\input zb-basic \input zb-ioport \iteman{io-port 05872431} \itemau{Kasten, Jens; Hotz, Ingrid; Noack, Bernd R.; Hege, Hans-Christian} \itemti{On the extraction of long-living features in unsteady fluid flows.} \itemso{Pascucci, Valerio (ed.) et al., Topological methods in data analysis and visualization. Theory, algorithms, and applications. Based on the 3rd workshop on topological methods in data analysis and visualization, Snowbird, UT, USA, February 23--24, 2009. Berlin: Springer (ISBN 978-3-642-15013-5/hbk; 978-3-642-15014-2/ebook). Mathematics and Visualization, 115-126 (2011).} \itemab Summary: 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 main long-living features employed in conceptual fluid mechanics models. Results are shown for periodic and chaotic vortex motion. \itemrv{~} \itemcc{} \itemut{} \itemli{} \end