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-   -   Difference between UEqn in sonicFoam & Theory (https://www.cfd-online.com/Forums/openfoam-programming-development/119333-difference-between-ueqn-sonicfoam-theory.html)

 sasanghomi June 14, 2013 18:13

Difference between UEqn in sonicFoam & Theory

Hi Foamers ,

I found a difference between UEqn in sonicFoam and Ferziger's book (Computational Methods for fluid Dynamics ).
In the code for UEqn in sonicFoam we have :
Code:

fvVectorMatrix UEqn (     fvm::ddt(rho, U)     + fvm::div(phi, U)     + turbulence->divDevRhoReff(U) ); solve(UEqn == -fvc::grad(p));
This code ensures below equation :

But in the Ferziger's book (generally in theory) there is another equation :

The last term in two equations are different . Please guide me that what is the origin of this difference ? And please correct me if I am wrong.

I appreciate any help from you.
Thanks and best regards,
Sasan.

 vwibaut June 26, 2013 09:20

Your ferziger's equation comes from incompressible theory, no? You haven't any term
dui/dxi

 immortality June 26, 2013 11:25

I think so ;)

 lfgmarc June 27, 2013 01:34

Dear sasanghomi,

I found the same incongruence some time ago. I think that is a bug, but if you have some time you can review the
Code:

 divDevRhoReff
member function to try to figure out this problem. I try to do this, but my conclusion was that it is an error, because the term

is missing :confused:

Best regards.

Felipe G.

 vwibaut June 27, 2013 02:54

Quote:
 Originally Posted by lfgmarc (Post 436240) Dear sasanghomi, I found the same incongruence some time ago. I think that is a bug, but if you have some time you can review the Code:  divDevRhoReff member function to try to figure out this problem. I try to do this, but my conclusion was that it is an error, because the term is missing :confused: Best regards. Felipe G.

The term with 2/3 rho k is present in the equation. It is taken into account in the pressure (in the equation p = ps + 2/3 rho k). The results you have, after simulations, give you a pressure wich contains the real pressure and turbulent kinetic energy.

 Chris Lucas June 27, 2013 03:09

Hi,

have a closer look at the function:

divDevRhoReff
and you will find your "missing" term

Regards,
Christian

 lfgmarc June 27, 2013 03:48

Hi to all,

Can you say me the OF version ? because at least in OF1.7.0 I can't find this term.

http://www.cfd-online.com/Forums/ope...sonicfoam.html

Maybe I looked in a wrong location in the code.

sonicFoam pressure equation.

Code:

rho = thermo.rho(); volScalarField rUA = 1.0/UEqn.A(); U = rUA*UEqn.H(); surfaceScalarField phid (     "phid",     fvc::interpolate(psi)   *(         (fvc::interpolate(U) & mesh.Sf())       + fvc::ddtPhiCorr(rUA, rho, U, phi)     ) ); for (int nonOrth=0; nonOrth<=nNonOrthCorr; nonOrth++) {     fvScalarMatrix pEqn     (         fvm::ddt(psi, p)       + fvm::div(phid, p)       - fvm::laplacian(rho*rUA, p)     );     pEqn.solve();     if (nonOrth == nNonOrthCorr)     {         phi = pEqn.flux();     } } #include "rhoEqn.H" #include "compressibleContinuityErrs.H"
divDevRhoReff kEpsilon :

Code:

 tmp<fvVectorMatrix> kEpsilon::divDevRhoReff(volVectorField& U) const {     return     (       - fvm::laplacian(muEff(), U)       - fvc::div(muEff()*dev2(fvc::grad(U)().T()))     ); }

Best regards.

 sasanghomi June 27, 2013 03:50

Hi Guys ,
I corrected the momentum equation in first post (Theory equation). But I think below term is missed.

Best regards
sasan.

 sasanghomi June 27, 2013 03:56

Quote:
 Originally Posted by vwibaut (Post 436257) The term with 2/3 rho k is present in the equation. It is taken into account in the pressure (in the equation p = ps + 2/3 rho k). The results you have, after simulations, give you a pressure wich contains the real pressure and turbulent kinetic energy.
Hi Valentin ,
Which file have this equation for pressure ?
Can you explain more and clarify the issue ?

Thanks and best regards,
Sasan.

 vwibaut June 27, 2013 04:12

You will not find this term because it is into the pressure term. More clearly, the pressure in your equation isn't the real pressure. It is the real pressure+ 2/3 rho k.

But it is close to the real pressure because k << p

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