[Paraview] Computation of gradients fails

[Martin Vymazal]

Hello,

  you're right. I didn't notice that some of the elements have  
negative volumes. Thank you very much.

  Martin Vymazal


Quoting "Moreland, Kenneth" <kmorel at sandia.gov>:

> After taking a look at your mesh, I see that the problem is that you  
> have some degenerate tetrahedra.  That is, there are tetrahedra  
> where all 4 vertices are in a plane.  In this case the tetrahedron  
> is self-intersecting and the gradient is ill-defined (over  
> constrained I believe), which is why the linear solver is failing to  
> compute it.  You can find the degenerate tetrahedra by running the  
> Mesh Quality filter to compute the volume of the tetrahedra and look  
> for any resulting "Quality" that is not strictly greater than 0.
>
> So, you can do one of two things.  The first is to ignore the  
> errors.  I don't think that any of the degenerate cells are part of  
> the surface (although I can't be sure), and since the end game is to  
> compute values on the surface it might not effect anything.
>
> The second thing you can do is to fix the mesh.  The best thing to  
> do is to go back to whatever created the original mesh, fix it, and  
> rerun the simulation.  There is a reasonable chance that these  
> degenerate tetrahedra also had a negative impact on the simulation  
> results.  If that's not practical, you can use the volume computed  
> by the Mesh Quality filter and the Threshold filter to remove any  
> tetrahedra that don't have a strictly positive volume.  That will  
> mess up the connectivity of the mesh though.
>
> -Ken
>
>
> On 9/22/10 9:30 AM, "Martin Vymazal" <martin.vymazal at vki.ac.be> wrote:
>
> Hello Ken,
>
>   I computed the x-velocity component using a calculator filter. Then
> I applied the 'Gradient of Unstructured Data Set' filter to the output
> of the previous filter. This is the place where the computation fails.
> I sent you the data in a private email.
>
> Best regards,
>
>   Martin Vymazal
>
>
> Quoting "Moreland, Kenneth" <kmorel at sandia.gov>:
>
>> More information please.
>>
>> How are you extracting the x-component of the velocity?  What filter
>> are you running to compute the gradient?  Would it be possible to
>> post some example data that exhibits the problem so that we can
>> replicate it?
>>
>> -Ken
>>
>>
>> On 9/22/10 7:12 AM, "Martin Vymazal" <martin.vymazal at vki.ac.be> wrote:
>>
>> Dear ParaView developers,
>>
>>   I'm trying to compute gradient of my x-component of velocity on a
>> grid of tetrahedra which has 55244 DOF. ParaView fails to compute the
>> gradients with the following error message:
>>
>> Generic Warning: In
>> /home/kitware/Dashboard/MyTests/ParaView-3-8/ParaView-3.8/ParaView/VTK/Common/vtkMath.cxx,
>> line
>> 1338
>> Unable to factor linear system
>>
>> The mesh does not have degenerate/ugly elements, but it has some sharp
>> edges (in the inlet/outlet area). Is this a problem? If not, is there
>> a way of avoiding the error?
>>
>> Thank you very much.
>>
>>   Martin Vymazal
>>
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>>
>>    ****      Kenneth Moreland
>>     ***      Sandia National Laboratories
>> ***********
>> *** *** ***  email: kmorel at sandia.gov
>> **  ***  **  phone: (505) 844-8919
>>     ***      web:   http://www.cs.unm.edu/~kmorel
>>
>>
>
>
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>
>    ****      Kenneth Moreland
>     ***      Sandia National Laboratories
> ***********
> *** *** ***  email: kmorel at sandia.gov
> **  ***  **  phone: (505) 844-8919
>     ***      web:   http://www.cs.unm.edu/~kmorel
>
>



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