
[Sponsors] 
October 21, 2013, 05:49 
Outflow vs Convective BC

#1 
Member
SM
Join Date: Dec 2010
Posts: 88
Rep Power: 7 
Hi,
I am having troubles with LES of jet flow of Re~22000 using FVM incompressible 2nd order code. In the streamwise direction at outlet I use outflow BC i.e. set velocity gradients to zero. I found same case simulated earlier with outlet placed at 20D. But in my case if I keep outlet 20D the run blows up few iterations after the flow reaches the outlet plane. I observed the same with 30D and now I am running with domain of 45D. The difference with literature is that they are using convective BC as common with compressible solver. My question is can the outlet BC have such a effect? If not what can be other reason? Thanks SM 

October 21, 2013, 15:04 

#2 
Member
Join Date: Oct 2012
Location: IITHyderabad, India
Posts: 42
Rep Power: 5 
Outflow boundary condition restricts your velocity components and the basic assumption for applying an outflow bc is that the flow must be fully developed at the outlet in question.
During LES, the outflw is not fully developed, hence we can't use outflow bc. Convective bcs have no as such physical significance, they are used for unsteady flows for being numerically stable for unsteady flows. Many papers from CTR stanford use convective bc at outflow for the above said reason. 

October 21, 2013, 15:17 

#3 
Senior Member
Filippo Maria Denaro
Join Date: Jul 2010
Posts: 2,518
Rep Power: 31 
I suggest a check for the pressure equation. It is the divergencefree constraint really ensured cellbycell at the outflow?


October 22, 2013, 08:07 

#4  
Member
SM
Join Date: Dec 2010
Posts: 88
Rep Power: 7 
Quote:
Also the codes from CTR are compressible mostly so that necessitates convective BC? 

October 22, 2013, 08:09 

#5  
Member
SM
Join Date: Dec 2010
Posts: 88
Rep Power: 7 
Quote:
There is a stretching of the Cartesian grid. Can aspect ratio be a issue? 

October 23, 2013, 00:57 

#6  
Member
Join Date: Oct 2012
Location: IITHyderabad, India
Posts: 42
Rep Power: 5 
Quote:
2. CTR's papers on incompressible flows also use convective BC, so its not only the compressible flows that have it. Even if you see the works from Aero dept. from IISc and IITK, they also extensively use convective BC for unsteady, transient and developing flows. 

Thread Tools  
Display Modes  


Similar Threads  
Thread  Thread Starter  Forum  Replies  Last Post 
Outflow boundary Control Volume SIMPLE Help !!!  Mizerable  Main CFD Forum  4  August 22, 2011 13:00 
Sponge layer for outflow BC  irc  OpenFOAM Running, Solving & CFD  13  April 8, 2009 04:18 
convective boundary condition for outflow BC  Pang Shengyong  Main CFD Forum  1  August 24, 2007 17:21 
Convective outflow boundary condition.  T. Gra.  Main CFD Forum  0  June 26, 2003 12:56 
Outflow Bocos  Romuald Skoda  Main CFD Forum  3  August 6, 1999 03:08 