liquid flow in distillation sieve tray
 

I am trying to simulate air-liquid system in a confined space (a section of distillation column) in which liquid is flowing from higher elevation to a lower one using VOF model.

Below is a simple illustration of the geometry setup. Liquid flowing in from the left (velocity inlet) and create a holdup until it reaches a certain height (higher than the outlet weir "marked by I") before it flows down to the lower region (pressure outlet at the right end)

__________I
................I
................I
................I_________

I managed to patch the water region in both elevations, however, how do I simulate liquid falling from higher elevation to the lower elevation? I am very concern on the liquid-air interface when the liquid is flowing down. I tried to follow a tutorial on tank sloshing, but couldn't get the desired observation.
Did I miss something?

Any suggestion would be greatly appreciated. Thanks