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