Isolating instability mechanisms affecting circular vortex pairs

Extensive research has utilized colliding vortex rings to examine fundamental dynamics comprising complex flows. Such rings are dominated by separate instabilities inducing long- and short-wavelength displacements of the vortex core. These mechanisms, however, are inherently coupled in studies of vortex pair interactions, and initiating such collisions with typical vortex ring generators constrains the achievable geometry of the resulting vortex pair. We address these limitations through a framework for generating circular vortex pairs congruent to systems of colliding vortex rings. Such pairs are generated computationally by selectively applying a non-conservative body force within the domain, enabling near-arbitrary pair geometries that isolate the instability mechanics. Experimentally, pairs are generated with a radial starting jet in a water tank, where the shape of the outlet seeds the perturbation. We show that the radially expanding vortex pair shares key characteristics with a similar head-on vortex ring collision and that long- and short-wavelength instability mechanisms can indeed be isolated. Furthermore, we computationally and experimentally map the dispersion relation of the instabilities, yielding the spatiotemporal scales on which they evolve with applications for understanding turbulent transition and flows dominated by initially coherent vortices.