Learning Forum

[eric1234598765]

Hi,

I'm new to Ansys static structure and trying to simulate the structure to find out if it could withstand the ambient pressure. Although most of the simulations(examples) don't consider ambient pressure ( 101325 Pa), it is important if there's pressure different between ambient pressure and container pressure as shown in 1st picture. Let's assume P1 is nearly vacuumed, and P2 is in ambient pressure , which is 101325 Pa. Then the structure below may be collapsed by air if the structure is weak. The second case is when P1 and P2 are the same, let's say 101325 Pa, than the object should not collapse.

Therefore, the question is , when I applied pressure to all surfaces with 101325 Pa, my structure deformed severely, this means that this structure cannot survive in ambient?(P.S: the structure is made of steel)

Obviously, it seems that I was wrong, therefore, I'm looking for help here about the effect of ambient pressure to the structure and the boundary condition setting of the picture below.

[Rameez_ul_Haq]

,why do you think you were wrong?

[eric1234598765]

Hi, In my opinion, structure should not collapse without any force acted on it except ambient air if it could be build when I set pressure to all surfaces, the results looks like there's other external force collapsing the structure, and therefore the structure deformed. Picture below is the simple geometry before calculating, and the next one is after calculating.

If I am correct, then this means that the structure would suddenly deform after entering ambient air, and that's the reason I felt the result is weird.
Hope this answer your question.

Thanks Eric

[Rameez_ul_Haq]

,I mean if you are applying some external pressure with no pressure inside the structure, then if the model is illustrating a failure, so I don't think the model in reality will behave otherwise. According to me, the structure will fail and collapse in reality as well if it is vacum from the inside and ambient pressure is applied on it.
One thing you can do is to increase the thickness of the structure, and then check if the stresses decrease and become lower than the critical value (this can be yield strength of material). If this happens, then it means that your original structure is indeed not strong enough to survive it.

[Rameez_ul_Haq]

And I don't know how are you coming to the conclusion that the structure is collapsing. Do you consider collasping as stresses being higher than the yield strength (or a factor of yield strength)? Because this is the only way you can use to reach to the conclusion if the structure is suffering from any failure or not. I don't use any other method to decide if the structure has failed or not.

[peteroznewman]

If this part was manufactured in a vacuum, then exposed to atmospheric pressure, the faces would move inward by a few nanometers. The model is correct. I don't see anything weird about the result. Perhaps you can't appreciate how small a nanometer is.

[eric1234598765]

Hi, all
If you read my boundary condition of the 3ird picture, then you will see that the structure is actually under pressure in ambient on all of the surfaces. The reason that I'm looking for deformation instead of structure strength is because changing the geometry of the shape does affect the aerodynamic of vehicle, therefore, the ideal condition is that the material geometry won't change much under different ambient pressure. Actually, the link below is what I am trying to express in this post, and the answer in that link is air pressure balance themselves or there's no much effect on the structure. Therefore, I cannot accept the result when I see structure deformed obviously when I consider ambient pressure into account. Pictures below are another geometry before and after calculation, respectively, which are simulating the structure under 1 atm.


As you can see, although the deformation of the contour is small, but the geometry changes obviously to me. Anyway, I'm still looking forward any idea about this topic.
Thank you and for giving advices.

Thanks!
Eric

[Rameez_ul_Haq]

Edit - some text removed at author's request.

Its your engineering judgement to decide if the deformations are big enough or not, if they can actually change the aerodynamics of the vehicle or not.

[peteroznewman]

the ideal condition is thatthe material geometry won't change much under different ambient pressure
To evaluate the change in shape due to changes in ambient pressure, you would apply the pressure change, not the total pressure.
If the part was made at a pressure of 101325 Pa then the pressure changed to 102325 Pa, the pressure to apply to the model would be 1,000 Pa and if the pressure changed to 100325, the pressure to apply to the model would be -1000 Pa.
When you apply a pressure of 101325 Pa, you are simulating making the part in a perfect vacuum chamber, then bringing it out to the atmosphere. If you were to apply 1,000 Pa, then the deformation would be 100 times smaller. That means the deformation would be 5e-10 m or 50 nm.
The diameter of a molecule of Nitrogen is 0.3 nm, but they are spaced about 9 diameters apart in air so the distance between two molecules of nitrogen would be 2.7 nm.
I don't expect deformation of the part surface by 18 molecules of gas would be enough for any change in aerodynamic performance.
The only reason you can see the shape of the deformation is because the display has multiplied the real deformation by a factor of several million to make it visible.

[Rameez_ul_Haq]

Edit more tidying at author's request.

The pressure is constant so if you increase the area, the force increases by the same proportion. Nevertheless, the force will exist, signifying that displacements must exist.

[eric1234598765]

Hi After reading all the advices, here's my conclusion about this case .
1.Ansys static structural model automatically adjust the scaling of deformation which is 1E+07 in my case, and that's why the object deform obviously in the animation. When I adjust the scaling to the true scale, no obvious deformation observed.


2.I'm agree with statement that the effect of ambient pressure apply on the object is the P0 -P1, where P0 is the ambient pressure where the deformation of the object is zero, which is usually 1 atm because most of the object live or be produced at that condition, and P1 is the different ambient pressure.

Last is the my feedback to the post:
Maybe the deformation caused by changing ambient pressure is small, (about nanometer), there's groups of people discussing this effect to crustal, which causes crustal deforms although I'm not expert in that field. The picture below is the phenomenon of atmospheric pressure loading and the reason why they concern.
You can refer to this page via http://rses.anu.edu.au/geodynamics/gps/atm/index.html

I'm appreciate for any support in this post, and please correct my statement in the conclusion if there're still something wrong.

Thanks you and again for sharing concept to me.

Eric