Three-dimensional pitching panel wake: Lagrangian analysis and momentum distribution from experiments

A majority of fish and other aquatic animals generate thrust by oscillating their fins and flukes, which creates a highly three-dimensional and complex wake that is characterized by spanwise compression, transverse expansion, wake breakdown, etc. The interaction between the oscillating fins and the surrounding fluid createsa complex system of wake structures that is a signature ofwhat canbean animal's markedly efficient swimming abilities. To interrogate the evolution of these flowfields, Lagrangian analysis using the finite-time Lyapunov exponentfield was carried out on experimentally recorded three-dimensional velocity data in the wake of an oscillating trapezoidal panel that models a fish caudal fin in a Strouhal number range of 0.17-0.56. The release of bounding Lagrangian saddles, identified as intersections of positive- and negative-time finite-time Lyapunov exponent ridges, from the panel trailing edge is showntobe related to the extrema of thrust and the evolution of the streamwise momentum field during an oscillation period. The location of vortex disintegration, which is observed downstream of the panel-fluid interaction, was observed to correspond with regions of low streamwise momentum, bifurcated wake structure, and the merger of Lagrangian saddles from two consecutive spanwise structures.