Free-Swimming Performance and Stability of Schooling Non-Uniformly Flexible Hydrofoils*

Recent tethered experiments have shown that non-uniform flexible hydrofoils not only have performance and energetic advantages over rigid and uniformly flexible ones, but a pair can also achieve high efficiency via schooling interactions. We present new experiments on a pair of freely-swimming non-uniformly flexible hydrofoils. We developed two wirelessly controllable air-bearing platforms equipped with a motion controller, a servomotor generating sinusoidal pitching motions of the foils, and a data acquisition system measuring energetics, swimming speed, and trajectories. Through video analysis the bending deformations of the foils are also acquired. We examine whether non-uniformly flexible foils can self-organize into stable formations, and their associated schooling benefits. We probe the fluid-structural coupling of the foils that can alter the self-organizational principles previously determined for rigid foils.