Stress-Induced Martensitic Phase Transformation and Fracture in Thin Sheets of Nitinol

Samantha Daly

Department of Mechanical Engineering
California Institute of Technology
Pasadena, CA

Nickel-Titanium, commonly referred to as nitinol, is a shape-memory alloy with numerous applications due to its superelastic nature and its ability to revert to a previously defined shape when deformed and then heated past a set transformation temperature. While the crystallography and the overall phenomenology are reasonably well understood, much remains unknown about the deformation and failure mechanisms of these materials. These latter issues are becoming critically important as nitinol is being increasingly used in medical devices and space applications.

The talk will describe the investigation of the deformation and failure of nitinol using an in-situ optical technique called Digital Image Correlation (DIC). With this technique, full-field quantitative maps of strain localization are obtained for the first time in thin sheets of nitinol under tension. These experiments provide new information connecting previous observations on the micro- and macro- scale. They show that martensitic transformation initiates before the formation of localized bands, and that the strain inside the bands does not saturate when the bands nucleate. The effect of rolling texture, the validity of the widely used resolved stress transformation criterion, and the role of geometric defects are examined. A detailed investigation of fracture will be presented, including the observed saturation and transformation zones around the cracktip, as well as a determination of the K_IC value for thin sheets of nitinol. A discussion of these results in the context of theoretical models will be provided.

Wednesday, January 10, 2007
3:30 PM
Seaver Science Library, Room 150 (SSL 150)

Refreshments will be served at 3:15 pm.

DSMC Modeling Of Near-Continuum Flows With Real Gas Effects

Yevgeny A. Bondar

Research Scientist 
Institute of Theoretical and Applied Mechanics
Novosibirsk 630090, Russia

The recent activity of Computational Aerodynamics Lab (ITAM, Novosibirsk, Russia) on statistical simulation of high-temperature near-continuum rarefied flows is reviewed. An accurate prediction of these flows, such as those behind the shock wave formed about a space vehicle at high altitudes, requires the use of adequate models of physical and chemical processes—so-called real gas effects—and effective numerical procedures. Current challenges and problems pertaining to the development, validation and application of such models are discussed.

A novel approach to statistical simulation of high-temperature nonequilibrium chemical reactions is described. Vibrationally specific dissociation cross sections are found as solutions of an integral equation whose right side contains a two-temperature reaction rate constant. The approach is illustrated by an example of the model of high-temperature dissociation of nitrogen. All stages of model implementation are considered in detail, namely, the mathematical basis, analysis of the model by comparisons with conventional models both at the level of cross sections and at the level of macroscopic reaction rates, and particular applications to computations of near-continuum reacting flows by the Direct Simulation Monte Carlo method.

Tuesday, January 16, 2007
11:00 AM
Laufer Library (RRB 208)

Characterization and Yield behavior of UFG, Nano-Twinned Copper

Andrea M. Hodge

Materials Scientist 
Nanoscale Synthesis and Characterization Laboratory
Lawrence Livermore National Laboratory
Livermore, CA

Yield point drops are a classic non-uniform plastic deformation process in solids. As stated by Johnston and Gilman in their classic work on single crystal Lithium Fluoride, the yield point drop phenomena in crystalline solids is clearly dependant on the availability of necessary mobile dislocations to support the plastic deformation process. In this talk, the presence of a yield point will be related to materials with nanocrystalline, ultrafine-grained (UFG) and evenly distributed nanoscale features (i.e. twins). Specifically, tensile tests performed on high purity (99.999%) copper foils (170 m thick), processed by magnetron sputtered multilayer technology, demonstrate reproducible observations of yield points. These type of materials present very low initial dislocation densities, a columnar grain structure (~ 0.55 m width), and uniformly distributed and spaced (? 45 to 50 nm) growth twins with an orientation parallel to the plane of deposition.

Wednesday, January 17, 2007
3:30 PM
Seaver Science Library, Room 150 (SLH 150)

Refreshments will be served at 3:15 pm.

In Search of Fast and Robust Adaptation

Naira Hovakimyan

Associate Professor 
Department of Aerospace and Ocean Engineering
Virginia Polytechnic Institute and State University
Blacksburg, VA 24061-0203
e-mail: nhovakim@vt.edu

The history of adaptive control systems dates back to early fifties, when the aeronautical community was struggling to advance aircraft speeds to higher Mach numbers.  In November of 1967, X-15 launched on what was planned to be a routine research flight to evaluate a boost guidance system, but it went into a spin and eventually broke up at 65,000 feet, killing the pilot Michael Adams. It was later found that the onboard adaptive control system was to be blamed for this incident.  Exactly thirty years later, fueled by advances in the theory of nonlinear control, Air Force successfully flight tested the unmanned unstable tailless X-36 aircraft with an onboard adaptive flight control system. This was a landmark achievement that dispelled some of the misgivings that had arisen from the X-15 crash in 1967. Since then, numerous flight tests of Joint Direct Attack Munitions (JDAM) weapon retrofitted with adaptive element have met with great success and have proven the benefits of the adaptation in the presence of component failures and aerodynamic uncertainties. However, the major challenge related to stability/robustness assessment of adaptive systems is still being resolved based on testing the closed-loop system for all possible variations of uncertainties in Monte Carlo simulations, the cost of which increases with the growing complexity of the systems. This presentation will give an overview of the limitations inherent to the conventional adaptive controllers and will introduce a new thinking for adaptive control  design that leads to fast and robust adaptation with provable control specifications and guaranteed stability/robustness margins. Various applications will be discussed throughout the presentation to demonstrate the tools and the concepts.

Wednesday, January 24, 2006
3:30 PM
Seaver Science Library, Room 150 (SSL 150)

Refreshments will be served at 3:15 pm.

Optimization in Complex Fluid Mechanics Problems Using the Surrogate Management Framework

Alison Marsden

Postdoctoral Fellow 
Stanford University
Stanford, CA

As computational tools mature in accuracy and ability to handle complex phenomena, their impact on solving significant engineering problems will grow. Along with the increase in fidelity of numerical simulations comes a need for development of optimization tools. Optimization applied to fluid mechanics encompasses some of the most challenging aspects of both sub jects, often requir-ing advanced numerical methods for fluid mechanics simulations, combined with non-traditional optimization methods. This talk will focus on new methodologies for optimization of airfoil shapes to reduce trailing-edge noise in turbulent flow. We will then briefly discuss how these optimization tools are being transferred to the field of cardiovascular bioengineering, where they have potential to impact surgical design for both congenital and acquired cardiovascular disease.

In optimization for aeroacoustics, or flow generated noise, time accurate computations such as large-eddy simulation (LES) are required to resolve the range of spatial and temporal flow scales relevant to noise generation. The large computational cost coupled with the difficulty in computing gradients of cost functions makes optimization using traditional methods particularly challenging. In this work, we have developed a methodology to optimize the shape of a hydrofoil trailing-edge in order to minimize the aerodynamic noise propagated to the far field. The optimization method applied in this problem is a tailored version of the surrogate management framework (SMF) (Booker et al., 1999). Several novel adaptations to this method have made it more suitable for the trailing- edge problem, particularly for constrained optimization.

Optimization has been performed to suppress the laminar vortex-shedding noise from acous-tically compact airfoils as well as the broadband noise from turbulent flow over an acoustically non-compact airfoil. For optimization in turbulent flow, LES is used for source field computations. Several optimal shapes have been identified, which result in significant reduction of trailing-edge noise in both laminar and turbulent flow with reasonable computational cost. The results of this study demonstrate the successful coupling of shape optimization to a time-accurate turbulent flow calculation, and validate the use of a novel methods for constrained optimization.

The SMF optimization method is currently being applied to optimize cardiovascular geometries that are representative of surgeries and diseased states. These problems share several challenges in common with the trailing-edge noise problem, particularly the importance of computing the unsteady flow field and a large computational cost. We will discuss how the tools that were developed for the trailing-edge problem can be effectively coupled to blood flow simulations in order to impact surgery design and improve understanding of cardiovascular disease. Finally, we will discuss future work in the area of optimization and simulation in cardiovascular medicine, including coronary artery bypass grafting, peripheral vascular disease, and the identification of principles of optimality in vessel branching patterns.

Wednesday, January 31, 2007
3:30 PM
Seaver Science Library, Room 150 (SSL 150)

Refreshments will be served at 3:15 pm.

Biodiesel Combustion and Emissions

André Boehman

Professor of Fuel Science 
Department of Energy and Geo-Environmental Engineering
Pennsylvania State University
University Park, PA 16802-5000

In this work, we consider the behavior of biodiesel fuels during diesel combustion, including the injection process, pollutant formation and the characteristics of the pollutants. Topics covered include the unique features of the ignition process for biodiesel fuels, the anomalous "NOx Effect" that is observed in diesel engines running on biodiesel and impacts of biodiesel on the characteristics of diesel soot. Past and ongoing work seeks to relate the nanostructure and oxidative reactivity of soot. This work shows that the initial structure alone does not dictate the reactivity of diesel soot and rather the initial oxygen groups have a strong influence on the oxidation rate. A comparison of the complete oxidation behavior and burning mode was made to address the mechanism by which biodiesel soot enhances oxidation. Diesel soot derived from neat biodiesel (B100) is far more reactive during oxidation than soot from neat Fischer-Tropsch diesel fuel (FT100). B100 soot undergoes a unique oxidation process leading to capsule-type oxidation and eventual formation of graphene ribbon structures. Incorporation of greater surface oxygen functionality in the B100 soot provides the means for more rapid oxidation and drastic structural transformation during the oxidation process. These characteristics of diesel soot have implications for the operation and regeneration of diesel particulate filters and, as a consequence of the coupling that can arise between particulate and NOx controls, for the operation of urea-selective catalytic reduction of NOx.

Wednesday, February 7, 2007
3:30 PM
Seaver Science Library, Room 150 (SSL 150)

Refreshments will be served at 3:15 pm.

Biological Materials, Biomaterials and Biomimetics

Ulrike G.K. Wegst

Max Planck Institute für Metallforschung
Stuttgart, Germany 
and 
Lawrence Berkeley National Laboratory
Berkeley, CA

Biological materials and their skilled use have played a key role in the development of mankind and technology, and the course of history. After millions of years, they are still of great importance today and used both as low cost, high volume materials and as materials for high-tech applications. One reason for their success is that they have properties which cannot easily be emulated by man-made materials, yet. Their striking mechanical efficiency is primarily due to their hierarchical structure which provides them with the potential of optimisation at each structural level, resulting in stiff, strong and tough composites even though, from a mechanical point of view, there is nothing very special about the individual components. The considerable advantage which we have over our ancestors today is that we cannot only use biological materials in their "native" state, but that we have the tools to investigate and test them at almost all levels of their structural hierarchy. With an informed evaluation of their structure, properties and function, principles of optimisation may thus be identified that allow for the development of new or improved man-made materials. Illustrated in this talk will be how the mechanical efficiency and optimisation of biological materials, ranging from bone to seaweed and from mollusc shell to bamboo, can be evaluated and compared with engineering materials. A variety of methods for the structural characterisation of biological materials and their hierarchical composite structure, ranging from synchrotron-based x-ray microtomography to a novel method for in situ mechanical testing in an SEM or FIB, will be presented. Finally, an example for a systematic knowledge transfer from nature to technology that resulted in the successful development of a biomimetic bone-substitute material will be given.

Ulrike G.K. Wegst studied Physics and Materials Science at the Georg-August-Universität, Göttingen, Germany and at the University of Cambridge, UK. She received her PhD from the University of Cambridge for her analysis of the Mechanical performance of natural materials. Until 2000 she worked as a Research Associate in the Engineering Design Centre of the Cambridge University Engineering Department on the development of a software-based methodology for the environmentally-conscious selection of materials and processes, since then implemented in the CES Eco-Selector software. From 2000 to 2001 Ulrike Wegst was a Visiting Scientist at the Institut National Polytechnique de Grenoble, France, where she started her work on the qualitative and quantitative characterization of biological materials using synchrotron-generated X-rays. Since 2001 Ulrike Wegst is a staff scientist at the Max Planck Institute for Metals Research in Stuttgart, Germany. Her research interests are primarily focussed on the question how structure and mechanical properties are combined in biological materials and surfaces to perform certain functions and how this may result in the formulation of new design paradigms for superior engineering materials. By combining her own and published research, she develops the Biomimetic Design Guide, a software based tool for the informed transfer of newly discovered principles of optimisation into engineering design. Since 2005, Ulrike Wegst is a Visiting Scientist at the Lawrence Berkeley National Laboratory; her project there is the development of novel nanocomposites for bone regeneration.

Thursday, February 8, 2007
3:30 PM
Von Kleinschmidt Center, Room 256 (VKC 256)

Refreshments will be served at 3:15 pm.

Nonlinear Dynamics of Multi-Mesh Gear Systems

Robert Parker

Professor 
Mechanical Engineering Department
Ohio State University
Columbus, OH 43210

Gear vibration dominates helicopter cabin noise, which can exceed 110 dB. Gear vibration is a major concern in numerous other applications including aerospace, automotive, wind turbines, high-speed machinery, manufacturing, and more. Despite gears' long history, scientific study of their dynamics has been concentrated in the last 40 years, and the pervasive impact of nonlinearities and parametric instability in gear vibration has been realized only in the last decade. Mathematical models are emerging to incorporate these critical aspects. Planetary gears and other systems having multiple interacting tooth meshes exhibit especially interesting dynamics that remain largely unexplored.

Nonlinearity from tooth contact loss and parametric instability from varying contact conditions as the gears rotate are essential features of complex phenomena observed in practice. After giving industrial examples motivating the research, the presentation will focus on modeling and analysis of the nonlinear dynamics of planetary gears using asymptotic and finite element/contact mechanics methods. In addition to illustrating and explaining the rich range of nonlinear dynamics that emerge, the analytical approximations generate results with clear practical implications. An ambitious $2.1M experimental gear dynamics program with specialized facilities that are unique worldwide will also be discussed.

Robert Parker has been at the Ohio State Department of Mechanical Engineering since 1996. He received his M.S. and Ph.D. degrees from the University of California, Berkeley. His research investigates problems on the dynamics, vibration, and stability of mechanical systems with particular focus on high-speed devices. He has held visiting research appointments at INSA Lyon (France), Risoe National Lab (Denmark), NASA Glenn Research Center, the University of Technology-Sydney, and Tokyo University. He worked for two years in the Spacecraft Dynamics division of The Aerospace Corporation in Los Angeles. He consults internationally on vibration problems in numerous industries.

Prof. Parker has received over $5M of research funding from the National Science Foundation, U.S. Army Research Office, NASA, National Rotorcraft Technology Center, General Motors, Ford, Boeing, Sikorsky, and other companies.

Prof. Parker is a Fellow of ASME and AAAS. He was one of a select group invited to National Academy of Engineering Frontiers of Engineering Symposia in the US and Germany. He received the Presidential Early Career Award for Scientists and Engineers (PECASE) in 1999, which is "the highest honor awarded by the U.S. government to scientific researchers early in their careers," as well as the NSF CAREER and Army Young Investigator Awards. Prof. Parker received the ASME Gustus Larson Award, ASEE Outstanding Faculty Award, SAE Ralph R. Teetor Educational Award, Ohio State Lumley Research Award (twice), and Ohio State Research Accomplishment Award (twice). He is an Associate Editor for the ASME Journal of Vibration and Acoustics, is active in international conference organization, and has given several keynote lectures at international conferences and universities.

Wednesday, February 14, 2007
3:30 PM
Seaver Science Library, Room 150 (SSL 150)

Refreshments will be served at 3:15 pm.

The Mechanics of Cell Migration and the Cytoskeleton

Juan Carlos del Alamo

Mechanical and Aerospace Engineering Department
University of California at San Diego
San Diego, CA

Motility of eukaryotic cells is essential for many biological processes such as embryonic development or tissue renewal, as well as for the function of the immune and nervous systems. If misregulated, motility plays an important part in diverse diseases such as cancer, osteoporosis, and mental retardation.

Cell migration over surfaces is an integrated chemical and physical process involving the cytoskeleton and its mechanical interaction with the substrate through discrete adhesion regions. Precise quantitative knowledge of the bio-physical processes involved in cell migration is limited. Better measurements are needed to ultimately build models with predictive capabilities. The free-living soil amoeba Dictyostelium has proven to be a valuable model system for the investigation of cell motility with extensive similarities to higher eukaryotes in general, and leukocytes in particular.

We present an improved force cytometry method and apply it to the analysis of the dynamics of the chemotactic migration of the amoeboid form of Dictyostelium discoideum. Our explicit calculation of the adhesion force field takes into account the finite thickness of the elastic substrate and improves the accuracy and resolution compared to previous methods. This enables us to quantitatively study the differences in the mechanics of the migration of wild-type and mutant cell lines up a chemoattractant gradient. The time evolution of the elastic energy exerted by the crawling cells on their substrate is quasi-periodic and can be used as a simple indicator of the different phases of the cell crawling cycle. We find that the period of the elastic energy cycle correlates strongly with the mean velocity of migration regardless of cell type. Furthermore, we show that when cells adhere to the substrate, the exert opposing pole forces that are orders of magnitude higher than the force required to overcome the resistance from their environment.

Tuesday, February 20, 2007
3:30 PM
Von Kleinschmidt Center, Room 101 (VKC 101)

Refreshments will be served at 3:15 pm.

Collective Motion and Decision-Making in Animal Groups

Iain Couzin

Royal Society University Research Fellow 
Department of Zoology
University of Oxford
Oxford, UK 
and
Visiting Research Fellow 
Pew Program in Biocomplexity
Princeton University
Princeton, NJ 08544

Our research focuses on understanding collective behavior; how large-scale biological patterns result from the actions and interactions of the individual components of a system. We study self-organised pattern formation in a wide range of biological systems, including ants, fish schools, bird flocks, locust / cricket swarms and human crowds.

Wednesday, February 21, 2007
3:30 PM
Seaver Science Library, Room 150 (SSL 150)

Refreshments will be served at 3:15 pm.

How Do You Count Individual Biological Bonds?

Todd Sulchek

Staff Scientist 
Biosecurity and Nanosciences Laboratory
Lawrence Livermore National Laboratory

My research program focuses on the measurement and prediction of how multiple individual biological bonds produce a coordinated function within molecular and cellular systems. In particular I focus on two complementary goals. The first is to understand the kinetics of multivalent pharmaceuticals during their targeting of disease markers. The second is to quantify the host cell signal transduction resulting from pathogen invasion. We develop and employ several tools to accomplish these goals. The primary platform for study is the atomic force microscope (AFM), which controls the 3D positioning of biologically functionalized micro- and nanoscale mechanical probes.

This talk will describe our method of using single molecule dynamic force spectroscopy to determine the binding strength of antibody- protein complexes as a function of binding valency in a direct and simple measurement. We used the atomic force microscope to measure the force required to rupture a single complex formed by the MUC1 protein, a cancer indicator, and therapeutic antibodies that target MUC1. We will show that nanomechanical polymer tethers can be used in a completely novel manner to count the number of biological bonds formed. Mechanical work (on the scale of a few kBT!) will disrupt these bonds and can quantify the overall kinetics. This ability to form, count and dissociate biological bonds with nanomechanical forces provides a powerful method to study the physical laws governing the interactions of the biological molecules.

Thursday, February 22, 2007
3:30 PM
Taper Hall of Humanities, Room 116 (THH 116)

Deformation and Failure of Composite Structures

Chiara Bisagni

Visiting Associate Professor 
Massachusetts Institute of Technology
Department of Mechanical Engineering
77 Massachusetts Avenue, Room 5-218
Cambridge, MA 02139
chiara@mit.edu 
and

Professore Associato 
Dipartimento di Ingegneria Aerospaziale
Politecnico di Milano
Via La Masa 34, 20156 Milano, Italy
chiara.bisagni@polimi.it

After a brief presentation of the research activities and experimental facilities of the Department of Aerospace Engineering of Politecnico di Milano, Italy, the lecture will focalize on the experimental and numerical investigations carried out by Chiara Bisagni during the last years concerning mainly composite structures under buckling and energy absorption requirements.

The first part of the presentation will consider the structural behavior of composite structures under buckling requirements. Some results on stringer stiffened composite panels subjected to buckling under compression and shear will be presented. In particular, the investigation of fuselage panels and of a helicopter tailplane performed during two European projects (POSICOSS, "Improved post-buckling simulation for design of fibre composite stiffened structures", and COCOMAT, "Improved MATerial Exploitation at Safe Design of COmposite Airframe Structures by Accurate Simulation of Collapse") will be presented. Also the first results of the research now under way will be presented. It considers cyclic buckling tests on composite boxes with combined loads, and the detection of damage propagation during the tests on stiffened buckling structures.

The second part of the presentation will consider energy absorption requirements. Indeed, crashworthiness related to composite materials has now become a serious issue, as composite structures have the possibilities to absorb an even superior amount of energy compared to metals, with contained costs. But the crash analysis of composite structures remains particularly challenging due to the complexity and diversity of failure modes that composites exhibit under crushing loads.

The experimental and numerical investigation on the energy absorbing capabilities of intersection elements for helicopter subfloor, and of Formula One car components will be presented. In particular, a building block approach has been used to calibrate the numerical model, analyzing at first coupon testing and tube crushing experiments, and then crash tests of the helicopter and Formula One car components.

Friday, February 23, 2007
2:30 PM
Stauffer Science Lecture Hall, Room 102 (SLH 102)

Nonlinear Analysis of Composite and Functionally Graded Shell Structures

J.N. Reddy

Advanced Computational Mechanics Laboratory
Texas A & M University
College Station, TX 77843-3123
jnreddy@tamu.edu

A tensor-based finite element formulation for the nonlinear analysis of laminated shell structures and through-thickness functionally graded shells will be discussed. A tensor-based finite element formulation is used to describe the deformation and 3-D constitutive laws of a shell in a natural and simple way by using curvilinear coordinates. In addition, a family of high-order elements with Lagrangian interpolations is used to avoid membrane and shear locking. A first-order shell theory with seven parameters is derived with exact nonlinear deformations and under the framework of the Lagrangian description. This approach takes into account thickness changes and, therefore, 3D constitutive equations are utilized. Numerical comparisons of the present results with those found in the literature for typical benchmark problems involving isotropic and laminated composite plates and shells as well as functionally graded plates and shells are found to be excellent and show the validity of the developed finite element model. Moreover, the simplicity of this approach makes it attractive for applications in contact mechanics and damage propagation in shells. A number of examples of applications to laminated composite shell structures are presented.

Keywords: Finite element model, nonlinear shell theory, multilayered composites, functionally graded shells, numerical examples.

Acknowledgement: The research results reported herein were obtained while the authors were supported by the Structural Dynamics Program of the Army Research Office (ARO) through Grant 45508EG.

References

R. A. Arciniega and J. N. Reddy, "Tensor-based Finite Element Formulation for Geometrically Nonlinear Analysis of Shell Structures," Computer Methods in Applied Mechanics and Engineering 196, Nos. 4-6, pp. 1048-1073, 2007.

R. A. Arciniega and J. N. Reddy, "Large deformation analysis of functionally graded shells," International Journal of Solids and Structures 44, pp. 2036-2052, 2007.

J.N. Reddy, Mechanics of Laminated Composite Plates and Shells: Theory and Analysis, 2nd edition, CRC Press, Boca Raton, Florida, 2004.

Wednesday, February 28, 2007
10:30 AM
Kaprielian Hall, Room 203 (KAP 203)

Refreshments will be served at 3:15 pm.

Ion Mobility Analysis of Gaseous and Particulate Pollutants

Anthony Wexler

Professor 
Department of Mechanical and Aeronautical Engineering
University of California at Davis
Davis, CA

USC faculty, staff and students have played a leading roll internationally in elucidating the physical and chemical constituents in the atmosphere and the health effects that they elicit. But like investigators everywhere, this work has been limited by the spatial resolution of the instruments that are available. Usually, a few measurements in a vast urban area such as Los Angeles must suffice due to instrument costs even though there are tremendous spatial inhomogeneities for many of the toxic pollutants. This talk will present a nascent effort at UC Davis to design and build an inexpensive, easily manufactured ion mobility spectrometer that is suitable for analyzing many common pollutants, especially the organic ones.

Wednesday, February 28, 2007
3:30 PM
Seaver Science Library, Room 150 (SSL 150)

Refreshments will be served at 3:15 pm.

Formulation of the k-omega Turbulence Model Revisited

David C. Wilcox

President 
DCW Industries, Inc.
La Cañada, CA

With the rapidly developing field of Detached Eddy Simulation (DES) has come renewed interest in classical Reynolds-averaged (RANS) models of turbulence. DES solves the exact Navier-Stokes equation for the largest eddies and uses a conventional turbulence model to determine Reynolds stresses in thin shear layers. The quality of the DES, of course, depends critically upon how accurate the RANS model is.

This seminar presents a new version of the author's k-omega model of turbulence, which is the most widely used turbulence model of its type for Computational Fluid Dynamics applications. The revisions include addition of just one new closure coefficient and a minor adjustment to the dependence of eddy viscosity on turbulence properties. The result is a model that applies to both boundary layers and free shear flows for all speed ranges from incompressible to hypersonic.

The modifications to the new k-omega model have been made using the methodology developed by Wilcox in his popular textbook entitled "Turbulence Modeling for CFD." In this methodology, boundary layers and free shear flows are first dissected and analyzed using perturbation methods and similarity solutions. All aspects of the model, including boundary conditions for rough surfaces and surfaces with mass injection, are then developed and validated. Finally, a series of computations is performed for approximately 100 different applications including free shear flows, attached boundary layers, backward-facing steps and separated flows. The test cases include flows from incompressible speeds to Mach numbers in excess of 10. All computations have been done with state-of-the-art numerical flow solvers.

The improvements to the k-omega model represent a significant expansion of its range of applicability. The new model, like preceding versions, provides accurate solutions for mildly-separated flows and simple geometries such as that of a backward-facing step. The model's improvement over earlier versions lies in its accuracy for even more complicated separated flows. This seminar demonstrates the enhanced capability for supersonic flow into compression corners and hypersonic shock/boundary-layer interactions. The excellent agreement is achieved without introducing any compressibility modifications to the turbulence model.

Wednesday, March 7, 2007
3:30 PM
Seaver Science Library, Room 150 (SSL 150)

Refreshments will be served at 3:15 pm.

Impacts of Fuel and Combustion Conditions on the Reactivity and Nanostructure of Diesel Soot

André L. Boehman

Professor of Fuel Science 
Department of Energy and Geo-Environmental Engineering
The Pennsylvania State University
University Park, PA 16802-5000 

Recent findings in our laboratory show that fuel formulation can affect the oxidative reactivity of the soot. The inclusion of biodiesel in the fuel lowers the ignition temperature of soot and consequently lowers the temperature required for regeneration of the diesel particulate filter (DPF) and this was attributed to the high surface oxygen content of biodiesel soot. In addition, the oxidation rate of biodiesel soot was found to be two times faster than that of diesel soot. This presentation includes a review of this recent work on biodiesel soot and other method to improve the regenerability of the DPF by enhancing the oxidative reactivity of diesel soot. Our latest results show that exhaust gas recirculation (EGR) also can be utilized to generate more reactive soot. The oxidative reactivity of diesel soot is shown to be strongly affected by simulated and actual EGR, where the simulated EGR was achieved by injection of carbon dioxide CO₂ in the intake air, which was used to simulate particle free and cold EGR.

Tuesday, March 20, 2007
1:00 PM
Laufer Library, RRB 208

Anatomy of Complex Reaction Systems. Combustion Reaction Mechanisms from Ignition Delay Times

Assa Lifshitz

Emeritus Faculty Member 
Department of Physical Chemistry
The Hebrew University of Jerusalem
Jerusalem, Israel

One of the very useful approaches to the understanding of the kinetics and mechanism of complex combustion systems is the measurement and modeling of the induction period that precedes the ignition of a fuel in a shock tube. When a mixture of a fuel and oxidant is subjected to shock heating, it ignites, following an induction period known as the ignition delay time. This delay is the outcome of the exponential character of the overall reaction rate resulting from various chain branching reactions and adiabatic temperature rise during the course of the reaction. The delay time which is a readily measurable quantity is a function of the initial temperature, pressure and composition of the reaction mixture. The measurement of its dependence on the reactant concentrations and temperature provides a very powerful basis for modeling and understanding the oxidation mechanism. The high potential of this methods was recognized by many combustion kineticists and a very large volume of experimental results and kinetics modeling have been published. The following picture is a typical pressure record showing the reflected shock heating and the ignition process.

It is useful to summarize the dependence of the ignition delay times on the composition of the system and on the temperature in a simple parametric relation that can later serve as a basis for computer modeling. It has been shown in the past in numerous ignition studies behind shock waves that the general relation between the induction times and the concentrations is very similar to the relation between a rate of a chemical reaction and the concentrations, that is,

where t_ignition is the ignition delay time, C_i is a concentration of a component i, and β_i is a parameter somewhat similar to an empirical reaction order. It has also been shown that the concentration independent parameter can be presented as,

an expression very similar to a rate constant (except that A decreases with temperature). The parameters E and β_i are determined by a complex kinetic scheme. They are experimentally determined quantities and provide a very useful means to summarize the experimental results in a quantitative manner.

After establishing an empirical relation as above and determining the parameters, one can perform computer experiments under conditions similar to the laboratory experiments and try, for a given reaction scheme to reproduce these parameters. One then arbitrarily varies the magnitude of the various rate constants in the kinetic scheme and examines the influence of such variations on the magnitude of the ignition delay time and its dependence on the concentrations and on the temperature. From the results of this type of experiments, the role of each reaction in the overall mechanism can be elucidated.

By employing such methods, many interesting combustion schemes were analyzed in the past and details of the kinetics and an understanding of the oxidation mechanisms were achieved. We will present and discuss several such systems.

Wednesday, March 21, 2007
3:30 PM
Seaver Science Library, Room 150 (SSL 150)

Refreshments will be served at 3:15 pm.

High-Angle Grain Boundaries and the Evolution of Texture During Severe Plastic Deformation (SPD) Processing

Terry R. McNelley

Professor 
Center for Materials Science and Engineering
Department of Mechanical and Astronautical Engineering
Naval Postgraduate School
Monterey, California

The production of highly refined microstructures in engineering alloys by application of novel SPD technologies may lead to dramatic property improvements, but realization of this potential will require improved understanding of microstructure control and microstructure - processing - property relationships. This presentation will examine high-angle boundary formation in microstructures after conventional and SPD processing of aluminum and its alloys, and the relationship between these boundaries and components of the texture. Recent orientation imaging microscopy investigations in this laboratory have revealed distinct, meso-scale band - or block-like features in processed materials. The lattice orientations within these features alternate between prominent texture orientations in a manner reminiscent of deformation banding in fcc metals. Analytical transmission electron microscopy has shown that the interfaces between these features are dislocation boundaries that may be precursors to disordered high-angle grain boundaries. Recent results on materials processed by large-strain extrusion machining will be included.

Terry McNelley is a native of Fort Wayne, Indiana. He received his BS in Metallurgical Engineering in 1967 from Purdue and his PhD in Materials Science and Engineering in 1973 from Stanford. For the period 1972-76 he was a faculty member in the Department of Mechanical Engineering at the University of Wyoming, and from 1976 - present he has been in the Mechanical Engineering Department at the Naval Postgraduate School, serving as Department Chair from 1996 - 2002. He has held visiting appointments at institutions in England (1980-81), Japan (1993) and Spain (1999). Professor McNelley's interests include microstructure - processing - property relationships in metallic materials; deformation processing, microstructures, recrystallization and superplasticity; and metal matrix composites. He was elected Fellow of ASMI in 2001.

Wednesday, March 28, 2007
3:30 PM
Seaver Science Library, Room 150 (SSL 150)

Refreshments will be served at 3:15 pm.

Highly Nonlinear Dynamics in Solids: A New Horizon in Wave Propagation

Prof. Chiara Daraio

Asst. Professor Aeronautics and Applied Physics 
California Institute of Technology
Pasadena, 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.

Professor 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.

Thursday, April 5, 2007
3:30 PM
Grace Ford Salvatori Hall, Room 222 (GFS 222)

Modeling the Flow Response of Severely Processed Metals: Application to Copper and Zirconium

Irene J. Beyerlein

Staff Scientist 
Theoretical Division
Los Alamos National Laboratory
Los Alamos, NM 87545
Irene@lanl.gov

Severe plastic deformation techniques have received considerable attention for their potential in producing nanocrystalline metals with outstanding properties. As the name suggests, these techniques involve deforming metals up to extremely large strains, from 100% to 1600%. To measure their mechanical performance, subsequent uniaxial tests or hardness measurements are conducted on the heavily processed samples. In most situations, the severely processed material is plastically anisotropic, meaning that the yield stress and hardening evolution depend on the strain mode and direction imposed by the test. So for example the tensile strength of the material could be stronger along the billet axis than transverse to it. The opposite may occur in compression. Furthermore, subsequent loading most often imposes a strain-path change to the material. Both strain-path changes and large plastic straining have for some time challenged development of models for metal deformation. We are currently developing micromechanical hardening laws and multi-scale models for the deformation behavior of metals with a large strain processing history. The resulting constitutive model accounts for contributions to anisotropy by texture and microstructural evolution in pre-straining and re-loading. In this talk, the predictions will be compared with the measured responses in copper and zirconium after they have been processed by equal channel angular extrusion. The model forecasts significant asymmetry in the tension and compression responses and directional dependence of these metals after ECAE, in agreement with observation.

Wednesday, April 11, 2007
3:30 PM
Seaver Science Library, Room 150 (SSL 150)

Refreshments will be served at 3:15 pm.

Reflections on the Turbulence Problem

Anatol Roshko

Theodore von Kármán Professor of Aeronautics 
Graduate Aeronautical Laboratories
California Institute of Technology
Pasadena, CA

The turbulence problem has been around a long time—since the latter part of the 19th century. Toward the end of his large book on Hydrodynamics, Edition 3, 1906, Lamb opens the section on "Turbulent Motion" with the statement, "It remains to call attention to the chief outstanding difficulty of our subject." Since then the importance has not diminished and the "difficulty" continues to get unprecedented attention. Because of its importance as an "unsolved problem" of physics and an ongoing problem for engineering, ideas about its solution and support for its clarification continue to develop. But just what is the "turbulence problem", or problems, and what might be the "solution", or solutions? This talk explores those questions in the historical background of the various developments, ideas and characters that have participated.

Wednesday, April 18, 2007
3:30 PM
Gerontology Auditorium

Distributed and Networked Systems: An Overview, and New Directions

Demetri Spanos

Graduate Student 
California Institute of Technology
Pasadena, CA

This talk will address some of the main themes in distributed and networked systems engineering, as well as some specific research results focused on a modeling and design framework for certain distributed systems.

We will begin with a (very) short history, and discuss why this field has experienced a revival in academia, industry, and government spending over the last decade. We will also present some less traditional views on future applications of distributed systems engineering, especially in the design of aerospace, mechanical, structural, and other "physically embedded" systems.

The latter part of the presentation will address a collection of recent research results, a modeling and design framework based on the concept of a "distributed gradient". As an application, we will discuss a widely studied architecture for control based on spatially averaged quantities, and show this to be a special case of the distributed gradient framework. If time permits, we will also discuss a wide variety of novel systems following from the principles of a distributed gradient system.

Monday, April 23, 2007
3:30 PM
Seaver Science Library, Room 150 (SSL 150)

Constrained Variation in Multiscale Simulations of Micro- and Nano-Fluidics and Subgrid-Scale Stress Model of Fluid Turbulence

Shiyi Chen

Alonzo G. Decker Jr. Chair in Engineering and Science 
Department of Mechanical Engineering
The Johns Hopkins University
Baltimore, MD

Finding physically consistent solutions in multiscale methods is crucial for various multiscale modeling and simulations. A framework for continuum and molecular dynamics hybrid multiscale method has been recently developed to simulate micro- and nano-fluid flows. In this approach, the continuum Navier-Stokes equation is used in one flow region and atomistic molecular dynamics in another. The spatial coupling between two methods is achieved through the constrained dynamics in an overlap region. The proposed multiscale method has been validated in simple fluid flows, including sudden-start Couette flow and channel flow with nano-scale wall roughness, showing quantitative agreement with results from analytical solutions and full molecular dynamics simulations. The hybrid method is then used to study the singularity problems in the driven cavity and moving contact lines. Following the stress over more than six decades in length in systems with characteristic scales of millimeters and milliseconds allows us to resolve the singularity and determine the force for the first time. The speedup over pure atomistic calculation is more than fourteen orders of magnitudes.

The similar idea of constrained variation has also been used for developing constrained dynamic subgrid-scale (C-SGS) stress model of fluid turbulence. In the C-SGS, we impose physical constraints in the dynamic procedure of calculating the SGS coefficients. In particular, we study dynamic mixed models with energy flux and helicity flux constraints. The comparison between the large eddy simulation results in steady and decaying isotropic turbulence using constrained and non-constrained SGS models and those from direct numerical simulation (DNS) will be presented. It is found that the C-SGS not only predicts the turbulent dissipation more accurately, but also shows a strong correlation between the model stress and the real stress from a priori test, which is a desirable feature combining the advantages of dynamic Smagorinsky and traditional mixed models.

Wednesday, April 25, 2007
3:30 PM
Seaver Science Library, Room 150 (SSL 150)

Refreshments will be served at 3:15 pm.