Probing dynamics of elliptical vortex rings via direct vorticity measurements with digital inline holography

Characterizing the dynamics of vortex structures is critical for understanding the fundamental mechanisms of the behavior of vortices in fluid flows across different scales, including high vorticity filaments in turbulence. Elliptical vortex rings, as one of the fundamental elements of vortical structures, exhibit intriguing temporal vorticity variation due to the compression and stretching of the vortex core through a process known as axis-switching. This phenomenon, while observed in numerical simulations, has remained experimentally elusive due to the constraints of spatial resolution in traditional measurement techniques. In this study, we employ the innovative method of digital inline holography (DIH) for direct vorticity measurement, enabling us to characterize the temporal evolution of vorticity in elliptical vortex rings with unprecedented detail. These rings are generated using a piston arrangement and elliptical nozzles with varying aspect ratios. We specifically quantify the temporal evolution of rings generated from a nozzle with an aspect ratio of 3 over two complete axis-switching cycles. Our DIH-based vorticimetry successfully resolves the differences in vorticity dynamics of elliptical vortex rings with varying aspect ratios. This study demonstrates the potential of our DIH-based vorticimetry for characterizing the intricate dynamics of small-scale vortex structures in turbulent flows.