Vortex Line Pair Instabilities in a Stable, Density Stratified Fluid *

We present an experimental study of the flow instabilities developed by a pair of counter-rotating vortices descending in a stable, density-stratified environment. The vortex line pair is generated by flapping two sharp-edged long foils in a way that both the vortex line Reynolds number and vortex line gap can be controlled. The instabilities developed can be captured by the shadowgraph imaging and refractive index matching methods. The results can shed new light on how the vortex wake in a stratified environment is affected by the Reynolds number and Froude number.