RoboTwin: A Platform to Study Hydrodynamic Interactions in Schooling Fish

By living and moving in groups, fish can gain many benefits, such as heightened predator detection, greater hunting efficiency, more accurate environmental sensing, and energetic saving. Although the benefits of hydrodynamic interactions in schooling fish have drawn growing interest in fields such as biology, physics, and engineering, and multiple hypotheses for how such benefits may arise have been proposed, it is still largely unknown which mechanisms fish employ to obtain hydrodynamic benefits, such as in increased thrust, or improved movement efficiency. One main bottleneck has been the difficulty in col- lecting detailed sensory information, corresponding locomotory responses and hydrodynamic information from real schooling fish. In this paper, we present the RoboTwin platform designed to aid such data collection: it allows us to replay the dynamic movements and body posture kinematics of real fish in fish- like robots, allowing us to measure the power cost, thrust, and detailed flow fields, all of which is extremely challenging for real animals. To mutually verify the capabilities of our platform, and our previously-proposed mechanism of energy saving (‘vortex phase matching’), we re-analyzed two goldfish (Carassius auratus) swimming in a flow tank, from which dynamic positions and corresponding body kinematics are quantified. By employing the RoboTwin system, we find there exist notable benefits to swimming together (for the kinematic patterns exhibited by real fish pairs) both in energy saving (approximately 8%) and in thrust enhancement (around 35%), compared to when swimming alone. Flow visualization through PIV (particle image velocime- try) shows that energetic savings arise due to vortex phase matching. Our results demonstrate the effectiveness of our design and highlight the potential of RoboTwin for future applications in exploring further hydrodynamic interactions among schooling fish.