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continuity source term in momentum Eq

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Old   February 23, 2015, 01:56
Default continuity source term in momentum Eq
  #1
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Nima Samkhaniani
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would you please help me to figure out following source codes:

in interPhaseChangeFoam, UEqn.H:

Code:
fvVectorMatrix UEqn
    (
        fvm::ddt(rho, U)
      + fvm::div(rhoPhi, U)
      - fvm::Sp(fvc::ddt(rho) + fvc::div(rhoPhi), U)
      + turbulence->divDevRhoReff(rho, U)
    );
in DPMFoam, UEqn.H:

Code:
fvVectorMatrix UcEqn
(
    fvm::ddt(alphac, Uc) + fvm::div(alphaPhic, Uc)
  - fvm::Sp(fvc::ddt(alphac) + fvc::div(alphaPhic), Uc)
  + continuousPhaseTurbulence->divDevRhoReff(Uc)
 ==
    (1.0/rhoc)*cloudSU
);

as much as i understand both "Sp" lines should be mathematically zero , they are continuity terms which multiply with U, my question is why they add to momentum equation and why it considered as negative source terms?
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Old   February 24, 2015, 02:09
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T Chourushi
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Quote:
Originally Posted by nimasam View Post
would you please help me to figure out following source codes:

in interPhaseChangeFoam, UEqn.H:

Code:
fvVectorMatrix UEqn
    (
        fvm::ddt(rho, U)
      + fvm::div(rhoPhi, U)
      - fvm::Sp(fvc::ddt(rho) + fvc::div(rhoPhi), U)
      + turbulence->divDevRhoReff(rho, U)
    );
in DPMFoam, UEqn.H:

Code:
fvVectorMatrix UcEqn
(
    fvm::ddt(alphac, Uc) + fvm::div(alphaPhic, Uc)
  - fvm::Sp(fvc::ddt(alphac) + fvc::div(alphaPhic), Uc)
  + continuousPhaseTurbulence->divDevRhoReff(Uc)
 ==
    (1.0/rhoc)*cloudSU
);

as much as i understand both "Sp" lines should be mathematically zero , they are continuity terms which multiply with U, my question is why they add to momentum equation and why it considered as negative source terms?
Dear Nima,

I guess the additional term is due to variable density (rho). Had it been incompressible fluid it would have been zero. Since, for compressible flows these variables cannot be ignored. Please rectify me if I miss anything as I am not well versed with compressible flows.

-
Best Regards!
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Old   March 5, 2015, 18:22
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Dongyue Li
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Hello guys!!

Pls see the image:


The reason maybe in this way, its conservative.

This is like twoPhaseEulerFoam's code. In openfoam 22x, this kind of equation is implemented in non-conservative way, but in OpenFOAM 23x, its implemented in conservative way. U can compare the code to check it out.

Best,
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Old   March 13, 2015, 05:39
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Ramon
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Hello everyone,

I am not sure but perhaps this has something to do with the diagonal dominance of the system? Although this would only make sense if the Sp term would be negative before converging to zero... if it would be slightly positive before converging to zero it would negatively affect the diagonal dominance...

Again, I am not sure about this, one of the remaining mysteries I am trying to figure out.

Kind regards,
Ramon
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