# Is pressure really modified pressure with 2/3k included?

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 December 22, 2009, 18:42 Is pressure really modified pressure with 2/3k included? #1 Member   Matthew J. Churchfield Join Date: Nov 2009 Location: Boulder, Colorado, USA Posts: 47 Rep Power: 7 I am trying to understand the formulation of the momentum equation in the solvers that use PISO. The divergence of the deviatoric part of the effective Reynold stress tensor is added into the left-hand implicit side of the momentum equations using divDevReff or divDevRhoReff for incompressible and compressible flow, respectively. The 2/3k delta_ij contribution to the Reynolds stress tensor has been removed since we are dealing with the deviatoric part. Where does this 2/3 k term go? Is the pressure really a modified pressure, p + 2/3k? If that is the case, is modified pressure written to the data files or is 2/3k removed and actual pressure written to file? I know that isn't so important in incompressible flow where the gradient of pressure is all that is needed, but it is important in compressible flow. P.S. If you well understand this subject, see my recently posted question added to the thread "Calculating divDevReff" started by jposunz. Any help would be appreciated. Thank you, Matt

January 6, 2010, 16:40
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Michał
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Quote:
 Originally Posted by mchurchf The 2/3k delta_ij contribution to the Reynolds stress tensor has been removed since we are dealing with the deviatoric part. Where does this 2/3 k term go? Is the pressure really a modified pressure, p + 2/3k?
As far as i know it is added like you wrote

Quote:
 If that is the case, is modified pressure written to the data files or is 2/3k removed and actual pressure written to file? I know that isn't so important in incompressible flow where the gradient of pressure is all that is needed, but it is important in compressible flow.
I would guess that the turbulent pressure (p+2/3k) is written. I don't think if models like RANS are ( directly ) appropriate for compressible flow
Quick search on this site gave me this, maybe helpful ( edge solves compressible RANS ):
http://www.foi.se/FOI/templates/Page____4568.aspx

 September 16, 2010, 04:47 #3 Member   Kim Yusik Join Date: Dec 2009 Posts: 39 Rep Power: 6 Hi, Matt Your detail explantion in openfoamwiki helps me a lot to understand the code in openfoam (especially turbulence model part). I also came across the same problems as yours. one is 'dev2' inside of the divDevBeff issue and the other is modified pressure (p+2/3k) . currently, I use incompressible pisoFoam (OF 1.7.0) so first one may affect little and it seems obvisouly 'dev2' rather than 'dev'. But second problem, I am not quite sure. based on solver itself, it seems it use only 'p' not 'p+2/3k'. So did you figure out this issue? or Do I misunderstand something? Thanks Yusik

September 27, 2010, 05:01
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Kim Yusik
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OpenFoam confirmed that 2/3 k is caculated as a part of the pressure.

http://www.openfoam.com/mantisbt/view.php?id=47

Quote:
 Originally Posted by impecca Hi, Matt Your detail explantion in openfoamwiki helps me a lot to understand the code in openfoam (especially turbulence model part). I also came across the same problems as yours. one is 'dev2' inside of the divDevBeff issue and the other is modified pressure (p+2/3k) . currently, I use incompressible pisoFoam (OF 1.7.0) so first one may affect little and it seems obvisouly 'dev2' rather than 'dev'. But second problem, I am not quite sure. based on solver itself, it seems it use only 'p' not 'p+2/3k'. So did you figure out this issue? or Do I misunderstand something? Thanks Yusik

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