Discussion Forum

I am uploading my model files with the hope that someone can help me resolve these issues.
Attached are four files:
mesh_refine.mph
mesh_refinetext.pdf
length_conv.mph
length_convtext.pdf

I wrote earlier "My FSI model converges only for two specific kinds of meshes. The same model does not converge when I refine either of the meshes. What can I do to achieve convergence for finer meshes?"
Please see the attached model file mesh_refine.mph. The log file associated with this model file is
mesh_refinetext.pdf. This model file converges for mesh11 but when I refine mesh11(the refined mesh is called
mesh12), the problem does not converge. Length of the pipe or artery is 10mm. Why is this happening?

I wrote earlier "Also, the FSI model consists of a 10mm long pipe with fluid flowing through it. As I mentioned earlier, I achieve convergence with 2 kinds of meshes. However if I increase the length of the pipe to 20mm, the problem does not convergence even with the meshes that worked for the pipe that was 10mm long. Can anybody tell me what this means?"
Please see the model file length_conv.mph and its associated log file length_convtext.pdf. This model file was
created using mesh_refine.mph. The mesh used here is mesh 11. Even though the problem converges for
mesh 11 when the length of the artery or pipe is 10mm, it fails to converge for other lengths >10mm (say 15 or
20mm). The log file shows an error in sweep. To me it looks like the error is related to the meshing but I don't
know why it is happening and how to fix it.

Thanks for the help.
Sirisha

I am still waiting for answer ............

Thanks for the help.
Sirisha

? Anyone?
Its important because I hit the same problem for a different kind of mesh too. Anybody have any suggestions?

Thanks,
Sirisha

Hi

you have a model that is getting quite complex as you have added many things, I would propose that you restart from scratch, with your geometry, consider the wall as fixed and get the fluid running, then add a "soft" wall think over if you need an additional muscle/fat layer to see the wall deform realistically and solve that, and so on.

currently your model seem to have a few BC not fully defined, Are you sure your mesh is fully and correctly constrained everywhere ?

also I would add a starting velocity profile and pressure drop to help the solver get along, starting at "0" is quite far away from the correct solution

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Good luck
Ivar

Dear Ivar,

I appreciate your response.

I did start by solving only the flow model. Then I solved the solid model. After I had success with both, I combined them and created the current model.

I currently use a parametric solver where the inlet BCs actually start at 0 and step up in steps of 10. So that should help the solver. Even otherwise, I did use the inlet conditions as the initial values. I noticed that it didnot make any difference from what I already did. I thought its because of the parametric solver.

Please let me know the BCs that you think are not fully defined.

I thought that the mesh was fully constrained. If it were not fully constrained then why is it working for some meshes and not for the others?

Are there any BCs that you think will help my case? I want the pipe to expand radially at the inlet and outlet and contract longitudinally at the outlet.

I understand that the problem is complex. But isn't that what COMSOL advertises that the software is capable of modeling realistic blood flows? The goal is to finish with time-dependent Bcs.

Thanks,
Sirisha

Hi

when I had a look, the output region seemed not to be constrained as I would have expected it (I'm not by my COMSOL coputer so I cannot reopen it just now so I cannot remember everything)

Check ech BC, are there a boundary or domain defined, are the values OK ... for each physics (incl ALE) do you have enough defined BC's ? Reminder, fluid flows across the mesh, the mesh is not supposed to exit via the outlet, even if the fluid is doing so in the simulation

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Good luck
Ivar

Reminder, fluid flows across the mesh, the mesh is not supposed to exit via the outlet, even if the fluid is doing so in the simulation

--
Good luck
Ivar

Hi

Do you mean the fluid mesh has to be fixed at the outlet region? Please confirm.
I want the outlet end to move freely along the axial and radial direction while maintaining the symmetry about the centerline axis. So I left the mesh to move freely.

Thanks,
sirisha

Hi

OK then you should not restrict it,

but why should the mesh move ? the mesh and the fluid movements are not directly 1:1 related

--
Good luck
Ivar

Ivar,

Because I want the pipe to expand radially and contract longitudinally, I left the outlet end to be free (for the solid domain) and I did not restrict the mesh in the fluid region. If I fix the mesh in the fluid region at the outlet, when the pipe expands, you will notice a tear at the interface of the solid and the fluid boundary.

Any other ideas to fix the problem of "the model converges for 2 specific meshes but not for others"........

Thanks!
sirisha