[Giorgitseli]

Description of my problem:
The geometry of the project I am currently working on includes an open vegetated rectangular channel, 19,5 m long. However, the number of volumes created in CFX-meshing makes it impossible to deal with. That is because the density of the cylinders simulating the vegetation is very high.

As a result I would like to desing just a fraction of my channel (1-2 meters for example) and apply "Translational periodicity" between the inlet and the outlet face, so as to ensure that the flow can be fully developed in the part of the channel designed.

Already applied boundary conditions:
I choose two parallel faces (inlet-outlet) to be the domain interfaces and - in order to define the discharge- I apply "Mass Flow Rate". I also apply wall boundary conditions for the sides and the bed, and free slip wall boundary condition for the free surface (single phase problem).
Nevertheless, it doesn't work.

How do I apply "translational periodicity"? Does this work in channel flows?

I am relatively new to cfx and any help you can offer would be highly appreciated, as I have to present my project in two weeks from now.
Thank you a priori !!