Froude number and flexibility effects on partially-submerged bio-inspired propulsors

Jumping archer fish are capable of sustained propulsion while partially-submerged and crossing the water-air interface. Inspired by such behavior, we characterize free surface interactions, total thrust production, and propulsive performance of a partially-submerged, flexible, finite-span heaving plate over a range of depths and propulsor stiffnesses. Using high-speed imaging, we determine how the plate bends in response to stroke dynamics, including the Froude number. Using multi-axis force and torque measurements, we characterize how the kinematics and resultant plate shape correlate with thrust and lateral loads. We examine the wake patterns using PIV and identify changes in vortex shedding patterns, wake strength, and the level of free surface interaction. By testing over a range of depths, we gain insight into how propulsive performance, including net thrust production and efficiency, evolves during water exit. We additionally identify cases where free surface effects are significant and cases where partially-submerged propulsion resembles fully-submerged behaviors.