Merging and Three-dimensional Instability in a Corotating Vortex Pair

Abstract

We investigate experimentally the interaction between two parallel laminar vortices of equal circulation. They are created by the impulsive displacement of two flat plates in a fluid initially at rest. The temporal evolution of the pair is analyzed with dye visualizations and quantitative measurements using Particle Image Velocimetry. For low Reynolds numbers, the vortices remain two-dimensional and merge into a single one. The size of the cores increases due to viscosity up to a certain limit, approximately a fourth of the separation distance, at which merging occurs. At higher Reynolds numbers, a three-dimensional instability is discovered, showing the characteristic features of an elliptic instability of the vortex cores. The two vortices still merge, but the final vortex is now turbulent.