Purpose This paper aims to develop a novel type of bionic underwater robot (BUR) with multi-flexible caudal fins. With the coordinate movement of multi-caudal fins, BUR will combine the undulation propulsion mode of carangiform fish and jet propulsion mode of jellyfish together organically. The use of Computational Fluid Dynamics (CFD) and experimental method helps to reveal the effect of caudal fin stiffness and motion parameters on its hydrodynamic forces. Design/methodology/approach First, the prototype of BUR was given by mimicking the shape and propulsion mechanism of both carangiform fish and jellyfish. Besides, the kinematics models in both undulation and jet propulsion modes were established. Then, the effects of caudal fin stiffness on its hydrodynamic forces were investigated based on the CFD method. Finally, an experimental set-up was developed to test and verify the effects of the caudal fin stiffness on its hydrodynamic forces under different caudal fin actuation frequency and amplitude. Findings The results of this paper demonstrate that BUR with multi-flexible caudal fins combines the hydrodynamic characteristics of undulation and jet propulsion modes. In addition, the caudal fin with medium stiffness can generate larger thrust force and reduce the reactive power. Practical implications This paper implies that robotic fish can be equipped with both undulation and jet propulsion modes to optimize the swimming performance in the future. Originality/value This paper provides a BUR with multi-propulsion modes, which has the merits of high propulsion efficiency, high acceleration performance and overcome the head shaken problem effectively.

SUMMARYThis paper proposed a novel type of bionic underwater robot (BUR). The undulation and jet propulsion modes on the self-propelled BUR were combined, and the kinematic characteristics of the two propulsion modes were thoroughly compared. First, the prototype and swimming strategy of the BUR were presented, and a dynamic model of the BUR was established based on several assumptions. Then, a central pattern generator (CPG) model allowing free adjustment of frequency and amplitude was employed to achieve the undulation propulsion of carangiform fish and the jet propulsion of jellyfish. Also, the kinematic characteristics of the two propulsion modes were investigated through experiments under different caudal fin actuation parameters. The experimental results indicate that the developed prototype can realize the undulation and jet propulsion by the means of the coordinated movement of the multi-caudal fins. By adjusting the CPG parameters, the BUR can switch the propulsion mode smoothly. Furthermore, the propulsion velocity of the BUR initially increased rapidly with the frequency and then slowed down when the frequency was greater than 0.8 Hz in both propulsion modes. The undulation propulsion velocity increased with the amplitude in the measurement ranges, however, the jet propulsion velocity initially increased quickly with the amplitude and then kept constant and even decreased when the amplitude was greater than 11 cm. Under the same caudal fin actuation parameters, the average velocity in undulation propulsion mode was higher than that in jet propulsion mode.

To perceive the static obstacles in still water, the flow field characteristics of a self-propelled robot fish approaching static obstacles were studied based on Artificial Lateral Line(ALL). The pressure distribution on the fish body surface was calculated with different separation between the robot fish and the obstacle boundary, obstacle size and undulating frequency. Subsequently, an ALL system was established and five obstacle perception models were studied to analyze the perceptual characteristics of the ALL. Finally, the experiments were conducted to further reveal the effects of obstacles and motion parameters on the body surface pressure of robot fish. The results indicate that the obstacles have a significant effect on the pressure distribution of the surface of the fish body. Namely the parameters of separation, obstacle size and undulating frequency will affect the peak value of the amplitude envelope of the pressure signals. The obstacle size and distance between the obstacles can be predicted using the time parameters of the amplitude envelope of the pressure signals. Moreover, the self-propelled robot fish with a medium undulating frequency approach to the large obstacles with small separation has better perceptual performance. The findings could offer some insight into understanding the perception of complex underwater environment based on ALL.

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