Why Fluent says that the periodic condition is not compatible with the VOF model ?
 

I realized a simulation of a free surface flow in the presence of periodic roughness with the VOF model. I remarked that when we impose the pressure gradient for the entire domain, the velocity of the air phase will be 100 x the velocity of the water phase.
Is it for this reason Fluent says that the periodic condition is not compatible with VOF?
To avoid this problem I used a UDF to impose this pressure gradient only in the water phase and I solved the problem.
Is the procedure I followed is logical?
How to contact Fluent for this problem?