Dynamic response of a semi-free flexible filament in the wake of a flapping foil.

Abstract The passive response of a semi-free flexible filament in the wake of a flapping foil is studied numerically Three distinct flapping states are observed for the filament in different vortex streets: flapping between Kármán vortices, flapping outside reversed Kármán vortices and static in compact Kármán vortices. Both the vertical dipole-induced velocity and the transport speed of the vortices determine the filament's flapping motion. A significant drag reduction is obtained for the filament when flapping outside reversed Kármán vortices compared to flapping between vortices. The pitching foil has decreased drag when the downstream filament is static. These passive flapping motions of the filament in different wakes may be beneficial for achieving a better understanding of fish swimming mechanics and behaviors in schools. Graphical abstract Highlights • Three flapping states are observed for the filament in different vortex streets. • The filament flaps between the vortices in the von-Kármán vortex streets. • The filament flaps outside the vortices in the reversed von-Kármán vortex streets. • The filament is static in the compact Kármán vortices. • Vertical dipole-induced velocity and vortex transportation speed affect the states. [ABSTRACT FROM AUTHOR]