Closed streamlines are a missing part in the visualization of vector
field topology. In this paper, we propose a method which detects
closed streamlines in a time-dependent two dimensional flow and
investigates the behavior of these closed streamlines over time. We
search in all timesteps for closed streamlines and connect them to
each other in temporal order to get a tube shaped visualization. As
a starting point for our investigation we look for changes of the
type of critical points that lead to the creation or vanishing of
closed streamlines (Hopf bifurcation). We follow the resulting limit
cycle over time. In addition, changes of the topological skeleton,
built by critical points and separatrices, are considered which may
start or terminate the life of a closed streamline.
