Rather than drafting, vortex capture dictates efficiency in three-hydrofoil schools

Three-dimensional experiments are presented on a school of three pitching hydrofoils. Two side-by-side leader foils maintain the same relative positions while the location of a third follower foil is varied. Force and flow measurements detail the mechanisms leading to collective thrust and efficiency 58 % and 24 % higher than isolated foils, respectively. In traditional drafting, a follower is positioned in the wake of an upstream object. In wakes with a net momentum deficit, drafting reduces drag by lowering oncoming flow speed. By contrast, wakes from oscillatory swimmers feature strong momentum surplus regions, which increases drag by increasing oncoming flow. Despite that, our results show that the best performance benefits occur for compact schools where the follower is directly in the vortex wake of a leader, whereas regions of reduced mean flow do not improve performance. The performance benefits are driven by vortex–body interactions that increase the thrust and efficiency of the follower and by body-to-body upstream interactions that reduce the power of the leaders. There is an optimal spatial phase to maximise the thrust and efficiency of the follower that depends upon the actual wake wavelength rather than the estimated wavelength used in previous literature. Moreover, wake breakdown, and its associated elimination of vortex–body performance benefits, is not observed within at least three chord lengths downstream of the leaders. Lastly, cross-stream stability measurements of the downstream foil indicate that compact, high-performance formations may require active control strategies in order to maintain their organisation and maximise the hydrodynamic benefits of schooling.