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Old   October 17, 2011, 08:20
Default Problem of negative volume
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Anam
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Hi!!!
I have to find the resistance of series 60 ship . I am using gambit for meshing the model. Every time when i mesh the model i get the "negative volume" with 2 or 3 inverted elements.So i can not use this mesh in fluent to get the result.Can any body tell me why i have the problem of "negative volume" and how can i fix this problem?
Sorry for my poor English

Please give me a quick response

thanks
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Old   October 17, 2011, 08:26
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What kind of elements are you using?
if your volume mesh is successfull (although you get error message), then you can go to examine mesh, and you can see where are those negative cells.
Pictures are welcomed
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Old   October 21, 2011, 07:15
Default Negative volume problem
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Hi!!!

Thanks for your help. I am trying to generate Hexahedral Mesh. The most of the elements creating problem are near the bow and stern region.

Now i can not use this meshed model in fluent for simulation because of this negative volume problem.

should i use Multi-Block Structured mesh scheme to get rid of this problem?
If not then how i can i fix this problem?

Sorry for my poor English

Once again thanks a lot.
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Old   October 21, 2011, 07:17
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ca you post pictures?
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Old   October 21, 2011, 07:57
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Can i post picture?

Sorry i could not understand?
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Old   October 21, 2011, 08:02
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Quote:
Originally Posted by Anam View Post
Can i post picture?
yes.
without pictures it is quite hard to imagine what you are talking about
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Old   October 28, 2011, 02:41
Default negative volume problem
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Hi!!

Thanks a lot for your help.

The criteria for meshing the surface of ship:
Mesh size=0.3% of length of water line (Length of water line=7m)

The criteria for meshing the domain:
Y+ Formula is used to mesh the domain.
Y+ =150 (As Y+ in my case is between 30 and 300)

When i meshed my model by the above criterion, i god the negative volume problem. By smooth inlet and outlet faces,i did not get the negative volume message again . when this meshed model was used in fluent for simulation , solution was diverged giving this message.

"Divergence detected in AMG solver: omega -> Increasing relaxation sweeps!
Error: Divergence detected in AMG solver: omega
Error Object: #f"

Now i am confused, why my solution was diverged. Is there still some problem in my mesh. If yes how i can fix this problem.Please give me some suggestion.

Once again thanks a lot.

The corresponding files are also uploaded, with the links given as following

Series 60 dbs: http://www.zshare.net/download/951799286333c979/
Series 60.jou:http://www.zshare.net/download/951799782f0a0056/
Series 60 Diverged.dat: http://www.zshare.net/download/95183110445bf2a3/
Series 60 Diverged.cas:http://www.zshare.net/download/95180180860b5efb/
Attached Images
File Type: jpg Series 60 Hull.jpg (22.6 KB, 43 views)
File Type: png Mesh Generated.png (62.8 KB, 78 views)
File Type: jpg Mesh Checking.jpg (30.6 KB, 44 views)
File Type: jpg Mesh Checking 1.jpg (25.3 KB, 35 views)
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Old   October 28, 2011, 02:54
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what says check Mesh in Fluent?
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Old   October 29, 2011, 11:53
Default Problem of negative volume
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Hi!!

From the mesh check in Fluent i got this warning

WARNING: Mesh check failed.
To get more detailed information about the mesh check failure increase the mesh check verbosity via the TUI command /mesh/check-verbosity.
Fluent can also try to fix the mesh check problems via the TUI command
/mesh/repair-improve/repair.

When i used the command "/mesh/repair-improve/repair" i did not get this warning again but solution was diverged after 1000 iteration.

Now i don't know what should i do to fix this problem?

Thank you for your help!
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Old   October 31, 2011, 02:27
Default negative volume
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Hi Anam

Is is a complete 3D model? Are you using any axi-symmetric condition on axis?
if yes, then keep axi-symmetry on x-axis. This is default condition. If your axisymmetry is lying below x-axis then move it on to the x-axis and then check mesh.

rgds,
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Old   November 3, 2011, 12:07
Default Problem of negative volume
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Thanks alot mvee for your useful suggestion. I have not used any axi-symmetric condition on axis.

I want to use the second order upwind schemes to improve my results. To get better result by this scheme, my mesh should be fine. When i tried to fine mesh i was getting the problem of negative volume and solution divergence.
I think (if i am not wrong) the problem for negative volume and solution divergence are due to the large number of skewed elements.

Now i am confused what should i do to improve my mesh and get better result.

Please help me and give me some suggestion how to improve the quality of mesh and get better results. I have spent almost three months in trying to fix this problem but no improvement.

Thank you once again.

Waiting for response.
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