How to maintain air at a constant pressure in multiphase flows?
 

Hello!
I am trying to reproduce the results given in a paper in Ansys Fluent using VOF model - it is basically a gravity-driven flow of a thin liquid film over an inclined plane. The inclined plane is hydrophobic in nature and is ribbed and contains grooves filled with air. The air above the liquid film as well as the air trapped in the grooves are both at a constant pressure of 1 atm.

I started the simulation by patching a desired height with the liquid film above the groove with air filling up the groove. Initially at t=0, the liquid-gas interface is flat. The initial pressure in the entire domain is 1 atm. According to the paper, the liquid-air interface should curve downwards. Unfortunately, when I start running the transient simulation, the pressure in the air in the groove starts increasing and due to the Laplace pressure difference, the interface curves upwards.

So my question is: Is there any way to keep the pressure in the air constant throughout the entire length of the transient simulation?