Direct numerical simulation of a two-manta-ray-model school is performed to study the effects of lateral distances on the hydrodynamic performance of the manta rays. An in-house numerical solver based on the immersed boundary method is adopted to perform this study. Clear trends in performance can be observed, with the highest cycle-averaged thrust enhancement of 32% on the tandem follower compared to that of a solitary swimmer. A leading-edge vortex (LEV) enhancement is found on the tandem follower at the moment of thrust enhancement. It is also found that within 1.5 span length (SL) in the lateral direction, the follower exhibits a propulsive advantage, while the leader’s performance remains nearly the same. Low surface pressure regions are observed at the front of the follower, which induces forward suction force to enhance the follower’s performance. The near-body vortex topology is studied, and the tandem follower’s circulation strength shows improvements along the spanwise direction in both the downstroke and upstroke. Far wake topology observations are made to compare all cases.

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