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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]

An experimental study has been conducted on the effects of distance variation on the impingement process of compressible vortex rings on a stationary solid wall. An experimental incident Mach number of 1.61 was used. Qualitative and quantitative studies compared the impingement and interaction flow characteristics of a compressible vortex ring with a stationary, solid, smooth surface at three distances: 1.66, 3.33, and 5.00 inner diameters. The three distances corresponded to an under-developed vortex ring (1.66 inner diameters), a vortex ring at its development threshold (3.33 inner diameters), and a fully developed one (5.00 inner diameters). Qualitative schlieren results showed that the surface distance affected the shock/vortex interaction process along with the impingement process of the vortex ring and the flow structure of its trailing jet. Quantitative data were extrapolated to evaluate the propagation velocity of the vortex ring prior to impingement. The boundary layer thickness was also estimated. Particle image velocimetry studies showed the main and secondary vortices to have opposite vorticity, with the magnitude of the vorticity of the secondary vortices being approximately half of that of the main vortex. Surface pressure results reveal the symmetrical properties of the impinging flow, along with a direct correlation between the maximum pressure measured at the instant the vortex ring impingement and an increase in surface distance. [ABSTRACT FROM AUTHOR]

Abstract: An experimental study has been conducted on the impingement of compressible vortex rings on stationary, inclined, smooth surfaces. Four surface angle inclinations were evaluated: 90° (perpendicular to the flow longitudinal axis), 75°, 60°, and 45°. The flow was assumed axisymmetric for the 90° benchmark case. A single pressure ratio of ∼4 was used, corresponding to an experimental incident shock wave Mach number of ∼1.31, generating a shock-free compressible vortex ring. Results showed two main flow processes: the interaction of the reflected shock wave with the approaching vortical structure, and the impingement of the vortex ring on the surface. At surface inclination angles other than 90°, these processes are asymmetrical and the shape of the reflected shock is modified by the approaching vortex ring. The lower outer section of the reflected shock wave is initially diffracted by the lower core of the vortex ring, its central section is then captured by the central section of the torus, and ultimately the upper outer section is diffracted by the upper core. The vortex ring subsequently impinges on the surface with the lower core impacting first, undergoing an asymmetrical stretching and deformation. Flow field studies showed an initial decrease in velocity magnitude and vorticity field as the flow transitioned from a symmetrical to an asymmetrical impingement, possibly due to the uneven formation of the boundary layer. The velocity magnitude and vorticity field of the upper core subsequently increase again, as a result of the upwards deflection of the flow. Oppositely, the lower core is seen to dissipate. Surface pressure measurements confirmed the asymmetry in the flow development. As the surface angle is varied, the pressure at the central measurement location is seen to decrease as the position of the stagnation point is altered. An interesting feature within the surface measurements is a strong lateral acceleration due to the formation of vortex lines which roll around the main vortex core. [Copyright &y& Elsevier]

An experimental study has been conducted on the formation and propagation of coaxial vortex loops using a shock tube facility. The study aimed at evaluating the flow characteristics of pairs of corotating vortex rings that generate the leapfrogging phenomenon. The driver and driven gas of the shock tube were air. Three driver pressures were used (4, 8, and 12 bars) with the driven gas being at ambient conditions. The Mach numbers of the shock wave generated inside the shock tube were 1.34, 1.54, and 1.66, respectively. The sudden expansion present at the diaphragm location effectively decreased the Mach number value of the traveling shock wave. Results showed that a pair of vortex rings staggered with respect to time and with the same direction rotation lead to leapfrogging. Results also indicated that the number of leapfrogging occurrences is related to the Reynolds number of the vortex ring pairs with a decrease in leapfrogs at higher Reynolds numbers. [ABSTRACT FROM AUTHOR]