Benefiting from the potential advantages of low noise, high efficiency and little disturbance, bionic propulsion has attracted wide attentions. Compared with the rigid structure, the performance of the elastic propulsion structure such as flexible caudal fin and passive compliant joint has been improved, yet the effective frequency range is limited due to the single mechanism. The optimal propulsion can only be produced in a certain frequency range. In this article, a biological passive peduncle joint integrated with the composite mechanism of elasticity and shear damping is proposed, solving the problem of the narrow frequency range of effective propulsive capacity. Through the optimal regulation of the elastic function at a certain frequency range and characterization of the damping function which increases with frequency, the response features of the passive joint are optimized over a wide range of frequencies, thereby improving the propulsive performance of the composite-mechanism bioinspired underwater vehicle (CMBUV). A dynamic model is built and the deformation analysis of the compliant caudal fin is carried out. The propulsive efficiency is characterized, and the results indicate that the compliant caudal fin modulates the power transmission for enhancing thrust production. Extensive simulations and experiments reveal that the CMBUV achieves both high swimming speed with 4.42 body length per second and low cost of transport with 90.33 J kg $^{-1}$ m $^{-1}$ . Bioinspired propulsion from this study takes advantage of undulating propulsion of natural fish, offering valuable insights into performing marine tasks in ocean environments.

Recent advances in underwater robotics highlight the potential of fish- like robots for efficient propulsion. However, their motion performance still lags behind real fish due to an incomplete understanding of fluid dynamics. This paper hypothesizes that chordwise-oriented vortices dominate the flow evolution of low-aspect-ratio flapping foils. Based on this, a quasi-three-dimensional hydrodynamic model, the Sliding Strip Discrete Vortex Method (SSDVM), is developed. SSDVM tracks chordwise-oriented vortex evolution along the chord, enabling hydrodynamic force calculations for heave and pitch motions across various parameters. The model closely agrees with Computational Fluid Dynamics (CFD) simulations while reducing computational cost. When integrated with a dynamic model, SSDVM accurately predicts robotic fish motion, with simulated speeds closely matching experimental results and achieving a Mean Absolute Percentage Error (MAPE) of 6.54%. SSDVM offers an analytical tool for robotic fish hydrodynamics, balancing accuracy and efficiency, with potential applications in optimizing bioinspired underwater propulsion.

The ability of fish to turn sharply is one of the key skills for surviving in water, allowing them to more effectively tackle multiple survival challenges, including evading predators and pursuing prey. Imitating the sharp turning maneuvers of fish to improve the maneuverability of robotic fish is extremely challenging, but important for practical applications. This article proposes a motion control strategy for sharp turning maneuvers of robotic fish, which combines fish-like C-turn behavior and a variable stiffness mechanism. Based on the developed dynamic model, with the optimization goal of minimizing turning time, the parameters of C-turn motion and fishtail stiffness are optimized to improve the turning efficiency of robotic fish. Simulation and experimental results demonstrate that the proposed turning strategy for robotic fish performs well in shortening turning time and improving turning efficiency. Compared with the strategies for the case of no C-turn and fixed stiffness, the case of no C-turn and adjustable stiffness, and the case of C-turn and fixed stiffness, the turning time for the proposed turning strategy is decreased by 53.2%, 41.1%, and 27.9%, respectively.

Biological tunas are exceptional open-ocean swimmers, maintaining superior swimming capabilities. A key to this achievement is their powerful contraction of the peduncle tendon muscle, which effectively transmits energy to the caudal fin. Drawing inspiration from this biomechanical principle, this article develops a robotic tuna prototype that integrates a lightweight magnetorheological (MR) damper that emulates caudal keel muscle functionality. An increase in the excitation current in the inner coil of the damper results in an increase in the shear stress of the MR fluid, thereby enhancing the output damping torque. Experimental results demonstrate a tunable damping torque range from 86.6 to 213.5 mN $\cdot$ m. A Lagrangian-based dynamic model is employed to analyze the effects of current intensity on propulsion performance. The load-bearing capacity of the peduncle structure is evaluated across three weight loads (300, 400, and 500 g), demonstrating improvements of 92.4%, 50.1%, and 31.64% with online current adjustment, respectively. Furthermore, swimming experiments reveal a maximum speed of 0.64 BL/s and a minimum cost of transport of 45.18 J/m/kg. Compared to a single elastic mechanism, the proposed MR damping modulation mechanism enhances propulsion performance, especially increasing speed by a factor of 2.38 and efficiency by a factor of 1.56 at a high frequency of 4.5 Hz.

Optimizing non-parametric flapping-foil motions for underwater propulsion remains challenging due to strong temporal coupling and nonlinear fluid–structure interactions. This study presents a non-parametric kinematics optimization framework based on the discrete adjoint method, built on a Morison-based low-order hydrodynamic model calibrated against high-fidelity Computational Fluid Dynamics (CFD) simulations. The model captures dominant periodic features and phase relationships of flapping-foil forces within a validated parameter regime. Non-parametric discrete adjoint optimization is performed under instantaneous power constraints. Results show that optimized motions substantially enhance propulsion performance over conventional sinusoidal motions, yielding non-sinusoidal, high-efficiency kinematics. In thrust-maximization cases, optimized kinematics achieve a 50.29% increase in mean thrust by redistributing heave and pitch amplitudes and timing. Optimization with different weighting factors reveals the trade-off between thrust and power consumption. CFD validation confirms that the Morison model predicts trends of forces and power for optimized motions, with deviations at high amplitudes or rapid oscillations. Overall, this study demonstrates the effectiveness of non-parametric discrete adjoint optimization and provides a practical framework for efficient underwater propulsion design.

Optimizing flapping-foil kinematics for underwater propulsion is challenging due to strong temporal coupling and nonlinear fluid–structure interactions. Most existing approaches rely on parameterized motion profiles, which restrict the accessible design space. A non-parametric kinematic optimization framework based on the discrete adjoint method is developed, enabling direct optimization of time-resolved motions without predefined functional forms. A Morison-based low-order hydrodynamic model, calibrated against Computational Fluid Dynamics (CFD), is employed for efficient evaluation within a validated regime. Results show that optimized motions substantially enhance propulsion performance over conventional sinusoidal motions, yielding non-sinusoidal, high-efficiency kinematics. In thrust-maximization cases, the optimized kinematics achieve a 50.29% increase in mean thrust by redistributing heave and pitch amplitudes and timing. Under balanced thrust–power conditions, the optimized motions consistently outperform sinusoidal counterparts. In power-minimization cases, a “generator-like” regime emerges, indicating a reversal of net energy transfer enabled by the non-parametric formulation. These results demonstrate that non-parametric optimization provides enhanced design flexibility and improved propulsion performance, offering a practical framework for biomimetic underwater propulsion design.