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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 721 records · Page 40
All sourcesNASA NTRSDOE OSTI
NASA NTRS2004

Spectroscopic Determination of the Physical Conditions in Hot Optically Thin Sources

Astrophysical Plasma Emission Database - APED consists of atomic data, primarily theoretical, needed to calculate X-ray through optical emission spectra of hot thermal plasmas. These data are supplemented by experimental values, such as laboratory plasma wave- lengths, and are validated by experiment only rarely. Thus the comparison of predicted spectra to astrophysical spectra serves as invaluable feedback on the quality of the models. Following up on unfavorable comparisons, we have communicated atomic data needs to atomic theorists and experimental plasma physicists so that improvements can be made to the database. The database is now reasonably complete from approx. 1 to 30 A, and from - 90 to 180 A, with significant gaps remaining in the range 30 to 90 A. Astrophysical Plasma Emission Code - APEC uses the APED data as inputs and computes level populations and line emissivities using a rate matrix solver. Tabulated models for a fine grid of temperatures and densities can be applied to observational data. The current public version of APEC output uses an input ionization balance model, assuming CIE; however, we have a working version to calculate the ionization state of the gas. We will begin testing non-equilibrium ionization (NEI) models over the next year - we are now finalizing the atomic database needed to perform the ionization state calculations. At that time, the code will be made public. We have developed optimized error codes, additional testing protocols and substantial documentation, in preparation for this public release. Secondarily, we have begun to investigate the use of APED and APEC for X-ray photoionized plasma, beginning with opacity modeling (e.g. warm absorbers). Application to bright AGN Chandra grating spectra suggests that the accurate wavelengths in APED are of great importance to fitting the data. We are exploring additional subroutines to APEC to extend its usefulness for X-ray photoionized plasma, e.g. to include collisional or photo-excitation of metastable levels for absorption.

Oliversen, Ronald J.↗
NASA NTRS1999

Optimal Frequency-Domain System Realization with Weighting

Several approaches are presented to identify an experimental system model directly from frequency response data. The formulation uses a matrix-fraction description as the model structure. Frequency weighting such as exponential weighting is introduced to solve a weighted least-squares problem to obtain the coefficient matrices for the matrix-fraction description. A multi-variable state-space model can then be formed using the coefficient matrices of the matrix-fraction description. Three different approaches are introduced to fine-tune the model using nonlinear programming methods to minimize the desired cost function. The first method uses an eigenvalue assignment technique to reassign a subset of system poles to improve the identified model. The second method deals with the model in the real Schur or modal form, reassigns a subset of system poles, and adjusts the columns (rows) of the input (output) influence matrix using a nonlinear optimizer. The third method also optimizes a subset of poles, but the input and output influence matrices are refined at every optimization step through least-squares procedures.

Juang, Jer-Nan↗
NASA NTRS1988

Continuous fiber ceramic matrix composites for heat engine components

High strength at elevated temperatures, low density, resistance to wear, and abundance of nonstrategic raw materials make structural ceramics attractive for advanced heat engine applications. Unfortunately, ceramics have a low fracture toughness and fail catastrophically because of overload, impact, and contact stresses. Ceramic matrix composites provide the means to achieve improved fracture toughness while retaining desirable characteristics, such as high strength and low density. Materials scientists and engineers are trying to develop the ideal fibers and matrices to achieve the optimum ceramic matrix composite properties. A need exists for the development of failure models for the design of ceramic matrix composite heat engine components. Phenomenological failure models are currently the most frequently used in industry, but they are deterministic and do not adequately describe ceramic matrix composite behavior. Semi-empirical models were proposed, which relate the failure of notched composite laminates to the stress a characteristic distance away from the notch. Shear lag models describe composite failure modes at the micromechanics level. The enhanced matrix cracking stress occurs at the same applied stress level predicted by the two models of steady state cracking. Finally, statistical models take into consideration the distribution in composite failure strength. The intent is to develop these models into computer algorithms for the failure analysis of ceramic matrix composites under monotonically increasing loads. The algorithms will be included in a postprocessor to general purpose finite element programs.

Tripp, David E.↗
DOE OSTI2015

Mueller-matrix mapping of biological tissues in differential diagnosis of optical anisotropy mechanisms of protein networks

We report a model of Mueller-matrix description of optical anisotropy of protein networks in biological tissues with allowance for the linear birefringence and dichroism. The model is used to construct the reconstruction algorithms of coordinate distributions of phase shifts and the linear dichroism coefficient. In the statistical analysis of such distributions, we have found the objective criteria of differentiation between benign and malignant tissues of the female reproductive system. From the standpoint of evidence-based medicine, we have determined the operating characteristics (sensitivity, specificity and accuracy) of the Mueller-matrix reconstruction method of optical anisotropy parameters and demonstrated its effectiveness in the differentiation of benign and malignant tumours. (laser applications and other topics in quantum electronics)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS1992

The effects of uneven fiber spacing on thermal residual stresses in a unidirectional SCS-6Ti-15-3 laminate

High residual stresses develop in SCS-6/Ti-15-3 composites during cooldown from the fabrication temperature; these residual stresses can effect the mechanical and physical properties of the composite. Discrete fiber-matrix finite element models were used to study the residual stresses due to the temperature change during the fabrication process, including the effects of uneven fiber spacing, the free surface, and increased fiber volume fractions. To accurately model the effects of the free surface, it is only necessary to model one fiber through the thickness. Below the first ply, the analysis predicts stress distributions that are identical to the infinite array predictions. For uneven fiber space less then 0.042 mm in an interior ply, the maximum hoop stress was predicted to occur between fibers within a ply and to increase as the fiber spacing decreased. The maximum hoop stress correlated well with the observed radial cracking between fibers. For the case of touching fibers, the analysis predicted tensile radial stresses at the fiber-matrix debonding during the fabrication cooldown. Identical trends were predicted for uneven fiber spacing in surface plies with slightly greater values of maximum stresses. The analysis predicted matrix yielding to occur upon cooldown when the edge-to-edge fiber spacing was less than or equal to 0.022 mm. The stress distributions predicted for increasing fiber volume fractions were similar to those predicted for decreasing the fiber spacing for two adjacent fibers within a ply.

Bigelow, C. A.↗
NASA NTRS1992

The effects of uneven fiber spacing on thermal residual stresses in a unidirectional SCS-6/Ti-15-3 laminate

High residual stresses develop in SCS-6/Ti-15-3 composites during cooldown from the fabrication temperature; these residual stresses can effect the mechanical and physical properties of the composite. Discrete fiber-matrix finite element models were used to study the residual stresses due to the temperature change during the fabrication process, including the effects of uneven fiber spacing, the free surface, and increased fiber volume fractions. To accurately model the effects of the free surface, it is only necessary to model one fiber through the thickness. Below the first ply, the analysis predicts stress distributions that are identical to the infinite array predictions. For uneven fiber space less than 0.042 mm in an interior ply, the maximum hoop stress was predicted to occur between fibers within a ply and to increase as the fiber spacing decreased. The maximum hoop stress correlated well with the observed radial cracking between fibers. For the case of touching fibers, the analysis predicted tensile radial stresses at the fiber-matrix debonding during the fabrication cooldown. Identical trends were predicted for uneven fiber spacing in surface plies with slightly greater values of maximum stresses. The analysis predicted matrix yielding to occur upon cooldown when the edge-to-edge fiber spacing was less than or equal to 0.022 mm. The stress distributions preidcted for increasing fiber volume fractions were similar to those predicted for decreasing the fiber spacing for two adjacent fibers within a ply.

Bigelow, Catherine A.↗
DOE OSTI2014

Coupled-cavity terahertz quantum cascade lasers for single mode operation

We demonstrate the operation of coupled-cavity terahertz frequency quantum-cascade lasers composed of two sub-cavities separated by an air gap realized by optical lithography and dry etching. This geometry allows stable, single mode operation with typical side mode suppression ratios in the 30–40 dB range. We employ a transfer matrix method to model the mode selection mechanism. The obtained results are in good agreement with the measurements and allow prediction of the operating frequency.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Strong thermal leptogenesis and the absolute neutrino mass scale

We show that successful strong thermal leptogenesis, where the final asymmetry is independent of the initial conditions and in particular a large pre-existing asymmetry is efficiently washed-out, favours values of the lightest neutrino mass m{sub 1}∼>10 meV for normal ordering (NO) and m{sub 1}∼>3 meV for inverted ordering (IO) for models with orthogonal matrix entries respecting |Ω{sub ij}{sup 2}|∼<2. We show analytically why lower values of m{sub 1} require a higher level of fine tuning in the seesaw formula and/or in the flavoured decay parameters (in the electronic for NO, in the muonic for IO). We also show how this constraint exists thanks to the measured values of the neutrino mixing angles and could be tightened by a future determination of the Dirac phase. Our analysis also allows us to place a more stringent constraint for a specific model or class of models, such as SO(10)-inspired models, and shows that some models cannot realise strong thermal leptogenesis for any value of m{sub 1}. A scatter plot analysis fully supports the analytical results. We also briefly discuss the interplay with absolute neutrino mass scale experiments concluding that they will be able in the coming years to either corner strong thermal leptogenesis or find positive signals pointing to a non-vanishing m{sub 1}. Since the constraint is much stronger for NO than for IO, it is very important that new data from planned neutrino oscillation experiments will be able to solve the ambiguity.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2020

A Monte Carlo Approach to Approximating the Effects of Pore Geometry on the Phase Behavior of Soil Freezing

Abstract Freezing in porous media is associated with a host of dynamic phenomena that stem from the presence and mobility of premelted liquid at subzero temperatures. Accurate assessments of the progressive liquid‐ice phase transition is required for predictive models of frost damage, glacier‐till coupling, and many other cold regions processes, as well as for evaluating the capacity for water storage in near‐surface extraterrestrial environments. We use a Monte Carlo approach to sample the pore space in a synthetic 3D packing of poly‐dispersed spherical particles and evaluate local geometrical constraints that allow us to assess changes in the relative proportions of pore fluid and ice. By approximating the phase boundary geometry in fine‐grained pores while considering both the curvature of the liquid‐ice interface and wetting interactions with matrix particles, our model predicts changes in phase equilibrium in granular media over a broad temperature range, where present accounting for the colligative effects of chloride and perchlorate solutes. In addition to formulating the constitutive behavior needed to better understand properties and processes in frozen soils, our results also provide insight into other aspects of phase equilibria in porous media, including the formation of methane hydrates in permafrost and marine sediments, and the partitioning between liquid water and vapor in the vadose zone.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2020

The Sylvester Graphical Lasso (SyGlasso)

This paper introduces the Sylvester graphical lasso (SyGlasso) that captures multiway dependencies present in tensor-valued data. The model is based on the Sylvester equation that defines a generative model. The proposed model complements the tensor graphical lasso that imposes a Kronecker sum model for the inverse covariance matrix by providing an alternative Kronecker sum model that is generative and interpretable. A nodewise regression approach is adopted for estimating the conditional independence relationships among variables. The statistical convergence of the method is established, and empirical studies are provided to demonstrate the recovery of meaningful conditional dependency graphs. Here we apply the SyGlasso to an electroencephalography (EEG) study to compare the brain connectivity of alcoholic and nonalcoholic subjects. Here, we demonstrate that our model can simultaneously estimate both the brain connectivity and its temporal dependencies.

Wang, Yu↗
DOE OSTI2015

Density matrix embedding in an antisymmetrized geminal power bath

Density matrix embedding theory (DMET) has emerged as a powerful tool for performing wave function-in-wave function embedding for strongly correlated systems. In traditional DMET, an accurate calculation is performed on a small impurity embedded in a mean field bath. Here, we extend the original DMET equations to account for correlation in the bath via an antisymmetrized geminal power (AGP) wave function. The resulting formalism has a number of advantages. First, it allows one to properly treat the weak correlation limit of independent pairs, which DMET is unable to do with a mean-field bath. Second, it associates a size extensive correlation energy with a given density matrix (for the models tested), which AGP by itself is incapable of providing. Third, it provides a reasonable description of charge redistribution in strongly correlated but non-periodic systems. Thus, AGP-DMET appears to be a good starting point for describing electron correlation in molecules, which are aperiodic and possess both strong and weak electron correlation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Phase identification using co‐association matrix ensemble clustering

Calibrating distribution system models to aid in the accuracy of simulations such as hosting capacity analysis is increasingly important in the pursuit of the goal of integrating more distributed energy resources. The recent availability of smart meter data is enabling the use of machine learning tools to automatically achieve model calibration tasks. This research focuses on applying machine learning to the phase identification task, using a co‐association matrix‐based, ensemble spectral clustering approach. The proposed method leverages voltage time series from smart meters and does not require existing or accurate phase labels. This work demonstrates the success of the proposed method on both synthetic and real data, surpassing the accuracy of other phase identification research.

24 POWER TRANSMISSION AND DISTRIBUTION↗
NASA NTRS2003

Optimal Substrate Preheating Model for Thermal Spray Deposition of Thermosets onto Polymer Matrix Composites

High velocity oxy-fuel (HVOF) sprayed, functionally graded polyimide/WC-Co composite coatings on polymer matrix composites (PMC's) are being investigated for applications in turbine engine technologies. This requires that the polyimide, used as the matrix material, be fully crosslinked during deposition in order to maximize its engineering properties. The rapid heating and cooling nature of the HVOF spray process and the high heat flux through the coating into the substrate typically do not allow sufficient time at temperature for curing of the thermoset. It was hypothesized that external substrate preheating might enhance the deposition behavior and curing reaction during the thermal spraying of polyimide thermosets. A simple analytical process model for the deposition of thermosetting polyimide onto polymer matrix composites by HVOF thermal spray technology has been developed. The model incorporates various heat transfer mechanisms and enables surface temperature profiles of the coating to be simulated, primarily as a function of substrate preheating temperature. Four cases were modeled: (i) no substrate preheating; (ii) substrates electrically preheated from the rear; (iii) substrates preheated by hot air from the front face; and (iv) substrates electrically preheated from the rear and by hot air from the front.

Ivosevic, M.↗
DOE OSTI2013

Nuclear matrix elements for 0νβ{sup −}β{sup −} decays: Comparative analysis of the QRPA, shell model and IBM predictions

In this work we report on general properties of the nuclear matrix elements involved in the neutrinoless double β{sup −} decays (0νβ{sup −}β{sup −} decays) of several nuclei. A summary of the values of the NMEs calculated along the years by the Jyväskylä-La Plata collaboration is presented. These NMEs, calculated in the framework of the quasiparticle random phase approximation (QRPA), are compared with those of the other available calculations, like the Shell Model (ISM) and the interacting boson model (IBA-2)

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
NASA NTRS2016

Electromagnetic Scattering by Fully Ordered and Quasi-Random Rigid Particulate Samples

In this paper we have analyzed circumstances under which a rigid particulate sample can behave optically as a true discrete random medium consisting of particles randomly moving relative to each other during measurement. To this end, we applied the numerically exact superposition T-matrix method to model far-field scattering characteristics of fully ordered and quasi-randomly arranged rigid multiparticle groups in fixed and random orientations. We have shown that, in and of itself, averaging optical observables over movements of a rigid sample as a whole is insufficient unless it is combined with a quasi-random arrangement of the constituent particles in the sample. Otherwise, certain scattering effects typical of discrete random media (including some manifestations of coherent backscattering) may not be accurately replicated.

Radiative transfer↗
DOE OSTI2015

Thermal aging modeling and validation on the Mo-containing Fe-Cr-Ni alloys

Thermodynamics of intermetallic phases in Fe-rich Fe-Cr-Ni-Mo alloys is critical knowledge to understand thermal aging effect on the phase stability of Mo-containing austenitic steels, which subsequently facilitates alloy design/improvement and degradation mitigation of these materials for reactor applications. Among the intermetallic phases, Chi (χ), Laves, and Sigma (σ) are often of concern because of their tendency to cause embrittlement of the materials. The focus of this study is thermal stability of the Chi and Laves phases as they were less studied compared to the Sigma phase. Coupled with thermodynamic modeling, thermal stability of intermetallic phases in Mo containing Fe-Cr-Ni alloys was investigated at 1000, 850 and 700°C for different annealing times. The morphologies, compositions and crystal structures of the precipitates of the intermetallic phases were carefully examined by scanning electron microscopy, electron probe microanalysis, X-ray diffraction, and transmission electron microscopy. Three key findings resulted from this study. First, the Chi phase is stable at high temperature, and with decreasing temperature it transforms into the Laves phase that is stable at low temperature. Secondly, Cr, Mo, Ni are soluble in both the Chi and Laves phases, with the solubility of Mo playing a major role in the relative stability of the intermetallic phases. Thirdly, in situ transformation from Chi phase to Laves phase was directly observed, which increased the local strain field, generated dislocations in the intermetallic phases, and altered the precipitate phase orientation relationship with the austenitic matrix. The thermodynamic models that were developed and validated were then applied to evaluating the effect of Mo on the thermal stability of intermetallic phases in type 316 and NF709 stainless steels.

36 MATERIALS SCIENCE↗
DOE OSTI2014

WE-E-18A-08: Towards a Next-Generation Electronic Portal Device for Simultaneous Imaging and Dose Verification in Radiotherapy

Purpose: This work forms part of an ongoing study to develop a next-generation electronic portal imaging device (EPID) for simultaneous imaging and dose verification in radiotherapy. Monte Carlo (MC) simulations were used to characterize the imaging performance of a novel EPID that has previously been demonstrated to exhibit a water-equivalent response. The EPID ' s response was quantified in several configurations and model parameters were empirically validated against experimental measurements. Methods: A MC model of a novel a-Si EPID incorporating an array of plastic scintillating fibers was developed. Square BCF-99-06A scintillator fibers with PMMA cladding (Saint-Gobain Crystals) were modelled in a matrix with total area measuring 150×150 mm{sup 2}. The standard electromagnetic and optical physics Geant4 classes were used to simulate radiation transport from an angled slit source (6 MV energy spectrum) through the EPID and optical photons reaching the photodiodes were scored. The prototype's modulation transfer function (MTF) was simulated and validated against experimental measurements. Several optical transport parameters, fiber lengths and thicknesses of an air gap between the scintillator and photodiodes were investigated to quantify their effects on the prototype's detection efficiency, sensitivity and MTF. Results: Simulated EPID response was more sensitive to variations in geometry than in the optical parameters studied. The MTF was particularly sensitive to the introduction of a 0.5–1.0 mm air gap between the scintillator and photodiodes, which lowered the MTF relative to that simulated without the gap. As expected, increasing the fiber length increased the detector efficiency and sensitivity while decreasing the MTF. Conclusion: A model of a novel water-equivalent EPID has been developed and benchmarked against measurements using a physical prototype. We have demonstrated the feasibility of this new device and are continuing to optimize the design to achieve an imaging response that warrants the development of a next-generation prototype.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗
DOE OSTI2023

Quantum dynamics simulations of the 2D spectroscopy for exciton polaritons

We develop an accurate and numerically efficient non-adiabatic path-integral approach to simulate the non-linear spectroscopy of exciton–polariton systems. This approach is based on the partial linearized density matrix approach to model the exciton dynamics with explicit propagation of the phonon bath environment, combined with a stochastic Lindblad dynamics approach to model the cavity loss dynamics. Through simulating both linear and polariton two-dimensional electronic spectra, we systematically investigate how light–matter coupling strength and cavity loss rate influence the optical response signal. Our results confirm the polaron decoupling effect, which is the reduced exciton–phonon coupling among polariton states due to the strong light–matter interactions. We further demonstrate that the polariton coherence time can be significantly prolonged compared to the electronic coherence outside the cavity.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗