The unsteady three-dimensional wake produced by a trapezoidal pitching panel
M. A. Green, C. W. Rowley, and A. J. Smits
Journal of Fluid Mechanics 685 : 117–145, October 2011.
Particle Image Velocimetry (PIV) is used to investigate the three-dimensional wakes of rigid pitching panels with a trapezoidal geometry, chosen to model idealized fish caudal fins. Experiments are performed for Strouhal numbers from 0.17 to 0.56 for two different trailing edge pitching amplitudes. A Lagrangian Coherent Structure (LCS) analysis is employed to investigate the formation and evolution of the panel wake. A classic reverse von Karman vortex street pattern is observed along the mid-span of the near wake, but the vortices realign and exhibit strong interactions near the spanwise edges of the wake. At higher Strouhal numbers, the complexity of the wake increases downstream of the trailing edge as the spanwise vortices spread transversely and lose coherence as the wake splits. This wake transition is shown to correspond to a qualitative change in the LCS pattern surrounding each vortex core, and can be identified as a quantitative event that is not dependent on arbitrary threshold levels. The location of this transition is observed to depend on both the pitching amplitude and freestream velocity, but is not constant for a fixed Strouhal number.
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