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

Non-reflective Boundary Condition divergence

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

Reply
 
LinkBack Thread Tools Display Modes
Old   December 18, 2012, 05:02
Default Non-reflective Boundary Condition divergence
  #1
New Member
 
Join Date: May 2009
Posts: 9
Rep Power: 7
frodooon is on a distinguished road
Hi all,

I'm currently trying to run a 2D axisymmetric calculation of a simplified engine exhaust on Fluent 14.5.
I have thus one velocity inlet at 200m/s, one pressure outlet (gauge pressure = 0 Pa), and one pressure inlet (also 0 Pa).
I ran it steady kwsst ==> ok
Then unsteady kwsst ==> ok
Then SAS ==> OK
Then I'm trying to switch to 2d LES with the TUI command, which works fine, and doesn't diverge (but shows a very more chaotic comportment compared to SAS).
Now, all of these runs were done with classic pressure BC. I now want to switch ON the non-reflective BC on the pressure outlet, still with the 2d LES. It diverges each time I try.

My solver settings are the default ones when switching to that 2d LES.
I tried dividing the time step by 5 (going to 1e-04s), reducing the courant number, going into more complex discretization schemes : it diverges always, and from what I see on the contour plots : it comes from the outlet.

the mesh is like I said 2D, perfectly squared cells (10cm) everywhere, overall length of the model is about 40m and "radius" of the engine is about 2m.

The starting point of the LES calculation is the SAS calculation, which seems stable and nice.

I now switched the inlet BC to a "mass-flow" BC instead of velocity, but it doesn't look promising...

I don't have any more ideas to try make it work, so can someone help ?

thank you,

Arnaud.

PS : It is the pressure based solver, coupled.
frodooon is offline   Reply With Quote

Old   December 18, 2012, 05:50
Default
  #2
Senior Member
 
Alex
Join Date: Jun 2012
Location: Germany
Posts: 1,034
Rep Power: 17
flotus1 will become famous soon enough
I hardly think that running an axisymmetric case with LES is physically valid.
So dont bother if it doesnt run. Switch to a 3D geometry if you want to run a LES.
flotus1 is offline   Reply With Quote

Old   December 18, 2012, 05:57
Default
  #3
New Member
 
Join Date: May 2009
Posts: 9
Rep Power: 7
frodooon is on a distinguished road
Yeah, you're right, but I'm not trying to get information really from this calculation, just trying to understand a little bit more the different models, so I'd like to run it 2d for starter. But of course I plan on going 3D after that.
frodooon is offline   Reply With Quote

Old   December 18, 2012, 12:45
Default
  #4
Senior Member
 
Alex
Join Date: Jun 2012
Location: Germany
Posts: 1,034
Rep Power: 17
flotus1 will become famous soon enough
In that case, a planar 2d simulation instead of an axisymmetric one might work better. I am not sure if the non-reflecting boundaries are suitable for axisymmetric cases.
flotus1 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
Boundary Condition for granular temperature Theta cheng1988sjtu OpenFOAM Programming & Development 1 June 2, 2012 00:15
Quarter Burner mesh with periosic condition SamCanuck FLUENT 2 August 31, 2011 12:34
Setting outlet Pressure boundary condition using CAFFA code Mukund Pondkule Main CFD Forum 0 March 16, 2011 05:23
Domain Imbalance HMR CFX 3 March 6, 2011 21:10
How exactly the "pressure outlet" bdry condition compute properties on the boundary? yating9901 FLUENT 3 June 28, 2010 13:26


All times are GMT -4. The time now is 08:12.