The influence of flow disturbances on the dynamics and propulsion of flexible oscillating foils

The environments that animals swim in are invariably complex, littered with vortices, gusts, and other disturbances. To understand how swimmers contend with these spatiotemporal heterogeneities, we explore the dynamics and propulsive performance of a flexible foil in disturbed flows. Flexibility is modeled via a torsional spring at the leading edge, which allows the foil to passively pitch in response to an applied heaving motion and fluid forces, calculated using a panel method. The foil is exposed to a variety of flow disturbances, including a wavy freestream, vortex streets, and periodic gusts. As the foil’s oscillations become large and the disturbances strengthen, the foil’s wake becomes increasingly non-planar, and the oncoming flow disturbances become deformed. This changes the bound circulation and induced velocity profiles, leading to deviations from predictions made by a thin airfoil theory. Impulse theory is employed to understand the deviations from linearity.