Effects of free surface deformation on thrust production by a partially-submerged fin at the water-air interface

Inspired by organisms that generate propulsion while partially-submerged, such as archer fish and dolphins, we investigate relationships between the free surface deformation and thrust produced by a flapping flexible partially-submerged propulsor. To observe free-surface effects, including air cavities, splashing, and waves, high-speed imaging is used to capture the movement of the propulsor through the water-air interface. Image processing techniques are utilized to isolate quantitative and qualitative characteristics of the free surface deformations. Relationships between these surface behaviors and wake structures are further examined using 2D PIV, revealing hydrodynamic patterns specific to propulsion at the water-air interface. Comparison of free surface deformations and their corresponding thrust profiles may reveal if parameter combinations that minimize surface disturbances perform more desirably. Results from these comparisons may also elucidate whether flapping propulsion across the water-air interface is a viable water-exit strategy for underwater vehicles, especially in sensitive environments where hydrodynamic disturbances should be minimized.