Hydrodynamic performance analysis of formations of dual three-dimensional undulating fins

The work seeks to understand the hydrodynamic performance and interaction mechanisms of double-fin formations. A high-resolution numerical technique based on the constraint immersed boundary method is used for simulating the 3D double-fin system. Comprehensive analysis is carried out, focusing on the hydrodynamic performance of formations with various arrangements, phase differences, and spatial distances. The result shows that the side-by-side formation can suppress the thrust of the fins while reducing energy consumption. The tandem formation notably enhances the thrust for the fin positioned at the rear, albeit at the cost of increased energy consumption. In the tandem formation with a small spacing, the following fin can obtain a thrust increase of 28% and an efficiency increase of 10%, independent of the phase difference. The thrust enhancement in the tandem formation is related to the suction-based propulsion mechanism induced by hydrodynamic interactions. However, the interaction mechanism between the two side-by-side fins resembles the so-called channelling effect. The unilateral channel flow perturbs the pressure equilibrium on either side of the fin, potentially imposing additional resistance on the undulating fin, with effects depending on the phase difference. The study offers new insights into understanding the collective behaviours of undulating fins.