Numerical simulation on hydrodynamic performance of manta ray under different motion modes

Aiming at the research deficiencies of existing manta ray-inspired propulsion models, which are mostly simplified to rigid flapping motion alone and ignore the actual torsional deformation of pectoral fins as well as the overall flexible traveling wave deformation, this paper establishes a three-degree-of-freedom coupled numerical model for the bionic manta ray integrating flapping, torsion and flexible traveling wave motions. The dynamic mesh technique is adopted to numerically solve the flow field with large deformation and multi-degree-of-freedom movements. The control variable method is utilized to set up six groups of comparative motion working conditions, so as to hierarchically decouple the hydrodynamic regulation differences among pure flapping motion, pure torsion motion, pure traveling wave motion, flapping coupled with torsion, flapping coupled with flexible traveling wave, and the flapping-torsion-traveling composited motion. Numerical results reveal that the manta ray fails to generate positive thrust merely through flapping, and positive thrust can only be produced when flapping is combined with torsion. The combination of flexible traveling wave and flapping exerts a negative effect on propulsion performance. The introduction of torsion homogenizes the distribution of vortex structures on the manta ray surface. By restraining the generation of small attached vortices and reducing momentum exchange, torsion improves thrust, suppresses the fluctuation of vertical lift, and accordingly enhances vertical stability. Superimposing flexible traveling wave on the flapping-torsion coupled motion further increases the thrust coefficient and propulsion efficiency, with varying improvement effects under different motion frequencies. Finally, flow field structures under typical working conditions are extracted for analysis. The underlying mechanisms accounting for discrepancies in propulsion performance under various motion modes are elaborated from perspectives including three-dimensional vortex topology, two-dimensional sectional vorticity, surface pressure distribution and shear stress distribution. This paper clarifies the complete physical mechanism of active flow control via torsion-flexibility synergy, which can provide theoretical basis and technical support for the design of bionic underwater propulsion devices with high stability.