Paul Krueger received his B.S. in Mechanical Engineering in 1997 from the University of California at Berkeley. He received his M.S. in Aeronautics in 1998 and his Ph.D. in Aeronautics in 2001, both from the California Institute of Technology (Caltech). In 2002 he joined the Mechanical Engineering Department at Southern Methodist University where he is currently an Associate Professor. He is a recipient of the Rolf D. Buhler Memorial Award in Aeronautics and the Richard Bruce Chapman Memorial Award for distinguished research in Hydrodynamics. In 2004 he received the Faculty Early Career Development Award (CAREER) from the National Science Foundation. His research interests are unsteady hydrodynamics and aerodynamics, vortex dynamics, bio-fluid mechanics, and pulsed-jet propulsion.

Abstract: Pulsed-Jet Propulsion at Large and Small Scales

Pulsed jets are distinct from other forms of jet propulsion due to the key role played by unsteadiness. For the extreme case of no jet flow between pulses (i.e., "fully-pulsed" jets) the effects can be dramatic. At large scales (jet Reynolds number greater than 1000) vortex rings are formed at each pulse. Vortex ring formation entails the acceleration of additional ambient fluid which in turn leads to more impulse per pulse than would be expected from the jet momentum alone (thrust augmentation). This effect is most pronounced for short pulses because for pulse length-to-jet diameter ratios greater than a critical value (the formation number), vortex ring formation stops mid-way through the pulse and the impulse benefit is diminished. For jets issuing into a co-flow (as is the case during propulsion), the impulse benefit of pulsing can still be expected because the co-flow surrounding the jet only weakly attenuates vortex ring formation as long as the co-flow velocity does not exceed a critical value. Indeed, recent in situ measurements of swimming squid have captured vortical flows similar to those observed in the laboratory with shorter pulses being associated with higher propulsive efficiencies. In addition, a self-propelled pulsed jet vehicle developed at SMU ("Robosquid") has recently demonstrated improved cruise speed compared to propulsion by a steady jet with the same average mass flux. At small scales, numerical simulations of starting jets with jet Reynolds number of 100 show the familiar vortical signature of pulsed jets, although the features are smoothed by viscosity. Likewise, recent observations of squid hatchlings (Doryteuthis pealeii) with body lengths on the order of a few millimeters and jet Reynolds numbers in the range 5 – 50 demonstrate vortex ring-like features with each pulse, but vorticity decay is more pronounced. In this regime the inferred impulse benefits from pulsing lead to respectable propulsive performance, including peak velocities of 10s of body lengths per second and propulsive efficiencies comparable to the high Reynolds number regime.

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