Fins in formation: hydrodynamic impact of median fins in in-line fish swimming

Median fins, including the dorsal and anal fins, influence fish propulsion by lowering body drag and increasing caudal fin thrust through active movement. While their role in solitary swimming is established, their impact on hydrodynamics within schooling environments remains unclear. Using high-fidelity computational fluid dynamics simulations of in-line fish pairs, we systematically varied median fin presence on leaders and followers to isolate neighbor-induced performance changes. When comparing the full-finned configuration to the finless configuration at a leader-follower streamwise spacing (<i>S</i>) of 1.1 body lengths (<i>l</i>), the follower's drag was reduced by 9.5%. A significant contribution of the total drag reduction, about 70%, was neighbor-induced, arising from wake-body interactions with the wake of a leader that had median fins, while the rest was attributed to adding the follower's own median fins. This neighbor-induced benefit arises from stronger leader-generated vortex structures that interact with the follower's body, lowering both shear and pressure drag. The neighbor-induced benefits persist across a range of spacings, diminishing only beyond<i>S</i>= 1.4 l. At higher Reynolds numbers, the neighbor-induced drag reduction also dominates the total drag reduction of the follower. These findings reveal that median fins can serve as hydrodynamic tools for enhancing group swimming performance through neighbor-induced effects, extending their recognized functional role beyond self-induced improvements in solitary swimming.