Dynamics of circular vortex dipoles*

Extensive research has examined colliding vortex rings as a way to isolate fundamental dynamical interactions comprising more complex flows. Initiating such collisions with vortex ring generators, however, constrains what types of rings are produced and requires precisely aligned generators. We address these limitations through a framework for generating circular vortex dipoles congruent to systems of colliding vortex rings following their initial approach. Such dipoles are generated computationally by selectively applying a non-conservative body force within the domain, enabling near-arbitrary dipole geometries that isolate mechanisms of interest. Experimentally, dipoles are generated with a radial starting jet in a water tank.

We show that a radially expanding vortex dipole shares key characteristics with a similar head-on vortex ring collision. Specifically, the base flow kinematics, linear perturbation dynamics, and local streamline topologies are consistent for both flows, confirming that our framework conveniently accesses the physics examined with vortex ring collisions. Furthermore, our method allows us to investigate a radially contracting vortex dipole akin to the vortex ring collision in reverse, revealing a distinct bifurcation in the dynamics that depends on the base flow.