Boundary Conditions

Mordorgieigrodrom

Hey,

I have a long cylinder filled with fluid to a certain height and I inject fluid into the cylinder from the top through a lance. Above the fluid there is a gas phase. I wish to look at the top part of the cylinder where I have a constant flow of new fluid (equal to the flow out of the cylinder). The system is affected by gravity.

I don't want to model the entire height of the cylinder (only the top part) and therefore I wish to employ some suitable boundary condition to shorten the domain.

What condition should I use to get a constant massflow out of the cylinder equal to the massflow into the cylinder (so that I maintain the same height of the fluid)?

(Should I neglect gravity since I know that the mass-outflow is constant, and then use the outflow condtition?)

Regards

M