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[ICEM] Mesh Quality

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Old   August 22, 2014, 06:38
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Robert Bischof
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Hello,

I have problems with my simulation in CFX.
I start a simulation with a rough mesh. The simulation starts, but there alternating variables (See the attachment).
With the results from my first simulation I think, that I have to refine my mesh.

After refining, the simulation stops after second iteration (" c_fpx_handler: Floating point exception: Overflow").
In my opinion the mesh has sufficient quality. But I'm a beginner with CFX and ICEM, so I can be wrong.
Can anybody help me ? Are there some more details required ?
Should I upload some mesh data ?

Thanks for your help

Greets biro
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File Type: jpg Auswahl_001.jpg (53.7 KB, 19 views)
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Old   August 22, 2014, 06:44
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Can You write about BC's which You use in simulation ?
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Old   August 22, 2014, 06:52
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I use the k-epsilon turbulence-model.
On the inlet I pretend a mass flow.
The outlet is executed as opening with pressure and temperature specification.

Are there more information you need ?
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Old   August 22, 2014, 08:16
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What is the problem with mesh? did you check quality metrics?
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Old   August 22, 2014, 08:44
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I don't know exactly the error.
With the rough mesh my simulation will work, but with the refine mesh the simulation doesn't work.
Based on the error message in CFX and the fact, that the simulation with the rough mesh is running, I thought its a problem with the mesh.

There are no bad angles and the quality is better 0.2.
The 3x3x3 determinant is better then 0.6 and when I checked the mesh in ICEM, there will be no errors.
I'm new with ICEM and maybe there are more facts, that could be responsible for the problem, but I don't know them.
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Old   August 22, 2014, 08:51
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Might be problem is transient. In bad mesh (coarse mesh) flow unsteady is not captured and hence no problem in convergence. But with fine mesh unsteadiness is creating problem for steady solver .


Past some pictures of mesh...
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Old   August 22, 2014, 09:14
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There are some pictures

mesh1 - mesh complete
mesh2_inlet - part of the inlet
mesh3 - intersection to box
mesh4 - intersection to pipe
mesh4 - intersection to pipe (zoom)
Attached Images
File Type: png mesh1_complete.png (8.3 KB, 18 views)
File Type: png mesh2_inlet.png (84.3 KB, 19 views)
File Type: jpg mesh3.jpg (48.4 KB, 14 views)
File Type: jpg mesh4.jpg (20.6 KB, 13 views)
File Type: jpg mesh5.jpg (68.6 KB, 13 views)
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Old   August 22, 2014, 09:45
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Quote:
Originally Posted by Far View Post
Might be problem is transient. In bad mesh (coarse mesh) flow unsteady is not captured and hence no problem in convergence. But with fine mesh unsteadiness is creating problem for steady solver .

How can I solve the problem ?
Are there any issues I have to follow by refining the mesh ?
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Old   August 22, 2014, 10:33
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what you are trying to model at the outlet with cylinder?
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Old   August 22, 2014, 10:44
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It's the geometry of the test-track.
At the end there is that cylinder
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Old   August 26, 2014, 02:06
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On the first step I will check this simulation for SST turbulence model.
In that kond of model a convergence criteria is more elastic.
When that change dont give sucess results - next case I will change outlet BCs for outlet with static pressure.
Best regards
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Old   September 5, 2014, 08:31
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I found a solution.
I have made a variation of element size. So I discovered, that the simulation works only in one case.
In this case the mesh is rough and at the wall its a bit more defined.

Then i can start the refined mesh by choosing the rough solution as start solution.
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Old   September 7, 2014, 17:52
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In cfx generally the alternating residuals during iteration process is due to a large timestep (false timestep if steady state simulation). You can try to reduce it one order and check how your iteration process go....

Best regards...
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Old   September 12, 2014, 14:46
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Thank you for you answer.

But I don't understand steady state and timestep? What does "timestep" means in this context ? And what are the criteria to choose it ?

Still any other question:
Is it better to use a previous simulation as initial values at all time, even if the simulation will run without initial values ?
Or should I start such simulation without initial values ?

Greets
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