Floworks won't do what your asking because you are dealing with air and a fluid. These are 2 different phases in a single cavity and are not supported.

The slushing effect could be approximated by assuming two fluids with one of the fluids having the density of air. Wait, sorry, forget that idea.

I would calculate the inner volume of the tank, then fill flow rate/tank volume will tell you the fill rate.

I would do a stress test to see if your ribs are doing anything. You may try accelerating the tank in it's 3 principle axis. I'd use shell elements if you have cosmos designer. If you only have cosmos express, then it will take some time to mesh.