Viscous stress
 

Hi there
I want to calculate the viscous stress that a body is experiencing in 3D, using Fluent.
From Fluent I can know the strain rate.

reviewing the concepts, I found that the newton's viscosity law is
Stress= miu x shear rate.

However Im a little confused since the viscous stress is
Stress = 2 miu x strain rate tensor.

I guess they are different because of the 3D.

i can obtain a strain-rate from fluent, and since it is an scalar I guess it should be the second invariant of this stress, therefore I guess I just need to make the product of this scalar by 2*viscosity.
What Im doing it writing a field function definition, so fluent makes the product of Strain rate x 2 miu

is this right?
thanks in advance