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High Speed Body Motion in Water

Inspired by the natural action of flapping in aquatic
locomotion, a dual flapping foil device was developed. The
performance of the device in providing propulsive and
maneuvering forces to small rigid axisymmetric bodies will be
detailed. Two modes of flapping were investigated: waving
and clapping. The clapping motion of wings is a common
mechanism for the production of lifi and thrust in the insect
world, particularly in butterflies and moths. Waving is similar
to the motion of the caudal fin of a fish. A model was built (1
m long, 7.6 cm diameter) with flapping foils at the end of the
tail cone and various measurements were performed in a water
tunnel. (In hindsight, the model can be described as a rigid-
bodied mechanical seal because seals have remarkably similar
dual flaps in their tails.) Time-dependent tests of thrust, drag,
and yawing moment were conducted for several flapping
frequencies commonly observed in relevant aquatic animals.
Phase-matched laser Doppler anemometry measurements of
the near wake were carried out and detailed vorticity-velocity
vector maps of the vortex shedding process have been
obtained for the axial and cross-stream planes. Dye
visualization of wake was documented and a video recording
was prepared of the entire dynamic process.
The ability of the dual flapping foil device to produce a net
thrust and maneuvering cross-stream forces has been
demonstrated, although the main body is rigid. Its wake,
which is composed of jets, is extremely wide, nonrotating, and
rapidly decaying. The thrust production greatly increases with
Strouhal number. The results have been compared with two-
dimensional inviscid flapping foil theories and measurements.
The effect of the rigid cylinder on the flapping performance is
extracted. The efficiency of thrust production generally
increases in the waving mode which mimics the side-to—sidc
head motion of a fish. Efficiency also tends to peak roughly in
the Strouhal number range popular among fish. Axial thrust
shows sensitivity to Strouhal number in the range popular
among fish. However, existing non-linear inviscid theories do
not capture this aspect and the strong viscous effects observed
also need to be included.