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Old   September 16, 2013, 05:31
Default Negative Volume IN CFX
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Hi,

I am doing a CFD analysis of a turbine blade. The blade is not rotating as wind tunnel tests are being validated.

The mesh has been generated in ICEM and only single passage with tip clearance has been meshed.

The pre mesh determinant 2x2x2 is above 0.5.
determinant 3x3x3 is above 0.5
angle is greater than 18 degree.
While creating geometry the CAD software had a tolerance of 1 e-5
model tolerance within ICEM was 1 e-20.

There are no negative volumes in ICEM CFD.

when i run simulation in CFX, first a negative sector volume is encountered. but solution runs and then a negative element volume is encountered and simulation is terminated.

Does anyone has an idea?

Regards
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Old   September 16, 2013, 06:20
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Are you using moving mesh? This is common error for moving mesh simulations. But you should not need moving mesh for this.

It is also possible to over-smooth a mesh in ICEM and end up with negative volume elements. It means you have to be more careful smoothing to not generate negative volume elements. This is because some metrics (eg the ones you have mentioned) do not include negative volume in them, so when you optimise for those paameters you might have a mesh with negative volumes.
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Old   September 16, 2013, 09:52
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Actually, Its a steady state analysis without any mesh motion.
I have not used any mesh smoothing option. I just checked the premesh parameters and then converted the mesh to unstructured and export it to CFX.

I have tried to increase the element size near wall and it solves the problem.
But it also produces undesirable y+.
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Old   September 16, 2013, 11:37
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How are you producing your inflation layer?

Are you generating a small amount of prism layers then splitting them and redistributing?

Is this a hex mesh and you are using edge parameters to generated near wall layers?

Are you running the ICEM Check Mesh Routine after you have done all your mesh processing (volume orientations will generally highlght these bad volumes).

I have found that in some cases ICEM will generate negative volumes in the inflation layer for very, very, thin elements, depending on how you build up the inflation layer.
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Old   September 17, 2013, 07:09
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Actually it is a structured hex mesh. I am using mesh distribution and i am not creating prism layers.

when i create unstructured block from structured block and check mesh. There are some volumes with orientation problems.
after a fix these volumes are fixed by icem. There are some penetrating elements as well but these penetrating elements appear whenever i give any edge distribution to resolve boundary layer.

The simulation runs even if i have penetrating elements
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ansys cfx 12, icem 3d, negative volume element

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