I have a model that outfalls into another pressurized system. I am wondering what is the best way to simulate this.

I tried creating a free outfall with invert elevation of 20ft. However, the outfall registered 0 flow.

Should I try using a control structure with an outlet set at elevation 20ft and the (free) outfall at 0

Also, how do I make sure that all of the pipes in my system are full of water/wastewater before the simulation?