The past decade has seen the rapid development of the robotic fish in many aspects. However, the velocity measurement problem has not been fully addressed, which limits the autonomy of the robotic fish. To this end, an artificial lateral line (ALL) sensor, inspired by the sensory organs of fish, is developed in this article. By measuring the deformation of the sensitive element, the local flow field around the robotic fish is sensed. According to the characteristics of fishlike motions, a fairing structure is proposed to suppress the turbulence noise and yaw motion noise caused by fishlike oscillation of the tail. This structure ensure that the flow measured by the ALL sensor is closer to laminar flow under viscous effects. Furthermore, to measure the magnitude and direction of the robotic fish velocity, an ALL sensor array is assembled by mounting multiple sensors on the robot's surface to sense the flow field distribution. Next, a kinematic-based fusion method is proposed for the array system, which obtained the real-time velocity vector of the robotic fish by solving overdetermined motion equations. The proposed ALL array system is tested on a freely swimming robotic fish, and our method achieves a mean absolute error of 0.018 m/s, a linearity ( $R^{2}$ ) of 0.951, and a position tracking error of 0.085 m. Additionally, the fairing structure is found to improve the signal-to-noise ratio by 116%.

Motivated by the high maneuverability and low power consumption requirements, using elastic components in the joints of robotic fish can improve thrust production and efficiency. However, the propulsive performance of the elastic mechanism cannot be satisfied over a wide range of frequencies. First, this article develops a robotic tuna with a novel passive flexible joint structure, which combines with the compliance features of the elastic component and the damper, so as to maintain an ideal amplitude and phase difference of the caudal fin, resulting in better swimming performance over multifrequency ranges. Moreover, a dynamic model for the robotic tuna is established based on Kane method. Thrust characteristic calculation and computational fluid dynamics method are applied to analyze the propulsion and vorticity characteristics. Simulated results show that the composite mechanism generates higher thrust production and stronger vortex structures. Finally, simulations and experiments validate the effectiveness of the proposed method. Experimental results demonstrate that the robotic tuna can achieve a maximum speed of 1.21 m/s (equivalent to 2.24 body lengths per second) and minimum cost of transport of 45.67 J/(m $\cdot$ kg).

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.

Scombrid fish possess superior-performance propulsion characteristics, attributed to the powerful contraction of the peduncle tendon muscle which effectively transmits energy to the caudal fin. Inspired by the bionic structure, we proposed a fish-like propeller consisting of a driving joint and a peduncle joint connected in series. The peduncle joint combines magnetorheological (MR) damping with elastic components and is actuated by a sine mechanism to replicate biological motion. The intensity of the damping mechanism enhances with frequency, which can compensate for the shortcomings of insufficient elastic intensity at high frequencies. The integration of both mechanisms aims to accomplish high propulsion across a wide frequency range. Then, an MR damper is constructed to imitate the tendon muscle of the peduncle. The shear stress of the MR fluid possesses online intensity adjustment capability by regulating the current of the inner coil, thereby affecting the damping torque output by the peduncle joint. Experimental results reveal that the controllable damping torque ranges from 49.3 to 134.4 mNm. A forward thrust dynamic model is built according to the Lagrangian method to explore the effect of the shear stress on propulsion. Simulated results indicate that at a specific frequency, an optimal shear stress exists that enables the system to achieve the best propulsion. Meanwhile, as frequency increases, the best propulsion gradually enhances, and the shear stress corresponding to the best propulsion also increases. Experimental results of four caudal fins with different aspect ratios (ARs) show that the shear stress should be further increased to accomplish best propulsion as AR decreases. The max average thrust of the fish-like propeller can reach 1.956 N at a frequency of 3 Hz.

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.