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how to add sourceterm in momentum equation for interFoam? 

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October 11, 2011, 09:10 
how to add sourceterm in momentum equation for interFoam?

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hi everybody
I'm trying to modify the interFoam equations by adding a source term to the momentum equation, i.e. UEqn.H. Whether it's better to change UEqn.H or pEqn.H (i.e. left or right side of equation (?)), I dont know, but since my source term is dependent on U, I assumed it would make sense to add to UEqn.H. the sourceterm that I want to add is s.th. like this (in the code I had the actual function +.. * ...  etc) (see attachment) f(alpha1) * g(rho1, rho2) * h(g, ddt(U)) my understanding is that the solver calculates the variables and their values at the cell faces rather than at the cellcenters?! If that's true, how do I incorporate that? (I tried ..::interpolate(...)) Every help, idea, hint... will be greatly apprecitated! ***EDIT:  so far I couldnt find an answer in this forum (and others), but maybe there is already a similar thread? a link to such a thread would be equally good for an answer/help Last edited by anon_g; October 12, 2011 at 06:04. 

October 12, 2011, 09:09 

#2 
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maybe someone has some experience with other solver/codemodifications?


October 13, 2011, 09:37 

#3 
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David Boger
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If you are adding a source term to the momentum equation, you will need to modify the momentum equation, which is the equation being solved in UEqn.H. If your source term depends on U, you will need to think carefully about whether to treat it explicitly or implicitly. See Patankar's "Numerical Heat Transfer and Fluid Flow" for implicit/explicit handling of source terms. If your source term is proportional to ddt(U) as you suggest in your post, then it seems like you should be modifying the existing time derivative.
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October 14, 2011, 09:43 

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1st: thx for replying...
Quote:
Quote:
i read through patankar, but i didnt find any passages, that would (explicitly) cover sourceterm handling as implicit/explicit. i dont expect it, but maybe you remember which chapter you were thinkin about? also, my problem seems to be more of a  programming/implementing one: when i add my source term and try to compile it, i get all sorts of error messages. i hoped someone could give me general advice on how to implement a term. Quote:
...and, it would be nice to be able to add/remove a source term without editing the original equation. any other ideas/suggestions/advice? help's much appriciated Last edited by anon_g; October 14, 2011 at 11:03. 

October 14, 2011, 14:59 

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David Boger
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Quote:
Quote:
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October 14, 2011, 15:04 

#6 
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Adhiraj
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What kind of a source term is it? Can you post the equation you want to code in?
If the term should be treated implicitly, I would use fvm, otherwise fvc. They are not interchangeable, as the return types are different. fvm returns the contribution to the coefficient matrix A(the Ax=b system yiou get after discretizing your equation) , while fvc returns a vector that you can just add to the right hand side of the equation. My understanding for implicit and explicit handling is that if by implicit handling the matrix A becomes easier to invert, then do it. Else, use explicit source term. 

October 17, 2011, 07:56 

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Quote:
But maybe i should start giving more details so that my problem becomes clearer. mathematically speaking: S = (1alpha1)*(rho2rho1)*(g  d(U)/dt) i figuered that my problems, i.e. the error messages upon compiling, arise as soon as a put the 'g' in there. the rest is implemented as is, except fvm::ddt(U) ^^ another question on the last post: Quote:


October 17, 2011, 09:27 

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David Boger
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I added the line you describe to the UEqn in UEqn.H and got no errors for both OpenFOAM 1.6ext and OpenFOAM 2.0.x. Maybe you need to attach your modified UEqn.H, tell us what version you are using, and most importantly, tell us what the error is that the compiler gives?
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October 18, 2011, 07:54 

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you are right
here's why it didnt work for me before: as i said i thought the equations where/are solved at the cellfaces... but the variables, being volScalarFields/VectorFields, represent as far as i know, please correct me if i'm wrong  the whole cell value, stored at the center of the cell. therefore i thought it would be necessary to project/interpolate the values to the surfaces. i tried this...: surfaceScalarField rhoF = fvc::interpolate(rho) i saw this in other parts of the code and thought it would be reasonable to implement it that way. but again, what do i know, i'm just at the first step of a high ladder when it comes to openfoam knowlegde. i would be curious though if my attempt was that bad at all and where the mistake in my "reasoning" exactly was. if somebody has the answer for this, please feel free to share anyway, when using my "created variables" (rhoF etc.) in the equation, compilation failed. 

October 18, 2011, 12:47 

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David Boger
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You're correct (now) that the dependent variables are usually stored as cellcentered values (volScalarFields and volVectorFields). But there are also many situations where the facecentered values (e.g., surfaceScalarFields) are required. I'd suggest finding Hrv Jasak's PhD dissertation (available online, check Google) for the explanation of how the equations are discretized to understand much of that reasoning.
In the future, I'd also suggest posting more detailed explanations of your problems. Saying that 'it failed' is not nearly as helpful as posting the exact error message and the portion of the code that the error refers to. But from what you've said, it sounds like the compiler might have been telling you that you couldn't mix and match your surfaceScalarFields and volScalarFields. For example, you wouldn't be allowed to add one to the other.
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interfoam, momentum equation, source term 
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