Hydrodynamic performance of two-dimensional undulating fins under flow excitation near the free surface in three-dimensional numerical tank

Abstract

This study explores the propulsion and hydrodynamics of an undulating fin near the free surface under various conditions. The fin, secured within the flow field, was actuated sinusoidally. A numerical technique, founded on the dynamic mesh and volume of fluid method, was employed to simulate the two-phase fluid-fin system. The outcomes demonstrate that, without the free surface taken into consideration, both thrust and efficiency are augmented by increasing wave amplitude and frequency. Notably, efficiency experiences a decline with escalating frequency, exhibiting a lack of sensitivity to wave amplitude. Incorporating the free surface in our analysis, the thrust is markedly augmented under low-frequency conditions, with this enhancement becoming more pronounced as the fin approaches the free surface. At high frequency, the fin away from the free surface behaved similarly to scenarios ignoring the free surface. Nevertheless, as the fin's proximity to the free surface decreased, thrust decreased gradually. This phenomenon attributed to the undulations of the free surface. At lower frequency, the free surface exhibited no significant wave patterns, and the reduced flow velocity within the flow field was found to be conducive to thrust generation. Conversely, at higher frequency and in close proximity to the free surface, distinct waveforms emerged on the free surface. The initiation and propagation of these waveforms were found to be highly correlated with the reduction in thrust on the fin surface. This study offers valuable insight for the design and optimization of undulating robots, particularly for applications in tasks conducted near the free surface.