Li-Ming Chao

Collective swimming is often attributed, at least in part, to hydrodynamic interactions between neighboring individuals. However, previous studies of passive hydrodynamic schooling have focused primarily on swimmers with identical kinematics. Here, we investigate whether bounded collective motion can emerge between swimmers with different undulatory wavelengths, frequencies, and amplitudes. We identify bounded schooling for several heterogeneous kinematic combinations within the sampled parameter space. Frequency matching is the predominant condition, although bounded states also occur for a short-wavelength, higher-frequency, and larger-amplitude follower. The bounded simulations exhibit several well-separated long-time spacing ranges. The close-spacing state promotes collective acceleration through a coherent unified wake, whereas the intermediate- and large-spacing states allow the follower to maintain nearly its isolated swimming speed while reducing its mechanical power expenditure. Flow-field analysis links these outcomes to near-field coupling, fragmented vortex-body interactions, and more coherent periodic wake encounters. Within the present two-dimensional, streamwise-constrained framework, these results demonstrate that passive hydrodynamic interactions can organize the streamwise relative motion of a heterogeneous in-line swimmer pair and produce distinct propulsive and energetic outcomes.

Self-propelled models are widely used to study fish-like swimming, yet most existing studies rely on two-dimensional simplifications and restricted degrees of freedom in the self-propelled motion. In this study, we systematically investigate how dimensionality and kinematic freedom influence the hydrodynamic performance of self-propelled swimmers by varying the undulation frequency, tailbeat amplitude, and fluid viscosity. The results show that two-dimensional models overestimate swimming speed, while increasing the rotational degrees of freedom reduces the swimming speed in both two- and three-dimensional cases. Generally, higher swimming speeds are consistently associated with higher power expenditure. Based on these results, we identify a data-driven parameter that can map two-dimensional results to their three-dimensional counterparts and clarify the limitations of such simplifications. These findings provide insight into the hydrodynamic performance of swimming fish and offer potential guidance for improving reduced-order models of fish-like locomotion.

Inspired by the water-entry strategies of amphibious birds, this study introduces a bio-inspired flying wing for efficient transition from air to water. A volume-of-fluid multiphase flow model coupled with the shear stress transport k–ω turbulence model is employed to investigate the two-phase flow dynamics during water entry. The stability, hydrodynamic loads, and energy efficiency of the proposed configuration are examined by systematically varying the contraction ratio, structural density, water-entry angle, and velocity. Results show that flying wings with smaller aspect ratios and lower densities, entering water at lower velocities and larger angles, exhibit slower attitude variations and improved stability, but at the cost of higher energy loss. Furthermore, increased wing contraction and reduced entry velocity significantly mitigate the impact loads. To describe these specific relationships, we define a coupling multi-parameter impact index and a comprehensive stability metric, demonstrating a strong linear correlation. These findings advance our understanding of biological water-entry behavior and provide valuable guidance for the design of bio-inspired flying wings in trans-media vehicles.

In aquatic environments, muscle activity in free-swimming fishes not only propels body undulations to generate thrust but also serves as proprioceptive sensors for detecting surrounding fluid dynamics. Testing the proprioceptive function of the muscle is challenging owing to its deep integration with swimming activity. To address this, we introduce an experimental platform that records up to 12-channel electromyography (EMG) signals synchronized with detailed kinematics in koi and carp. We first apply various neural networks to map densely collected EMG signals to synchronized video-based body kinematics, thereby validating our EMG collection system. We then compare EMG data from fishes swimming in various laminar flows and within Kármán vortices. Our results show that the phase of muscle activity consistently precedes body kinematics in various laminar flows. While within Kármán vortices, we observe a mixed phase relationship, where muscle activity sometimes leads and at other times lags behind body kinematics. This suggests that fishes may use muscle proprioceptive sensing when interacting with complex flows, such as nearby vortices. Our research not only introduces novel methods for biological EMG studies but also offers insights that could influence the design of bio-inspired underwater sensory systems.

Fish have evolved remarkable underwater turning maneuverability, primarily under active control. This allows them to execute turns within confined spaces, such as during C-start rapid turning. In our study, conducted through computational fluid dynamics simulations of a self-propelled swimmer, we revealed that burst-and-coast swimming patterns can generate various turning behaviors purely through passive fluid–body interactions. The burst-and-coast swimming is characterized by the alternating tail movements between continuous undulating burst phases (bp) and non-undulating or gliding coast phases (cp). Through extensive systematic three-dimensional (3D) simulations, we found that both the burst-and-coast duty cycle—the ratio of burst duration to the total cycle duration—and the swimmer's undulation frequency inhibit turning maneuverability, which is quantified by the curvature of swimming trajectories. We also found there is an optimal Reynolds number that maximizes turning maneuverability. Further analysis suggests that the turning maneuverability is probably due to the persistent presence of the Wagner effect during burst phases and the Magnus effect during coast phases, which differs from the mechanism of actively generating lateral forces by asymmetric continuous flapping. These insights not only advance our understanding of fish locomotion control mechanisms but also provide guidelines for designing underwater robots with improved navigational capabilities.

Recent studies have shown that superimposing rhythmic perturbations to oscillating tailbeats could simultaneously enhance both the thrust and efficiency (Lehn et al., Phys. Rev. Fluids, vol. 2, 2017, p. 023101; Chao et al., PNAS Nexus, vol. 3, 2024, p. 073). However, these investigations were conducted with a tethered flapping foil, overlooking the self-propulsion intrinsic to real swimming fish. Here, we investigate how the high-frequency, low-amplitude superimposed rhythmic perturbations impact the self-propelled pitching and heaving swimming of a rigid foil. The swimming-speed-based Reynolds number ranges from 1400 to 2700 in our study, depending on superimposed perturbations and swimming modes. Numerical results reveal that perturbations significantly increase swimming speeds in both pitching and heaving motions, while enhancing efficiency exclusively in the heaving motion. Further derived scaling laws elucidate the relationships of perturbations with speeds, power costs and efficiency, respectively. These findings not only hypothesise the potential advantages of perturbations in biological systems, but also inspire designs and controls in biomimetic propulsion and manoeuvring within aquatic environments.