Cyber-Physical Experiments with Bio-Inspired Swimming Protocols

Abstract

Bio-inspired swimming protocols have shown much potential in recent years to improve upon existing marine propulsion systems. Optimization studies in this field could benefit greatly from the use of cyber-physical controllers, which combine measured and virtual forces to alter the dynamics of a system. To take advantage of the new parameter spaces accessible through these controllers, a cyber-physical apparatus was designed and built for an existing bio-inspired propulsion system involving a heaving and pitching foil in a water channel. An actuation system was constructed to move the foil in the streamwise direction with a high degree of precision. Controllers were developed to prescribe these movements according to thrust-force readings, and their capabilities were verified in experiments. The cyber-physical system was used to impose a free-swimming condition through streamwise motion, since much of the research in this field has assumed that achieving self-propelled swimming is necessary to study more accurately the fluid mechanics of real-world systems. Comparisons with experiments conducted with a propulsor fixed in the streamwise direction showed that introducing the free-swimming condition had no significant effect on thrust production. Further, these findings were generalized to all streamwise velocity oscillations, as experiments demonstrated that neither the mean thrust force nor the wake structure was affected by streamwise velocity oscillations up to 37.5 percent of the mean flow velocity. The free-swimming condition therefore proved to be nonessential for accurate swimming experiments with heaving and pitching foils. These results were submitted for publication, and cleared the way for a simple model of intermittent swimming to be developed. In addition, the free-swimming controllers were adapted to create a second cyber-physical system, in which a virtual torsional spring was imposed on a heaving foil to produce passive pitch. This controller was fully characterized in experiments, and was applied to problems regarding the effects of resonance on swimming performance. Overall, the designs of the streamwise actuation system and the cyber-physical control schemes proved highly versatile, providing a robust, well-documented set of tools for a wide variety of cyber-physical swimming experiments.