The role of the remnant vortex in the wake instability of a heaving and pitching foil in quiescent fluid

Bistable wake switching is identified experimentally and numerically in an elliptical airfoil undergoing combined heaving and pitching in quiescent fluid. A history-dependent “remnant vortex impingement” mechanism drives the instability: A lingering secondary leading-edge vortex (LEV) from the preceding stroke triggers premature detachment of the nascent LEV, whose subsequent merger with the trailing-edge vortex forms an obliquely ejecting wake dipole. Aperiodic reversal of dipole direction switches the lift asymmetry despite symmetric kinematics, manifesting as nonlinear spectral sidebands in the lift spectrum. Between switching events, a vortex-splitting process facilitates a metastable symmetric reverse Kármán vortex street until disrupted by ambient disturbances.