A note on the generation of a compressible vortex rings using helium as driver gas.

An experimental study has been conducted on the generation and propagation of compressible vortex rings using helium as a driver gas, with the aim of evaluating the effects of multi-gas operations for real-life applications. The advantage of such system, when compared to a constant gas system based on ambient air, is to effectively increase the Mach number while keeping the pressure ratio constant. Three pressure ratios of ∼4, 8 and 12 were set, corresponding to experimental Mach numbers of approximately 1.50, 1.81 and 2.05. The increase in incident Mach number resulted in the variation of the vortex ring and trailing jet structure, and an increase in both the velocity magnitude and vorticity field. Results showed a transition from the regular-reflection shock-cell system at the experimental Mach number approximately 1.50 to the presence of a Mach reflection with a central Mach disc, which grew in size with further increase in incident Mach number. The presence of the Mach disc resulted in the formation of a subsonic jet, internal to the main trailing jet. Its velocity was measured to be in the order of magnitude of 550 m/s, with the speed of sound of helium at 1005 m/s. Results also demonstrated that shear layers formed between the subsonic and main trailing jet have opposing vorticity, with that of the subsonic jet being approximately half in magnitude. Secondary counter-rotating vortex rings were generated ahead of the main vortex and orbited around it. The analysis of the vorticity field has shown that these secondary vortices have a magnitude approximately half of that of the vorticity of the main vortex, and has confirmed that they have an opposite direction of rotation. [ABSTRACT FROM AUTHOR]