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

Boundary condition & reverse flows

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

Reply
 
LinkBack Thread Tools Display Modes
Old   May 8, 2011, 14:02
Question Boundary condition & reverse flows
  #1
New Member
 
Join Date: Jan 2011
Posts: 25
Rep Power: 6
zhenglun.wei is on a distinguished road
I met a problem on my current simulation. It always give me a reverse flow problem.
It is a 3D problem with a major inlet on the front boundary. The flow is allowable to go out from the back boundary. There is a minor inlet on the top boundary which is very closed to the back boundary. The flow comes from this minor boundary is very small.
The problem is, if I use 'out flow' as the boundary condition of the back boundary, the FLUENT gives me a lot of warning about 'reverse flow' and the simulation will not converge to even 10^-3. Then, I tried the 'velocity inlet' and set a constant velocity 'outward' of the back boundary. It can converge to 10^-4, but I know it is not real, specially I put a turbulence model on the whole simulation. Then, I tried the 'pressure-outlet', it still gives me the 'reverse flow' warning, but it can converge to 10^-4. Personally, I prefer to work with 'pressure-outlet' boundary, since, I think, it is more physical and understandable. But, I do not quite understand this BC.
Any suggestions on this?

thanks in advance.
zhenglun.wei is offline   Reply With Quote

Old   May 9, 2011, 11:32
Default
  #2
New Member
 
Join Date: Sep 2010
Posts: 9
Rep Power: 6
farshadexp is on a distinguished road
Hi there....

you can also use outlet-vent in your simulation, remember that according to my experience P-outlet many times leads to R-flow. By the way, I have similar problem now, but in different simulation case and I'll be really happy if you find better answer and inform me

Regards...
farshadexp is offline   Reply With Quote

Old   May 9, 2011, 13:35
Default
  #3
New Member
 
Join Date: Jan 2011
Posts: 25
Rep Power: 6
zhenglun.wei is on a distinguished road
Dear Farshadexp,
thanks for your reply. Fluent manual says that the P-outlet can help us to make the simulation converge better when RF exists. However, in my opinion, it does not say that the P-outlet can help us to eliminate the reverse flow. As I tested, the P-outlet indeed help me to converge the simulation to 10^-4 (outlet BC can not even converge to 10^-3); however, the RF warning is still showing over there. I'll do some tests on your suggestions and let you know if it can fit my problem.

best,
zhenglun.wei is offline   Reply With Quote

Old   October 20, 2011, 13:10
Default
  #4
New Member
 
Choon Hwai Yap
Join Date: Oct 2011
Posts: 4
Rep Power: 5
choonhwai is on a distinguished road
Hi,

I have the same problem. I have 1 inlet and 4 outlets. Both outflow and outlet-vent BC did not correct the non-convergence and the slowly growing number of reverse flow faces. I tried refining the mesh and re-meshing with aggressive smoothing, but neither worked.

will appreciate any suggestions..
choonhwai is offline   Reply With Quote

Old   October 25, 2011, 17:38
Default problem is due to meshing?
  #5
New Member
 
Choon Hwai Yap
Join Date: Oct 2011
Posts: 4
Rep Power: 5
choonhwai is on a distinguished road
Hi guys,

I have some update: I think meshing may be at least partially responsible for the non-convergence of the multiple out-flow BC simulations.

I meshed a model with 1 inlet and 4 outflow BC with ICEM CFD and with Gambit. The one from ICEM CFD does not converge (I tried re-meshing to get finer mesh and also tried outlet vent BC), but the one from Gambit has no stability problems.

Does anyone know what is the difference in the meshing approach of ICEM CFD and Gambit?
choonhwai is offline   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
Inlet turbulent boundary condition: external flows ROOZBEH FLUENT 3 Yesterday 15:34
External Radiation Boundary Condition (Two sided wall), Grid Interface CFD XUE FLUENT 0 July 8, 2010 06:49
problem with boundary condition??? smn CFX 5 November 24, 2009 07:37
how to set up a wall boundary condition according to calculated wall shear stress? gameoverli OpenFOAM Pre-Processing 1 May 21, 2009 08:28
New topic on same subject - Flow around race car Tudor Miron CFX 15 April 2, 2004 06:18


All times are GMT -4. The time now is 17:49.