Results of simulations of air injection into a liquid-filled (water or liquid lead) closed vessel with a high pressure difference are presented. The simulations reveal a big difference in processes of air volume formation and evolution of its boundaries during air injection into air or liquid lead. A comparison of periodic pressure pulsations in the gas volume (simulation results) with pressure pulsations described by a quasi-stationary 1D model demonstrates conservatism of the simplified model. The results of destruction of gas cavities with pressure pulsations for water and liquid lead are compared. Other conditions being identical, the gas volume in water decays faster than that in lead.