Erik Kostson
Ansys Employee
No worries.
"I was talking about "Radiation Boundary 2" which I cant see at the picture you posted."

That is not needed (was just something else that we do not need) - so we only need one radiation boundary, on the same place as the surface velocity excitation (so at the inlet of the duct). See below:

The fluid resistivity is a material property and is often given by the manufacturer say of the foam or mineral wool etc. It can be measured with physical testing, but not sure if there is an easy way of calculating it - I would search the web to see if it is possible, but unfortunately that is all that I can help with since we do not have such possibility (to estimate fluid resistivity), since it is a material input from the user.

All the best