CFD Online Discussion Forums - pimpleDymFoam -- Courant Number increasing and Case crashing after few time steps

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

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

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! :)

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

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

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 ??

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

Hi Jan,

Thank you for your reply.

I shall try that :)