Streamwise and lateral maneuvers of a fish-inspired hydrofoil

Fish are highly maneuverable compared to human-made underwater vehicles. Maneuvers are inherently transient, so they are often studied via observations of fish and fish-like robots, where their dynamics cannot be recorded directly. To study maneuvers in isolation, we designed a new kind of wireless carriage whose air bushings allow a hydrofoil to maneuver semi-autonomously in a water channel. We show that modulating the hydrofoil's frequency, amplitude, pitch bias, and stroke speed ratio (pitching speed of left vs right stroke) produces streamwise and lateral maneuvers with mixed effectiveness. Modulating pitch bias, for example, produces quasi-steady lateral maneuvers with classic reverse von Kármán wakes, whereas modulating the stroke speed ratio produces sudden yaw torques and vortex pairs like those observed behind turning zebrafish. Our findings provide a new framework for considering in-plane maneuvers and streamwise/lateral trajectory corrections in fish and fish-inspired robots.