Meshing problem for transient analysis

arun1994 · July 5, 2016, 09:17

I am trying to make transient analysis for a simplified piston model with single intake and single outlet. I have shown it in picture 1 .

I have a problem with "negative volume " . I know my mesh is very bad . The surface mesh is pretty good to look from outside but inside using section plane ,it looks very bad as in pic 2.

How do I make a hexagonal mesh which are same in size ? Is it possible that I can "extrude " mesh from the base till the top ?
What are best practices to make hexa mesh ?
Also is there a comprehensive tutorial/PDF to read for transient analysis ?

Probably there are a lot of posts on this but I have less time now to search all. It will be nice to get specific answer for my problem.

Cheers,
Arun

CFDLAB_IUT · July 5, 2016, 09:59

I think your question must be in meshing forum:
http://www.cfd-online.com/Forums/ansys-meshing/

maybe you need to review some of examples and always the point is not to have a 'hexagonal mesh which are same in size'...You must be aware of how to evaluate your grid structure and check it.

some Properties of mesh you should check:
Skewness
boundary layer mesh (Inflation)
angels
growth rate
...

arun1994 · July 5, 2016, 10:39

Hello !

thanks for your reply. Can I know how should these parameters look like so that I get correct mesh? rather what are optimum values?

ghorrocks · July 5, 2016, 18:53

Your mesh is really bad and it is never going to work like that. I recommend you do the meshing tutorials to find out the basics of meshing.

In this case as you have a cylindrical object (which I presume is the piston) with some ports attached, I recommend you:
1) Separate the ports and cylinder. Mesh them separately. You should then get a very good mesh for the cylinder as it will just be a simple sweep mesh.
2) Connect them up using GGI interfaces. As they are going to move they will need to be Transient Rotor-Stator.
3) The ports can be stationary mesh, the cylinder moving mesh.
4) As the cylinder is just a simple linear squashing of the mesh the mesh motion stuff should work well on this.

If you need some help, have a look at the FAQ: http://www.cfd-online.com/Wiki/Ansys..._went_wrong.3F

arun1994 · July 6, 2016, 04:38

Thank you very much for the reply ! One quick question... where can I find the tutrials?

ghorrocks · July 6, 2016, 05:54

The ANSYS Community web page.

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July 5, 2016, 09:59		#2
CFDLAB_IUT New Member Join Date: Jul 2016 Posts: 1 Rep Power: 0	I think your question must be in meshing forum: http://www.cfd-online.com/Forums/ansys-meshing/ maybe you need to review some of examples and always the point is not to have a 'hexagonal mesh which are same in size'...You must be aware of how to evaluate your grid structure and check it. some Properties of mesh you should check: Skewness boundary layer mesh (Inflation) angels growth rate ...

July 5, 2016, 10:39		#3
arun1994 Member Arun subramanian Join Date: Jun 2016 Location: Florence,Italy Posts: 48 Rep Power: 9	Hello ! thanks for your reply. Can I know how should these parameters look like so that I get correct mesh? rather what are optimum values?

July 5, 2016, 18:53		#4
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	Your mesh is really bad and it is never going to work like that. I recommend you do the meshing tutorials to find out the basics of meshing. In this case as you have a cylindrical object (which I presume is the piston) with some ports attached, I recommend you: 1) Separate the ports and cylinder. Mesh them separately. You should then get a very good mesh for the cylinder as it will just be a simple sweep mesh. 2) Connect them up using GGI interfaces. As they are going to move they will need to be Transient Rotor-Stator. 3) The ports can be stationary mesh, the cylinder moving mesh. 4) As the cylinder is just a simple linear squashing of the mesh the mesh motion stuff should work well on this. If you need some help, have a look at the FAQ: http://www.cfd-online.com/Wiki/Ansys..._went_wrong.3F

July 6, 2016, 04:38		#5
arun1994 Member Arun subramanian Join Date: Jun 2016 Location: Florence,Italy Posts: 48 Rep Power: 9	Thank you very much for the reply ! One quick question... where can I find the tutrials?

July 6, 2016, 05:54		#6
ghorrocks Super Moderator Glenn Horrocks Join Date: Mar 2009 Location: Sydney, Australia Posts: 17,703 Rep Power: 143	The ANSYS Community web page.