I realized a simulation of a free surface flow in the presence of periodic roughness with 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?