Pulsed-Jet Propulsion at Large and Small Scales

Paul S. Krueger

Department of Mechanical Engineering

Southern Methodist University

Dallas, TX 75275

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 causes the acceleration of additional ambient fluid which in turn leads to more impulse per pulse than would be expected from the jet momentum alone. If the jet pulses are long, the impulse benefit is diminished as steady jet behavior is approached, but jet pulses near the transition between vortex dominated and jet dominated flow appear to optimize the average thrust per pulse for jets issuing into quiescent fluid. For jets issuing into a co-flow (as is the case when the jet apparatus is propelled), 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, the characteristic vortical flow of pulsed jets has been observed in flow-field measurements behind swimming squid. In addition, a self-propelled pulsed jet vehicle developed at SMU (“Robosquid”) has recently demonstrated substantially improved cruise speed compared to propulsion by a steady jet with the same average mass flux. At small scales (jet Reynolds number in the range of 5 - 20), the familiar vortical signature of pulsed jets is still present. Recent observations of several species of squid hatchlings with body lengths on the order of a few millimeters demonstrate rather typical vortex ring formation with each pulse, although shorter pulses are preferred and vorticity decay is more pronounced. In this regime the inferred impulse benefits from pulsing lead to respectable propulsive performance despite the small size of the animals. Accelerations and peak swimming speeds near 2 g and 10 body lengths/sec, respectively, are typical.

Bio: 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.