Experimental Discovery of Early-Stage "Mushroom" Instability in Counter-Rotating Vortex pair in stratified fluid

A pair of parallel counter-rotating vortices represents one of the most elementary flow configurations found in the far wake of any lifting devices. In this study, we experimentally observe and analyze the dynamics and instabilities of two counter-rotating vortex lines in a stratified fluid. We report the first observation of a "mushroom" instability at the early stage of vortex movement when the Froude number around 1, characterized by a row of mushroom-like structures with a spacing of 0.37 times the initial distance between the vortex lines. Initial instability wavelength was set by the elliptic instability, which helps develop baroclinic torque that finally results in flow structure with lighter fluids invading the heavier fluids. Those mushrooms resemble the structures developed in Rayleigh Taylor instability. Additionally, we examine the effect of the density gradient on the characteristics of the elliptical instability, providing insights into the interplay between stratification and vortex dynamics.