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pimpleDymFoam -- Courant Number increasing and Case crashing after few time steps

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Old   September 16, 2015, 09:21
Default pimpleDymFoam -- Courant Number increasing and Case crashing after few time steps
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Sravan Kumar
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Hi Foamers,

I am trying to simulate transient case of Centrifugal Pump for which I have taken initial condition from last time step of steady case using mapFields. I have not changed the Initial conditions and polymesh folder where I have my mesh files and boundary conditions. I have used the fvschemes , fvSolutions files from propeller and mixervesselAMI2D cases.

Also I have checked with Dynamic Mesh in paraview and it looks working well.

The problem is that initially the courant number is less than one but in next 4-5 iterations it is increasing rapidly and case is crashing.

In initially I have taken the time step of 1e-3 and went till 1e-8 but still its not solving the problem. Hence I suppose the problem is not with timestep control.

Also I have tried using first order divergence schemes for velocity ( Gauss upwind ). But even that doesnt turned out well.

I have tried out changing the tolerence values but no effect.

I am attaching the log file for further reference.

Please let me know if any other information is required

Thanks in Advance
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File Type: zip logfile.zip (166.9 KB, 25 views)
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Old   September 16, 2015, 09:30
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what does "checkMesh" give out? in my experiance with OF so far this behavior because of a bad mesh, to high order schemes in the beginning, or to little number of correctors. But mostly the mesh has flaws.

Else try to set a upper limit for Co in controldict.

hope i could help!
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Old   September 16, 2015, 09:42
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Please find the checkMesh result. I have one error but I do not think it is the cause of the problem. This mesh has worked perfectly for steady case.

Code:
Create time

Create polyMesh for time = 7000

Time = 7000

Mesh stats
    points:           311575
    faces:            889864
    internal faces:   846488
    cells:            289392
    faces per cell:   6
    boundary patches: 12
    point zones:      0
    face zones:       2
    cell zones:       2

Overall number of cells of each type:
    hexahedra:     289392
    prisms:        0
    wedges:        0
    pyramids:      0
    tet wedges:    0
    tetrahedra:    0
    polyhedra:     0

Checking topology...
    Boundary definition OK.
    Cell to face addressing OK.
    Point usage OK.
    Upper triangular ordering OK.
    Face vertices OK.
   *Number of regions: 2
    The mesh has multiple regions which are not connected by any face.
  <<Writing region information to "7000/cellToRegion"
  <<Writing region 0 with 178544 cells to cellSet region0
  <<Writing region 1 with 110848 cells to cellSet region1

Checking patch topology for multiply connected surfaces...
    Patch               Faces    Points   Surface topology                  
    OUTFLOW             1184     1229     ok (non-closed singly connected)  
    PIPE                3432     3520     ok (non-closed singly connected)  
    CASING              9676     9870     ok (non-closed singly connected)  
    IF2BACKCHANNEL      1208     1359     ok (non-closed singly connected)  
    IF2IMP-AMI1         4228     4379     ok (non-closed singly connected)  
    PASSAGE-HUB         4786     5090     ok (non-closed singly connected)  
    PASSAGE-SHROUD      4786     5090     ok (non-closed singly connected)  
    PASSAGE-OUTFLOW-AMI22688     2856     ok (non-closed singly connected)  
    BLADE               5472     5814     ok (non-closed singly connected)  
    INBLOCK-HUB         2142     2244     ok (non-closed singly connected)  
    INBLOCK-SHROUD      2142     2244     ok (non-closed singly connected)  
    INBLOCK-INFLOW      1632     1734     ok (non-closed singly connected)  

Checking geometry...
    Overall domain bounding box (-0.252091 -0.232021 -0.526682) (1.15 0.274332 0.0749815)
    Mesh (non-empty, non-wedge) directions (1 1 1)
    Mesh (non-empty) directions (1 1 1)
    Boundary openness (-5.44028e-17 -2.3469e-17 -9.54647e-17) OK.
    Max cell openness = 1.04888e-15 OK.
    Max aspect ratio = 97.4788 OK.
    Minimum face area = 8.72446e-08. Maximum face area = 0.000588455.  Face area magnitudes OK.
    Min volume = 1.29892e-10. Max volume = 1.56608e-06.  Total volume = 0.0428573.  Cell volumes OK.
    Mesh non-orthogonality Max: 77.509 average: 24.5895
   *Number of severely non-orthogonal (> 70 degrees) faces: 597.
    Non-orthogonality check OK.
  <<Writing 597 non-orthogonal faces to set nonOrthoFaces
    Face pyramids OK.
 ***Max skewness = 4.01477, 2 highly skew faces detected which may impair the quality of the results
  <<Writing 2 skew faces to set skewFaces
    Coupled point location match (average 0) OK.

Failed 1 mesh checks.
Also I have atttached the fvSchemes and fvSolutions files in zip file for your reference
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File Type: zip system folder.zip (1.5 KB, 10 views)
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Old   September 21, 2015, 04:59
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Code:
  *Number of regions: 2     The mesh has multiple regions which are not connected by any face.   <<Writing region information to "7000/cellToRegion"   <<Writing region 0 with 178544 cells to cellSet region0   <<Writing region 1 with 110848 cells to cellSet region
There could be an issue, never had two regions in any of my meshes.

Code:
  <<Writing 597 non-orthogonal faces to set nonOrthoFaces     Face pyramids OK.  ***Max skewness = 4.01477, 2 highly skew faces detected which may impair the quality of the results   <<Writing 2 skew faces to set skewFaces     Coupled point location match (average 0) OK.
skewness is not necessarily a problem...BUT in my experience even one misshaped cell in the wrong spot can make the difference.
try to visualize the mesh quality in paraview. maybe that gives you a clue
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Old   September 22, 2015, 04:09
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Hi Sway

Thank you for your valuable tips, I shall work on them, and will let you know if anything works good

I have one doubt in your point. You have told that you have never had 2 regions in any of your meshes. I have not got your point.

I have two cell zones one for rotor and other for stator. In DynamicMeshDict I have given the rotor cell zone.

Any mistake I have done ??
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Old   September 23, 2015, 08:11
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Hi,

the logfile in post 1 indicates, that your case is running without diverging?

Anyway, with these nonOrtho-values from checkMesh you should use at least one nonOrtho corrector.

Best regards,
Jan
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Old   September 23, 2015, 11:43
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Hi Jan,

Thank you for your reply.

I shall try that
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