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 Anam October 17, 2011 08:20

Problem of negative volume

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

 -mAx- October 17, 2011 08:26

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

 Anam October 21, 2011 07:15

Negative volume problem

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.

 -mAx- October 21, 2011 07:17

ca you post pictures?

 Anam October 21, 2011 07:57

Can i post picture?

Sorry i could not understand?

 -mAx- October 21, 2011 08:02

Quote:
 Originally Posted by Anam (Post 328883) Can i post picture?
yes.
without pictures it is quite hard to imagine what you are talking about

 Anam October 28, 2011 02:41

negative volume problem

4 Attachment(s)
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

 -mAx- October 28, 2011 02:54

what says check Mesh in Fluent?

 Anam October 29, 2011 11:53

Problem of negative volume

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!

 mvee October 31, 2011 02:27

negative volume

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,
mvee

 Anam November 3, 2011 12:07

Problem of negative volume

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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