[MNoCeZ]

Hello together,

i want to calculate the stress tensor with the following definition:



my first approach:

calcTau = 2 *mu *(gradU + gradU.T()+1/3*diag(U));

the problem is diag(U), because it does not return the required format?

I would be very grateful for your help.

[alexvaleije]

Hi

I'm not 100% sure, because I'm taking code that I wrote years ago, but give it a try to this:

twoSymm(fvc::grad(U))

In another thread, I have also found this definition, in case you want to try:

UEqn.diag()

Regards,
Alex

[EmadTandis]

Hello
you can use "tr(fvc::grad(U)) I" instead of diag(U). fvc::div(U) is correct as well but the first is better since it is calculated more correctly at boundaries.

[MNoCeZ]

Hello,

i solved it with:

Tau = 2 *mu *(gradU + gradU.T()-(1/3.0)*diag_gradU);

where diag_gradU is

volTensorField diag_gradU = gradU;

forAll(diag_gradU,cellI)
{
diag_gradU[cellI].xy()=0;
diag_gradU[cellI].xz()=0;
diag_gradU[cellI].yx()=0;
diag_gradU[cellI].yz()=0;
diag_gradU[cellI].zx()=0;
diag_gradU[cellI].zy()=0;
}