PET Bottle static Crush test - How to simulate the air or water inside?

[SergioP]

Hi, I'm doing some FEA of PET bottles, but I want to take in count first the air trapped inside (as if the test were with the empty closed bottle), second with water inside, and after with a mix of water and air.

I have tried to apply a pressure inside the model, but it inflates that doesen't make sense to me. After I try to put the same pressure inside and outside, but results are the same as if it not have pressure at all.

Regards