Pulsed Jetting in the Mechanical and Biological Worlds

Paul S. Krueger Associate ProfessorDepartment of Mechanical EngineeringSouthern Methodist UniversityDallas, TX 75275 Nature boasts a wide array of organisms utilizing jet propulsion, or more properly, pulsed-jet propulsion. Squid and jellyfish are some of the more well-known members of this group. It is often assumed that this form of locomotion requires high velocity, inefficient jets to be effective. Studies of mechanically generated fully-pulsed jets (pulsed jets with a period of no flow between pulses), on the other hand, have revealed a spectrum of possible flows ranging from vortex rings for short jet pulses to vortex rings followed by trailing jets for longer pulses. Jet pulses producing isolated vortex rings obtained a thrust benefit from over-pressure at the nozzle exit plane during vortex ring formation, a result with potential benefits for biological and/or mechanical pulsed-jet propulsion. In this talk the propulsive efficiency of brief squid (juveniles and adults), longfinned squid (hatchlings), and a self-propelled, mechanical pulsed-jet vehicle ("Robosquid") will be assessed using direct measurement of the jet hydrodynamics with digital particle image velocimetry (DPIV). The results for the juvenile and adult squid show that they utilize the spectrum of jet flows available with the propulsive efficiency of isolated vortex rings being significantly greater than the longer jet pulses. The performance of Robosquid mirrors these results with propulsive efficiency increasing as pulse duration decreases for pulse durations that produce isolated vortex rings. Surprisingly, squid hatchlings outperformed their larger counterparts, a result which is attributed to a range of factors including the hatchlings' preference for shorter pulses and their proportionately larger funnel diameters. A simple model for propulsive efficiency of pulsed jets incorporating nozzle exit over-pressure associated with the unsteady flow physics will be presented. The model explains the key experimental results in terms of over-pressure effects associated with vortex ring formation and predicts efficiencies that increasingly outperform steady jets as scale (i.e., Reynolds number) is reduced for pulsed jets with short, high-frequency pulses. --------------------------------------------------------------------------------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 (CAREER) Award from the National Science Foundation and he was elected the ASME North Texas Section Young Engineer of the Year in 2009. His research interests include unsteady hydrodynamics and aerodynamics, vortex dynamics, vortex-boundary interactions, bio-fluid mechanics, and pulsed-jet propulsion.

Location: Seaver Science Library, Rm 150

Audiences: Everyone Is Invited

Contact: April Mundy