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Diffusion term in OpenFOAM

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Old   October 4, 2013, 08:00
Default Diffusion term in OpenFOAM
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Matthias Stammen
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Hello,

checking the term turbulence->divDevReff(U) I came up with the following question:

The diffusion term for incompressible flows should look somehow like this (here, I ignored the viscosity):
fvc::div(fvc::grad(U)+fvc::grad(U)().T())
And this can be transformed to this:
fvc::laplacian(U)
But when I implement that into OpenFOAM, those terms give different results. I assume it is connected to the discretization schemes, but even when I use linear for all schemes, it gives me different results.
So, what do I do wrong? What do I miss? Is it because of wall-functions or boundary conditions?
Any comment is appreciated...

Regards,
Matze
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Old   February 12, 2014, 10:23
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Joachim Herb
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I think this wiki page might explain what is going on here:
http://openfoamwiki.net/index.php/Bu...DevReff.28U.29

Also have a look at this thread (the difference between compressible and incompressible models goes away if the continuity equation is fulfilled after convergence of the solution):
Calculating divDevReff
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Old   February 12, 2014, 10:43
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Bernhard
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Quote:
Originally Posted by MatzeS View Post
Hello,

checking the term turbulence->divDevReff(U) I came up with the following question:

The diffusion term for incompressible flows should look somehow like this (here, I ignored the viscosity):
fvc::div(fvc::grad(U)+fvc::grad(U)().T())
And this can be transformed to this:
fvc::laplacian(U)
But when I implement that into OpenFOAM, those terms give different results. I assume it is connected to the discretization schemes, but even when I use linear for all schemes, it gives me different results.
So, what do I do wrong? What do I miss? Is it because of wall-functions or boundary conditions?
Any comment is appreciated...

Regards,
Matze
You ignored viscosity in your derivation. The turbulent viscosity is not constant, so you can't pull it through the differential.
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Old   February 12, 2014, 10:55
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Matthias Stammen
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Hello,

thank you for your reply, it was very helpful!

Regards,
Matze
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