Conferences, Lectures, & Seminars
Events for October
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Microscopic Mechanisms of Deformation in Amorphous Solids
Wed, Oct 03, 2007 @ 03:30 PM - 04:30 PM
Aerospace and Mechanical Engineering
Conferences, Lectures, & Seminars
Mo Li Associate Professor School of Materials Science and EngineeringGeorgia Institute of Technology While the fundamental deformation mechanisms in crystalline materials, namely, the dislocation-based process, have long been understood and put into use, our understanding of the microscopic deformation mechanisms in amorphous solids still remains in its infancy despite tremendous efforts made in the past forty years. Amorphous solids contribute to a large fraction of materials used today, including metallic glasses and amorphous semiconductors, granular matters, and many geological materials. They are characterized by metastability and the lack of long-range order, which poses great challenges for experimentalists as well as theorists to have a detailed understanding of how deformation occurs at the atomic or molecular level. In this talk, I will give a brief introduction to the mechanical properties of amorphous solids and in particular, metallic glasses with special emphasis on shear localization or shear banding. I will present the results from extensive atomistic modeling of the changes in the local atomic structure, volume, and mechanical properties in several model systems subjected to various external loadings. The results led us to the establishment of a new model, an extended Ginzburg-Landau theory. We conclude, from these studies, that the microscopic deformation mechanism in amorphous metals is through the process starting from local volume change and then local shear softening to the final breakdown. The implications derived from this study and its applications to other disordered systems such as granular matters and nanocrystalline materials will be briefly mentioned. --------------------------------------------------------------------------------Mo Li received his Ph.D. in applied physics in 1994 from California Institute of Technology. He joined Morgan Stanley & Co. in New York after a brief stays as a postdoctoral fellow at Caltech and the Argonne National Laboratory. From 1998 to 2001 he was an assistant professor at the Johns Hopkins University. Currently he is an associate professor at Georgia Institute of Technology. His research focuses on mechanical properties of amorphous solids and nano-scaled materials, phase transitions in metastable systems, interfaces, and statistical physics and its applications. The approaches used in his research are a blend of those from statistical physics, solid state physics, materials science, metallurgy, mechanics and computational methods. His research focuses on algorithm development, simulation, and theoretical analysis.
Location: Stauffer Science Lecture Hall, Rm 102
Audiences: Everyone Is Invited
Contact: April Mundy
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On Instability Waves and the Noise Generated by Turbulent Jets
Wed, Oct 10, 2007 @ 03:30 PM - 04:30 PM
Aerospace and Mechanical Engineering
Conferences, Lectures, & Seminars
Tim Colonius Mechanical Engineering Department California Institute of TechnologyPasadena, CA Reduction of jet noise remains an important goal for commercial and military aircraft, as well as a challenging problem for experimental and computational fluid dynamics. We present analysis and results from recent experiments that suggest that the pressure fluctuations associated with large-scale structures in turbulent jets are well modeled by linear instability waves of the mean velocity profile. An 80 microphone phased-array was used to measure pressure fluctuations just outside the jet shear layers of turbulent jets over a range of subsonic Mach numbers and temperature ratios. Measured pressures are decomposed into azimuthal modes and compared to predictions of linear instability theory based using measured and/or predicted mean flow profiles. Agreement in terms of streamwise evolution, phase speed, and radial decay are demonstrated. The near-field pressure measurements are also projected to the far-field using a Kirchhoff surface approach and compared with the directly measured far-field. We also analyze serrated (chevron) nozzles in an attempt to understand how they reduce low frequency noise. Work supported by the Aeroacoustics Research Consortium and the Naval Air Systems Comma
Location: Stauffer Science Lecture Hall, Rm 102
Audiences: Everyone Is Invited
Contact: April Mundy
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Highly Nonlinear Dynamics in Solids: a new Horizon in Wave Propagation
Wed, Oct 17, 2007 @ 03:30 PM - 04:30 PM
Aerospace and Mechanical Engineering
Conferences, Lectures, & Seminars
Chiara Daraio Assistant Professor of Aeronautics and Applied PhysicsCalifornia Institute of TechnologyPasadena, CA The discovery of novel highly nonlinear dynamic phenomena in multiscale artificial composite systems (metamaterials) will be presented. Emphasis will be given to the new tunable properties provided by the high nonlinearity in the specific cases of granular materials and carbon nanotubes. This research was conducted for designing and constructing optimized macro-, micro- and nano-scale structural configurations of materials and for studying their nonlinear acoustic behavior. Variation of composite arrangements of the fundamental elements with different elastic properties in a linear 1-D chain-of-spheres, Y-junction or 3-D configurations led to a variety of novel physical phenomena and interesting wave properties. Potential applications can be found in the area of mechanical, structural and biomedical engineering as well as security and communications systems. The characterization of mechanical and electronic properties of carbon nanostructures with different atomic arrangements and microstructures, exhibiting an exciting highly nonlinear behavior, will also be discussed. --------------------------------------------------------------------------------Chiara Daraio's interests reside at the interface of materials science, condensed matter physics and solid mechanics, particularly in the design, development and testing of multi-scale metamaterials; phononic crystals; responsive soft matter; highly nonlinear solitary waves; mechanical and electronic properties of nano and biomaterials. http://www.daraio.caltech.edu She received her Laurea degree (Equivalent to a master degree) in Mechanical Engineering from the Universita' di Ancona, Universita' Politecnica delle Marche, Ancona, Italy (2001). She received her M.S. (2003) and Ph.D. degrees (2006) in Materials Science and Engineering from the University of California, San Diego. She has been a guest researcher at the Lawrence Berkeley National Laboratories, NCEM, since 2003 and won several awards. Among these, she is a Gold Medal winner of the MRS Graduate Student Award (2005) and winner of the AIM young investigator award (2006). She published over 30 peer reviewed papers, one book chapter and one patent.
Location: Stauffer Science Lecture Hall, Rm 102
Audiences: Everyone Is Invited
Contact: April Mundy
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Applying Realistic Chemistry in Direct Numerical Simulations
Wed, Oct 31, 2007 @ 03:30 PM - 04:30 PM
Aerospace and Mechanical Engineering
Conferences, Lectures, & Seminars
Tianfeng LuResearch AssociateMechanical and Aerospace Engineering DepartmentPrinceton UniversityDirect numerical simulations (DNS) of reacting flows invoking realistic chemistry
constitute the ultimate approach to produce results of high fidelity. This would allow for the solution of a broad range of problems from first principle such as fuel utilization, pollutant emissions, climate change, as well as biochemical cycles and human health. However, DNS with detailed chemistry has been computationally unaffordable due to the high dimension of variables and the mandatory fine resolutions that are required both spatially and temporally. Recently, DNS of turbulence reactive flows with realistic hydrocarbon fuels have been successfully carried out on supercomputers in collaboration with the Sandia National Laboratories, following a significant reduction in both the dimension and the stiffness of the involved detailed kinetics. Most of the major difficulties being faced in the reduction process, such as variable elimination from largescale
nonlinear systems and analytic solution of quasi steady state equations, have been plaguing the scientific community for decades. To solve each of these problems satisfactorily, we have developed a suite of new techniques involving graph theory, binary integer programming, singular perturbation, and spectral analysis. In this talk, representative components of these methods will be discussed and their potential impacts on other fields will be outlined.
Location: Stauffer Science Lecture Hall, Rm 102
Audiences: Everyone Is Invited
Contact: April Mundy