Origins of Dual Hairpin-Horseshoe Vortex Arrangement in the Wake of Oscillating Foils

Three dimensional wake evolution of an oscillating foil in coupled heaving and pitching motion is evaluated numerically at range of chord-based Strouhal number (0.32 \(\le St_c \le \) 0.56), while the phase-offset ( \(\phi ^\circ \) ) is fixed at \(\phi =\) 90 \(^\circ \) . The increase in \(St_c\) reveals growth of a supplementary horseshoe-like system in the wake. These large scale structures evolve along with the secondary hairpin arrangement that has been already identified in wakes. The horseshoe-like system forms on account of bending strains of the primary leading edge vortex (LEV) in the presence of a developing trailing edge vortex (TEV). The bending is further accelerated at higher \(St_c\) , which coincides with increasing circulation strength of developing TEVs. This promoted emergence of the horseshoe-like system early on in the wake.