  We revisit the two vortex merger problem (symmetric and asymmetric) for the
Navier-Stokes equations using the core growth model for vorticity evolution
coupled with the passive particle field and an appropriately chosen
time-dependent rotating reference frame. Using the combined tools of analyzing
the topology of the streamline patterns along with careful tracking of passive
fields, we highlight the key features of the stages of evolution of vortex
merger, pinpointing deficiencies in the low-dimensional model with respect to
similar experimental/numerical studies. The model, however, reveals a far
richer and delicate sequence of topological bifurcations than has previously
been discussed in the literature for this problem, and at the same time points
the way towards a method of improving the model.
