CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > Software User Forums > ANSYS > FLUENT

Mesh dependency - possible that refinement = worse results?

Register Blogs Members List Search Today's Posts Mark Forums Read

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   November 14, 2012, 08:46
Default Mesh dependency - possible that refinement = worse results?
  #1
New Member
 
Join Date: Nov 2011
Posts: 3
Rep Power: 15
crisgc is on a distinguished road
Hi everyone,

I am simulating a simple jet breakup, laminar flow, issuing from a capillary nozzle applying an oscillatory boundary condition to achieve breakup. The study is 2D axisymmetric. It seems that I obtain good results with a fine mesh (aprox. 40000 elements) when I compare them with experimental cases but if I refine the grid even more (100000 elements or further) the results are worse and I cannot even produce the breakup and the interface seems quite different to what it should look like (tiny ripples appear).

Any suggestion? Is it possible that refining the mesh led to worse results?

Thank you for your help!
crisgc is offline   Reply With Quote

Old   November 14, 2012, 09:35
Default
  #2
Senior Member
 
RodriguezFatz's Avatar
 
Philipp
Join Date: Jun 2011
Location: Germany
Posts: 1,297
Rep Power: 27
RodriguezFatz will become famous soon enough
Yes, this is possible. You must generally be careful if your result changes when you refine the grid. This is a problem of "solving the equations right".
When your results - compared to experiments - become worse due to refinement, this can be a problem of "solving the right equations". In your case, it is possible that your results are "accidentally correct". The numerical error cancels the model error. This does not mean, that the results are good. It is just some kind of random incident.
RodriguezFatz is offline   Reply With Quote

Old   November 14, 2012, 09:43
Default
  #3
New Member
 
Join Date: Nov 2011
Posts: 3
Rep Power: 15
crisgc is on a distinguished road
Wow! Curious!

Thank you RodriguezFatz for your reply! One question, what do you mean by "the numerical error cancels the model error"? That I am using the wrong solver / algorithms but the error associated to the numerical method used makes the solution to seem right?

What makes me wonder (and become a bit nervious) about how to solve it! I am using a VOF (usually + Level Set) and I wouldn't know what other model to use for that kind of problem. Could you recommend me anything?

Thank you once more!
crisgc is offline   Reply With Quote

Old   November 14, 2012, 09:53
Default
  #4
Senior Member
 
RodriguezFatz's Avatar
 
Philipp
Join Date: Jun 2011
Location: Germany
Posts: 1,297
Rep Power: 27
RodriguezFatz will become famous soon enough
More or less: I meant, that a bad grid (as a source of numerical error) can cancel the error that the model (as a source of error compared to reality) produces. Then, the solution seems to be right, but isn't.

Can you afford even more refinement? If it is possible, you should try at least two stages of refinement of the grid (allways!). Now have a look at how your relevant values change over the three grids. If you are allready in a convergent region of the grid, all relevant values should change monotonic (over the refinement). The difference between the coarse and medium grid has to be larger than the difference between the medium and fine grid.
Do you have a copy of http://www.cfd-online.com/Books/show_book.php?book_id=4
?
Everything is explained in there!
RodriguezFatz is offline   Reply With Quote

Old   November 14, 2012, 13:16
Default
  #5
New Member
 
Join Date: Nov 2011
Posts: 3
Rep Power: 15
crisgc is on a distinguished road
Hmm.. OK, I will try to check again with three grids at different refinement stages as you've recommended me and let's see what happens.

I will also try to obtain a copy of the book.

Thank you for your help!
crisgc is offline   Reply With Quote

Reply

Tags
grid, grid dependence, jet breakup

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
[snappyHexMesh] How to Do External Mesh for Airfoil sHM msuaeronautics OpenFOAM Meshing & Mesh Conversion 1 September 23, 2012 05:00
[snappyHexMesh] Layers:problem with curvature giulio.topazio OpenFOAM Meshing & Mesh Conversion 10 August 22, 2012 10:03
[snappyHexMesh] external flow with snappyHexMesh chelvistero OpenFOAM Meshing & Mesh Conversion 11 January 15, 2010 20:43
Convergence moving mesh lr103476 OpenFOAM Running, Solving & CFD 30 November 19, 2007 15:09
basic of mesh refinement arya CFX 4 June 19, 2007 13:21


All times are GMT -4. The time now is 08:23.