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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 1,603 records · Page 89
All sourcesNASA NTRSDOE OSTI
DOE OSTI2026

Deep learning‑based metal artefact reduction in X-ray computed tomography of TRISO fuel compacts

TRISO (TRi-structural ISOtropic) – compact-type micro-particle fuels – are next generation nuclear fuel compacts designed with safety in mind. Structural integrity and characterisation before and after irradiation are important to determine the performance of the fuel compacts under real reactor conditions. X-ray computed tomography can be an important tool for non-destructive evaluation of these fuel compacts. The fuel particles are highly attenuating for X-rays which creates metal artefact, rendering the images unusable. Our artefact correction can mitigate these artefacts significantly. The proposed method works by first segmenting highly-attenuating structures (metal) and forward projecting to localise the source of artefacts in the projection domain, then, using a traditional or U-net-based deep learning architecture, contextually interpolating those regions to remove the artefacts. Finally the reconstruction from the modified projection is fused with the segmented metal. The proposed method shows a significant improvement in the image quality while significantly reducing the reconstruction time compared to the standard technique.

Rahman, Obaid [ORNL] (ORCID:0000000277810840)↗
DOE OSTI2025

A physics-constrained neural ordinary differential equations approach for robust learning of stiff chemical kinetics

The high computational cost associated with solving for detailed chemistry poses a significant challenge for predictive computational fluid dynamics (CFD) simulations of turbulent reacting flows. While deep learning techniques have been explored to develop faster surrogate models, they often fail to integrate reliably with CFD solvers. This instability arises because traditional deep learning approaches optimize for training error without ensuring compatibility with ordinary differential equation (ODE) solvers, resulting in accumulation of errors over time. Recently, neuralODE (NODE) based approaches have been shown to be a promising technique to emulate and accelerate detailed chemistry computations. Here, in the present work, we extend this NODE framework for stiff chemical kinetics by incorporating mass conservation constraints directly into the loss function during training. This ensures that the total mass as well as the individual elemental species masses are conserved in an a-posteriori manner. Proof-of-concept studies are performed with the novel physics-constrained NODE (PC-NODE) approach for homogeneous autoignition of hydrogen-air mixture over a range of composition and thermodynamic conditions. It is demonstrated that the PC-NODE framework not only improves the physical consistency of the resulting data-driven model with respect to mass conservation criteria, but also improves training efficiency. PC-NODE is shown to achieve 2–100× speedup relative to the hydrogen-air detailed chemical mechanism depending on the type of the ODE solver (implicit or explicit) used during autoregressive inference tests. Lastly, a-posteriori studies are performed wherein the trained PC-NODE model is coupled with a CFD solver. It is shown that higher accuracy is achieved with PC-NODE relative to the purely data-driven NODE approach. Moreover, PC-NODE also exhibits robustness and generalizability to unseen initial conditions from within (interpolative capability) as well as outside (extrapolative capability) the training regime.

computational combustion↗
DOE OSTI2019

GeoAI: spatially explicit artificial intelligence techniques for geographic knowledge discovery and beyond

Recent progress in Artificial Intelligence (AI) techniques, the large-scale availability of high-quality data, as well as advances in both hardware and software to efficiently process these data, are transforming a range of fields from computer vision and natural language processing to autonomous driving and healthcare. For example, the availability of high-resolution geographic data and high-performance computing techniques together with deep learning fuel progress in fast and accurate object detection. Recent examples of GeoAI work include the detections of terrain features and densely-distributed building footprints, information extraction from scanned historical maps, semantic classification (e.g. LiDAR point clouds), novel methods for spatial interpolation, and advances in traffic forecasting. Similarly, machine learning and natural language processing are facilitating the extraction of geographic information from unstructured (textual) data, such as news articles and Wikipedia as well as the matching of natural features in multiple gazetteers.

96 KNOWLEDGE MANAGEMENT AND PRESERVATION↗
DOE OSTI2013

SPECTROSCOPIC METALLICITY DETERMINATIONS FOR W UMa-TYPE BINARY STARS

This study is the first attempt to determine the metallicities of W UMa-type binary stars using spectroscopy. We analyzed about 4500 spectra collected at the David Dunlap Observatory. To circumvent problems caused by the extreme spectral line broadening and blending and by the relatively low quality of the data, all spectra were subject to the same broadening function (BF) processing to determine the combined line strength in the spectral window centered on the Mg I triplet between 5080 A and 5285 A. All individual integrated BFs were subsequently orbital-phase averaged to derive a single line-strength indicator for each star. The star sample was limited to 90 W UMa-type (EW) binaries with the strict phase-constancy of colors and without spectral contamination by spectroscopic companions. The best defined results were obtained for an F-type sub-sample (0.32 < (B - V){sub 0} < 0.62) of 52 binaries for which integrated BF strengths could be interpolated in the model atmosphere predictions. The logarithmic relative metallicities, [M/H], for the F-type sub-sample indicate metal abundances roughly similar to the solar metallicity, but with a large scatter which is partly due to combined random and systematic errors. Because of the occurrence of a systematic color trend resulting from inherent limitations in our approach, we were forced to set the absolute scale of metallicities to correspond to that derived from the m{sub 1} index of the Stroemgren uvby photometry for 24 binaries of the F-type sub-sample. The trend-adjusted metallicities [M/H]{sub 1} are distributed within -0.65 < [M/H]{sub 1} < +0.50, with the spread reflecting genuine metallicity differences between stars. One half of the F-sub-sample binaries have [M/H]{sub 1} within -0.37 < [M/H]{sub 1} < +0.10, a median of -0.04 and a mean of -0.10, with a tail toward low metallicities, and a possible bias against very high metallicities. A parallel study of kinematic data, utilizing the most reliable and recently obtained proper motion and radial velocity data for 78 binaries of the full sample, shows that the F-type sub-sample binaries (44 stars with both velocities and metallicity determinations) have similar kinematic properties to solar-neighborhood, thin-disk dwarfs with space velocity component dispersions: {sigma}U = 33 km s{sup -1}, {sigma}V = 23 km s{sup -1} and {sigma}W = 14 km s{sup -1}. FU Dra with a large spatial velocity, V{sub tot} = 197 km s{sup -1} and [M/H]{sub 1} = -0.6 {+-} 0.2, appears to be the only thick-disk object in the F-type sub-sample. The kinematic data indicate that the F-type EW binaries are typical, thin-disk population stars with ages about 3-5.5 Gyr. The F-type binaries that appear to be older than the rest tend to have systematically smaller mass ratios than most of the EW binaries of the same period.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

PANCHROMATIC HUBBLE ANDROMEDA TREASURY. XII. MAPPING STELLAR METALLICITY DISTRIBUTIONS IN M31

We present a study of spatial variations in the metallicity of old red giant branch stars in the Andromeda galaxy. Photometric metallicity estimates are derived by interpolating isochrones for over seven million stars in the Panchromatic Hubble Andromeda Treasury (PHAT) survey. This is the first systematic study of stellar metallicities over the inner 20 kpc of Andromeda’s galactic disk. We see a clear metallicity gradient of −0.020 ± 0.004 dex kpc{sup −1} from ∼4–20 kpc assuming a constant red giant branch age. This metallicity gradient is derived after correcting for the effects of photometric bias and completeness and dust extinction, and is quite insensitive to these effects. The unknown age gradient in M31's disk creates the dominant systematic uncertainty in our derived metallicity gradient. However, spectroscopic analyses of galaxies similar to M31 show that they typically have small age gradients that make this systematic error comparable to the 1σ error on our metallicity gradient measurement. In addition to the metallicity gradient, we observe an asymmetric local enhancement in metallicity at radii of 3–6 kpc that appears to be associated with Andromeda’s elongated bar. This same region also appears to have an enhanced stellar density and velocity dispersion.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

DELO-BEZIER FORMAL SOLUTIONS OF THE POLARIZED RADIATIVE TRANSFER EQUATION

We present two new accurate and efficient methods to compute the formal solution of the polarized radiative transfer equation. In this work, the source function and the absorption matrix are approximated using quadratic and cubic Bezier spline interpolants. These schemes provide second- and third-order approximations, respectively, and do not suffer from erratic behavior of the polynomial approximation (overshooting). The accuracy and the convergence of the new method are studied along with other popular solutions of the radiative transfer equation, using stellar atmospheres with strong gradients in the line-of-sight velocity and in the magnetic-field vector.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

Modeling the near-ultraviolet band of GK stars. III. Dependence on abundance pattern

We extend the grid of non-LTE (NLTE) models presented in Paper II to explore variations in abundance pattern in two ways: (1) the adoption of the Asplund et al. (GASS10) abundances, (2) for stars of metallicity, [M/H], of –0.5, the adoption of a non-solar enhancement of α-elements by +0.3 dex. Moreover, our grid of synthetic spectral energy distributions (SEDs) is interpolated to a finer numerical resolution in both T {sub eff} (ΔT {sub eff} = 25 K) and log g (Δlog g = 0.25). We compare the values of T {sub eff} and log g inferred from fitting LTE and NLTE SEDs to observed SEDs throughout the entire visible band, and in an ad hoc 'blue' band. We compare our spectrophotometrically derived T {sub eff} values to a variety of T {sub eff} calibrations, including more empirical ones, drawn from the literature. For stars of solar metallicity, we find that the adoption of the GASS10 abundances lowers the inferred T {sub eff} value by 25-50 K for late-type giants, and NLTE models computed with the GASS10 abundances give T {sub eff} results that are marginally in better agreement with other T {sub eff} calibrations. For stars of [M/H] = –0.5 there is marginal evidence that adoption of α-enhancement further lowers the derived T {sub eff} value by 50 K. Stellar parameters inferred from fitting NLTE models to SEDs are more dependent than LTE models on the wavelength region being fitted, and we find that the effect depends on how heavily line blanketed the fitting region is, whether the fitting region is to the blue of the Wien peak of the star's SED, or both.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

TTVFast: An efficient and accurate code for transit timing inversion problems

Transit timing variations (TTVs) have proven to be a powerful technique for confirming Kepler planet candidates, for detecting non-transiting planets, and for constraining the masses and orbital elements of multi-planet systems. These TTV applications often require the numerical integration of orbits for computation of transit times (as well as impact parameters and durations); frequently tens of millions to billions of simulations are required when running statistical analyses of the planetary system properties. We have created a fast code for transit timing computation, TTVFast, which uses a symplectic integrator with a Keplerian interpolator for the calculation of transit times. The speed comes at the expense of accuracy in the calculated times, but the accuracy lost is largely unnecessary, as transit times do not need to be calculated to accuracies significantly smaller than the measurement uncertainties on the times. The time step can be tuned to give sufficient precision for any particular system. We find a speed-up of at least an order of magnitude relative to dynamical integrations with high precision using a Bulirsch-Stoer integrator.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Flexible and scalable methods for quantifying stochastic variability in the era of massive time-domain astronomical data sets

We present the use of continuous-time autoregressive moving average (CARMA) models as a method for estimating the variability features of a light curve, and in particular its power spectral density (PSD). CARMA models fully account for irregular sampling and measurement errors, making them valuable for quantifying variability, forecasting and interpolating light curves, and variability-based classification. We show that the PSD of a CARMA model can be expressed as a sum of Lorentzian functions, which makes them extremely flexible and able to model a broad range of PSDs. We present the likelihood function for light curves sampled from CARMA processes, placing them on a statistically rigorous foundation, and we present a Bayesian method to infer the probability distribution of the PSD given the measured light curve. Because calculation of the likelihood function scales linearly with the number of data points, CARMA modeling scales to current and future massive time-domain data sets. We conclude by applying our CARMA modeling approach to light curves for an X-ray binary, two active galactic nuclei, a long-period variable star, and an RR Lyrae star in order to illustrate their use, applicability, and interpretation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

The physical parameters of the retired a star HD 185351

We report here an analysis of the physical stellar parameters of the giant star HD 185351 using Kepler short-cadence photometry, optical and near infrared interferometry from CHARA, and high-resolution spectroscopy. Asteroseismic oscillations detected in the Kepler short-cadence photometry combined with an effective temperature calculated from the interferometric angular diameter and bolometric flux yield a mean density ρ{sub *} = 0.0130 ± 0.0003 ρ{sub ☉} and surface gravity log g = 3.280 ± 0.011. Combining the gravity and density we find R {sub *} = 5.35 ± 0.20 R {sub ☉} and M {sub *} = 1.99 ± 0.23 M {sub ☉}. The trigonometric parallax and CHARA angular diameter give a radius R {sub *} = 4.97 ± 0.07 R {sub ☉}. This smaller radius, when combined with the mean stellar density, corresponds to a stellar mass 1.60 ± 0.08 M {sub ☉}, which is smaller than the asteroseismic mass by 1.6σ. We find that a larger mass is supported by the observation of mixed modes in our high-precision photometry, the spacing of which is consistent only for M {sub *} ≳ 1.8 M {sub ☉}. Our various and independent mass measurements can be compared to the mass measured from interpolating the spectroscopic parameters onto stellar evolution models, which yields a model-based mass M {sub *,} {sub model} = 1.87 ± 0.07 M {sub ☉}. This mass agrees well with the asteroseismic value, but is 2.6σ higher than the mass from the combination of asteroseismology and interferometry. The discrepancy motivates future studies with a larger sample of giant stars. However, all of our mass measurements are consistent with HD 185351 having a mass in excess of 1.5 M {sub ☉}.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Discovery of a ∼5 day characteristic timescale in the Kepler power spectrum of Zw 229–15

We present time series analyses of the full Kepler data set of Zw 229–15. This Kepler light curve—with a baseline greater than 3 yr, composed of virtually continuous, evenly sampled 30 minute measurements—is unprecedented in its quality and precision. We utilize two methods of power spectral analysis to investigate the optical variability and search for evidence of a bend frequency associated with a characteristic optical variability timescale. Each method yields similar results. The first interpolates across data gaps to use the standard Fourier periodogram. The second, using the CARMA-based time-domain modeling technique of Kelly et al., does not need evenly sampled data. Both methods find excess power at high frequencies that may be due to Kepler instrumental effects. More importantly, both also show strong bends (Δα ∼ 2) at timescales of ∼5 days, a feature similar to those seen in the X-ray power spectral densities of active galactic nuclei (AGNs) but never before in the optical. This observed ∼5 day timescale may be associated with one of several physical processes potentially responsible for the variability. A plausible association could be made with light-crossing dynamical or thermal timescales depending on the assumed value of the accretion disk size and on unobserved disk parameters such as α and H/R. This timescale is not consistent with the viscous timescale, which would be years in a ∼10{sup 7} M {sub ☉} AGN such as Zw 229–15. However, there must be a second bend on long (≳ 1 yr) timescales and that feature could be associated with the viscous timescale.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Dynamical modeling of tidal streams

I present a new framework for modeling the dynamics of tidal streams. The framework consists of simple models for the initial action-angle distribution of tidal debris, which can be straightforwardly evolved forward in time. Taking advantage of the essentially one-dimensional nature of tidal streams, the transformation to position-velocity coordinates can be linearized and interpolated near a small number of points along the stream, thus allowing for efficient computations of a stream's properties in observable quantities. I illustrate how to calculate the stream's average location (its 'track') in different coordinate systems, how to quickly estimate the dispersion around its track, and how to draw mock stream data. As a generative model, this framework allows one to compute the full probability distribution function and marginalize over or condition it on certain phase-space dimensions as well as convolve it with observational uncertainties. This will be instrumental in proper data analysis of stream data. In addition to providing a computationally efficient practical tool for modeling the dynamics of tidal streams, the action-angle nature of the framework helps elucidate how the observed width of the stream relates to the velocity dispersion or mass of the progenitor, and how the progenitors of 'orphan' streams could be located. The practical usefulness of the proposed framework crucially depends on the ability to calculate action-angle variables for any orbit in any gravitational potential. A novel method for calculating actions, frequencies, and angles in any static potential using a single orbit integration is described in the Appendix.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Generating synthetic magnetic field intermittency using a Minimal Multiscale Lagrangian Mapping approach

The Minimal Multiscale Lagrangian Mapping procedure developed in the context of neutral fluid turbulence is a simple method for generating synthetic vector fields. Using a sequence of low-pass filtered fields, fluid particles are displaced at their rms speed for some scale-dependent time interval, and then interpolated back to a regular grid. Fields produced in this way are seen to possess certain properties of real turbulence. This paper extends the technique to plasmas by taking into account the coupling between the velocity and magnetic fields. We examine several possible applications to plasma systems. One use is as initial conditions for simulations, wherein these synthetic fields may efficiently produce a strongly intermittent cascade. The intermittency properties of the synthetic fields are also compared with those of the solar wind. Finally, studies of cosmic ray transport and modulation in the test particle approximation may benefit from improved realism in synthetic fields produced in this way.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

ESTIMATING BLACK HOLE MASSES IN HUNDREDS OF QUASARS

We explore the practical feasibility of active galactic nucleus (AGN) broadband reverberation mapping and present first results. We lay out and apply a rigorous approach for the stochastic reverberation mapping of unevenly sampled multi-broadband flux measurements, assuming that the broad-line region (BLR) line flux is contributing up to 15% in some bands, and is directly constrained by one spectroscopical epoch. The approach describes variations of the observed flux as the continuum, modeled as a stochastic Gaussian process, and emission line contribution, modeled as a scaled, smoothed, and delayed version of the continuum. This approach can be used not only to interpolate in time between measurements, but also to determine confidence limits on continuum—line emission delays. This approach is applied to Sloan Digital Sky Survey observations in Stripe 82 (S82), providing flux measurements that are precise to 2% at ∼60 epochs over ∼10 yr. The strong annual variations in the epoch sampling prove a serious limitation in practice. In addition, suitable redshift ranges must be identified where strong, broad emission lines contribute to one filter, but not to another. By generating and evaluating problem-specific mock data, we verify that S82-like data can constrain τ{sub delay} for a simple transfer function model. In application to real data, we estimate τ{sub delay} for 323 AGNs with 0.225 < z < 0.846, combining information for different objects through the ensemble-scaling relationships for BLR size and black hole mass. Our analysis tentatively indicates a 1.7 times larger BLR size of Hα and Mg II compared to Kaspi et al. and Vestergaard, but the seasonal data sampling casts doubt on the robustness of the inference.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

CHARACTERIZING THE COOL KOIs. VI. H- AND K-BAND SPECTRA OF KEPLER M DWARF PLANET-CANDIDATE HOSTS

We present H- and K-band spectra for late-type Kepler Objects of Interest (the {sup C}ool KOIs{sup )}: low-mass stars with transiting-planet candidates discovered by NASA's Kepler Mission that are listed on the NASA Exoplanet Archive. We acquired spectra of 103 Cool KOIs and used the indices and calibrations of Rojas-Ayala et al. to determine their spectral types, stellar effective temperatures, and metallicities, significantly augmenting previously published values. We interpolate our measured effective temperatures and metallicities onto evolutionary isochrones to determine stellar masses, radii, luminosities, and distances, assuming the stars have settled onto the main sequence. As a choice of isochrones, we use a new suite of Dartmouth predictions that reliably include mid-to-late M dwarf stars. We identify five M4V stars: KOI-961 (confirmed as Kepler 42), KOI-2704, KOI-2842, KOI-4290, and the secondary component to visual binary KOI-1725, which we call KOI-1725 B. We also identify a peculiar star, KOI-3497, which has Na and Ca lines consistent with a dwarf star but CO lines consistent with a giant. Visible-wavelength adaptive optics imaging reveals two objects within a 1 arcsec diameter; however, the objects' colors are peculiar. The spectra and properties presented in this paper serve as a resource for prioritizing follow-up observations and planet validation efforts for the Cool KOIs and are all available for download online using the ''data behind the figure'' feature.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

LEAVING THE DARK AGES WITH AMIGA

We present an Analytic Model of Intergalactic-medium and GAlaxy (AMIGA) evolution since the dark ages. AMIGA is in the spirit of the popular semi-analytic models of galaxy formation, although it does not use halo merger trees but interpolates halo properties in grids that are progressively built. This strategy is less memory-demanding and allows one to start modeling at sufficiently high redshifts and low halo masses to have trivial boundary conditions. The number of free parameters is minimized by making a causal connection between physical processes usually treated as independent of each other, which leads to more reliable predictions. However, the strongest points of AMIGA are the following: (1) the inclusion of molecular cooling and metal-poor, population III (Pop III) stars with the most dramatic feedback and (2) accurate follow up of the temperature and volume filling factor of neutral, singly ionized, and doubly ionized regions, taking into account the distinct halo mass functions in those environments. We find the following general results. Massive Pop III stars determine the intergalactic medium metallicity and temperature, and the growth of spheroids and disks is self-regulated by that of massive black holes (MBHs) developed from the remnants of those stars. However, the properties of normal galaxies and active galactic nuclei appear to be quite insensitive to Pop III star properties due to the much higher yield of ordinary stars compared to Pop III stars and the dramatic growth of MBHs when normal galaxies begin to develop, which cause the memory loss of the initial conditions.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Pfirsch–Schlüter neoclassical heavy impurity transport in a rotating plasma

In this paper, we extend previous analytic theories for the neoclassical transport of a trace heavy impurity in a rotating plasma in the Pfirsch-Schl¨uter regime. The complete diffusive and convective components of the ambipolar particle flux are derived. The solution is valid for arbitrary impurity charge and impurity Mach number and for general geometry. Inclusion of finite main ion temperature gradient effects is shown in the small ion Mach number limit. A simple interpolation formula is derived for the case of high impurity charge and circular geometry. While an enhancement of the diffusion coefficient is found for order one impurity Mach number, a reduction due to the rotation-driven poloidal asymmetry in the density occurs for very large Mach number.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2020

Quantifying uncertainties on fission fragment mass yields with mixture density networks

Probabilistic machine learning techniques can learn both complex relations between input features and output quantities of interest as well as take into account stochasticity or uncertainty within a data set. In this initial work, we explore the use of one such probabilistic network, the mixture density network (MDN), to reproduce fission yields and their uncertainties. In this paper, we study mass yields for the spontaneous fission of 252 Cf, exploring the number of training samples needed for converged predictions, how different levels of uncertainty propagate from the training set to the MDN predictions, and how well physical constraints of the yields—such as normalization and symmetry—are upheld by the algorithm. Finally, we test the ability of the MDN to interpolate between and extrapolate beyond samples in the training set using energy-dependent mass yields for the neutron-induced fission on 235 U. The MDN provides a reliable way to include and predict uncertainties and is a promising path forward for supplementing sparse sets of nuclear data.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗