Beyond the Kármán gait: knifefish swimming in periodic and irregular vortex streets

Nonlinear Dynamics Research of Ground Undulatory Fin Robot with Flexible Deformation and Frictional Contact

The unique rigid-flex connection between the fin-rays and fin-surface in a bionic undulatory fin robot endows the fin-surface with both active flexibility and load-bearing capacity, enabling this robot to perform amphibious motions in underwater, terrestrial, and even marshy environments. However, investigations into dynamic modeling problems for the undulatory fin robot, considering the impact of nonlinear deformation and frictional contact on ground locomotion performance, are scarce. Given this, based on the absolute nodal coordinate formulation (ANCF), this paper presents an efficient and accurate nonlinear dynamic model for this robot to elucidate the fin's flexible deformation and motion law. This model considers material, geometric, and boundary nonlinearities, utilizing ANCF thin plate elements and reference nodes to individually describe the fin-surface and fin-rays of the undulatory fin. Then, by using the master-slave technique, a frictional contact formulation for the fin and the ground is proposed. Furthermore, we conduct in-depth research and analysis on the formation and undulatory motion of the undulatory fin, encompassing its static deformation, static contact deformation, and frictional contact motion, and successfully obtain its responses under various conditions. Research indicates that during fin-surface motion, longitudinal sliding or a tendency for sliding at the contact points results in the undulatory fin moving in a crawling gait. The proposed theoretical model correctly captures the fin's complex nonlinear deformations and frictional characteristics and reveals its ground locomotion mechanism, whose effectiveness and superiority are validated through numerical examples and experiments.

Towards the yin and yang of fish locomotion: linking energetics, ecology and mechanics through field and lab approaches

Most of our understanding of fish locomotion has focused on elementary behaviors such as steady swimming and escape responses in simple environments. As the field matures, increasing attention is being paid to transient and unsteady behaviors that characterize more complex interactions with the environment. This Commentary advocates for an ecologically relevant approach to lab studies. Specific examples have brought new understanding to the energetic consequences of fish swimming, such as (1) station holding around bluff bodies, which departs drastically from steady swimming in almost all aspects of kinematics, muscle activity and energetics, and (2) transient behaviors such as acceleration and feeding, which are critical to survival but often neglected because of challenges in measuring costs. Beyond the lab, a far richer diversity of behaviors is available when fish are given enough space and time to move. Mesocosm studies are poised to reveal new insights into fish swimming that are inaccessible in laboratory settings. Next-generation biologgers that incorporate neural recordings will usher in a new era for understanding biomechanics in the wild and open the door for a more mechanistic understanding of how changing environments affect animal movement. These advances promise to allow insights into animal locomotion in ways that will mutually complement and accelerate laboratory and field studies in the years to come.