Attracting fixed points for heavy particles in the vicinity of a vortex pair

We study the behaviour of heavy inertial particles in the flow field of two like-signed vortices. In a frame co-rotating with the two vortices, we find that stable fixed points exist for these heavy inertial particles; these stable frame-fixed points exist only for particle Stokes number $St<St_{cr}$. We estimate $St_{cr}$ and compare this with direct numerical simulations, and find that the addition of viscosity increases the $St_{cr}$ slightly. We also find that the fixed points become more stable with increasing $St$ until they abruptly disappear at $St=St_{cr}$. These frame-fixed points are between fixed points and limit cycles in character.