Global visualization and quantification of compressible vortex loops

The physics of compressible vortex loops generated due to the rolling up of the shear layer upon the diffraction of a shock wave from a shock tube is far from being understood, especially when shock-vortex interactions are involved. This is mainly due to the lack of global quantitative data available which characterizes the flow. The present study involves the usage of the PIV technique to characterize the velocity and vorticity of compressible vortex loops formed at incident shock Mach numbers ofM=1.54 and1.66. Another perk of the PIV technique over purely qualitative methods, which has been demonstrated in the current study, is that at the same time the results also provide a clear image of the various flow features. Techniques such as schlieren and shadowgraph rely on density gradients present in the flow and fail to capture regions of the flow influenced by the primary flow structure which would have relatively lower pressure and density. Various vortex loops, namely, square, elliptic and circular, were generated using different shape adaptors fitted to the end of the shock tube. The formation of a coaxial vortex loop with opposite circulation along with the generation of a third stronger vortex loop ahead of the primary with same circulation direction are of the interesting findings of the current study

Collision of vortex-rings upon V-walls

A study on and 4000 vortex rings colliding with V-walls with included angles of to 120 has been conducted. Along the valley plane, higher Reynolds numbers and/or included angles of lead to secondary/tertiary vortex-ring cores leapfrogging past the primary vortex-ring cores. The boundary layers upstream of the latter separate and the secondary/tertiary vortex-ring cores pair up with these wall-separated vortices to form small daisy-chained vortex dipoles. Along the orthogonal plane, primary vortex-ring cores grow bulbous and incoherent after collisions, especially as the included angle reduces. Secondary and tertiary vortex-ring core formations along this plane also lag those along the valley plane, indicating that they form by propagating from the wall surfaces to the orthogonal plane as the primary vortex ring gradually comes into contact with the entire V-wall. Circulation results show significant variations between the valley and orthogonal plane, and reinforce the notion that the collision behaviour for is distinctively different from those at larger included angles. Vortex-core trajectories are compared to those for inclined-wall collisions, and secondary vortex-ring cores are found to initiate earlier for the V-walls, postulated to be a result of the opposing circumferential flows caused by the simultaneous collisions of both primary vortex-ring cores with the V-wall surfaces. These circumferential flows produce a bi-helical flow mode (Lim, Exp. Fluids, vol. 7, issue 7, 1989, pp. 453-463) that sees higher vortex compression levels along the orthogonal plane, which limit vortex stretching along the wall surfaces and produce secondary vortex rings earlier. Lastly, vortex structures and behaviour of the present collisions are compared to those associated with flat/inclined walls and round-cylinder-based collisions for a more systematic understanding of their differences.Ministry of Education (MOE)The authors acknowledge the support for the study by the School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore, Singapore Ministry of Education AcRF Tier-2 grant (MOE2014-T2-1-002) and National Science Foundation of China grant (grant number: 11772197)