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

multiple solutions using Fluent

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

Reply
 
LinkBack Thread Tools Display Modes
Old   August 15, 2006, 10:14
Default multiple solutions using Fluent
  #1
Hanjie Lee
Guest
 
Posts: n/a
Hi,

I wonder if anybody have experiences of getting different solutions by using different mesh in Fluent?

I have a quite simple pipe geometry with about 90 degree bend and rapid expansion. The diameter ratio on both ends of the bend is slightly larger 3. The tetra mesh predicted a flow field with strong swirl component but the prism/hex mesh did not predict such swirl. Pressure loss is certainly very diferent. All three cases start from zero and converge well. I am almost certain that this difference is not caused by mesh density. I checked with the tetra mesh case. Questions I have in mind are: 1) if the two different solutions are merely caused by mesh type, should I trust one by hex mesh or by tetra mesh? Why? 2) if there exist multiple solutions of the governing equation set for this case, is there a way to determine their stability in Fluent?

Thank you!
  Reply With Quote

Old   August 15, 2006, 12:15
Default Re: multiple solutions using Fluent
  #2
cadaei
Guest
 
Posts: n/a
This is quite a difficult question to answer. Need a bit more detail on the problem first.

Is this a tubulent simulation? What discretisation schemes are you using, 1st, 2nd order etc? Is the tet mesh a hybrid mesh (do you use hex in boundary layer)?

In general tet meshs are more prone to numerical diffusion and it is difficult to resolve boundary layers with tet meshes. However, if the model is setup correctly, Fluent should yield solutions with very small discrepancy. I would recommend using the hex mesh if possible because its less likely to converge on unrealistic solutions.
  Reply With Quote

Old   August 15, 2006, 15:38
Default Re: multiple solutions using Fluent
  #3
Hanjie Lee
Guest
 
Posts: n/a
Thank you for your reply. Yes. Turbulence is modeled by k-epsilon model. All discretizations are 1st order upwind scheme. There is no boundary layer mesh. I am pretty sure the model has been set up correctly because we have done many of them but didn't notice this issue until recently. Maybe this is just for this geometry, I don't know. I tend to trust solutions by hex/prism mesh because they preserve the symmetry quite well (the ProE geometry has a plane of symmetry). Tetra mesh solution, howerver, showed a strong swirl. The sense of rotation is probably, I guess, caused by the slight imperfection of the ProE model.
  Reply With Quote

Old   August 15, 2006, 17:21
Default Re: multiple solutions using Fluent
  #4
cadaei
Guest
 
Posts: n/a
you have to use at least 2nd order discretisation schemes with an unstructured grid. This is probably the cause of the rotation.

You should also probably put about 10 layers of hex cells on the boundary layer to capture the sharp gradients and fill the rest with tet cells.
  Reply With Quote

Reply

Thread Tools
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 On
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
how to set periodic boundary conditions Ganesh FLUENT 13 January 22, 2014 05:11
Two questions on Fluent UDF Steven Fluent UDF and Scheme Programming 4 September 20, 2013 16:30
What the differences flow equation of Fluent 6.3 and Fluent 12.1 opehterinar81 FLUENT 0 August 19, 2011 11:55
Starting Fluent on multiple machine diamondx FLUENT 0 July 22, 2011 16:41
running multiple Fluent parallel jobs Michael Bo Hansen FLUENT 8 June 7, 2006 08:52


All times are GMT -4. The time now is 03:03.