Hydrodynamic Performance of 2D Undulating Fins Under Flow Excitation Near the Free Surface

This study investigates the propulsion principle and hydrodynamic performance of an undulating fin under varying hydrodynamic conditions near the free surface. The fin was tethered in the flow field and actuated sinusoidally. A high-resolution numerical technique based on the dynamic mesh and volume of fluid method is utilized to simulate the two phases fluid-fin system. The numerical results indicate that the efficiency of the fin exhibits a monotonically bounded decreasing trend as the nondimensional phase speed grows, and is basically independent of the amplitude regardless of the presence of free surface. In the free surface conditions, an upward net lift appears, which remains zero without considering the free surface effect. And the average lift decreases as the fin approach the free surface. In both conditions, the average thrust increases as the amplitude and phase speed grows. But in the cases of low phase speed, the average thrust will increase with the decrease of the depth, and shows a significant increase compared to the situation without the free surface. This research provides valuable insights for designing and optimizing undulating propulsion robots, especially for those performing tasks near the free surface.