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February 15, 2013, 06:51 
MRFSimpleFoam implementation questions

#1 
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Florian
Join Date: Nov 2009
Posts: 59
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Hello,
I''m digging into the MRF implementation of MRFSimpleFoam. For understanding the mathematics I've found [1] and [2]. The MRFSimpleFoam source code [3]. The rhs of the momentum equation is added a sources term for coriolis forces. pEqn.H calls mrfZones.relativeFlux(phi); which "makes the given absolute mass/vol flux relative within the MRF region". So we are working with relative velocity, correct? In MRFZone.C [4] Foam::MRFZone::addCoriolis(fvVectorMatrix& UEqn) is called. It adds the source term Usource[celli] = V[celli]*(Omega ^ U[celli]); to every cell within the MRF zone. U is relative velocity, V is the cell volume, Omega the rotating axis/frequency, ^ the cross product.  If we're using relative velocity, doesn't the coriolis term needs to be 2 * omega ^ Urel ?  Where is the centripetal force term omega ^ omega ^ r? Is is contained in fvm::laplacian(rAU, p) in pEqn?  Why the cell volume? The compressible addCoriolis function uses V * rho which is mass... Thanks for any clarifications! Florian [1] http://openfoamwiki.net/index.php/Se...RF_development [2] http://openfoamwiki.net/index.php/Si...ry_MRF_Library [3] http://foam.sourceforge.net/docs/cpp...e79dad982.html [4] http://foam.sourceforge.net/docs/cpp/a03948_source.html 

February 22, 2013, 04:42 

#2 
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Florian
Join Date: Nov 2009
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Dare to bump that...


February 22, 2013, 17:25 

#3  
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Daniel P. Combest
Join Date: Mar 2009
Location: St. Louis, USA
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Florian,
Glad you asked this question, it made me look at it MRF solvers a little more closely. Quote:
Quote:
Quote:
Last edited by chegdan; February 22, 2013 at 17:44. Reason: added more information 

February 25, 2013, 09:09 

#4  
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Florian
Join Date: Nov 2009
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Quote:
The equation used is Eqn. 6 at http://openfoamwiki.net/index.php/Se...RF_development . Correct? There is the expression The dyadic product of relative and absolute velocity. I still don't really understand where this expression has gone in the source code? Thanks! Florian 

February 25, 2013, 11:37 

#5 
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Daniel P. Combest
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Location: St. Louis, USA
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Florian,
Glancing at other CFD packages documentation and the formulation for MRF solvers, I do see this term as well and seems that it would make sense that the advection term , should in fact be in there somewhere. If the absolute velocity is the variable of interest, then U_R is related to phi and coupled explicitly through some phi adjustment. If you look further in the MRFSimpleFoam, you will see the pEqn.H. In that file there is a method that is called that adjusts the flux for this: Code:
mrfZones.relativeFlux(phi) Code:
$FOAM_SRC/finiteVolume/cfdTools/general/MRF Code:
relativeFlux() 

December 19, 2013, 10:07 

#6 
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Gerhard Holzinger
Join Date: Feb 2012
Location: Austria
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I have one question regarding the unsteady term of Eq. 6 in [1]. There, the relative velocity appears in the time derivative.
Am I correct, that OpenFOAM computes the absolute velocity in all zones? If that is the case: why is the relative velocity in the unsteady term? Maybe one of you can elaborate. Cheers, [1] http://openfoamwiki.net/index.php/Se...RF_development 

March 14, 2014, 05:58 

#7 
New Member
Join Date: Oct 2011
Posts: 27
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Hi guys.
I have a question regarding MRF. I have printed the output of UEqn.source() (meaning Usource[celli] = V[celli]*(Omega ^ U[celli]) ) before and after adding coriolis term ( mrfzones.addCoriolis(rho, UEqn) ), and it prints the same. Why isnt the UEqn being updated?? Thanks 

March 15, 2014, 10:59 

#8  
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Julian Langowski
Join Date: May 2011
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Quote:
Quote:
Best regards Julian
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