Experimental investigation of the instabilities of a pair of vortex lines in a stably density-stratified environment

A pair of parallel counter-rotating vortices represents one of the most elementary flow configurations found in the far wake of any lifting devices. When a single vortex line remains isolated, it can persist over a long distance in the absence of instability. However, when two counter-rotating vortex lines are in close proximity, their mutually induced flows can trigger a range of complex three-dimensional instabilities, from short-wavelength elliptic instability to long-wavelength Crow instability that eventually leads to the demise of the vortex lines. These instabilities can potentially be modulated by the density-stratified environment. In this work, to understand the interaction between buoyancy and flow instability, we designed a high-speed volumetric scanning system capable of performing stereoscopic particle image velocimetry (stereo PIV) and planar laser-induced fluorescence (PLIF). This allows us to measure the three-dimensional fluid velocity field and density field simultaneously. By systematically varying the density stratification (the Froude number) and the inertia of the vortex pair (the Reynolds number), we investigated various instabilities in a controlled way and generated a unique dataset that helps us study the parametric dependence of the routes to turbulence originating from these different instabilities.