Navier-Stokes solver convergence issues

Extension of Elmer in computational glaciology

Navier-Stokes solver convergence issues

Postby LizR » 11 Jan 2016, 08:52

Hello,

I am having some problems with the Navier-Stokes solver. I would like to perform a simple diagnostic test to apply pressure to the top of a cube of ice and get the stresses as an output. I have modified the ForceToStress test case to create a simple example. However, if I replace the ForceToStress solver with the Navier-Stokes solver it won't run. I keep getting this error - ERROR:: IterSolve: Failed convergence tolerances.

I have tried increasing the convergence tolerance but if I make it too large the answers won't mean anything and I don't think tight tolerances are the cause of the problem.

I have been trying to do essentially the same thing in a few different ways with different .sif files and different meshes but I keep hitting this problem. What am I missing?
Attachments
simple.sif
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cube.grd
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LizR
 
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Re: Navier-Stokes solver convergence issues

Postby raback » 11 Jan 2016, 10:28

Hi

Your problem has a large null space. Assume a solution and add constant velocity to that and it will also be a solution. Hence you cannot have a solution for a linear system. You should add some BCs which determine the velocity field uniquely.

Apart from that I don't really understand the setting. You seem to solve the N-S in a 2D plate where also the external pressure is given. It would seem more natural to give the BC for a lower dimensional entity.

-Peter
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Re: Navier-Stokes solver convergence issues

Postby LizR » 15 Feb 2016, 09:21

Since the last post I have made the boundary conditions and the mesh geometry more realistic and I understand a lot more about what the boundary conditions do now.
However, I still can't seem to get the Navier-Stokes solver to do a diagnostic run. Using the ElasticSolver and equivalent BCs works just fine.
I don't understand what the difference is, and why the Navier-Stokes solution will not converge.
Any help would be much appreciated.
-Liz
Attachments
Tests.zip
Contains elastic and NS solver versions of the same problem
(422.1 KiB) Downloaded 101 times
LizR
 
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Location: ACE CRC Tasmania

Re: Navier-Stokes solver convergence issues

Postby raback » 18 Feb 2016, 02:23

Hi Liz

Elasticity solver has a Lagrangian formulation and the displacements are directly linked to the shape of the domain. One solver is therefore sufficient even with a moving surface.

The N-S solver is naturally written in Eulerien frame of reference. Velocity is not directly associated with the shape of the domain. Instead you need a free surface solver, a mesh adaptation solver, and an ALE formulation in the N-S solver. There are many examples among tests, and the Elmer/Ice community can probably provide you with real cases.

-Peter
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Re: Navier-Stokes solver convergence issues

Postby LizR » 18 Feb 2016, 02:30

Hi Peter
Thank you very much for telling me this. It is good to know that the issue was things that were missing, not things that were incorrect per se. I will continue working on it.
-Liz
LizR
 
Posts: 6
Joined: 14 Dec 2015, 08:39
Location: ACE CRC Tasmania


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