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endlessly assembling matrices in transient solver

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Hello,

after fruitlessly trying almost every possible way to solve a Navier-stokes equation with volumic forces in 3D with very few ram (2GB), and thus putting my software to all the stresses possible, it ended up that now it just refuses to go beyond the first time step in transient analysis, but not because the solver runs out of memory, but because, after solving "successfuly" the first linear system, the solver keeps going by endlessly "assembling matrices", with percentages always in the same set (46-93-97-100 etc.), and this for minutes long, with the time parameter never taking off 0.actually this first happened when, after several trials, I tried UMFPACK again, knowing that it should not succeed and run out of memory around 60%, because that already happened before with same model and same mesh, but I tried it nonetheless, and it surprisingly succeeded going all the way to 100%, and there, it started the endless "matrix assembly". And, I noticed there a weird fact: I always monitor the RAM usage by comsol, and there it appears that UMFPACK allocated itself a lot more ram than usual, which is of course good for me, and is why probably it succeeded to "solve the system", but I still don't understand why it came to this problem of endless assembly. At this run, the "memory allocation factor" was set to 1, and after the bug I lowered it to 0.8, while restarting comsol: there, UMFPACK allocated the same few memory as usual (at least 3 times less), and ran out of it. then I tried Pardiso, and SPOOLES. And SPOOLES succeeded again, even with a restart, but again, I got my endless "assemblies".
Another point: before that I was trying to get something from the GMRES solver: after an incomplete LU, it solves correctly its first 2 systems, then is unable to converge on the third one. At first I thought the third system was for one of the variables, and I immediately thought about the pressure, which was completely unspecified, so I specified it on one vertice. But it did not solve the problem: GMRES is still not converging on the third system. That was why I re-tried the direct solvers, with all the weird things listed above...

Please, have anybody seen any similar behavior, like endlessly assembling matrices in transient navier-stokes analysis in 3D? because I have never seen that before...

Thank you very much for your help, of any kind

Jean-Pierre

6 Replies Last Post Jun 5, 2017, 2:18 p.m. EDT
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Hello Jean-Pierre Matan

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Posted: 1 decade ago Jul 22, 2011, 6:28 a.m. EDT
I am also having this issue while trying to solve eigenfrequency analysis. I do not have very much experience with COMSOL, but have been doing essentially this same calculation, but with different geometries. Now, I cannot seem to get passed the assembling matrices step.
I hope someone with some more knowledge notices our two posts soon.
I am also having this issue while trying to solve eigenfrequency analysis. I do not have very much experience with COMSOL, but have been doing essentially this same calculation, but with different geometries. Now, I cannot seem to get passed the assembling matrices step. I hope someone with some more knowledge notices our two posts soon.

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Posted: 1 decade ago Aug 3, 2011, 6:59 p.m. EDT
did you find the solution to this problem?

Thanks.
did you find the solution to this problem? Thanks.

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Posted: 1 decade ago Feb 10, 2012, 5:14 a.m. EST
I also have the problem now in Structure-Fluid interaction modeling. Do you know how to solve it now? Thanks!
I also have the problem now in Structure-Fluid interaction modeling. Do you know how to solve it now? Thanks!

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Posted: 9 years ago Apr 10, 2016, 8:36 a.m. EDT
It seems this problem is machine specific. I have two installations on two PC. The problem occurs to one but not the other one.

Cheers!
--
Pu, ZHANG
DTU Fotonik
It seems this problem is machine specific. I have two installations on two PC. The problem occurs to one but not the other one. Cheers! -- Pu, ZHANG DTU Fotonik

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Posted: 7 years ago Jun 5, 2017, 11:26 a.m. EDT
I have the same problem when I am doing heat transfer simulation. Can anyone find the solution? Thanks.
I have the same problem when I am doing heat transfer simulation. Can anyone find the solution? Thanks.

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Posted: 7 years ago Jun 5, 2017, 2:18 p.m. EDT
I did some research on this problem and I found that my endlessly assembling matrices problem in transient solver is because my model is a high nonlinear problem. What I did is I applied a time-dependent solver to see what's going and this worked!

Please follow this link for more instructions: community.cmc.ca/docs/DOC-1453
I did some research on this problem and I found that my endlessly assembling matrices problem in transient solver is because my model is a high nonlinear problem. What I did is I applied a time-dependent solver to see what's going and this worked! Please follow this link for more instructions: https://community.cmc.ca/docs/DOC-1453

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