My task is to investigate the flow from a circular canal into a filter and analyse the outgoing pressure/velocity from the filter. The pressurelosskoefficient is known. How do I define the filter. As a porous media? How do I work with the media in Gambit?

A porous medium should work for you. First, you need to determine the permeability (a measure of free area) and inertial resistance factor for the medium from your pressure loss data. Check the FLUENT docs for the format of the additional sink term on the momentum equation. Second, just create a separate fluid region in Gambit. Once you import the mesh to FLUENT, you can define the fluid region as a porous medium through the boundary conditions tab.

Thank you Evan! I will try that.