# Outflow vs Convective BC

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 October 21, 2013, 05:49 Outflow vs Convective BC #1 Member   SM Join Date: Dec 2010 Posts: 90 Rep Power: 8 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: IIT-Hyderabad, India Posts: 42 Rep Power: 7 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: 3,681 Rep Power: 41 I suggest a check for the pressure equation. It is the divergence-free constraint really ensured cell-by-cell at the outflow?

October 22, 2013, 08:07
#4
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SM
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Quote:
 Originally Posted by samurai_01 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.
I understand that flow may not be fully developed but at the same time if the outlet is sufficiently far away is this condition violated severely?
Also the codes from CTR are compressible mostly so that necessitates
convective BC?

October 22, 2013, 08:09
#5
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SM
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Quote:
 Originally Posted by FMDenaro I suggest a check for the pressure equation. It is the divergence-free constraint really ensured cell-by-cell at the outflow?
I will check this but I have no reason to suspect it otherwise.
There is a stretching of the Cartesian grid. Can aspect ratio be a issue?

October 23, 2013, 00:57
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Quote:
 Originally Posted by canopus I understand that flow may not be fully developed but at the same time if the outlet is sufficiently far away is this condition violated severely? Also the codes from CTR are compressible mostly so that necessitates convective BC?
1.Actually yes, even if the outlet is far away, the error starts traveling backwards and affects the inflow. So even if you use an outlet far away and the flow is not fully developed, it shall affect the solution.

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 IIT-K, they also extensively use convective BC for unsteady, transient and developing flows.

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