Jamie M. Anderson1,2, Knut Streitlien2, David Barrett2, Michael S. Triantafyllou2
1Charles Stark Draper Laboratory, Cambridge, MA 02139, USA
2Department of Ocean Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
Tóm tắt
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 Kármán 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.
