  The impact of spanwise flexibility on the propulsion performance of two foils
arranged in tandem and subjected to a prescribed sinusoidal heaving motion has
been studied at a Reynolds number of 100. This comprises a wide range of
natural frequencies of the flexible foil, covering pre-resonance, resonance,
and post-resonance regimes, with respect to the heaving frequency. The
propulsion performance of the foils under different Strouhal numbers, with
either fixed reduced frequency or fixed natural frequency, has also been
investigated. We observed that the propulsion efficiency, thrust, and power
consumption drop, but the tip deformation coefficient increases to its maximum
when operating at resonance. The propulsion efficiency of the flexible dual
foils increases significantly in the pre-resonance regime at small Strouhal
numbers. Both thrust force and power consumption increase with the reduced
frequency during the pre- and post-resonance regimes. An instability phenomenon
is observed in the instantaneous tip deformation coefficient of the flexible
dual foils in the post-resonance regimes. We also find that the dual heaving
foils, with a larger resonance Strouhal number, achieve a greater thrust
increment compared with that of rigid dual heaving foils when working at the
pre-resonance regimes. Meanwhile, dual heaving foils with a smaller resonance
Strouhal number can achieve a larger propulsion efficiency increment compared
with that of rigid dual heaving foils when working at small Strouhal numbers.
