  We show how phasing between tandem bioinspired fins flapping at high-stroke
amplitudes modulates rear fin thrust production and wake characteristics. Load
cell thrust measurements show that the rear fin generates 25% more thrust than
the front fin when it lags the latter by a quarter cycle, and performs 8% worse
when it leads the front fin by the same amount. The flow interactions between
the fins responsible for these observations are analyzed using two-dimensional
particle image velocimetry measurements and three-dimensional computational
fluid dynamics simulations. Distributions of velocity elucidate variations in
the effective flow induced on the rear fin for different phase offsets. Vortex
structure interactions and particle rakes reveal the contributions of the
leading- and trailing-edge vortices shed by the front fin in modulating the
suction at the rear fin leading edge. Furthermore, the wake structure far
downstream of the fins changes in its coherence, axial and radial extents for
the different phase offsets. These findings are relevant for the design and
performance optimization of various unmanned underwater vehicles that utilize
such tandem systems.
