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Freestream boundary condition

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Old   October 4, 2011, 23:03
Default Freestream boundary condition
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Lee Yin Jen
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Dear FOAMers,

Hi, I'm pretty new to OpenFOAM and CFD. One question I have is regarding Freestream boundary condition - how exactly does it work?

Specifically, I tried to run an external aerodynamics case, using fixedValue for velocity, nut and nuTilda at outer boundary, zeroGradient for pressure at outer boundary; simpleFOAM, S-A turbulence. The results are rather unphysical and diverging.

However, following the airfoil example for simpleFoam and using freestream boundary condition, the simulation seems to be more reasonable and without divergence. My question is, how does freestream boundary condition differ from fixed values?

Thanks, FOAMers!
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Old   October 7, 2011, 10:54
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Frederic Collonval
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Hello Lee,

The freestream BC has the type inletOutlet meaning that it looks locally (for every face of the patch) at the mass flow rate. And if the flow is going outside the boundary will be locally zerogradient, if it is going inside the boundary will be locally fixedValue.
The freestreampressure BC is a zeroGradient BC but it fixes the flux on the boundary to be rho*Sf*freestreamValue.

Good luck

Frederic
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Old   October 8, 2011, 10:37
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Lee Yin Jen
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Thanks, Frederic! Think I get the point now...
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Old   December 11, 2012, 12:29
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Malik
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Hi,
Thank you for the explanation. If i am right the difference between a freestream BC and a fixedValue BC is that for fixed value there are constraints on each vector of the velocity field, while with freeStream we have a constraint only for the flux.

This is why the solution sounds more physical ?
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Old   December 19, 2012, 22:18
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Lee Yin Jen
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Hi, malaboss

Freestream BC is like a hybrid fixedValue and zeroGradient boundary condition. It behaves like a zeroGradient when fluid is flowing out of the boundary face, but behaves like a fixedValue when fluid is not flowing out.

So, instead of fixedValue that imposes its constant value regardless of situation, freestream is more flexible, doing whatever is more physically realistic, so to say.
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Old   October 8, 2014, 07:05
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Mieszko Młody
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Hi,
I am using currently freestream BC for the flow in the tunnel (channel).
Is it an appropriate BC for such flow ?

In short, there is large tunnel (3m in diameter) flow is from left to right (inlet, outlet with freestream BC) but inside the tunnel there is additional small inlet with some mass flow specified.

thanks
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Old   May 5, 2015, 04:00
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Hi everyone,

Is it possible to use the freestream BC (to simulate a external hydrodynamic case) in 3D.
In this case what is the good boundary condition to use for the frontAndBack ?

Thanks for answers
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Old   June 4, 2015, 08:03
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Quote:
Originally Posted by fcollonv View Post
Hello Lee,

The freestream BC has the type inletOutlet meaning that it looks locally (for every face of the patch) at the mass flow rate. And if the flow is going outside the boundary will be locally zerogradient, if it is going inside the boundary will be locally fixedValue.
The freestreampressure BC is a zeroGradient BC but it fixes the flux on the boundary to be rho*Sf*freestreamValue.

Good luck

Frederic
Hi all,

Could you explain me what is Sf.

In advance, many thanks
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Old   July 7, 2015, 02:04
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Junshin Park
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Quote:
Originally Posted by ASo View Post
Hi all,

Could you explain me what is Sf.

In advance, many thanks
As far as I know, Sf means the surface area of a face of the cell.
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