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-   -   Physical model: MRF RANS (http://www.cfd-online.com/Forums/openfoam-solving/129249-physical-model-mrf-rans.html)

 Ruli January 31, 2014 18:11

Physical model: MRF RANS

Dear Foamers,
again I have a question. I am simulating a wind turbine, using SIMPLE, the MRF approach with a 120° wedge and cyclic boundaries and RANS (kOmegaSST) turbulence modelling.

Currently I am trying to figure out the exact physical equations I am actually using.
I do know and understand:
- the Navier Stokes equation and its RANS formulation
- the formulation of the Navier Stokes equation in its MRF formuation (from here: http://openfoamwiki.net/index.php/Se...RF_development)

What I don't understand:
- How do both formulations look like in combination?
- Does MRFSource in fvOptions only add the Coriolis term?
- Would the MRF approach work for unsteady flows? As I understand, for unsteady flow there have to be considered further time-dependent terms.

If somebody knows a helpful reference or already has figured it out, I would be greatful for any help. I am not very experienced in reading the source code, but I am also thankful for hints, where in the code to look.

Best regards
Julian

 Ruli February 5, 2014 11:22

Hi,
my current thoughts:

The formula looks like the one described in the link above, but instead of kinematic viscosity (nu) I simply have (nu + nut).

Does that sound right?

Best regards
Julian

 farhhad February 5, 2014 11:25

as far as I know you can use pimpleFoam (an unsteady solver) with fvOption where you'll be able to add MRF zones. I believe you can use any turbulence model available with pimple.

best

 Ruli February 15, 2014 09:21

thanks for your reply. Good to know, that fvOptions does work with pimple, too.

Any other suggestions, how the Navier Stokes equation looks in the combined MRF + RANS formulation?

Best regards
Julian

 GerhardHolzinger February 17, 2014 07:14

Quote:
 Originally Posted by Ruli (Post 472836) Dear Foamers, What I don't understand: - Does MRFSource in fvOptions only add the Coriolis term? Best regards Julian
Hi,

the file MRFZone.C contains a method called correctBoundaryVelocity(). There the velocity at all boundaries within the MRFZone are corrected, i.e. the solid body rotation is applied.

Code:

```void Foam::MRFZone::correctBoundaryVelocity(volVectorField& U) const {     const vector Omega = this->Omega();     // Included patches     forAll(includedFaces_, patchi)     {         const vectorField& patchC = mesh_.Cf().boundaryField()[patchi];         vectorField pfld(U.boundaryField()[patchi]);         forAll(includedFaces_[patchi], i)         {             label patchFacei = includedFaces_[patchi][i];             pfld[patchFacei] = (Omega ^ (patchC[patchFacei] - origin_));         }         U.boundaryField()[patchi] == pfld;     } }```

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