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Investigating vorticity dynamics provides an effective way for understandingthe fundamental mechanisms of fluid flows across diverse scales. However,experimental vorticity measurements often suffer from limited spatial andtemporal resolution, hindering our capability to probe into small-scaledynamics in various flows, particularly turbulence. In Li et al. (EXIF, 2022,vol. 63, 161), we introduced a novel holographic vorticimetry technique fordirect vorticity measurements by tracking the three-dimensional rotations oftracers with internal markers. This study further extends it to investigate theintricate vorticity dynamics during the evolution of elliptical vortex ringswith different aspect ratios. Based on the shadowgraph imaging quantifying theaxis-switching cycles and vortex ring deformation, holographic vorticimetry isapplied to measure the vorticity distribution within the millimeter-size coreof elliptical vortex rings during their evolution. Specifically, our methodresolves an even vorticity spread near the core center that rapidly decayswithin a few hundred microns. Additionally, our results reveal the intricatevorticity fluctuations associated with the folding-unfolding behaviors duringthe vortex ring evolution. These subtle vorticity changes informed bysimulations have not been captured by previous experiments due to limitedresolution. Furthermore, we find that higher aspect ratios yield larger initialvorticity and vorticity fluctuations but also prompt earlier inception ofinstabilities, causing vortex core distortion. These opposing effects result ina non-monotonic vorticity evolution trend. Overall, our measurementsdemonstrate the efficacy of holographic vorticimetry by measuring the intricatevorticity variations in unsteady vortex flows, paving the way for capturing thevorticity dynamics of small-scale turbulence structures.

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