and with the value of 3.5 m.

Figure 14 shows the pressure distribution curves on the vehicle surface with different barrier porosity rates. The pressures on the windward side of the vehicle surface (points 1 to 11) increase remarkably with the increasing porosity rates.

Figure 15 show the distribution curves of the tri-com- ponent coefficients versus barrier porosity rates. The aero-dynamic coefficients of the vehicle are more sensitive than

Figure 14 Pressure distribution on vehicle surface in case 4.

Figure 15 Tri-component coefficients of train-bridge system vs. porosity rate . (a) Vehicle; (b) deck.

Guo W W, et al. Sci China Tech Sci February (2015) Vol.58 No.2 225

those of the deck to the porosity rate. Both the side-force and rolling-moment coefficients of the vehicle obviously increase with the porosity rate. Therefore, the porosity rate of the barrier should be controlled in order to ensure the shelter effect.

The tri-component coefficients of the vehicle and the deck at zero wind-attack angle are listed in Table 6. Unlike the vehicle, the drag force and moment coefficients of the deck decrease slowly with the porosity rate, due to the de-crease of windward area.

5 Conclusions

We investigate the aerodynamic effect of a wind barrier on a train-bridge system by means of wind tunnel tests on 1:32 scale models. Our main conclusions are summarized as fol-lows.

(1) Due to the protection of the wind barrier, the aerody-namic coefficients of the vehicle are efficiently reduced. With a single-side wind barrier of 3.5 m height and 30% porosity rate, the side-force and rolling-moment coefficients of the vehicle are reduced by 71.4% and 78.7%, respective-ly.

(2) Although it is helpful to the vehicle, the wind barrier has a negative influence on the bridge. This is because it enlarges the windward area of the structure, and thus re-markably increases the drag and moment forces on the deck.

(3) Train arrangement on the deck will influence the aerodynamic properties of the train-bridge system, because it changes the flow pattern around the deck. The side-force and rolling-moment coefficients of the vehicle on the windward track are larger than on the leeward track due to the higher wind velocity at the windward side.

(4) The height and porosity rate of the wind barrier are two important factors that influence the windbreak effect. According to our tests, the optimal height of the barrier is

Table 6 Tri-component coefficients of train-bridge system in case 4

Case 4 Deck Vehicle

CD CL CM CD CL CM

=0 2.739 –0.191 0.070 –0.086 0.032 –0.012

=10% 2.728 –0.127 0.068 –0.030 0.181 –0.006

=30% 2.700 0.095 0.065 0.382 0.325 0.044

=50% 2.607 0.367 0.051 0.775 0.276 0.120

3.5 m for this train-bridge system. The porosity rate of the barrier should be controlled to ensure the shelter effect.

This work was supported by the National Basic Research Program of Chi-na (“973” Project) (Grant No. 2013CB036203), the National Natural Science Foundation of China (Grant No. 51308034), and the “111” Pro-ject (Grant No. B13002).

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