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Simulational studies of the Farley-Buneman in the equatorial electrojet

Description: The Farley-Buneman instability in the equatorial electrojet current system in the E-region of the ionosphere has been identified as the cause of the observed Type I electron density irregularities. The goal of this work was to study the instability in the equatorial region.
Date: July 1, 1995
Creator: Otani, N.; Seyler, C. & Kelley, M.
Partner: UNT Libraries Government Documents Department

Self-Consistent Multiscale Theory of Internal Wave, Mean-Flow Interactions

Description: This is the final report of a three-year, Laboratory Directed Research and Development (LDRD) project at Los Alamos National Laboratory (LANL). The research reported here produced new effective ways to solve multiscale problems in nonlinear fluid dynamics, such as turbulent flow and global ocean circulation. This was accomplished by first developing new methods for averaging over random or rapidly varying phases in nonlinear systems at multiple scales. We then used these methods to derive new equations for analyzing the mean behavior of fluctuation processes coupled self consistently to nonlinear fluid dynamics. This project extends a technology base relevant to a variety of multiscale problems in fluid dynamics of interest to the Laboratory and applies this technology to those problems. The project's theoretical and mathematical developments also help advance our understanding of the scientific principles underlying the control of complex behavior in fluid dynamical systems with strong spatial and temporal internal variability.
Date: June 3, 1999
Creator: Holm, D.D.; Aceves, A.; Allen, J.S.; Alber, M.; Camassa, R.; Cendra, H. et al.
Partner: UNT Libraries Government Documents Department

An Improved Linear Tetrahedral Element for Plasticity

Description: A stabilized, nodally integrated linear tetrahedral is formulated and analyzed. It is well known that linear tetrahedral elements perform poorly in problems with plasticity, nearly incompressible materials, and acute bending. For a variety of reasons, linear tetrahedral elements are preferable to quadratic tetrahedral elements in most nonlinear problems. Whereas, mixed methods work well for linear hexahedral elements, they don't for linear tetrahedrals. On the other hand, automatic mesh generation is typically not feasible for building many 3D hexahedral meshes. A stabilized, nodally integrated linear tetrahedral is developed and shown to perform very well in problems with plasticity, nearly incompressible materials and acute bending. Furthermore, the formulation is analytically and numerically shown to be stable and optimally convergent. The element is demonstrated to perform well in several standard linear and nonlinear benchmarks.
Date: April 25, 2005
Creator: Puso, M
Partner: UNT Libraries Government Documents Department

Use of response surface metamodels for damage identification of a simple nonlinear system.

Description: The need for low order models capable of performing damage identification has become apparent in many structural dynamics applications where structural health monitoring and damage prognosis programs are implemented. These programs require that damage identification routines have low computational requirements and be reliable with some quantifiable degree of accuracy. Response surface metamodels (RSMs) are proposed to fill this need. Popular in the fields of chemical and industrial engineering, RSMs have only recently been applied in the field of structural dynamics and to date there have been no studies which fully demonstrate the potential of these methods. In this thesis, several RSMs are developed in order to demonstrate the potential of the methodology. They are shown to be robust to noise (experimental variability) and have success in solving the damage identification problem, both locating and quantifying damage with some degree of accuracy, for both linear and nonlinear systems. A very important characteristic of the RSMs developed in this thesis is that they require very little information about the system in order to generate relationships between damage indicators and measureable system responses for both linear and nonlinear structures. As such, the potential of these methods for damage identification has been demonstrated and it is recommended that these methods be developed further.
Date: January 1, 2003
Creator: Cundy, A. L. (Amanda L.); Hemez, F. M. (Fran├žois M.); Inman, D. J. & Park, G. H. (Gyu Hae)
Partner: UNT Libraries Government Documents Department

Nonlinear Dynamics of Parametrically Excited Gyroscopic Systems

Description: The primary objective of this project is to determine how some of the powerful geometric methods of dynamical systems can be applied to study nonlinear gyroscopic systems. We proposed to develop techniques to predict local and global behavior and instability mechanisms and to analyze the interactions between noise, stability, and nonlinearities inherent in gyroscopic systems. In order to obtain these results we use the method of normal forms, global bifurcation techniques, and various other dynamical systems tools.
Date: June 1, 2001
Creator: Namachchivaya, N. S.
Partner: UNT Libraries Government Documents Department

Ultrafast and nonlinear optical characterization of optical limiting processes in fullerenes

Description: The authors present recent results of broadband femotosecond (fs) transient absorption (TA) and broadband nanosecond (ns) optical limiting (OL) studies of C{sub 60} and derivatized C{sub 60}. Improvements in measurement techniques for fs TA spectra provide sensitivity to 10{sup {minus}5} in differential transmission, allowing detailed comparison of excited-state spectra with established energy level diagrams, as well as comparison of the ratio of triplet to singlet excited-state absorption cross sections from TA spectra with those obtained by modeling time transients at different wavelengths. For derivatized fullerenes, which provide enhanced solubility and a ground-state absorption extended into the infrared compared with C{sub 60} there is no spectral region where the triplet absorption cross section dominates the singlet as strongly as demonstrating broadband limiting in all 6, 6 mono-adducts and neat C{sub 60}. The authors report new approaches to processing sol-gel encapsulated fullerenes to improve the OL performance of solid-state materials to approach the response of solution limiters.
Date: October 1, 1997
Creator: Kohlman, R.; Klimov, V. & Shi, X.
Partner: UNT Libraries Government Documents Department

Unified treatment of collective instabilities and nonlinear beam dynamics

Description: Nonlinear dynamics deals with parametric resonances and diffusion, which are usually beam-intensity independent and rely on a particle Hamiltonian. Collective instabilities deal with beam coherent motion, where the Vlasov equa-tion is frequently used in conjunction with a beam-intensity dependent Hamiltonian. We address the questions: Are the two descriptions the same? Are collective instabilities the results of encountering parametric resonances whose driv-ing force is intensity dependent? The space-charge domi-nated beam governed by the Kapchinskij-Vladimirskij (K-V) envelope equation [1] is used as an example.
Date: April 19, 1999
Creator: Lee, K.Y. Ng and S.Y.
Partner: UNT Libraries Government Documents Department

Annual progress report

Description: The nonlinear analysis of plasma instabilities in the threshold regime is discussed. The emphasis remains on carrying out calculations in realistic geometry and making comparison with experiment. Attention has shifted entirely to fully ionized plasma. An analysis was made of the collisional drift-wave instability. Considerable progress was made in explaining the explosive nature of the mirror-flute instability and in understanding the flute instability in the presence of both density and temperature gradients. The application of the threshold method to parametric instabilities in nonuniform plasma is discussed. Details of the results and comparison with experiment are given. (auth)
Date: September 18, 1973
Creator: Simon, A.
Partner: UNT Libraries Government Documents Department

Automatic differentiation tools in optimization software.

Description: The authors discuss the role of automatic differentiation tools in optimization software. We emphasize issues that are important to large-scale optimization and that have proved useful in the installation of nonlinear solvers in the NEOS Server. Our discussion centers on the computation of the gradient and Hessian matrix for partially separable functions and shows that the gradient and Hessian matrix can be computed with guaranteed bounds in time and memory requirements.
Date: January 15, 2001
Creator: More, J. J.
Partner: UNT Libraries Government Documents Department

Test-Analysis Correlation and Finite Element Model Updating for Nonlinear Transient Dynamics

Description: This research aims at formulating criteria for measuring the correlation between test data and finite element results for nonlinear, transient dynamics. After reviewing the linear case and illustrating the limitations of modal-based updating when it is applied to nonlinear experimental data, simple time-domain, test-analysis correlation metrics are proposed. Two implementations are compared: the conventional least-squares technique and the Principal Component Decomposition that correlates subspaces rather than individual time-domain responses. Illustrations and discussions are provided using the LANL 8-DOF system, an experimental testbed for validating nonlinear data correlation and model updating techniques.
Date: February 8, 1999
Creator: Hemez, F. M. & Doebling, S. W.
Partner: UNT Libraries Government Documents Department

COPS: Large-scale nonlinearly constrained optimization problems

Description: The authors have started the development of COPS, a collection of large-scale nonlinearly Constrained Optimization Problems. The primary purpose of this collection is to provide difficult test cases for optimization software. Problems in the current version of the collection come from fluid dynamics, population dynamics, optimal design, and optimal control. For each problem they provide a short description of the problem, notes on the formulation of the problem, and results of computational experiments with general optimization solvers. They currently have results for DONLP2, LANCELOT, MINOS, SNOPT, and LOQO.
Date: February 10, 2000
Creator: Bondarenko, A.S.; Bortz, D.M. & More, J.J.
Partner: UNT Libraries Government Documents Department

Final Technical Report

Description: The Final Technical Report summarizes research accomplishments and Publications in the period of 5/1/99 to 4/30/03 done on the grant. Extensive progress was made in the period covered by this report in the areas of chemical kinetics of non-linear systems; spatial structures, reaction - diffusion systems, and thermodynamic and stochastic theory of electrochemical and general systems.
Date: April 30, 2003
Creator: Ross, John
Partner: UNT Libraries Government Documents Department

Studies of impurity mode and ITG mode in toroidal plasmas

Description: The impurity mode and {eta}{sub i} mode driven by impurity ions with outwardly peaked density profiles, just as it is at the boundary of tokamak plasmas, and the ion temperature gradient, respectively, are studied in high temperature toroidal plasmas. The gyrokinetic theory is applied and finite Larmor radius effects of both hydrogenic and impurity ions are included. It is found that the impurity mode is enhanced by the ion temperature gradient. In addition, the impurity ions with outwardly peaked density profiles are demonstrated to have destabilizing effects on the {eta}{sub i} mode. These two modes are strongly coupled to each other so that it is impossible to distinguish between them when both the driving mechanisms axe strong enough to drive the corresponding mode unstable independently. The correlation of the results with nonlinear simulations and the experimental observations are discussed.
Date: April 1, 1995
Creator: Dong, J.Q. & Horton, W.
Partner: UNT Libraries Government Documents Department

Variational particle scheme for the porous medium equation and for the system of isentropic Euler equations

Description: We introduce variational particle schemes for the porous medium equation and the system of isentropic Euler equations in one space dimension. The methods are motivated by the interpretation of each of these partial differential equations as a 'steepest descent' on a suitable abstract manifold. We show that our methods capture very well the nonlinear features of the flows.
Date: December 10, 2008
Creator: Westdickenberg, Michael & Wilkening, Jon
Partner: UNT Libraries Government Documents Department


Description: The existence of multipole components in the dipole and quadrupole magnets is one of the factors limiting the beam stability in the RHIC operations. So, a realistic non-linear model is crucial for understanding the beam behavior and to achieve the ultimate performance in RHIC. A procedure is developed to build a non-linear model using the available multipole component data obtained from measurements of RHIC magnets. We first discuss the measurements performed at different stages of manufacturing of the magnets in relation to their current state in RHIC. We then describe the procedure to implement these measurement data into tracking models, including the implementation of the multipole feed down effect due to the beam orbit offset from the magnet center. Finally, the field quality analysis in the RHIC interaction regions (IR) is presented.
Date: June 25, 2007
Creator: BEEBE-WANG,J. & JAIN, A.
Partner: UNT Libraries Government Documents Department

Modeling mesoscopic phenomena in extended dynamical systems

Description: This is the final report of a three-year, Laboratory-Directed Research and Development project at the Los Alamos National Laboratory (LANL). We have obtained classes of nonlinear solutions on curved geometries that demonstrate a novel interplay between topology and geometric frustration relevant for nanoscale systems. We have analyzed the nature and stability of localized oscillatory nonlinear excitations (multi-phonon bound states) on discrete nonlinear chains, including demonstrations of successful perturbation theories, existence of quasiperiodic excitations, response to external statistical time-dependent fields and point impurities, robustness in the presence of quantum fluctuations, and effects of boundary conditions. We have demonstrated multi-timescale effects for nonlinear Schroedinger descriptions and shown the success of memory function approaches for going beyond these approximations. In addition we have developed a generalized rate-equation framework that allows analysis of the important creation/annihilation processes in driven nonlinear, nonequilibiium systems.
Date: August 1, 1997
Creator: Bishop, A.; Lomdahl, P.; Jensen, N.G.; Cai, D.S.; Mertenz, F.; Konno, Hidetoshi et al.
Partner: UNT Libraries Government Documents Department

Preconditioning Newton-Krylor Methods for Variably Saturated Flow

Description: In this paper, we compare the effectiveness of three preconditioning strategies in simulations of variably saturated flow. Using Richards' equation as our model, we solve the nonlinear system using a Newton-Krylov method. Since Krylov solvers can stagnate, resulting in slow convergence, we investigate different strategies of preconditioning the Jacobian system. Our work uses a multigrid method to solve the preconditioning systems, with three different approximations to the Jacobian matrix. One approximation lags the nonlinearities, the second results from discarding selected off-diagonal contributions, and the third matrix considered is the full Jacobian. Results indicate that although the Jacobian is more accurate, its usage as a preconditioning matrix should be limited, as it requires much more storage than the simpler approximations. Also, simply lagging the nonlinearities gives a preconditioning matrix that is almost as effective as the full Jacobian but much easier to compute.
Date: January 7, 2000
Creator: Woodward, C. & Jones, J
Partner: UNT Libraries Government Documents Department

Time Series Based Model Updating in Nonlinear Systems using Singular Value Decomposition

Description: The problem considered is the use of time series data to do model updating in nonlinear structural systems for which the mathematical form of the system nonlinearities is known ahead of time. This work is a departure from most classical model updating work, which utilizes model data to update linear structural dynamics models. In the present application a singular value decomposition (SVD) of the measured data (e.g., m of the N coordinates are measured at n sampling times) is the basis of the updating. The SVD produces a representation of the data as a linear combination of the so-called principal components, which are analogous to modal coordinate time histories in a linear system. The structural dynamics model parameters are updated by minimizing the differences in the SVD's of the experimental data and the model simulations. This method, proposed by Hasselman et al (IMAC 1998), has been applied to both simulated and actual experimental data for low degree of freedom spring-mass systems with cubic nonlinearity and light damping. The main results that will be presented are the following: (1) the SVD updating is robust in the presence of noise, (2) SVD based updating is effective for both linear and nonlinear systems, and (3) in some cases the nonlinear updating problem is actually easier to do than the linear problem because of the additional ''information'' contained in the harmonics produced by the nonlinearity. A possible limitation of the approach is the computing time needed to do the parameter optimization.
Date: June 27, 1999
Creator: Hemez, F.M.; Beardsley, P.; Rhee, W. & Burton, T.D.
Partner: UNT Libraries Government Documents Department