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July 8, 2010, 05:36 
fvc::reconstruct( ) algorithm

#1 
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Join Date: Jul 2010
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Hi Foamers,
I am trying to figure out how the fvc::reconstruct( ) algorithm works. My first thought was: Sum up the facefluxes multiplied with the facenormal over a cell and divide it by the sum of the magnitude of the facevector components of every face of the cell. The algorithm in the "fvcReconstruct.C" file seems far more complex then my thought: inv(surfaceSum(sqr(mesh.Sf())/mesh.magSf())) & surfaceSum((mesh.Sf()/mesh.magSf())*ssf) The second line is what I expected, but the first line is a little mystery to me. Can anybody explain this to my, please? Rainer 

July 8, 2010, 10:31 

#2 
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David Gaden
Join Date: Apr 2009
Location: Winnipeg, Canada
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Just a guess... it looks like it might be nonorthogonal correction...?


July 9, 2010, 12:15 

#3 
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Kevin Maki
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Rainer, I agree with your description. The first line finds one over the square of the components of the area. The second line has the the components of the area in the numerator, so the result is just one over the area times the sum of what ever you are reconstructing. This is what you want for a faceareaweighted average.
There is another post by Henry which is relevant here: http://www.cfdonline.com/Forums/ope...integrate.html Kevin 

March 16, 2011, 13:27 

#4 
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SungEun Kim
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I am also puzzled by what fvc:reconstruct does. Has anyone here figured what it does? It seems it is not based on any mathematical identity (like Gauss theorem), but some sort of approximation.


January 16, 2012, 13:06 
fvc::reconstruct explaination

#5 
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Thibault Pringuey
Join Date: Mar 2009
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Hello,
I have had a look at the fvc::reconstruct method. Please find attached an attempt to explain it. Cheers, Thibault 

January 17, 2012, 05:00 

#6 
Senior Member

Thibault, excellent! Thank you for you work! Currently I am working on improving my model introducing a reconstruction for them... So hope to find some advantage on using it, will post some general conclusions.
__________________
Best regards, Dr. Alexander VAKHRUSHEV Christian Doppler Laboratory for "Advanced Process Simulation of Solidification and Melting" Simulation and Modelling of Metallurgical Processes Department of Metallurgy University of Leoben http://smmp.unileoben.ac.at 

August 27, 2014, 14:56 

#7 
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Charlie
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Hi makaveli_lcf,
Did you find any advantage using fvc::reconstruct ? In my experience, using fvc::reconstruct seems too dissipative? need to find out what cause the dissipation, maybe due to the faceweighted averaging ? Just curious, did you find anything similar? 

July 23, 2015, 00:41 

#8 
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Xavier Pivan
Join Date: May 2015
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Hi Cheng,
Did you figure out why renconstruct was too dissipative ? I'm trying to reproduce an experiment with settlingFOAM everything work except that my system dissipate too much and then the velocities are not strong enough. reconstruct might be a start. Thanks 

July 27, 2015, 00:11 

#9 
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Kyle Mooney
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There could be several sources of numerical dissipation. To start make sure you have achieved mesh convergence and using 2nd order discretization.


September 22, 2015, 06:37 

#10 
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Michael
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I think that fvc::reconstruct is dissipative because it is first order accurate.
It assumes Sum_f n_f ( S_f * u_p) = Sum_f n_f ( S_f * u_f) which is accurate up to irst order. I have another question: What is the difference between the method used to reconstruct the velocity at the cell center from the face center used in fvc::reconstruct and simply calculating it by Sum_f 1/S_f phi_f n_f ? 

October 27, 2015, 06:33 

#11 
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Michael
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I have to add that the fvc::reconstruct is second order accurate for orthogonal grids. It can be seen by a Taylorseries expansion of the face velocity, i.e., U_f = U_c + dU/dx_c delta x + ... and inserting it into the relation sum n_f (S_f * u_c) = sum n_f (S_f * u_f)


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