  Symmetric flapping foils are known to produce deflected jets at high
frequency-amplitude combinations even at a zero mean angle of attack. This
reduces the frequency range of useful propulsive configurations without side
force. In this study, we numerically analyse the interaction of these deflected
jets for tandem flapping foils undergoing coupled heave to pitch motion in a
two dimensional domain. The impact of the flapping Strouhal number, foil
spacing and phasing on wake interaction is investigated. Our primary finding is
that the back foil is capable of cancelling the wake deflection and mean side
force of the front foil, even when located up to 5 chord lengths downstream.
This is achieved by attracting the incoming dipoles and disturbing their
cohesion within the limits of the back foil's range of flapping motion. We also
show that the impact on cycle averaged thrust varies from high augmentation to
drag generation depending on the wake patterns downstream of the back foil.
These findings provide new insights towards the design of biomimetic tandem
propulsors, as they expand their working envelope and ability to rapidly
increase or decrease the forward speed by manipulating the size of the shed
vortices.
