# Diffusion term in OpenFOAM

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 October 4, 2013, 08:00 Diffusion term in OpenFOAM #1 New Member   Matthias Stammen Join Date: Oct 2010 Posts: 6 Rep Power: 7 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

 February 12, 2014, 10:23 #2 Senior Member   Joachim Herb Join Date: Sep 2010 Posts: 298 Rep Power: 10 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

February 12, 2014, 10:43
#3
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Bernhard
Join Date: Sep 2009
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Quote:
 Originally Posted by MatzeS 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.

 February 12, 2014, 10:55 #4 New Member   Matthias Stammen Join Date: Oct 2010 Posts: 6 Rep Power: 7 Hello, thank you for your reply, it was very helpful! Regards, Matze

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