Alright, so we have a splitter box with a single inlet that comes in the top of the box, and two outlets coming out of the box.
These orifices are separated by a weir, in which the fluid flow underneath one guide, and then over the next.
However, one section of the box (not an outlet) is vented to open air. It is the area above the two outlet valves.
We know from experience that at various different flow rates, the water level remains constant, and we are trying to simulate this with Flow Simulation.
But I have no idea how to simulate this open air condition, and then how to determine from the simulation what the water height is.
I have attached two images for reference (let me know if additional images will be helpful).
From the front view, I am sketched in basic red flow lines to show direction, and the blue line indicates a potential water level.
The only strategy I've tried so far has been to model the "air" as a solid, and run a goal optimization where I am searching for atmospheric pressure on the surface of that solid. You'll see this in the attached images. If you think this idea is a good modelling strategy, please also let me know.
It sounds to me like you are trying to model water as a dynamic free surface. If this is the case, you are out of luck because Flow Sim does not have this capability. All fluid domains in Flow Sim have fixed rigid boundaries for all simulation time.