calculating forces coeff
 

Hi
I want to calculate forces for viscoelastic fluid flow. According to what I found out, just I should change the "fT" , i.e. tangential force. in forces.C code we can see:

-etaS*dev(twoSymm(fvc::grad(U)));

for shear stress.

I have three questions:

1- why did it use deviatoric form of twoSymm(fvc::grad(U)) tensor? As you know the shear stress tensor is calculated by:
-etaS*twoSymm(fvc::grad(U))
why deviatoric?!

2- why did it use minus in formula?

3- if I want to add it polymer stress tensor, what should I do? is it enough to write:
(+ or - ??)tau - etaS*dev(twoSymm(fvc::grad(U)));

thanks