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FSI/MFX: folded mesh after changing Air to Water

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Old   February 8, 2012, 11:07
Default FSI/MFX: folded mesh after changing Air to Water
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Dear all:

I've been trying to simulate an FSI problem with Ansys MFX without much luck. I'll be fortunate to have your help on spotting my error in the analysis.

I have followed closely the FSI example of piezoelectric-actuated pump.
(The piezoelectric effect is simply applied as a displacement boundary condition in voltage on nodes)

The example uses AIR as fluid medium, and the analysis runs without problems.
When I changed the fluid medium into WATER, however, the analysis does not converge.
I kept on having folded mesh problems after trying following options:

1. mesh size in fluid domain (from 4e-5 to 1e-5)
2. time step (from 5e-6 to 0.1e-6)
3. maintain uniform mesh size (all kept at 1e-5)
4. increase or decrease 'model exponent' in 'mesh stiffness' (from 1 to 20)
5. try options of 'increase near boundaries' or 'increase near small volumes' in 'mesh stiffness'
6. try 'first order backward Euler' or 'second order backward Euler' in 'transient scheme option' of solver control

I am wondering if there is anything I've done incorrectly so the analysis does not run? and are there other options I can try to have a convergent solution? The attached are the input files following the Ansys example.
Thank you very much for your help!


piezo.txt (to generate solid model and input file 'pfsi-solid.cdb')
CFXfluid.txt (to generate fluid model and mesh file 'fluid.cdb')
cfx_mfxexample.txt (definition file for CFX. Need to change file extension to .ccl; reload mesh 'fluid.cdb' is needed)
MFX sequence.txt (to generate MFX procedure 'mfxexample.dat')
Attached Images
File Type: jpg cfx_mfxexample.jpg (48.5 KB, 26 views)
Attached Files
File Type: txt cfx_mfxexample.txt (19.1 KB, 10 views)
File Type: txt CFXfluid.txt (2.0 KB, 6 views)
File Type: txt MFX sequence.txt (1.1 KB, 7 views)
File Type: txt piezo.txt (3.1 KB, 4 views)
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Old   February 8, 2012, 11:52
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Edmund Singer P.E.
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Without putting too much thought into this (and I admit I haven't looked over the details of your outline), but did you make the water slightly compressible in this analysis or did you use imcompressible water.

If you used incompressible water, try making it slightly compressible.
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Old   February 8, 2012, 15:25
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If you have access to the ANSYS Customer Portal search their knowledge base for solution 2022119. It has a video that walks through correcting this type of problem.
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Old   April 24, 2012, 09:17
Nick Cleveland
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Hi. I am excited to find this post, because I am having the same problem: strange deformation when I change the fluid properties (increase viscosity).

However, I searched the for sol# 2022119 and it was not there. Other similar numbers like 2022117 were. I am a student, is it possible that students have limited resource access to these solutions?
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Old   April 24, 2012, 09:26
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Ok, never mind I found 2022119. I was searching in al CFX posts, but it was under "all" posts
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