1. Introduction
Inflatable membrane structure include gas-bearing structure and air-ribbed structure. The inflatable structure has the advantages of short molding time, low construction cost and light structure weight, which has been gradually attracted worldwide attention in recent years.
^{[1-4]} The U.S. military has developed various inflatable hangars, which have improved the aircraft attendance rate and reduced the cost of the aircraft's whole-life maintenance costs. The long-span inflatable structure is greatly affected by wind load, many accidents occurredunder the influence of typhoons. This paper presents a simulation analysis of the long-span gas-rib membrane structure. The modeling and simulation is carried out in different windward angles and wind speeds, and the worst condition is carried out, this simulation can help provide helpful reference for the construction of membrane structure.
2. CFD Modeling
In this paper, the wind load characteristics of long-span gas-ribbed film are analyzed by means of CFD.
The model is established every ten degrees from 0° to 90°, model coordinates are shown in Figure 1.
Theturbulence model adopts k-ωSST model, the boundary condition are shown in Table 1
Table 1. The boundary condition
Name |
Boundary type |
inlet |
velocity-inlet |
outlet |
pressure-oulet |
inwall |
wall |
outwall |
wall |
Figure 1. Initial model of CFD calculation
3. Simulation Results
The distribution of wind pressure under different angles and wind speeds of the same model is calculated respectively, wind speed increased from 5 meters per second to 25 meters per second，with a spacing of 5 meters per second.
Figure 2. Drag force of X direction under different angles and wind speeds
Figure 3. Lift force of Y direction under different angles and wind speeds
Figure 4. Drag force of Z direction under different angles and wind speeds
The angle of maximum drag and lift force at the same wind speed is different in different length-width ratio. Under the wind speed of 20 meter per second, the wind load of the same span ratio with different width models is shown in Figures below.
Figure 5. Drag force of X direction under different angles and length-width ratios
Figure 6. Lift force of Y direction under different angles and length-width ratios
Figure 7. Drag force of Z direction under different angles and length-width ratio
4. Simulation Results Analysis
The wind load of the long-span arch film structure includes horizontal drag force and lift force, both the drag and the lift are influenced by the structure and wind direction.
4. 1 Effect of Windward Angle on Surface Wind Pressure
From the calculation above, wind direction has a great influence on the lift and drag. The length-width ratio of the initial model is 0. 484, drag force of X direction reaches maximum in the angle of 20 degrees of different wind speeds in Figure 2, lift force of Y direction reaches maximum in the angle of 30 degrees in Figure 3, and the drag force of Z direction reaches maximum in the angle of 60 degrees in Figure 4. Maximum force degree is not related to wind speed but to the wind direction according to the calculation.
4. 2 Effect of Length-Width Ratio on Surface Wind Pressure
The result indicates that length-width has a great influence on the surface wind pressure. Take the inlet wind speed as 20 m/s, Figures above show the influence of the length-width ratio. When the ratio is small, the worst working condition may be less affected by the windward angle as the Figures shows above, the changes of lift force and drag force in X direction is relatively smooth when the length-width ratio is 0. 226. As the ratio increases, the effect of wind load becomes more obvious.
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Figure 5 shows that the develop of the ratio increase the maximum angle of drag force in X direction, the same rule is shown in lift force and drag force in Z direction.
5. Conclusion
The air flow can pass through the membrane structure, with the increase of windward angle, drag force changes rapidly. The increase in lift force caused by wind angle changes is relatively small, however the drop is obvious after reached the maximum.
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The drag force of X to the drag of Z is approximately symmetrical. However the distribution is different at 0°and 90°, because the force of Z direction is close to zero.
The worst working condition is 60 degrees in the wind direction. The air flows along the oblique, the outer of the windward side and the inner of the lee side are subjected to higher wind pressure, when the air bypassed the gas-rib structure, turbulence generated, thus the inner of the windward side and the outer of lee side are subjected the negative pressure.
In which the drag force reaches the maximum, while the lift force is relatively small, in this case, membrane structure is more likely to collapse.
In the case of equal wind speed, maximum force angle increase as the length-width ratio, however if the ratio is too small, the change is not so obvious, as the calculation shows.
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