Robotic Models for Studying Undulatory Locomotion in Fishes

Many fish swim using body undulations to generate thrust and maneuver in three dimensions. The pattern of body bending during steady rectilinear locomotion has similar general characteristics in many fishes and involves a wave of increasing amplitude passing from the head region toward the tail. While great progress has been made in understanding the mechanics of undulatory propulsion in fishes, the inability to control and precisely alter individual parameters such as oscillation frequency, body shape, and body stiffness, and the difficulty of measuring forces on freely swimming fishes have greatly hampered our ability to understand the fundamental mechanics of the undulatory mode of locomotion in aquatic systems. In this paper, we present the use of a robotic flapping foil apparatus that allows these parameters to be individually altered and forces measured on self-propelling flapping flexible foils that produce a wave-like motion very similar to that of freely swimming fishes. We use this robotic device to explore the effects of changing swimming speed, foil length, and foil-trailing edge shape on locomotor hydrodynamics, the cost of transport, and the shape of the undulating foil during locomotion. We also examine the passive swimming capabilities of a freshly dead fish body. Finally, we model fin-fin interactions in fishes using dual-flapping foils and show that thrust can be enhanced under correct conditions of foil phasing and spacing as a result of the downstream foil making use of vortical energy released by the upstream foil.