Using hyperbolic Lagrangian coherent structures to investigate vortices in bio-inspired fluid flows
M. A. Green, C. W. Rowley, and A. J. Smits
Chaos 20 : 017510, January 2010.
We use Direct Lyapunov Exponents to identify Lagrangian Coherent Structures in a bio-inspired fluid flow: the wakes of rigid pitching panels with a trapezoidal planform geometry, chosen to model idealized fish caudal fins. When compared with commonly used Eulerian criteria, the Lagrangian method has previously exhibited the ability to define structure boundaries without relying on a preselected threshold. In addition, qualitative changes of the LCS have previously been shown to correspond to physical changes in the vortex structure. For this paper, Digital Particle Image Velocimetry experiments were performed to obtain the time-resolved velocity fields for Strouhal numbers of 0.17 and 0.27. A classic reverse von Kármán vortex street pattern was observed along the mid-span of the near wake at low Strouhal number, but at higher Strouhal number the complexity of the wake increased downstream of the trailing edge. The spanwise vortices spread transversely across the wake and lose coherence, and this event was shown to correspond to a qualitative change in the LCS at the same time and location.
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