Title: Flow speed has little impact on propulsive characteristics of oscillating foils,
Abstract: Experiments are reported on the performance of a pitching and heaving
two-dimensional foil in a water channel in either continuous or intermittent
motion. We find that the thrust and power are independent of the mean
freestream velocity for two-fold changes in the mean velocity (four-fold in the
dynamic pressure), and for oscillations in the velocity up to 38\% of the mean,
where the oscillations are intended to mimic those of freely swimming motions
where the thrust varies during the flapping cycle. We demonstrate that the
correct velocity scale is not the flow velocity but the mean velocity of the
trailing edge. We also find little or no impact of streamwise velocity change
on the wake characteristics such as vortex organization, vortex strength, and
time-averaged velocity profile development---the wake is both qualitatively and
quantitatively unchanged. Our results suggest that constant velocity studies
can be used to make robust conclusions about swimming performance without a
need to explore the free-swimming condition.