Ground effect on the hydrodynamic performance of a flexible hinge-connected fin

Abstract

The swimming performance of a flexible fin under ground effect is investigated with computational method in this paper. The flexible fin consists of two parts with different flexibility and performs a pitching motion. Its hydrodynamic force and fluid response are detailed investigated with various gap/distance between the fin and ground. It is found that smaller gap ratio indicates larger thrust force peaks and directly results in larger average thrust force in which the presence of the ground can enhance the average thrust force up to 64% for the rigid-flexible fin from gap ratio of 2 to 0.25. For each gap ratio, the rigid-flexible case can achieve larger force amplitude and larger phase difference because of the interaction of ground effect and fin flexibility. It is also found that when the gap ratio is larger than 1, the ground effect is negligible for rigid-rigid case and rigid-medium flexible case, while the ground effect is non-ignorable even with gap ratio of 2 for rigid-flexible case which suggests that the zone of ground effect of flexible fin is much larger than rigid fin. Meanwhile, two different thrust force peaks in a motion cycle are achieved for the near-ground swimming, and the larger force peak is obtained when the fin moves away from the ground. Besides, it is found that the benefits of ground effect is more notable for higher frequency suggesting that the influence of ground effect may be restricted to higher frequency regimes. These results will be useful for the force generating and control of underwater propulsor.