This paper proposes a Galilean invariant generalization of critical points of vector field topology for 2D time-dependent flows. The approach is based upon a Lagrangian consideration of fluid particle motion. It extracts long-living features, like saddles and centers, and filters out short-living local structures. This is well suited for analysis of turbulent flow, where standard snapshot topology yields an unmanageable large number of topological structures that are barely related to the few

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... in long-living features employed in conceptual fluid mechanics models. Results are shown for periodic and chaotic vortex motion.