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Creatures with longer bodies in nature like snakes and eels moving in water commonly generate a large swaying of their bodies or tails, with the purpose of producing significant frictions and collisions between body and fluid to provide the power of consecutive forward force. This swaying can be idealized by considering oscillations of a soft beam immersed in water when waves of vibration travel down at a constant speed. The present study employs a kind of large deformations induced by nonlinear vibrations of a soft pipe conveying fluid to design an underwater bio-inspired snake robot that consists of a rigid head and a soft tail. When the head is fixed, experiments show that a second mode vibration of the tail in water occurs as the internal flow velocity is beyond a critical value. Then the corresponding theoretical model based on the absolute nodal coordinate formulation (ANCF) is established to describe nonlinear vibrations of the tail. As the head is free, the theoretical modeling is combined with the computational fluid dynamics (CFD) analysis to construct a fluid-structure interaction (FSI) simulation model. The swimming speed and swaying shape of the snake robot are obtained through the FSI simulation model. They are in good agreement with experimental results. Most importantly, it is demonstrated that the propulsion speed can be improved by 21% for the robot with vibrations of the tail compared with that without oscillations in the pure jet mode. This research provides a new thought to design driving devices by using nonlinear flow-induced vibrations.

Flow-induced vibration of fuel rods subjected to axial flows frequently appears in nuclear engineering, which has been a significant scientific problem still unsolved. This paper simplifies the fuel rod as a slender cylinder with loose support in axial flows, and explores nonlinear dynamics of the slender cylinder through theoretical modeling. The dynamical model is constructed with consideration of impact and friction forces attributed by the loose support. The results show that the flutter critical flow velocity and wear rate are dependent on clearance size and position of the loose support. The flow velocity range of buckling becomes narrower while the range for flutter becomes wider with the increase of clearance size. The flow velocity range for buckled behavior is widened, the flutter flow speed range is reduced as the clearance position is varied from upward end to downward end of the cylinder. It is indicated that there are optimal values for clearance size and position of the loose support where the flutter critical flow velocity is much higher and the wear rate is lowest. The present study can provide a theoretical basis for predicting flow-induced vibrations and designing the loose support for fuel rods in the nuclear engineering.