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Published June 2010 | public
Journal Article

Vortex modes in the wake of an oscillating long flexible cylinder combining POD and fuzzy clustering

Abstract

A method combining proper orthogonal decomposition (POD) and Fuzzy Clustering (FC) is used as a pattern recognition technique, in order to identify vortex modes in digital particle image velocimetry (DPIV) data, obtained in the wake of a long flexible circular cylinder undergoing vortex-induced vibrations. The POD allows a low-dimensional description of the wake, so the the fuzzy c-means algorithm can be used for clustering in a reduced order problem. The output is a set of well-defined flow clusters representing the vortex patterns found in the wake. This methodology provides an alternative, easier to automate when dealing with large amounts of data, to instantaneous or phase averaged representations of vortex wakes. Phase averaging becomes difficult and tedious when applied as in this case, to wakes of bluff bodies undergoing non-periodic motions. The DPIV data were obtained at two elevations along the length of a long flexible circular cylinder model, which had an aspect ratio (length over diameter) of about 94. The experiments were carried out in a water channel with flow speeds up to 0.75 m/s, giving Reynolds numbers, based on the external diameter of the cylinder, in the range from 1,200 to 12,000. The set-up allowed changes in the fundamental natural frequencies, which resulted in reduced velocities based on that frequency (velocity divided by frequency and external diameter), up to 15. The mass ratio of the model (mass divided by mass of displaced fluid) was around 1.8.

Additional Information

© Springer-Verlag 2009. Received: 27 January 2009. Revised: 28 October 2009. Accepted: 30 October 2009. Published online: 26 November 2009. Special thanks are due to David Jeon and Mory Gharib for providing one of his DPIV data sets from Jeon and Gharib (2001). The experiments were funded by Universitat Rovira i Virgili (URV) grants 2006AIRE-03 and 2007ACCES-14 under a collaboration with Prof. P. W. Bearman and the Department of Aeronautics of Imperial College London. FJHH is Assistant Prof. at URV but during 2008–2010 he works as a Marie Curie Research Fellow (PIOF-GA-2008-219429) at Caltech, so funding from the EC is gratefully acknowledged.

Additional details

Created:
August 19, 2023
Modified:
October 20, 2023