[yzf1215]

For example in SimpleFoam, within U equation there is
+ turbulence->divDevReff(U)
I assume this is the deviate part of Reynolds stress. So my question is that where is the isotopic part?

And in OpenFOAM turbulence models, say K epsilon of k omega, there are R() and divDevRhoReff(). I think R() is the Reynolds stress, why to use divDevRhoReff() instead of R()?

I am not sure if I understand it correctly. I looked at OpenFOAM source code and found that in RANS models the Reynolds stress comes into momentum equation only has deviate part bus the LES sub grid stress comes into momentum equation is non trace-free.

Could anyone explain to me why? I am a little confused here.