Dynamics and locomotion of flexible foils in a frictional environment

Abstract: Over the past few decades, oscillating flexible foils have been used to study the physics of organismal propulsion in different fluid environments. Here we extend this work to a study of flexible foils in a frictional environment. When the foil is oscillated by heaving at one end but not allowed to locomote freely, the dynamics change from periodic to non-periodic and chaotic as the heaving amplitude is increased or the bending rigidity is decreased. For friction coefficients lying in a certain range, the transition passes through a sequence of $N$-periodic and asymmetric states before reaching chaotic dynamics. Resonant peaks are damped and shifted by friction and large heaving amplitudes, leading to bistable states. When the foil is allowed to locomote freely, the horizontal motion smoothes the resonant behaviors. For moderate frictional coefficients, steady but slow locomotion is obtained. For large transverse friction and small tangential friction corresponding to wheeled snake robots, faster locomotion is obtained. Traveling wave motions arise spontaneously, and and move with horizontal speed that scales as transverse friction to the 1/4 power and input power that scales as transverse friction to the 5/12 power. These scalings are consistent with a boundary layer form of the solutions near the foil's leading edge.