I am modeling a blast protection system, which is essentially a heavy plate with orifices backed by a volume of water and a gas cushion. I am modeling the impact of a projectile with the plate, which compresses the water (and gas) and forces it out of the orifices. I would like to include the inertia of the water in the system energy balance, but since it is flowing out, the fluid inertia is variable. Is there any way to capture this using the existing modeling toolbox, or would a custom block of some kind be required? Incidentally, mechanical masses are fixed only as well. What is the best way to simulate a time-varying mass?