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Outflow boundary condition for LES/DNS

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Old   February 18, 2014, 16:06
Default Outflow boundary condition for LES/DNS
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I came across a statement that use of zero gradient boundary conditions for LES and DNS would result in unphysical results.

Can anyone please explain why.
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Old   February 18, 2014, 16:13
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Originally Posted by Harish050887 View Post
I came across a statement that use of zero gradient boundary conditions for LES and DNS would result in unphysical results.

Can anyone please explain why.
zero gradient BC can be adopted for outflow provided that is fixed far from the main zone... it can be improved by higher approximations
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Old   February 19, 2014, 04:55
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Thank you..but can you also explain why does this happen ?
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Old   February 19, 2014, 05:59
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Thank you..but can you also explain why does this happen ?
because you arbitrarily fix some components of the velocity gradient to vanish. That can be quite acceptable for some conditions but not in general
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Old   February 28, 2014, 11:40
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Had to ask one more thing, why is it then ok to use the zero gradient bc for RANS ?
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Old   February 28, 2014, 12:12
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Had to ask one more thing, why is it then ok to use the zero gradient bc for RANS ?

RANS solves for the statistically steady state, d/dn=0 is much more acceptable
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Old   March 2, 2014, 16:17
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Quote:
Originally Posted by Harish050887 View Post
I came across a statement that use of zero gradient boundary conditions for LES and DNS would result in unphysical results.

Can anyone please explain why.
zero gradient may be acceptable if the flow is established before it crosses the outlet boundary. This would certainly be not true for turbullent flows (LES/DNS) or only if the computational domain is quite large or in case where you know that the turbulence is damped by the viscosity in this region. However for RANS methods since the flow is time averaged then this BC becomes suitable. For LES/DNS use Orlansky boundary conditions that is:
dphy/dt + U*dphy/dn=0 (derivatives here are partial derivatives and U is a velocity which may be defined by different means, (it can be the mean velocity for example). phy stands for each componnent of the velocity vector.
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Old   March 4, 2014, 04:42
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Thank you very much guys. I get the idea.
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Old   May 17, 2014, 01:31
Default Orlansky Boundary condition
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How can i put Orlanksy boundary condition in FLUENT ?
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Old   January 16, 2019, 08:37
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Does someone know the Orlansky boundary condition or convective boundary condition in Openfoam? I want use this boundary in DNS simulation. Thank you.
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Old   January 16, 2019, 09:41
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Does someone know the Orlansky boundary condition or convective boundary condition in Openfoam? I want use this boundary in DNS simulation. Thank you.

I think it's already implemented as the advective BC for foam.
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Old   January 16, 2019, 10:04
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Yes. it is called "advective" in openFoam and does:

du/dt + Udu/dn=0, at the outflow boundary. For more theoretical aspects on this condition you can look at ferzinger and peric book, chapter 9.
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Old   January 16, 2019, 10:10
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Thanks to AliE and LuckyTran ! I will have a try later!
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