Hello,
I am having some problems with the NavierStokes 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 NavierStokes 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?
NavierStokes solver convergence issues
NavierStokes solver convergence issues
 Attachments

 simple.sif
 (3.12 KiB) Downloaded 212 times

 cube.grd
 (743 Bytes) Downloaded 204 times

 Site Admin
 Posts: 3394
 Joined: 22 Aug 2009, 11:57
 Antispam: Yes
 Location: Espoo, Finland
 Contact:
Re: NavierStokes solver convergence issues
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 NS 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
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 NS 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
Re: NavierStokes solver convergence issues
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 NavierStokes 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 NavierStokes solution will not converge.
Any help would be much appreciated.
Liz
However, I still can't seem to get the NavierStokes 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 NavierStokes 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 181 times

 Site Admin
 Posts: 3394
 Joined: 22 Aug 2009, 11:57
 Antispam: Yes
 Location: Espoo, Finland
 Contact:
Re: NavierStokes solver convergence issues
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 NS 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 NS solver. There are many examples among tests, and the Elmer/Ice community can probably provide you with real cases.
Peter
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 NS 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 NS solver. There are many examples among tests, and the Elmer/Ice community can probably provide you with real cases.
Peter
Re: NavierStokes solver convergence issues
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
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