Boundary Condition - Conserve mass, oscillating tank
 

Hello!

I am working on a simulation of an oscillating tank that is filled with water; a concentration of another substance is introduced to the system at the top of the liquid level. The frame oscillates vertically (parallel with gravity) and the bottom of the tank pushes liquid water up before allowing it to move downward. The system is a batch process, flow regime is laminar, thermal condition is isothermal. Movement is applied to the frame via zone_motion udf.

I am in need of a boundary condition for the top of the liquid level that maintains the concentration in the fluid at the top while not pushing the fluid when the tank moves downward (I don't want a solid wall as this will push the fluid down and contribute to the velocity). Originally, the system had a nitrogen blanket at the top; to simplify my geometry I have attempted to remove this portion since it doesn't have any effect on my fluid. The simplification was preferable since the next step in my simulation is to apply the oscillating tank to an oil-water emulsion with population balance, then to add a reaction.

Is there such a boundary condition that I may apply to the top of the liquid level? I am currently trying a pressure outlet but there's much strangeness occurring with the flow at the boundary (partial reversing of flow and such when there shouldn't be).

EDIT: avoided the issue; included the gaseous phase at the top of the column with course mesh.