The influence of mach number and over-pressure on the structure of supersonic gas jets

The Mach number is the primary parameter in determining the intrinsic driving properties, and effect on the environment of a supersonic jet. In the most basic form, a jet is released when a wall of a large high pressure gas reservoir is punctured. The resulting high-pressure jet contains a configuration of shocks which continue to disturb the environment after the initial bow shock has passed. Here we perform numerical simulations to determine the properties attributable to pure adiabatic hydrodynamic effects, taking a uniform stream out of a circular nozzle. We take a range of Mach numbers that, along with the jet over-pressure, determine the flow pattern and shock locations. We distinguish conditions which generate Mach shock discs rather than a diamond pattern of oblique regular reflections. Potential observational diagnostics explored include the disc size, the distance from the nozzle, and oscillations in shock positions. Rapid oscillations occur in the divergent-convergent pattern through a feedback/hysteresis effect promoted by the ambient medium. The underlying flow patterns are independent of relative jet density but heavy jets display both lower amplitude and lower frequency oscillations. We also study the energy transferred into the environment. Over-pressured jets may contribute to noise and sound wave generation through screeching. However, these oscillations in the near-field are not sufficiently significant to regulate star and galaxy formation. We expect that upcoming high dynamic range and resolution observations will increasingly detect the shock patterns as jet gas transits from protostellar and galactic cores.