Hi Wolfgang,

thank you for your reply. I will look at the extensions of step-22, as well 
as the parallelization of the code detailed in step-32.

Cheers,


Alex

El divendres, 26 juny de 2020 10:24:20 UTC-6, Wolfgang Bangerth va escriure:
>
>
> Alex, 
>
> > I am solving the Stokes flow problem in three dimensions using OpenFCST 
> > <
> https://nam01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fwww.openfcst.mece.ualberta.ca%2F&data=02%7C01%7CWolfgang.Bangerth%40colostate.edu%7Ccd3182fe6dfc4066e6b208d81897e47a%7Cafb58802ff7a4bb1ab21367ff2ecfc8b%7C0%7C1%7C637286387952996923&sdata=5F%2BU95iUemz1oaDHrEaZ8C3xevwIzmoSqrI3UMU%2F2%2FA%3D&reserved=0>,
>  
>
> > which is an open-source platform for fuel cell simulations and it is 
> based on 
> > deal.II v8.4.1. The code that I use is mostly based on the code in 
> Step-22, 
> > and I have noticed that my simulations fail when my mesh has more than 
> 20 
> > million DOFs. In order to check if the problem comes from deal.ii and 
> not from 
> > OpenFCST, I did the following: 
> > 
> > 1. Modified slightly the code in step-22 (see file "step-22.cc" 
> attached)  to 
> > consider a domain in 3D that is of size 100x100x50 and it has 50 
> divisions per 
> > direction. 
> > 2. Applied the same boundary conditions that are applied in step-22 
> > 4. Ran the simulation both in deal.II v8.4.1 and v.9.0.1 with less than 
> 20 
> > million DOFs, and I was able to obtain a solution (see file 
> "DealII_test.png" 
> > attached). 
> > 5. Ran the simulation in deal.II v8.4.1 -> it resulted in a 
> "Segmentation 
> > fault" error (see file "output_v8_4_1.out"). 
> > 5. Ran the simulation in deal.II v9.0.1 -> it resulted in a "nan 
> residual" 
> > error (see file "output_v9_0_1.out"). 
> > 
> > I was wondering if someone could please tell me why this issue is 
> appearing 
> > when I consider a mesh with more than 20 million DOFs, and if there is a 
> > solution to this. 
>
> 20 million unknowns is quite a large number, especially if you are using 
> the 
> solver used in step-22. My suspicion is that in both cases, the error 
> message 
> really just points out that you're run out of memory. 
>
> It would be interesting to see how your program's memory use increases as 
> you 
> go from smaller to large problems. I suspect that you will see that for 20 
> million unknowns on a single machine, you will need ~100GB of memory and 
> that 
> that exceeds what the operating system is willing to give you. 
>
> 20 million unknowns is solidly in the region where you need (i) a parallel 
> machine, and (ii) a better linear solver than the one used in step-22. 
> Both 
> exist in deal.II: a better linear solver is discussed in the 
> "Possibilities 
> for extensions" of step-22, and is implemented in a parallel fashion in 
> step-32 among others. 
>
> Best 
>   W. 
>
> -- 
> ------------------------------------------------------------------------ 
> Wolfgang Bangerth          email:                 bang...@colostate.edu 
> <javascript:> 
>                             www: http://www.math.colostate.edu/~bangerth/ 
>
>

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