ind oad parameters and the w behavior for wind fences of...

RESEARCH ON THE WIND LOAD PARAMETERS AND THE WIND FENCES BEHAVIOR FOR WIND FENCES OF RAILWAY

BRIDGE Shi-xiong Zheng

Professor, School of Civil Engineering, Southwest Jiaotong University Chengdu Sichuan 610031, PR China, [email protected]

ABSTRACT

In southern Xingjiang autonomous region of China, the wind fence is installed on the railway bridge to keep the operation safety and on time of train in strong winds. Wind fences can reduce wind loads acting on train and increase critical wind velocity of overturning, but increase the wind load on the bridge too. In this paper, the wind load parameters of wind-fence, trains and beams are studied with the method of CFD and wind tunnel model test. The effects of the parameters such as height and porosity of wind fences are investigated. The operation safety analysis of train is performed.

KEYWORDS: WIND FENCES, COMPUTATIONAL FLUID DYNAMICS, WIND TUNNEL TEST

Introduction The southeast coast region and Xinjiang autonomous region of China are two main

areas where always be attacked by strong wind. In Xinjiang autonomous region, the strong wind occurs in spring and autumn because of the cold air. However, in the southeast coast region, the strong wind does in summer and autumn due to the typhoon. The basic wind speed of the two areas may be greater than 40m/s. Recently, the railways of Xinjiang are often hit by gale and lead to big hazards. The trains especially box wagon are derailed or constrained and even turn over due to strong wind. In order to ensure safe operation of trains and enhance the capacity of trains passing through, setting up wind fence on roadbeds or on bridge is the most effective measures[J.D.Holmes 2001 and S. Charuvisit et al 2004].

On the railway from Lanzhou to Xinjiang, the railway wind fences structure is composed of windbreaks and wind fences. The windbreaks have been used to the roadbed. The wind fences are always employed to the bridge. The wind fences were installed on the wind-fence-bridge. The wind-fence-bridge is the special bridge only used to bearing the wind fences. It stands alone, located at the windward of the bridge of railway and parallel to the bridge of railway. This wind fences measure is economical for the low bridge with height less than 10m and needs the good geological conditions. For the new or high bridge, it is much more expensive to build the wind-fence-bridge and gives rise to poor visual effects. So, installing the wind fences on the railway bridge is the economical and practical measure for the new railways.

To guarantee the train running safety and reducing the number of train stopping due to strong wind, it is the most effective measures to install the wind fences on the railways bridge. The bridge wind fences can improve the aerodynamic stability of trains, but increase the wind load on the bridge and affect the design of bridge. In this paper, systematically analysis and wind tunnel model testing were carried out. The more reasonable bridge wind fences structure

溫素嵐

印章

was researched. The wind-load parameters and wind fences behavior are investigated[K.G. Ranga Raju 1976].

The research of bridge wind fences behavior can be carried out in accordance with the following two methods: computational fluid dynamics (CFD) numerical simulation and wind tunnel model testing[Simiu,E.et al.1986] . The method of CFD may be lower precision than wind tunnel model testing method, but it is low cost and can analyze the full-scale structure. The wind tunnel model test is only for the scale model. In this paper, both of methods are adopted.

No.2 railway line from Turofan to Kuerle in South-Xinjiang passes through three draught areas, where the basic wind velocity is larger than 40m/s. Toudao river bridge which is 59m high is the highest bridge in the draught areas. By the methods of CFD simulation and wind tunnel-test, the wind load parameters of bridge wind fences, train and bridge were systematically studied. The wind shield effect for difference height and porosity are analyzed.

In this paper, Toudao river bridge is taken as example of the case study. It crosses perpendicularly to the valley of Toudao river. The records of the strong wind velocity at the bridge site shows, at the height of 10m, the largest 10min mean wind speed for return period 50 years is 47.0m/s , and 50.5m/s for return period 100 years. The basic wind speed for Toudao river bridge is 50.5m/s. The path and direction of the strong wind are very stable above Beaufort scale 8. The changing of wind direction is very small and always flowing along the river valley. So only the cross-bridge wind is taken into account.

The Toudao river bridge is composed of many spans of simple supported. The spans are all 32m. The railway beam is composed of four T-section beams and the railway is two-lane railway layout.

By analysis the terrain roughness of the site of Toudao river bridge, the terrain roughness height and its characters were acquired. The terrain roughness height Z0≈0.25～0.52mm. The roughness is closed to the snow surface with the depth of 20cm or more. So the terrain roughness is classified to category of A. The exponent of the power law profile of the wind speed contour lineα=0.073 and at the bridge site of 60m height, the largest mean wind speed averaged over a period of 10min is 57.3m/s with the return period 50 years, the largest mean wind speed averaged over a period of 10min is 61.6m/s with the return period of 100 years. At the height of 20m, the mean wind speed averaged over a period of 10min V10 and the instantaneous wind speed Vs is satisfied the following formula

Vs=1.1051V10+3.063 (1) and the correlative coefficient is 0.93.

Calculation the Wind Load Parameters of Wind fences with CFD By the method of Computational Fluid Dynamics, the wind load parameters of wind

fences are calculated [J.D.Holmes 2001 and C.W. Letchford el al 1994]. The software of Fluent 6.0 is used, which is composed of pre-processor module Gambit, solver module Fluent and other auxiliary module. The analysis process is as follows. Firstly, building computation models with Gambit; then, defining boundary conditions, flowing field characteristics, flowing field parameters, analysis parameters and so on with Fluent; finally, calculating and analyzing.

The coordinate system is defined as follows: x-axis is the same as the transverse direction of bridge, the y-axis is vertical upward; the z-axis is longitudinal along the bridge at the bottom of rails. Accordingly, tri-component forces, which are drag force, lift force and pitching moment, of trains, beam, wind fences can be expressed as xF 、 yF and zM respectively:

The Seventh Asia-Pacific Conference on Wind Engineering, November 8-12, 2009, Taipei, Taiwan

xx HBCVF 2

21 ρ= ; zy HBCVF 2

21 ρ= ; Mzz BCHVM 22

21 ρ=

Where ρ is air density; V is wind speed; H and B are structural characteristic scales. For

simplification, H or B for the trains, beam,wind fences can be same, and H is 3m , B is 1m in this paper.

xC 、 yC 、 MzC are the force coefficients which are respectively drag coefficients,lift coefficients and

pitching moment coefficient. When caculating, the passenger train is taken as an example, the wind comes in the

cross-bridge direction, the wind speed is 40m/s.

The different porositys and heights of wind fences are considered. The bridge is composed of 4 T-beam. 12 studied cases are performed. Show in table 1.

The Fig.1 and Fig. 2 show the wind speed distributional and the total pressure distributional in the region of calculation for the NO.3 studied case respectively. Table. 2 illustrates the force coefficients of trains, beam, wind fences under each studied case.

Fig.1. Nephogram of Wind Speed Fig.2. Nephogram of Total Pressure

It can be see from table 2. After setting up the wind fences, the coefficients of drag force and pitching moment of train reduce significantly and the lift force coefficients of train change greaterly. When wind fences is 3m high and the porosity is 21.6%, the drag force and pitching moment of train are only about 22% and 32% of the condition of no wind fences respectively. When wind fences wall is 3m high and the porosity is 30%, the drag force and pitching moment of train are only about 29% and 28% of the condition of no wind fences respectively.When wind fences is 3.5m high and the porosity is 30%, the drag force and pitching moment of train are only about 22% and 23% of the condition of no wind fences respectively. So, setting up wind fences wall will can improve the safe operation of trains significantly.

Comparing the studied case No. 2, No.4 and No. 5, the height of wind fences are 3.0m,2.5m and 3.5m respectively, and their porosity are same. It can be seen, the drag force, lift force and pitching moment of trains decrease with the height of wind fences increases. drag force and pitching moment of wind fences increase slightly with the height of wind fences increases.


Table 1 Studied Case for CFD Number of

Studied Case Height of

Wind-fence(m) porosity of Wind-fence

Rail line Located by Train

1 none none Windward side 2 3.0 30% Windward side 3 3.0 21.6% Windward side 4 2.5 30% Windward side 5 3.5 30% Windward side 6 3.0 21.6% none 7 3.0 21.6% Leeward side 8 3.0 0% Windward side 9 3.0 30% Leeward side

10 3.0 30% none 11 3.5 30% Leeward side 12 3.5 30% none

Table 2 Force Coefficients of train ,Wind fences and beam

Force Coefficients Force Coefficients Studied Case

Structural CX CY CMZ

Studied Case

Structural CX CY CMZ

beam 1.4112 0.8872 0.1630 beam 1.3017 -1.2304 -0.0782Wind fences

/

/

/

Wind fences

1.1689

0.1619

-0.5673

1

train 2.3714 -0.0191 -1.9159

7

train 0.3536 0.6930 -0.3922beam 1.4794 -1.6643 -0.3275 beam 1.3041 -1.5400 -0.1684Wind fences

1.0249

0.2151

-0.2577

Wind fences

1.2852

0.0535

-0.4545

2

train 0.6919 0.4224 -0.5372

8


1.1592 0.0969 -0.5142

Wind fences

1.0135

0.1152

-0.3657

3

train 0.5157 0.3633 -0.4682

9


0.6103

0.0917

-0.2427

Wind fences

1.2321

0.1214

-0.5717

4

train 1.2043 0.2881 -0.6872

10

train - - - beam 1.5367 -1.2734 -0.3201 beam 1.4913 -1.2712 -0.2726Wind fences

1.1239

0.0820

-0.2782

Wind fences

1.1546

0.1651

-0.3527

5

train 0.5111 0.3307 -0.4373

11


1.2465

0.1677

-0.5892

Wind fences

1.3564

0.1873

-0.6143

6

train - - -

12

train - - -

For the studied case No.2, No.3 and No.8, they have the same height of wind fences but the porosity are 30%,21.6% and 0% respectively. the drag force, lift force and pitching moment of trains decrease with the porosity decreases.

It can also see, train on or not on the bridge has slight effect on wind load of wind fences and the drag forces of beam. The wind load of wind fences and bridge have little relation with whether train is at the windward side of rails or the leeward side od rails. The the drag force and pitching moment of train at windward side are greater than that at leeward side. So, when the tain runs at windward side rails, it will be more unfavourable.


Fig.3 illustrates the force coefficients of train changing with different passing speeds. They can be get by CFD dynamic grid technology. In this paper, Only when the height of wind fences is 3.0m and its porosity is 30% is considered.

20 40 60 80 100 120 140 160

0.40

0.45

0.50

0.55

0.60

0.65

0.70

0.75

0.80

Forc

e co

effc

ient

s

Speed of train(km/h)

Cx Cy C

Mz

Fig. 3. Force Coefficients of train changing with Train Speed

Wind Tunnel Model Test for the Wind Load Parameters Wind tunnel model test is done to measure the force coefficients of wind fences,

bridge and train. Considering the geometrical size of train, height of bridge beam, sectional size of wind tunnel, requirement of obstructive degree and simulative need on the whole, the geometric scale of wind tunnel test model (including wind fences, bridge, train and so on) is taken 1:30. The model of train or bridge is made of high-quality timber and bridge fences is made of 2mm thickness aluminous plate punched with holes.

MODEL AND TESTING EQUIPMENT The wind tunnel model test aims at the engineering condition of height of 3m and

ventilate rate of 30%. The whole length of wind tunnel testing model is 2.1m,height of wind fences wall model is 100mm and to the bridge model,the width is 303m, the height is 108mm.

The test was done in the wind tunnel at Southwest Jiaotong University(XNJD-1). The wind tunnel section is 2.4m width and 2.0m height. The test wind speed in the section can change from 0.5m/s to 45m/s.

The test is done in the condition of uniform flow. Hot-wire wind speeding probes are installed at windward side of model to measure wind speed. testing wind speed is 20m/s,30m/s,40m/s. The coordinate system of model is same as above CFD calculation.

THE TESTING RESULTS AND ANALYSIS Wind tunnel test case and testing results under each case are showed in Tabel3. In

accordance with CFD calculation, the characteristic sizes of train, beam,wind fences are taken uniformly as H=3m and B=1m when caculating the force coefficients .

The test results shows that wind loads of train,wind fences and beam under three test wind speed are in good agreement. That is to say, the force coefficients has little dependence on the wind speed.


It can be conclusion, when wind fences is 3.0m height and porosity rate of 30% , the drag forces coefficient of train is 30% times greater than that of no wind fences and pitch moment coefficient is about 33.5% times greater than that of no wind fences.

Comparing results of wind tunnel with that of CFD, we can see that the force coefficients from wind tunnel is in good accordance with that from CFD.

TABLE. 3. Force Coefficient of beam,Wind fences and Train under Each test case

Force Coefficient Test Case Characteristics of Wind Fences

Location

of Train

Structure

CX CY CMZ

beam 1.532 0.788 0.153 wind fences / / /

1

No fences wall

windward

side train 2.217 0.005 -1.864 beam 1.456 -1.356 -0.354 wind fences 1.178 0.165 -0.287

2

3.0m height and

30% porosity

windward


3

3.0m height and

30% porosity

leeward


4

3.0m height and

30% porosity

no train

train - - -

Analysis the Operation Safety of Train

The CFD calculating results and the results form wind tunnel test are all show that it is move unsafely when the train running at the railway of windward side. So the operation safety analysis of train is focused on this state.

The method of quasi-static is used to operation safety analysis. Ignoring the influences of coupling gear force causing by other vehicle, disregarding the oscillating load of train, the wind loads are regarded as static load. The effects of crooked curve or orbit and swing load of train are ignored. Only single vehicle is considered saftly.

The passenger train which is called YZ25k is taked into considered. Its net weight is 45.6t. The distance between two contacting spot of two wheels at each sides of train is 1.5m. The train overturning coefficient is taken as 0.8 and the moment of self-stability is 273.6kN.m.

Table 4 shows the relation between train speed ,dangerous turning over wind speed (instantaneous wind speed) and 10min average wind speed. It can be see, when the train is at the state of motionless, the overturning instantaneous wind speed is 56m/s. that is to say, when the transient wind speed is larger than 56m/s,the overturning moment causing by wind will be larger than stable moment. And the train is in dangerous state. The wind speed is called dangerous wind speed. According to the formula (1), the 10min average wind speed can also be get. Also shows in Table 4.


It can also be seen. with the train speed increases, the dangerous wind speed will decreases. When the train runs under the speed of 160km/h, the average turning over wind speeds at the height of bridge are all greater than the largest wind speed of Beaufort scale 12 wind. The threshold value of wind velocity for safety operation of train can be enhanced to above Beaufort scale 12 wind with rational wind fences.

Table 4 Relation of Train Speed, Dangerous Turning Over Wind Speed and Wind

Speed(10min) Speed of train

（km/h） 0 30 50 80 100 120 160

Dangerous turning over wind speed (transient, m/s)

56 54.5 54.0 52.5 52.0 51.0 49.5

wind speed(10min average m/s) 47.9 46.5 46.1 44.7 44.3 43.4 42.0

Conclusion

By wind tunnel model testing and CFD simulation to the wind load parameters of wind fences, bridge and train, the following conclusion can be made:

1) The force coefficient results from wind tunnel test and from CFD are in good accordance. According to the CFD calculating, the wind pressure coefficient of wind fences distributes evenly along the height.

2) The drag-force, overturning moment and other wind loads of train decrease obviously after setting up wind fences. When the wind-fences is 3.0m height and the porosity is 21.6%, the drag force coefficient of train is 23% of the coefficient without setting wind fences, and the overturn moment coefficient of train is 25% of the coefficient without setting wind fences. When the wind-fences is 3.0m height and the porosity is 30%, the drag force coefficient of train is 30% of the coefficient without setting wind fences, and the overturn moment coefficient of train is 33.5% of the coefficient without setting wind fences. When the wind-fences is 3.5m height and the porosity is 30%, the drag force coefficient of train is 22% of the coefficient without setting wind fences, and the overturn moment coefficient of train is 23% of the coefficient without setting wind fences.

3) With the increasing of wind fence height, coefficients of drag force and pitch moments of train decrease slightly, but the coefficients of lift force of train increase.

4) With the porosity of wind fence decreasing, the coefficients of drag force and pitch moments of train decrease, but the coefficients of lift force of train increase. The changing of the porosity of wind fences (limited to 20%-30%) has little effect on the coefficients of drag force and pitch moments.

5) The train running at the windward side railway or at leeward side railway, it has little affect on the wind loads of wind fences, but has greatly affect on the wind load of train itself. The wind load of train is larger when the train running at the windward side of railway.

6) It is proved a good wind shield effect can be expected with wind fences of 3m or 3.5m high and the porosity of 30%. The higher of the wind fences, the larger wind load of the bridge. The threshold value of wind velocity for safety operation of train can be enhanced to above Beaufort scale 12 wind with rational wind fences.


References

Simiu,E., and Scanlan, R.H.(1986),“Wind effects on structures＂, 2d Ed,. John Wiley and Sons, New York, N.Y..

J.D.Holmes(2001), “Wind loading of parallel free-standing walls on bridges, cliffs, embankments and ridges”[J], Journal of Wind Engineering and Industrial Aerodynamics ,89,1397–1407.

S. Charuvisit, K. Kimura, Y. Fujino(2004), “Effects of wind fence on a vehicle passing in the wake of a bridge tower in cross wind and its response”[J], Journal of Wind Engineering and Industrial Aerodynamics,92, 609–639.

K.G. Ranga Raju, J. Loeser, E.J. Plate(1976), “Velocity profiles and fence drag for a turbulent boundary layer along smooth and rough flat plates”[J], J. Fluid Mech. 76, 383–399.

C.W. Letchford, J.D. Holmes (1994), “Wind loads on free-standing walls in turbulent boundary layers”[J], J.Wind Eng. Ind. Aerodyn. 51,1–27.


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