Three-dimensional wake of a biologically-inspired propulsor

The wakes of rigid pitching panels with a trapezoidal planform geometry, chosen to model idealized fish caudal fins, were investigated using planar Digital Particle Image Velocimetry (DPIV). Experiments were performed for Strouhal numbers of 0.33 and 0.55. The three-dimensional unsteady wake downstream of the panel trailing edge was reconstructed from spatially- and temporally-resolved two-component data. A Lagrangian Coherent Structure (LCS) analysis was employed in addition to Eulerian vortex identification criteria to investigate the formation and evolution of the wake. A classic reverse von Kármán vortex street pattern was observed along the mid-span of the near wake, but the complexity and three-dimensionality of the wake increases away from the mid-span as streamwise vortices interact with the swept edges of the panel. Downstream of the trailing edge, the wake was observed to shrink in the spanwise direction at both Strouhal numbers. In addition, a quantitative bifurcation in the LCS coincided with a qualitative transition of the wake structure observed at the higher Strouhal number.