Hydrodynamic Characteristics and Experimental Investigations of Underwater Vehicles with Multiple Motion Modes

Abstract

To adapt to the complex marine environment, a submersible vehicle with multiple motion modes was designed. Referring to the motion characteristics of three typical marine organisms (rainbow eel, sea snake, and tuna), kinematic equations are established to correspond to three motion modes: uniform-amplitude curve model (UCM), variable-amplitude curve model (VCM), and triple-segment curve model (TCM). The hydrodynamic performance and wake flow characteristics of the submersible vehicle are numerically investigated. Meanwhile, the effect of the nondimensional vertical spacing ratio Dy(defined as the vertical distance divided by the fin length) between adjacent flexible fins on the propulsion performance of the vehicle was analyzed, where three representative spacing ratios of 0.35, 0.50 and 0.65 were employed. The results indicate that, across all motion modes, in-phase flapping of dual parallel flexible fins produces alternately skewed wake vortices, whose mutual interference weakens the directional propulsive jet. In contrast, anti-phase flapping generates a symmetric and stable central jet, effectively improving thrust output and propulsive efficiency. Quantitative force analysis reveals distinct spacing-dependent performance characteristics for three motion modes. For the UCM mode, the maximum average thrust of 0.37 N is achieved at a vertical fin spacing of 0.50 under anti-phase motion, representing the optimal working condition. For the TCM mode, the highest average thrust of 0.67 N occurs at a vertical spacing of 0.65. In comparison, the vertical spacing exhibits a minor influence on the thrust performance of the VCM mode, with relatively limited variation in average thrust under different spacing configurations and phase states. Finally, the feasibility of the multi-modal underwater vehicle design scheme and the correctness of the numerical results were verified through experiments.