## Problems with basal sliding when applying Porous Solver

Extension of Elmer in computational glaciology
glebcher
Posts: 12
Joined: 07 Apr 2018, 17:58
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### Problems with basal sliding when applying Porous Solver

Dear Elmer users,

I am trying to introduce basal sliding into a thermo-mechanically coupled model for a polythermal mountain glacier. I use snow/firn rheology (Porous Solver). If I apply no-slip conditions at the bedrock, the case works. But when I set the basal boundary condition as described in Elmer/Ice Wiki (http://elmerfem.org/elmerice/wiki/doku. ... ers:porous), no convergence is seen even after 10 000 linear system solver iterations. The error message is:

Code: Select all

      9990 0.9374E+00
10000 0.9391E+00
10001 0.9391E+00
NUMERICAL ERROR:: IterSolve: Too many iterations was needed.
Note: The following floating-point exceptions are signalling: IEEE_DENORMAL

The sif and mesh files are attached.
porous_thermomech_slide.sif
Data.rar
Also, it may worth mentioning that the solver is restarted from the previous solution (unfortunately, the restart file is too large to be attached). I will also provide additional information, if needed. I do not know what is incorrect and how to obtain convergence. So, I would be very grateful for any help or advice.

Regards,
Gleb
Last edited by glebcher on 23 Nov 2020, 18:16, edited 3 times in total.
fgillet
Posts: 46
Joined: 30 Sep 2010, 16:58

### Re: Problems with basal sliding when applying Porous Solver

Hello;
if the iterative solver is not converging you can try to use a direct solver (umfpack in serial or Mumps in parallel) this should always work (at least if you have enough memory).

If the direct method is too slow you may still try to use another iterative solver and/or preconditionner.
glebcher
Posts: 12
Joined: 07 Apr 2018, 17:58
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### Re: Problems with basal sliding when applying Porous Solver

Hello, fgillet!

Thank you very much for your reply and recommendations. I will try to do everything that you suggested. Unfortunately, I have not managed to attach any files, although not all of them were too large. I will try to fix it somehow.

Regards,
Gleb
glebcher
Posts: 12
Joined: 07 Apr 2018, 17:58
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### Re: Problems with basal sliding when applying Porous Solver

An update to the post: the sif and mesh files are now attached and available.

Regards,
Gleb
glebcher
Posts: 12
Joined: 07 Apr 2018, 17:58
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### Re: Problems with basal sliding when applying Porous Solver

Dear Elmer users,

the problem with convergence has been finally solved by applying the GCR method with the restart parameter equal to 300 and the ILU0 preconditioner in the Porous Solver section. Thanks fgillet for the useful recommendations!

Regards,
Gleb
glebcher
Posts: 12
Joined: 07 Apr 2018, 17:58
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### Re: Problems with basal sliding when applying Porous Solver

Dear Elmer users,

my new question concerns the friction law associated with the Porous Solver (http://elmerfem.org/elmerice/wiki/doku. ... ers:porous). In the basal boundary conditions, friction is introduced as follows:

Code: Select all

Boundary Condition 2
Target Boundaries  = 2
Normal-tangential Porous = True
Porous 1 = Real 0.0
Porous Slip Coeff 2 = Real 0.1
Porous Slip Coeff 3 = Real 0.1
End

I would like to know, what is the analytical form of the friction law used here? How it can be expressed by an explicit formula? I would really appreciate an explanation.

Regards,
Gleb
gagliar
Posts: 77
Joined: 04 Sep 2009, 16:34
Location: LGGE - Grenoble
Contact:

### Re: Problems with basal sliding when applying Porous Solver

Dear Gleb,

The form of the friction law here is linear, i.e. \tau_nt = C u_t where C is the "Porous Slip Coef t", for t = 1 and 2. If you want to use more complex friction laws, you should have a look on the wiki in User Functions (Weertman, Coulomb, Budd).

Hope it helps,
Olivier
glebcher
Posts: 12
Joined: 07 Apr 2018, 17:58
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Dear Olivier,