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-   -   Advective BC: local velocity? (http://www.cfd-online.com/Forums/openfoam-solving/66030-advective-bc-local-velocity.html)

dany July 3, 2009 08:48

Advective BC: local velocity?
 
Dear OpenFoamers,

I want to simulate a high-speed atomizing liquid jet with a high density ratio using the lesInterFoam code (OpenFoam 1.5) setting the LESModel to laminar (= "DNS"). The outlet BC gives me a headache. Unfortunately the advective BC is, as the simulation shows, not the right condition to use.

What I understand of the advective boundary condition (/OpenFOAM/OpenFOAM-1.5/src/finiteVolume/fields/fvPatchFields/derived/advective) is that (without defining fieldInf and lInf and lets say using the Euler ddtScheme) the refValue is the old velocity field of the BC.
This refValue is used in the mixedFvPatchField.C file (in /OpenFOAM/OpenFOAM-1.5/src/finiteVolume/fields/fvPatchFields/basic/mixed/) where the new LOCAL velocity is calculated switching between fixedValue and zeroGradient BC (or a blend between this two BC's).

As I see it, I must admit to be a noob in OF, this LOCAL velocity is used in the advective term of the equation:

delta U / delta t = - U_LOCAL * (delta U / delta x)

which tends to be more unstable as to use the MEAN velocity:

delta U / delta t = - U_MEAN * (delta U / delta x)

.) Am I wrong with my assumption that the code uses a LOCAL velocity in the advective term?
.) If not, is it the easiest way just to overwrite the solution of the LOCAL velocity in the mixedFvPatchField.C with a new calculated MEAN velocity?

Thx for your help.

With best regards


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