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-   -   Freestream boundary condition (http://www.cfd-online.com/Forums/openfoam/93093-freestream-boundary-condition.html)

 YJ Lee October 5, 2011 00:03

Freestream boundary condition

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!

 fcollonv October 7, 2011 11:54

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

 YJ Lee October 8, 2011 11:37

Thanks, Frederic! Think I get the point now...

 malaboss December 11, 2012 12:29

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 ?

 YJ Lee December 19, 2012 22:18

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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