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Using a custom algorithm for cell center calculations 

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June 25, 2013, 09:37 
Using a custom algorithm for cell center calculations

#1 
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Andrej
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Hello,
I am not sure if it is not implemented already, but I haven't found any straight possibility to fully customly choose cell center coordinates for calculations. Maybe I am wrong? If they are calculated during preprocessing once, and then are used for ALL calculations as a part of mesh  then it looks simple, just change it as a part of preprocessing. But do I need to calculate face centers aswell or it has nothing to do with cell centers for calculations? If they are calculated dynamically as a part of scheme implementation, where should I look for it? I saw somewhere in sourcecode somesort of cell center giving function: it looked complicated, as splitting volume into pyramids and then averaging its weighted centers, but, I guess, it has nothing to do with the centers, that are used for calculations, since in comments it was told that it is deprecated. Maybe i should change scheme realization algorithm, but all my tryings to clarify the place, at which that code intervention should be made, failed, so I don't know where to start yet, any hints to do that would be awesome ... I really need it, because i've constructed a mesh with triangle prisms, with acute triangles  so using Voronoy points as cell centers would solve problems with errors because of nonorthogonality. So forcing OpenFOAM to use my custom cell centers in ALL calculations  is it possible? And at wich point of code should I implement that modification? Thank you regards, Andrej Last edited by Zvex; June 25, 2013 at 11:50. 

June 26, 2013, 10:42 

#2 
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David Gaden
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Cell centres are calculated and stored by the fvMesh object, and automatically used by the discretization schemes. If you want to force OpenFOAM to use your custom cell centres for some reason, it would be a pretty big job.
You may be able to get away with defining a new set of discretization schemes within the finiteVolume library, or you may have to create your own library finiteVolumeWithBizzareCellCentres. Why do you want to use your own custom cell centres if OF already calculates them?
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June 26, 2013, 11:01 

#3 
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Andrej
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we don't achieve a convergence of an order 1/N, where N is the number of volumes ... We have made a handmade solver on pure c++, and achieve it by taking centers as Voronoy points.
So we need it, because we've constructed a mesh with triangle prisms, with acute triangles  so using Voronoy points as cell centers would solve problems with errors because of nonorthogonality. "are calculated and stored by the fvMesh object"  is it done only once after reading a mesh? As a see for now a syntaxis mesh.C()[cell_index].x(); let's to access the cell center coordinate, there are two methods: returning it as a const, and just returning by reference. So the second one should let us to change it. But I am not sure if it will be enough and it will be calculating everything correctly 

June 26, 2013, 11:12 

#4 
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David Gaden
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The cell centres are created when they are first requested (lazy evaluation). For a regular fvMesh, they are never recalculated. dynamicFvMesh must recalculate them, but I haven't looked at the code. I guess you could just create a copy of the finiteVolume library and change the cell centre calculation code, but, as you say, it may not be enough.
Will offcentre cell centres theoretically impact other parts of the code that just assume centroid locations? For instance, body forces would now also have a moment that are not accounted for. Would this also confuse nonorthogonality corrections and inverse distance calculations? Off the top of my head, I don't know.
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June 26, 2013, 11:22 

#5  
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Andrej
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Quote:
I guess I'll just try to track the cell centers and check if they will change during calculations. 

June 26, 2013, 11:28 

#6 
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David Gaden
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fvMesh will never let the cell centres change.
Also, the finiteVolume library is littered with nonorthogonality corrections. I'd suggest you copy and rename this library, modify the cell centre calculations, and go through all the discretization schemes, stripping nonorthogonality corrections. They're easy to spot... any discretization or interpolation that looks more complicated than the theory says, it's likely nonorthogonality correction doing it. Then copy a solver, e.g. simpleFoam, and rename it, link it instead to your custom fv library.
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June 26, 2013, 12:02 

#7  
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Andrej
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Quote:


June 30, 2013, 05:01 

#8 
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Andrej
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I've just added a new method, that brakes encapsulation rules and allows to change centers. So all OpenFOAM functionality is kept, but inside my program I can add a second postrocessing after the mesh is read, just by reading center points from my file. It is a dirty solution, but very cheap, and easy to "turn on/off" original OpenFOAM functionality, so for our purposes this solution is pefect.
Results: 1) turning on/off any nonorthogonality corrections has zero effect on results 2) I've got the same convergence speed as in handmade solver 3) Mesh centers are read from meshs tructure, and modifying it was enough. You were right, it is constant during calculations Thank you for the tips again, my problems are solved . 

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