Compressible vortex loops and their interactions

Vortex loops are compact toroidal structures wherein fluid rotation forms a closed loop around a fictitious axis, manifest in many natural occurrences. These phenomena result from brief impulses through vents or apertures in fluid systems, such as in caves, volcanic crusts, downbursts, or the descent of liquid droplets. The majority of naturally occurring and laboratory-generated vortex loops, studied for fundamental research on their formation, growth, instability, and dissolution, are classified as incompressible. This categorisation denotes negligible alterations in thermodynamic properties within the vortex loop. However, a distinct category of vortex loops emerges from processes involving artillery, shock tubes, explosions, chemical interactions, and combustion. This class primarily constitutes compressible vortex loops. Their presence in flow fields spans over a century, and they have been observed since the application of open-ended shock tubes to explore phenomena like diffracting shock waves, blast wave interactions with objects, and noise mitigation. The study and comprehension of compressible vortex loops and their interactions have historically relied heavily on optical techniques, lacking comprehensive insight into the intricate flow dynamics. Nevertheless, the advancements in flow visualisation tools and computational capabilities in the 21st century have significantly aided scientists in scrutinising and characterising these vortex loops and their interactions in intricate detail. Unfortunately, a comprehensive review of the literature addressing compressible vortex loops originating from shock tubes, their evolution, and interactions with shockwaves and various objects, including walls, appears lacking. This review article aims to address this gap.