We carry out numerical simulations to investigate the complex flow features in the near-field of under-expanded gas jets. OpenFOAM, an open source, computational fluid dynamics (CFD) tool is used to obtain the results. Reynolds averaged, 3 dimensional Navier-Stokes equations are solved coupled with k- Omega SST turbulence model. The new solver has been constructed to compute additional features such as transport properties of multi-species mixture based on kinetic theory. Solver is validated with the experimental and simulation data for density of a helium jet expanding in air atmosphere. We report the location and diameter of the Mach disk for helium jet and compare with the analytical results. Investigation is extended for air, argon, H₂ and MNH/N₂O₄ bi-propellant fuel jet gases expanding in low pressure air atmosphere. Velocity and temperature flowfield of these gases is analyzed. Shock geometry for different jet gases is found to be weakly dependent on the specific heat ratio of jet gas medium. Location of Mach disk is farthest from nozzle exit for H₂ jet, and it also generates high temperature zones in the surrounding. The current study is important from the perspective of aerospace applications where sonic jets are often encountered.