Outflow vs Convective BC
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 
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 suggest a check for the pressure equation. It is the divergencefree constraint really ensured cellbycell at the outflow?

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Also the codes from CTR are compressible mostly so that necessitates convective BC? 
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There is a stretching of the Cartesian grid. Can aspect ratio be a issue? 
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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. 
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