FSI coupling

General discussion about Elmer
Post Reply
emB
Posts: 12
Joined: 20 Jul 2022, 10:22
Antispam: Yes

FSI coupling

Post by emB »

I am trying to utilize the block solver to couple the Helmholtz and StressSolver for an FSI problem as demonstrated in the group of tests/ShoeboxFSI group of test cases.

I have two questions.

This test case was introduced as strong FSI coupling, does this mean two-way coupling(or fully coupling)?

Is there a reason why convergence is not possible(or too many iterations needed) at some frequencies?
raback
Site Admin
Posts: 4812
Joined: 22 Aug 2009, 11:57
Antispam: Yes
Location: Espoo, Finland
Contact:

Re: FSI coupling

Post by raback »

Yes, the test cases correspond to strong coupling.

You could solve the system as a monolithic one. Probably the block preconditioning does not work so well near resonance.

-Peter
emB
Posts: 12
Joined: 20 Jul 2022, 10:22
Antispam: Yes

Re: FSI coupling

Post by emB »

Thank you Peter.

But I don't know what you mean by "strong coupling" and "the system as a monolithic one".

Some definitions, the way I understand them:

Single-way coupling : The structure is deformed, and the deformation is applied to the fluid as boundary condition. No iterations.
...versus:
Two-way coupling : The deformation of the structure perturbs the fluid, and the fluid perturbs back the structure, and so on until convergence.

Fully coupled: All the unknowns (pressures and displacements) are solve at the same time in the same matrix
...versus:
Segregated: Treats each physics sequentially, and iterates until convergence.

So, by "strong coupling" do you mean "Two-way coupling" ??

Thanks again
raback
Site Admin
Posts: 4812
Joined: 22 Aug 2009, 11:57
Antispam: Yes
Location: Espoo, Finland
Contact:

Re: FSI coupling

Post by raback »

Hi,

Sorry for being ambiguous. These have two-way coupling that is basically solved from the same matrix equation except the matrix is treated as a block matrix and block matrix preconditioning is used whereas the outer iteration minimizes the residual of the full system using GCR. There is a possibility to solve the system also as fully monolithic one which will allow the use of direct solvers but consumes more memory and has worse scalability.

-Peter
emB
Posts: 12
Joined: 20 Jul 2022, 10:22
Antispam: Yes

Re: FSI coupling

Post by emB »

Thank you peter , it's clear now.

I'm comparing the result of Comsol and Elmer, but it's not perfect yet.

How can I fix my sif file to get better results?
Attachments
case.sif
(7.8 KiB) Downloaded 46 times
emB
Posts: 12
Joined: 20 Jul 2022, 10:22
Antispam: Yes

Re: FSI coupling

Post by emB »

I'm testing the B.SIF file, Do you think the results will be different from A.SIF file?

B takes too long to converge, so I think it will take too long to compare with A.
Attachments
B.sif
(7.46 KiB) Downloaded 45 times
A.sif
(6.78 KiB) Downloaded 50 times
kevinarden
Posts: 2237
Joined: 25 Jan 2019, 01:28
Antispam: Yes

Re: FSI coupling

Post by kevinarden »

Better results most likely will come with mesh changes not sif changes. Many structural codes automatically use higher order elements even if you don't create them. Commercial codes put a lot of development in their element formulations and they keep them proprietary. Use a slightly finer mesh on the elmer problem and see if the answer changes. If it does than the mesh is not converged. If you do this on tests/ShoeboxFSI you will see that the results change.
emB
Posts: 12
Joined: 20 Jul 2022, 10:22
Antispam: Yes

Re: FSI coupling

Post by emB »

Thank you for your advice. I'll try it.
Post Reply