Experimental study on the underwater propulsion performance of bionic flexible flapping fins

Due to their exceptional efficiency, maneuverability, and stealth capabilities, bionic underwater vehicles have garnered worldwide attention. Although numerous theoretical and simulation studies have been conducted, there have as yet been insufficient experimental studies focused on the multi-parameter coupling problem of flexible flapping fins. In this work, a three-degree-of-freedom underwater flapping experimental system was developed, along with flexible fins of different stiffness. Based on this system, experiments were conducted to investigate the impact of the flap and pitch amplitudes, motion frequency, Reynolds number, and model flexibility on the thrust and propulsion efficiency. Specifically, this was facilitated by the introduction of a hybrid design of experiments based on orthogonal arrays and the surrogate models using Gaussian process regression. Experimental force and moment measurements were acquired, and this was followed by data processing and analysis. The experimental results demonstrate that increased fin flexibility enables higher propulsion efficiency at lower flapping frequencies with larger amplitudes under maintained thrust, while appropriate pitch motion significantly boosts efficiency at large flap amplitudes. Furthermore, moderately stiff fins achieve superior efficiency, but highly flexible fins become preferable at lower Reynolds numbers.