Erik Kostson
Ansys Employee

happy to help

"But the trend of it seems weird, it oscillates a lot (see picture). Is it normal or is there some other problem? "
This is hard to say why it does that.
"The solution interval is 50 Hz. I see you have set second Radiation Boundary condition but I dont know where. Could this cause the problem?"
As shown in the image I showed and as explained, the radiation boundary is on the same place as the surface velocity excitation (so at the inlet of the duct).

So in theory with this workflow (port, surface velocity and radiation boundary on excitation inlet) we should be able to get a reasonable curve see below:

All the best