  Fish swimming style and schooling behaviour are two important aspects of
underwater locomotion. How swimming style affects fish schooling is
investigated by a representative problem setup consisting of two tethered
NACA0012 hydrofoils undulating in a free stream flow at various wavelengths $
\lambda^* = 0.8 - 8 $, front-back distance $ D = 0,\ 0.25,\ 0.5,\ 0.75 $, phase
difference $ \phi/\pi = 0,\ 0.5,\ 1,\ 1.5 $, and lateral gap distance $ G =
0.25,\ 0.3,\ 0.35 $ with fixed Reynolds number $ Re = 5000 $, Strouhal number $
St = 0.4 $, and maximum amplitude $ A_{max} = 0.1 $. In total, 336 cases were
simulated by open-source software IBAMR based on immersed boundary method. The
increase in wavelength generally leads to higher thrust and lateral force,
consistent with a previous study regarding a single swimmer. The highest Froude
efficiency is obtained at intermediate wavelength $ \lambda^* = 2 $ for either
individual or two foils as a group. At side-by-side arrangement $ D = 0 $, the
thrust force upon the two foils can be equivalent, indicating a stable
formation. At staggered arrangement, $ D > 0 $, the follower can take
significant advantage of the leader in locomotion performance by tuning phase
difference. The follower's benefit decreases with relative distance. Various
combinations of wavelength and relative distance can lead to distinct flow
structures, indicating a tunable stealth capacity of the schooling swimmers.
