How is the eigenframe of the rate-of-strain tensor perturbed by density gradients?

We present an experimental study on a variable-density turbulent jet. The jet's three-dimensional density and velocity fields have been reconstructed using a time-resolved simultaneous stereoscopic PIV and planar LIF system together with the \textit{Taylor's frozen turbulence hypothesis. }The obtained dataset is then analyzed to understand how density gradients perturb the eigenframe of the rate-of-strain tensor whose alignments with the vorticity vector govern the key turbulence process of vortex-stretching.