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Uncovering large-scale coherent structures in natural and forced turbulent wakes by combining PIV, POD, and FTLE

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Abstract

Planar velocity data of the unsteady separated flow in the turbulent wake of a circular cylinder obtained by particle image velocimetry (PIV) are analyzed in order to visualize the large-scale coherent structures associated with alternating vortex shedding at a Reynolds number of 2,150. Two different cases are examined: unforced vortex shedding in the natural wake and vortex lock-on incited by forced perturbations superimposed in the inflow velocity. Proper orthogonal decomposition (POD) is employed to reconstruct the low-order wake dynamics from randomly sampled snapshots of the velocity field. The reconstructed flow is subsequently used to determine the evolution of the finite-time Lyapunov exponent (FTLE) fields which identify the Lagrangian coherent structures. The results demonstrate that the combination of methods employed offers a powerful visualization tool to uncover large-scale coherent structures and to exemplify vortex dynamics in natural and forced bluff-body wakes.

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Correspondence to E. Konstantinidis.

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Kourentis, L., Konstantinidis, E. Uncovering large-scale coherent structures in natural and forced turbulent wakes by combining PIV, POD, and FTLE. Exp Fluids 52, 749–763 (2012) doi:10.1007/s00348-011-1124-0

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Keywords

  • Vortex
  • Particle Image Velocimetry
  • Coherent Structure
  • Proper Orthogonal Decomposition
  • Proper Orthogonal Decomposition Mode