Thanks for your answer,
My model has hydraulic oil as a working fluid. That is why I take it as incompressible fluid. For both cases (air and oil) the solution converges. However, for the case of oil, converged result is not I expected to happen in the system. There maybe another numerical solution which the program is converged to as you explanied. The boundary conditions must be pressure type. I cannot change that. However maybe I can change something else to get expected results.
What should I change to get expected results? After I posted my question, I have tried that I started the simulation for air and after the results were converging I switched to hydraulic oil (in terms of density). Nevertheless, at the moment I changed the density, results were completely off and at the end the simulation I got the same result as I used oil density at the beginning of the simulation.