Investigate the dynamics of vortex ring using holographic vorticimetry*

Characterizing vortex structures and associated vorticity dynamics is vital to enhance our fundamental understanding of turbulence beyond conventional statistical averaging and stochastic dynamics frameworks. We have recently developed a method for direct vorticity measurement in fluid flows based on digital inline holography^[1]. This approach was shown to be able to simultaneously measure the Lagrangian rotation and translation of multiple tracers and obtain the vorticity in regions less than 100 µm. Following up this study, here we use this approach to examine the dynamics of a vortex ring in a tank filled with refractive-index-matched fluid. The vortex ring is generated using a piston arrangement through a circular nozzle and seeded with specially made tracers with internal markers under a variety of conditions (i.e., piston velocity and stroke ratio). Using our approach, we can reconstruct the rotation of tracers trapped in the vortex core and determine the spatial distribution of vorticity and the change of vorticity of vortex ring as it evolves from its generation to its final stage of dissipation. This study paves the way for implementing our approach to characterize the dynamics of small-scale vortex structures in turbulent flows.