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undesired rolling during a symmetric deformation
Posted Mar 2, 2012, 6:02 p.m. EST Version 4.0, Version 4.0a, Version 4.1, Version 4.2, Version 4.2a 5 Replies
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Hi,
I am simulating the effect of pressure change inside a flexible tube (cylinder) that causes it to radially stretch. This is not an axisymmetric problem, but is a 2D problem (look at it from the circular section of that and parallel to the axis of the tube). Due to symmetry, I only have simulated the upper half of the cylinder. On the inner boundary I have a pressure load and the two walls cut sections are considered symmetry. Although the deformation seems to be symmetric, the deformed geometry translates (rolls) to one side; for example, if the center of the cylinder before pressurizing it is at (x,y)=(0,0) after the symmetric pressure load it moves to (x,y)=(0.5,0) rather than staying at (0,0). In addition, when I repeated the solution, I noticed that the amount of rolling differs from the previous run (x,y)=(0.25,0). This means that there is not a unique solution for the problem or probably the problem is not very well-defined. I do not think that if I wanted to simulate a full cylinder I would get this translation, but there is a justified reason that I opted for a half cylinder.
Also, the mesh is pretty fine, although it is not symmetric because it is of an unstructured type. So I do not think them mesh has any effect on it.
I wonder why there is no unique solution, and how can I restrict the undesired translation without fixing any point on the domain.
Your input is appreciated.
Thanks,
Jessica
I am simulating the effect of pressure change inside a flexible tube (cylinder) that causes it to radially stretch. This is not an axisymmetric problem, but is a 2D problem (look at it from the circular section of that and parallel to the axis of the tube). Due to symmetry, I only have simulated the upper half of the cylinder. On the inner boundary I have a pressure load and the two walls cut sections are considered symmetry. Although the deformation seems to be symmetric, the deformed geometry translates (rolls) to one side; for example, if the center of the cylinder before pressurizing it is at (x,y)=(0,0) after the symmetric pressure load it moves to (x,y)=(0.5,0) rather than staying at (0,0). In addition, when I repeated the solution, I noticed that the amount of rolling differs from the previous run (x,y)=(0.25,0). This means that there is not a unique solution for the problem or probably the problem is not very well-defined. I do not think that if I wanted to simulate a full cylinder I would get this translation, but there is a justified reason that I opted for a half cylinder.
Also, the mesh is pretty fine, although it is not symmetric because it is of an unstructured type. So I do not think them mesh has any effect on it.
I wonder why there is no unique solution, and how can I restrict the undesired translation without fixing any point on the domain.
Your input is appreciated.
Thanks,
Jessica
5 Replies Last Post Mar 15, 2012, 4:53 p.m. EDT
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Posted:
1 decade ago
Hi
ifyou do nt constrain enough DoF your final object might move around, often one can get rid of such effects by adding a symmetry boundary, or splitting the object along the symmetry bounary, mesh one half and mirror the mesh onto the other symmetric part (at least this should reduce the motion).
But in real life if you blow up a baloon, it will move left and right as you bow it up, due to small inhomogeniousities in the air flow, turbulence, material non homogeinities, so in some sens its physical, but can be unwanted for the modelling, I agree
--
Good luck
Ivar
ifyou do nt constrain enough DoF your final object might move around, often one can get rid of such effects by adding a symmetry boundary, or splitting the object along the symmetry bounary, mesh one half and mirror the mesh onto the other symmetric part (at least this should reduce the motion).
But in real life if you blow up a baloon, it will move left and right as you bow it up, due to small inhomogeniousities in the air flow, turbulence, material non homogeinities, so in some sens its physical, but can be unwanted for the modelling, I agree
--
Good luck
Ivar
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Posted:
1 decade ago
I agree, for this problem I would use a 2D asymmetric simulation with the centre of the tube as the edge of symmetry.
Simulation wise the centre would be impossible to move.
Simulation wise the centre would be impossible to move.
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Posted:
1 decade ago
I agree, for this problem I would use a 2D asymmetric simulation with the centre of the tube as the edge of symmetry.
Simulation wise the centre would be impossible to move.
Hi Jaap,
Thanks for the your comments. In fact, my problem is not axisymmetric but it is a 2D problem. Imagine you are looking at the circular section of the tube.
Jessica.
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Posted:
1 decade ago
Hi
ifyou do nt constrain enough DoF your final object might move around, often one can get rid of such effects by adding a symmetry boundary, or splitting the object along the symmetry bounary, mesh one half and mirror the mesh onto the other symmetric part (at least this should reduce the motion).
But in real life if you blow up a baloon, it will move left and right as you bow it up, due to small inhomogeniousities in the air flow, turbulence, material non homogeinities, so in some sens its physical, but can be unwanted for the modelling, I agree
--
Good luck
Ivar
Hi Ivar,
Thanks for your comments.
I completely agree with you about the actual physical conditions that may result in non-symmetric deformation and displacement. I believe that I have sufficiently constrained my model by assigning symmetry condition. I also tried a structured (mapped) mesh to eliminate any chance of asymetric results due to asymetric unstructured mesh. This method didn't work either. I am basically simulating an axially cut blood vessel which has been already opened from 360 degrees to 140 degrees. Now I want to close it again and see how much stress will be developed when I close it. For reducing the simulation time and simplifying formulation I am considering just half of the cut vessel (from 70 degrees to 140 degrees). Then I want to pressurize it and see what happens.
So I may want to try only to simulate one quarter of the tube and see what happens.
Thanks again.
Jessica
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Posted:
1 decade ago
I have an update on this:
In my 2D half cylinder, I added a line from the inner to the outer boundary, exactly half way from the two ends. COMSOL automatically considers it an interior boundary, where the continuity is also met, and I did not need to add any boundary condition for it. Then I constrained that interior boundary by setting the Prescribed Displacement in x direction equal to zero, and it worked very well as the attached picture shows.
Jessica
In my 2D half cylinder, I added a line from the inner to the outer boundary, exactly half way from the two ends. COMSOL automatically considers it an interior boundary, where the continuity is also met, and I did not need to add any boundary condition for it. Then I constrained that interior boundary by setting the Prescribed Displacement in x direction equal to zero, and it worked very well as the attached picture shows.
Jessica
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