| Summary: | Thrust-producing harmonically oscillating foils are studied through force and power
measurements, as well as visualization data, to classify the principal characteristics of
the flow around and in the wake of the foil. Visualization data are obtained using
digital particle image velocimetry at Reynolds number 1100, and force and power
data are measured at Reynolds number 40 000. The experimental results are compared
with theoretical predictions of linear and nonlinear inviscid theory and it is found
that agreement between theory and experiment is good over a certain parametric
range, when the wake consists of an array of alternating vortices and either very
weak or no leading-edge vortices form. High propulsive efficiency, as high as 87%, is
measured experimentally under conditions of optimal wake formation. Visualization
results elucidate the basic mechanisms involved and show that conditions of high
efficiency are associated with the formation on alternating sides of the foil of a
moderately strong leading-edge vortex per half-cycle, which is convected downstream
and interacts with trailing-edge vorticity, resulting eventually in the formation of a
reverse Karman street. The phase angle between transverse oscillation and angular
motion is the critical parameter affecting the interaction of leading-edge and trailing-edge
vorticity, as well as the efficiency of propulsion.
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