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Graduate Aeronautical Laboratories
Caltech
Abstract-
Although jet propulsion is commonly associated with aeronautics, the concept
of jet propulsion was pioneered by aquatic creatures such as squid and
jellyfish. In this setting, jet propulsion consists of periodic ejection of
fluid from an orifice or tube, which is commonly associated with the formation
of vortex rings. The recent work of Gharib, Rambod, and Shariff [JFM,
360
(1998)] has shown that vortex rings formed by ejecting fluid from a tube using
a piston-cylinder mechanism stop growing and pinch off from their generating
jet for sufficiently large values of the piston stroke to diameter ratio
(L/D), suggesting a maximization principle may exist for propulsion utilizing
starting jets. The analysis of Weihs [Fortschr. der Zoo., 24
(1977)] suggests
propulsive benefits may also be obtained from rapid ejection of pulses due to
the increased translational velocity of the resulting vortex rings derived
from their mutual interaction. To study the significance of these issues to
propulsion, a piston-cylinder mechanism attached to a force balance is used to
investigate the impulse and thrust generated by starting jets at L/D ratios in
the range 2 to 8. Fully-pulsed jets comprised of a periodic series of
starting jets are also considered for non-dimensional frequencies,
StL, in the
range 0.1 to 0.97, where 1.0 is the maximum value attainable by a fully-pulsed
jet. Measurements of the impulse associated with vortex ring formation show
it to be much larger than that expected from the jet velocity alone and
proportionately larger than that associated with a trailing jet for L/D large
enough to observe pinch off. The latter result leads to a local maximum in
the average thrust during a pulse at L/D values associated with vortex rings
whose circulation has been maximized. The characteristics of vortex ring
formation leading to this optimal condition will be discussed with the aid of
energy and circulation measurements obtained using DPIV. Measurements of the
time averaged thrust of fully-pulsed jets confirm the relative significance of
vortex ring formation to propulsion over that of a trailing jet for all
StL,
but indicate that the thrust benefit obtained is generally reduced as
StL
increases. Mechanisms by which preceding pulses interfere with vortex ring
formation will be presented to explain this reduction with
StL. The
propulsive benefits predicted by Weihs are not observed, and the flow
characteristics governing this unexpected result will be discussed.
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