Underwater supersonic gas jets are widely utilized in submerged propulsion systems such as missiles launched from submarines. Unlike the single phase supersonic jets (gas jet in air), supersonic jets which are submerged underwater can give rise to high pressure pulses moving back into the nozzle assembly. This study is focused on the numerical simulation of the initial transients of underwater supersonic air jets. The results from the numerical simulations are used to visualize the oscillation of pressure, Mach number and jet interface under varying operating parameters to envisage the physics of the flow field. Several aspects of the submerged jets are compared with those of the single phase jets to understand the major differences in the multiphase environment. Subsequently, the submerged jet will be studied more elaborately under varying operating parameters. Thereby, this work attempts to investigate and understand the different physical aspects of the development of the flow structure of a supersonic jet submerged in water. © 2021, American Institute of Aeronautics and Astronautics Inc, AIAA. All rights reserved.