358 Matching Results

Search Results

Advanced search parameters have been applied.

F Process: Final Report

Description: This report follows an investigation on Great Lakes Carbon with the objectives to demonstrate their ability to make and F process purify reactor carbon on a commercial scale; and construct an F process pilot plant and use it in a survey of U.S. petroleum cokes to discover additional supplies of raw coke suitable for making reactor grade carbon.
Date: March 15, 1951
Creator: Leppla, Paul Warren & Markel, R. F.
Partner: UNT Libraries Government Documents Department

Stochastic pump effect and geometric phases in dissipative and stochastic systems

Description: The success of Berry phases in quantum mechanics stimulated the study of similar phenomena in other areas of physics, including the theory of living cell locomotion and motion of patterns in nonlinear media. More recently, geometric phases have been applied to systems operating in a strongly stochastic environment, such as molecular motors. We discuss such geometric effects in purely classical dissipative stochastic systems and their role in the theory of the stochastic pump effect (SPE).
Date: January 1, 2008
Creator: Sinitsyn, Nikolai
Partner: UNT Libraries Government Documents Department

Regional Stochastic Generation of Streamflows using an Arima (1, 0, 1) Process and Disaggregation

Description: From abstract: An ARIMA (1, 0, 1) model is used to generate annual flow sequence at three sites in the Juniata River basin, Pennsylvania. The study was designed to analyze low-flow frequency characteristics of a basin. The model preserves the mean, variance, and cross-correlations of the observed station data.
Date: May 1979
Creator: Armbruster, Jeffrey T.
Partner: UNT Libraries Government Documents Department

Emergence of Complexity from Synchronization and Cooperation

Description: The dynamical origin of complexity is an object of intense debate and, up to moment of writing this manuscript, no unified approach exists as to how it should be properly addressed. This research work adopts the perspective of complexity as characterized by the emergence of non-Poisson renewal processes. In particular I introduce two new complex system models, namely the two-state stochastic clocks and the integrate-and-fire stochastic neurons, and investigate its coupled dynamics in different network topologies. Based on the foundations of renewal theory, I show how complexity, as manifested by the occurrence of non-exponential distribution of events, emerges from the interaction of the units of the system. Conclusion is made on the work's applicability to explaining the dynamics of blinking nanocrystals, neuron interaction in the human brain, and synchronization processes in complex networks.
Date: May 2008
Creator: Geneston, Elvis L.
Partner: UNT Libraries

Two-Fold Role of Randomness: A Source of Both Long-Range Correlations and Ordinary Statistical Mechanics

Description: The role of randomness as a generator of long range correlations and ordinary statistical mechanics is investigated in this Dissertation. The difficulties about the derivation of thermodynamics from mechanics are pointed out and the connection between the ordinary fluctuation-dissipation process and possible anomalous properties of statistical systems is highlighted.
Date: December 1998
Creator: Rocco, A. (Andrea)
Partner: UNT Libraries

COMPLEXITY&APPROXIMABILITY OF QUANTIFIED&STOCHASTIC CONSTRAINT SATISFACTION PROBLEMS

Description: Let D be an arbitrary (not necessarily finite) nonempty set, let C be a finite set of constant symbols denoting arbitrary elements of D, and let S and T be an arbitrary finite set of finite-arity relations on D. We denote the problem of determining the satisfiability of finite conjunctions of relations in S applied to variables (to variables and symbols in C) by SAT(S) (by SATc(S).) Here, we study simultaneously the complexity of decision, counting, maximization and approximate maximization problems, for unquantified, quantified and stochastically quantified formulas. We present simple yet general techniques to characterize simultaneously, the complexity or efficient approximability of a number of versions/variants of the problems SAT(S), Q-SAT(S), S-SAT(S),MAX-Q-SAT(S) etc., for many different such D,C ,S, T. These versions/variants include decision, counting, maximization and approximate maximization problems, for unquantified, quantified and stochastically quantified formulas. Our unified approach is based on the following two basic concepts: (i) strongly-local replacements/reductions and (ii) relational/algebraic represent ability. Some of the results extend the earlier results in [Pa85,LMP99,CF+93,CF+94O]u r techniques and results reported here also provide significant steps towards obtaining dichotomy theorems, for a number of the problems above, including the problems MAX-&-SAT( S), and MAX-S-SAT(S). The discovery of such dichotomy theorems, for unquantified formulas, has received significant recent attention in the literature [CF+93,CF+94,Cr95,KSW97]
Date: January 1, 2001
Creator: Hunt, H. B. (Harry B.); Marathe, M. V. (Madhav V.) & Stearns, R. E. (Richard E.)
Partner: UNT Libraries Government Documents Department

Accelerating DSMC data extraction.

Description: In many direct simulation Monte Carlo (DSMC) simulations, the majority of computation time is consumed after the flowfield reaches a steady state. This situation occurs when the desired output quantities are small compared to the background fluctuations. For example, gas flows in many microelectromechanical systems (MEMS) have mean speeds more than two orders of magnitude smaller than the thermal speeds of the molecules themselves. The current solution to this problem is to collect sufficient samples to achieve the desired resolution. This can be an arduous process because the error is inversely proportional to the square root of the number of samples so we must, for example, quadruple the samples to cut the error in half. This work is intended to improve this situation by employing more advanced techniques, from fields other than solely statistics, for determining the output quantities. Our strategy centers on exploiting information neglected by current techniques, which collect moments in each cell without regard to one another, values in neighboring cells, nor their evolution in time. Unlike many previous acceleration techniques that modify the method itself, the techniques examined in this work strictly post-process so they may be applied to any DSMC code without affecting its fidelity or generality. Many potential methods are drawn from successful applications in a diverse range of areas, from ultrasound imaging to financial market analysis. The most promising methods exploit relationships between variables in space, which always exist in DSMC due to the absence of shocks. Disparate techniques were shown to produce similar error reductions, suggesting that the results shown in this report may be typical of what is possible using these methods. Sample count reduction factors of approximately three to five were found to be typical, although factors exceeding ten were shown on some variables under some techniques.
Date: October 1, 2006
Creator: Gallis, Michail A. & Piekos, Edward Stanley
Partner: UNT Libraries Government Documents Department

Dynamics of density fluctuations in a non-Markovian Boltzmann- Langevin model

Description: In the course of the past few years, the nuclear Boltzmann-Langevin (BL)model has emerged as a promising microscopic model for nuclear dynamics at intermediate energies. The BL model goes beyond the much employed Boltzmann-Uehling-Uhlenbeck (BUU) model, and hence it provides a basis for describing dynamics of density fluctuations and addressing processes exhibiting spontaneous symmetry breaking and catastrophic transformations in nuclear collisions, such as induced fission and multifragmentation. In these standard models, the collision term is treated in a Markovian approximation by assuming that two-body collisions are local in both space and time, in accordance with Boltzmann`s original treatment. This simplification is usually justified by the fact that the duration of a two-body collision is short on the time scale characteristic of the macroscopic evolution of the system. As a result, transport properties of the collective motion has then a classical character. However, when the system possesses fast collective modes with characteristic energies that are not small in comparision with the temperature, then the quantum-statistical effects are important and the standard Markovian treatment is inadequate. In this case, it is necessary to improve the one-body transport model by including the memory effect due to the finite duration of two-body collisions. First we briefly describe the non-Markovian extension of the BL model by including the finite memory time associated with two-body collisions. Then, using this non-Markovian model in a linear response framework, we investigate the effect of the memory time on the agitation of unstable modes in nuclear matter in the spinodal zone, and calculate the collisional relaxation rates of nuclear collective vibrations.
Date: March 1, 1996
Creator: Ayik, S.
Partner: UNT Libraries Government Documents Department

Polarization of atomic radiation in stochastic plasma fields

Description: When a laser pulse of certain polarization or an electron beam excites atoms in a plasma, the atomic spectrum of the radiation emitted by the atoms exhibits differently polarized line core and line wings. This unusual effect, which is predicted to occur under a variety of conditions, can be accompanied by the appearance of the forbidden component in the spectrum, with polarization opposite to that of the exciting laser pulse.
Date: May 12, 1997
Creator: Savchenko, V.I. & Fisch, N.J.
Partner: UNT Libraries Government Documents Department

Algorithms for fusion of multiple sensors having unknown error distributions

Description: The authors presented recent results on a general sensor fusion problem, where the underlying sensor error distributions are not known, but a sample is available. They presented a general method for obtaining a fusion rule based on scale-sensitive dimension of the function class. Two computationally viable methods are reviewed based on the Nadaraya-Watson estimator, and the finite dimensional vector spaces. Several computational issues of the fusion rule estimation are open problems. It would be interesting to obtain necessary and sufficient conditions under which polynomial-time algorithms can be used to solve the fusion rule estimation problem under the criterion. Also, conditions under which the composite system is significantly better than best sensor would be extremely useful. Finally, lower bound estimates for various sample sizes will be very important in judging the optimality of sample size estimates.
Date: June 1, 1997
Creator: Rao, N. S. V.
Partner: UNT Libraries Government Documents Department

Structural model uncertainty in stochastic simulation

Description: Prediction uncertainty in stochastic simulation models can be described by a hierarchy of components: stochastic variability at the lowest level, input and parameter uncertainty at a higher level, and structural model uncertainty at the top. It is argued that a usual paradigm for analysis of input uncertainty is not suitable for application to structural model uncertainty. An approach more likely to produce an acceptable methodology for analyzing structural model uncertainty is one that uses characteristics specific to the particular family of models.
Date: September 1, 1997
Creator: McKay, M.D. & Morrison, J.D.
Partner: UNT Libraries Government Documents Department

Linear kinetic theory and particle transport in stochastic mixtures. Third year and final report, June 15, 1993--December 14, 1996

Description: The goal in this research was to continue the development of a comprehensive theory of linear transport/kinetic theory in a stochastic mixture of solids and immiscible fluids. Such a theory should predict the ensemble average and higher moments, such as the variance, of the particle or energy density described by the underlying transport/kinetic equation. The statistics studied correspond to N-state discrete random variables for the interaction coefficients and sources, with N denoting the number of components in the mixture. The mixing statistics considered were Markovian as well as more general statistics. In the absence of time dependence and scattering, the theory is well developed and described exactly by the master (Liouville) equation for Markovian mixing, and by renewal equations for non-Markovian mixing. The intent of this research was to generalize these treatments to include both time dependence and scattering. A further goal of this research was to develop approximate, but simpler, models from any comprehensive theory. In particular, a specific goal was to formulate a renormalized transport/kinetic theory of the usual nonstochastic form, but with effective interaction coefficients and sources to account for the stochastic nature of the problem. In the three and one-half year period of research summarized in this final report, they have made substantial progress in the development of a comprehensive theory of kinetic processes in stochastic mixtures. This progress is summarized in 16 archival journal articles, 7 published proceedings papers, and 2 comprehensive review articles. In addition, 17 oral presentations were made describing these research results.
Date: May 1, 1997
Creator: Pomraning, G.C.
Partner: UNT Libraries Government Documents Department

Microscopic modeling of travel-demand: Approaching the home-to-work problem

Description: In this article the results are described that have been found when tackling the problem of the assignment of employees to their working places (destination assignment) by using a truly microscopic approach, whose output is suitable for a microscopic traffic simulation. This problem is dealt with a microscopic stochastic analogue of the gravity ansatz of transportation planning, described in this article. However, the computation of the travel destinations is only the first step in a sequence of simulation steps. Its output will be used to compute a simulation-based dynamic traffic assignment (route assignment), resulting in travel times needed from home to work for any traveler. Those travel times will be used in a further reassignment step, where any traveler whose travel-time has exceeded a certain limit is subject to re-assigning a new working place (destination). This creates a sequence of re-assignment and re-routing processes, whose results will be reported in this article. The results obtained show that approach presented in this article is capable of describing the destination and route choices microscopically.
Date: December 31, 1998
Creator: Wagner, P. & Nagel, K.
Partner: UNT Libraries Government Documents Department

The roles of shear and cross-correlations on the fluctuation levels in simple stochastic models. Revision

Description: Highly simplified models of random flows interacting with background microturbulence are analyzed. In the limit of very rapid velocity fluctuations, it is shown rigorously that the fluctuation level of a passively advected scalar is not controlled by the rms shear. In a model with random velocities dependent only on time, the level of cross-correlations between the flows and the background turbulence regulates the saturation level. This effect is illustrated by considering a simple stochastic-oscillator model, both exactly and with analysis and numerical solutions of the direct-interaction approximation. Implications for the understanding of self-consistent turbulence are discussed briefly.
Date: November 3, 1999
Creator: Krommes, J.A.
Partner: UNT Libraries Government Documents Department

Self-organized criticality, long-time correlations, and the standard transport paradigm

Description: Some aspects of low-frequency, long-wavelength fluctuations are considered. A stochastic model is used to show that power-law time correlations need not arise from self-organized criticality. A formula for the frequency spectrum of uncorrelated, overlapping avalanches is shown to be a special case of the spectral balance equation of renormalized statistical turbulence theory. It is argued that there need be no contradiction between the presence of long-time correlations and the existence of local transport coefficients.
Date: February 11, 2000
Creator: Krommes, J.A.
Partner: UNT Libraries Government Documents Department

Final Report: Climate Variability, Stochasticity and Learning in Integrated Assessment Models, September 15, 1996 - September 14, 1999

Description: The focus of the work has been on climate variability and learning within computational climate-economy models (integrated assessment models--IAM's). The primary objective of the research is to improve the representation of learning in IAM's. This include's both endogenous and exogenous learning. A particular focus is on Bayesian learning about climate damage. A secondary objective is to improve the representation of climate variability within IAM's.
Date: September 14, 1999
Creator: Kolstad, Charles D.
Partner: UNT Libraries Government Documents Department

Long-time tails do not necessarily imply self-organized criticality or the breakdown of the standard transport paradigm

Description: Numerical measurements and analytical studies are performed on a stochastic model with features relevant to plasma confinement. Although the model lacks crucial features of self-organized criticality (SOC) and its transport can be computed by standard techniques, it nevertheless exhibits intermittency and algebraic time correlations. This suggests that SOC need not be the explanation for observed long-time tails in experimental fluctuation data. Arguments based on the renormalized spectral balance equation, and simulation of a standard nonlinear paradigm, predict a range of Hurst exponents in reasonable agreement with the observations without invoking submarginal dynamics.
Date: January 18, 2000
Creator: Krommes, J.A. & Ottaviani, M.
Partner: UNT Libraries Government Documents Department

Stochastic particle acceleration by plasma waves in AGN jets

Description: The free energy stored in the stressed magnetic fields in AGN jets could be dissipated via generating turbulent plasma waves. The authors review several key wave-particle resonant interactions and point out the importance of a broad wave spectrum. Under several idealized assumptions, they show that the transit-time damping process can accelerate electrons to TeV energies in an AGN jet environment, and present a preliminary calculation on the evolution of plasma wave, electron, and photon distributions. The authors especially emphasize several open questions on particle acceleration by waves, and argue that a plausible scenario is to energize electrons out of the thermal background via transit-time damping and further accelerate them by the parallel propagating right-handed waves.
Date: November 1, 1997
Creator: Li, Hui; Colgate, S.A. & Miller, J.A.
Partner: UNT Libraries Government Documents Department

Monte Carlo solution of a semi-discrete transport equation

Description: The authors present the S{sub {infinity}} method, a hybrid neutron transport method in which Monte Carlo particles traverse discrete space. The goal of any deterministic/stochastic hybrid method is to couple selected characters from each of the methods in hopes of producing a better method. The S{sub {infinity}} method has the features of the lumped, linear-discontinuous (LLD) spatial discretization, yet it has no ray-effects because of the continuous angular variable. They derive the S{sub {infinity}} method for the solid-state, mono-energetic transport equation in one-dimensional slab geometry with isotropic scattering and an isotropic internal source. They demonstrate the viability of the S{sub {infinity}} method by comparing their results favorably to analytic and deterministic results.
Date: September 1, 1999
Creator: Urbatsch, T.J.; Morel, J.E. & Gulick, J.C.
Partner: UNT Libraries Government Documents Department