Conferences, Lectures, & Seminars
Events for February
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On Focusing of Shock Waves
Wed, Feb 11, 2009 @ 03:30 PM - 04:30 PM
Aerospace and Mechanical Engineering
Conferences, Lectures, & Seminars
Veronica Eliasson Postdoctoral ScholarGALCIT California Institute of TechnologyPasadena, CA In this project we study converging shocks in gas, both experimentally and numerically. The interest in converging shocks stems from their ability to concentrate energy in a small volume. However, it has proven difficult to experimentally obtain a stable cylindrical converging shock wave because initial shape perturbations are amplified during the nonlinear focusing event. In this talk, we address the issue of generating and studying stable converging shocks with various geometrical shapes.
A shock tube is used to transform an initially planar shock into a cylindrical ring-shaped shock. These cylindrical shock waves are then further transformed into different geometrical shapes during the focusing phase by two methods. One method consists of changing the shape of the outer boundary of the test section of the shock tube, while the other introduces cylindrical obstacles in specific patterns inside the test section. As a result, a polygonal shape is most often obtained and depending on the number of sides of the shock, either a Mach or regular reflection occurs at the corners during the focusing event.
The shock wave focusing is also studied numerically using Euler equations of gas dynamics for a gas obeying the ideal gas law with constant specific heats with a high-order accurate Godunov method. The governing equations are discretized on body-fitted overlapping structured grids, and adaptive mesh refinement is used to dynamically track the shocks and contact surfaces. Two problems are analyzed; an axisymmetric model of the shock tube used in the experiments and a cylindrical shock wave diffracted by cylinders in a two-dimensional test section.
Location: Seaver Science Library (SSL) - Rm 150
Audiences: Everyone Is Invited
Contact: April Mundy
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Pulsed Jetting in the Mechanical and Biological Worlds
Wed, Feb 18, 2009 @ 03:30 PM - 04:30 PM
Aerospace and Mechanical Engineering
Conferences, Lectures, & Seminars
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
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Microstructure as Derived from X-Ray Line Broadening
Wed, Feb 25, 2009 @ 03:30 PM - 04:30 PM
Aerospace and Mechanical Engineering
Conferences, Lectures, & Seminars
Tamás Ungár Professor Department Materials Physics Eötvös University Budapest, Hungary X-ray diffraction peaks broaden either when crystallites become small or if the crystal is distorted by lattice defects. Size broadening is independent of diffraction order, however, strain broadening increases with diffraction order. Planar defects, especially stacking faults or twin boundaries produce a mixture of size and strain. hkl dependence of the different microstructure features can be very different allowing separation. A brief summary of the principles and specific case studies will be presented for nanomaterials and conventional grain size structural materials, including the effect of twinning in hexagonal metals.
Location: Seaver Science Library (SSL) - Rm 150
Audiences: Everyone Is Invited
Contact: April Mundy