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continuity equation rhoEqn.H

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Old   November 6, 2017, 07:54
Default continuity equation rhoEqn.H
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Fridrik Magnusson
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Hallo everyone
im trying to learn about OpenFoam, and by that i would like to explore the continuity equation. But my source-code-reading-skills is not the best. This post is made to disect the continuity equation from an absolut beginners point of view. Where the continuity equation is:

\frac{\partial\rho}{\partial t}+\nabla\cdot\left(\rho\overline{u}\right)=0

I have posted the rhoEqn.H below, which might be the most generally implementation of the continuity equation. Where it is unknown
what the line 7, 9, 10 and 11 does.

Code:
1{
2 fvScalarMatrix rhoEqn //Class called rhoEqn
3 (
4 fvm::ddt(rho) //generates the implicit (fvm) first order time derivative (ddt)
5 + fvc::div(phi) // adds the explicit (fvc) divergence of phi, where phi = rho*u
6 ==
7 fvOptions(rho) // compares to the acceptable error in fvSolution ?
8 );
 
9 fvOptions.constrain(rhoEqn); // Give solution criteria ?
 
10 rhoEqn.solve(); // Acturally solves the equation ?
 
11 fvOptions.correct(rho); // rewrite the code ?
12 }
Hopefully this can later be condensed into an introduction for other new users.

line 4-8 is somewhat defined in the ProgrammersGuide
http://foam.sourceforge.net/docs/Gui...mmersGuide.pdf
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Old   November 10, 2017, 09:18
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Dave
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fvOptions allows one to manipulate certain fields outside of the solver. Line 7 is a direct source/sink term, lines 9 and 11 impose a constraint on the equation (before solution) or a correction on the field (after solution). These lines won't do anything unless the user has written some fvOptions code and placed it in constant/.
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Old   November 14, 2017, 04:24
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Fridrik Magnusson
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dhumbird, thanks for getting me one step closer to the answer

your answer leads to further questions ?


line 7: fvOptions(rho) - pulls infomation from fvSolution and fvSchemes through the function fvOptions ?

line 9: What kind of constraint?

line 11: What kind of correction ?
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Old   November 14, 2017, 23:16
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Dave
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Fridrik,
At each of the fvOptions lines, the user can insert code without re-writing the solver. The code goes in a new file called constant/fvOptions. This is *advanced* OpenFOAM, so from an "absolute beginner's" perspective, these lines can be ignored. (Line 7 will return a 0, Lines 9 and 11 do nothing.) If/when you need this functionality, there are lots of hints here in the forum.
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