Conferences, Lectures, & Seminars
Events for January
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Stress-Induced Martensitic Phase Transformation and Fracture
Wed, Jan 10, 2007 @ 03:30 PM - 04:30 PM
Aerospace and Mechanical Engineering
Conferences, Lectures, & Seminars
Samantha DalyCalifornia Institute of TechnologyAbstractNickel-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.
Location: Seaver Science Library (SSL), Rm 150
Audiences: Everyone Is Invited
Contact: April Mundy
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DSMC modeling of near-continuum flows
Tue, Jan 16, 2007 @ 11:00 AM - 12:00 PM
Aerospace and Mechanical Engineering
Conferences, Lectures, & Seminars
Ye.A. BondarInstitute of Theoretical and Applied Mechanics, Novosibirsk 630090, RussiaThe 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.
Location: Laufer Library, RRB 207
Audiences: Everyone Is Invited
Contact: April Mundy
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Characterization and Yield behavior of UFG, Nano-Twinned Copper
Wed, Jan 17, 2007 @ 03:30 PM - 04:30 PM
Aerospace and Mechanical Engineering
Conferences, Lectures, & Seminars
Andrea M. Hodge Materials ScientistNanoscale Synthesis and Characterization
LaboratoryLawrence 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.Location: Seaver Science Library, Room 150
Audiences: Everyone Is Invited
Contact: April Mundy
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In Search of Fast and Robust Adaptation
Wed, Jan 24, 2007 @ 03:30 PM - 04:30 PM
Aerospace and Mechanical Engineering
Conferences, Lectures, & Seminars
Dr. Naira HovakimyanDepartment of Aerospace and Ocean EngineeringVirginia Polytechnic Institute and State UniversityAbstract: 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.
Location: Seaver Science Library, Room 150
Audiences: Everyone Is Invited
Contact: April Mundy
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Optimization in Complex Fluid Mechanics Problems Using the Surrogate Management Framework
Wed, Jan 31, 2007 @ 03:30 PM - 04:30 PM
Aerospace and Mechanical Engineering
Conferences, Lectures, & Seminars
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.
Location: Seaver Science Library (SSL) Rm 150
Audiences: Everyone Is Invited
Contact: April Mundy