Three-dimensional instability during vortex merging

The interaction of two parallel vortices of equal circulation is observed e xperimentally. For low Reynolds numbers (Re), the vortices remain two dimen sional and merge into a single one, when their time-dependent core size exc eeds approximately 30% of the vortex separation distance. At higher Re, a t hree-dimensional (3-D) instability is discovered, showing the characteristi cs of an elliptic instability of the vortex cores. The instability rapidly generates small-scale turbulent motion, which initiates merging for smaller core sizes and produces a bigger final vortex than for laminar 2-D flow. ( C) 2001 American Institute of Physics.