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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 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
DOE OSTI2012

TURBULENT MAGNETIC FIELD AMPLIFICATION FROM SPIRAL SASI MODES: IMPLICATIONS FOR CORE-COLLAPSE SUPERNOVAE AND PROTO-NEUTRON STAR MAGNETIZATION

We extend our investigation of magnetic field evolution in three-dimensional flows driven by the stationary accretion shock instability (SASI) with a suite of higher-resolution idealized models of the post-bounce core-collapse supernova environment. Our magnetohydrodynamic simulations vary in initial magnetic field strength, rotation rate, and grid resolution. Vigorous SASI-driven turbulence inside the shock amplifies magnetic fields exponentially; but while the amplified fields reduce the kinetic energy of small-scale flows, they do not seem to affect the global shock dynamics. The growth rate and final magnitude of the magnetic energy are very sensitive to grid resolution, and both are underestimated by the simulations. Nevertheless, our simulations suggest that neutron star magnetic fields exceeding 10{sup 14} G can result from dynamics driven by the SASI, even for non-rotating progenitors.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

Tomography of massive stars from core collapse to supernova shock breakout

Neutrinos and gravitational waves are the only direct probes of the inner dynamics of a stellar core collapse. They are also the first signals to arrive from a supernova (SN) and, if detected, establish the moment when the shock wave is formed that unbinds the stellar envelope and later initiates the optical display upon reaching the stellar surface with a burst of UV and X-ray photons, the shock breakout (SBO). We discuss how neutrino observations can be used to trigger searches to detect the elusive SBO event. Observation of the SBO would provide several important constraints on progenitor structure and the explosion, including the shock propagation time (the duration between the neutrino burst and SBO), an observable that is important in distinguishing progenitor types. Our estimates suggest that next-generation neutrino detectors could exploit the overdensity of nearby SNe to provide several such triggers per decade, more than an order-of-magnitude improvement over the present.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

The core collapse supernova rate from the SDSS-II supernova survey

We use the Sloan Digital Sky Survey II Supernova Survey (SDSS-II SNS) data to measure the volumetric core collapse supernova (CCSN) rate in the redshift range (0.03 < z < 0.09). Using a sample of 89 CCSN, we find a volume-averaged rate of 1.06 ± 0.19 × 10{sup –4}((h/0.7){sup 3}/(yr Mpc{sup 3})) at a mean redshift of 0.072 ± 0.009. We measure the CCSN luminosity function from the data and consider the implications on the star formation history.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Collapse and fragmentation of magnetic molecular cloud cores with the Enzo AMR MHD code. II. Prolate and oblate cores

We present the results of a large suite of three-dimensional models of the collapse of magnetic molecular cloud cores using the adaptive mesh refinement code Enzo2.2 in the ideal magnetohydrodynamics approximation. The cloud cores are initially either prolate or oblate, centrally condensed clouds with masses of 1.73 or 2.73 M {sub ☉}, respectively. The radial density profiles are Gaussian, with central densities 20 times higher than boundary densities. A barotropic equation of state is used to represent the transition from low density isothermal phases, to high density optically thick phases. The initial magnetic field strength ranges from 6.3 to 100 μG, corresponding to clouds that are strongly to marginally supercritical, respectively, in terms of the mass to magnetic flux ratio. The magnetic field is initially uniform and aligned with the clouds' rotation axes, with initial ratios of rotational to gravitational energy ranging from 10{sup –4} to 0.1. Two significantly different outcomes for collapse result: (1) formation of single protostars with spiral arms, and (2) fragmentation into multiple protostar systems. The transition between these two outcomes depends primarily on the initial magnetic field strength, with fragmentation occurring for mass to flux ratios greater than about 14 times the critical ratio for prolate clouds. Oblate clouds typically fragment into several times more clumps than prolate clouds. Multiple, rather than binary, system formation is the general rule in either case, suggesting that binary stars are primarily the result of the orbital dissolution of multiple protostar systems.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2013

AXISYMMETRIC AB INITIO CORE-COLLAPSE SUPERNOVA SIMULATIONS OF 12-25 M{sub Sun} STARS

We present an overview of four ab initio axisymmetric core-collapse supernova simulations employing detailed spectral neutrino transport computed with our CHIMERA code and initiated from Woosley and Heger progenitors of mass 12, 15, 20, and 25 M{sub Sun }. All four models exhibit shock revival over {approx}200 ms (leading to the possibility of explosion), driven by neutrino energy deposition. Hydrodynamic instabilities that impart substantial asymmetries to the shock aid these revivals, with convection appearing first in the 12 M{sub Sun} model and the standing accretion shock instability appearing first in the 25 M{sub Sun} model. Three of the models have developed pronounced prolate morphologies (the 20 M{sub Sun} model has remained approximately spherical). By 500 ms after bounce the mean shock radii in all four models exceed 3000 km and the diagnostic explosion energies are 0.33, 0.66, 0.65, and 0.70 Bethe (B = 10{sup 51} erg) for the 12, 15, 20, and 25 M{sub Sun} models, respectively, and are increasing. The three least massive of our models are already sufficiently energetic to completely unbind the envelopes of their progenitors (i.e., to explode), as evidenced by our best estimate of their explosion energies, which first become positive at 320, 380, and 440 ms after bounce. By 850 ms the 12 M{sub Sun} diagnostic explosion energy has saturated at 0.38 B, and our estimate for the final kinetic energy of the ejecta is {approx}0.3 B, which is comparable to observations for lower mass progenitors.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2021

Long-term variability of root production in bioenergy crops from ingrowth core measurements

Long-term determination of root biomass production upon land-use conversion to biofuel crops is rare. To assess land-use legacy influences on belowground biomass accumulation, we converted 22-year-old Conservation Reserve Program (CRP) grasslands and 50+-year-old agricultural (AGR) lands to corn (C), switchgrass (Sw) and restored prairie (Pr) biofuel crops. We maintained one CRP grassland as a reference (Ref). We hypothesized that land-use history and crop type have significant effects on root density, with perennial crops on CRP grasslands having a higher root biomass productivity, while corn grown on former agricultural lands produce the lowest root biomass. Methods The ingrowth core method was used to determine in situ ingrowth root biomass, alongside measurements of aboveground net primary productivity (ANPP). Ancillary measurements, including air temperature, growing season length and precipitation were used to examine their influences on root biomass production. Important Findings Root biomass productivity was the highest in unconverted CRP grassland (1716 g m-2 yr-1) and lowest in corn fields (526 g m-2 yr-1). All perennial sites converted from CRP and AGR lands had lower root biomass and ANPP in the first year of planting but peaked in 2011 for switchgrass and a year later for restored prairies. Ecosystem stability was higher in restored prairies (AGR-Pr: 4.3 ± 0.11; CRP-Pr: 4.1 ± 0.10), with all monocultures exhibiting a lower stability. Root biomass production was positively related to ANPP (R2 = 0.40). Overall, attention should be given to root biomass accumulation in large-scale biofuel production as it is a major source of carbon sequestration.

Environmental Sciences & Ecology↗
DOE OSTI2011

Nanostructures of Liquid Crystal Phases in Mixtures of Bent-core and Rod-shaped Molecules

We report small angle x-ray scattering (SAXS) studies of isotropic, nematic, and smectic mesophases formed by binary mixtures of bent-core (BC) and rod-shaped (RS) molecules. While optical studies indicate that the components are fully miscible, SAXS reveals fascinating structures that are consistent with segregation on a nanoscopic scale. We find that tilted smectic clusters, which have been previously reported in both the nematic and isotropic states of the pure BC materials, are also present in mixtures with up to 50 wt% of the RS compound; this is consistent with previous dielectric and flexoelectric studies on such mixtures. Unexpectedly in this concentration range the clusters are present in the isotropic and in the induced smectic phase range, as well as throughout the nematic phase. The results in the smectic phase also reveal complex layering phenomena, providing important insight into the interaction between bent and rod-shaped molecules. These studies will be crucial in the design of promising new functional nanomaterials.

36 MATERIALS SCIENCE↗
DOE OSTI2014

MIC-SVM: Designing A Highly Efficient Support Vector Machine For Advanced Modern Multi-Core and Many-Core Architectures

Support Vector Machine (SVM) has been widely used in data-mining and Big Data applications as modern commercial databases start to attach an increasing importance to the analytic capabilities. In recent years, SVM was adapted to the field of High Performance Computing for power/performance prediction, auto-tuning, and runtime scheduling. However, even at the risk of losing prediction accuracy due to insufficient runtime information, researchers can only afford to apply offline model training to avoid significant runtime training overhead. To address the challenges above, we designed and implemented MICSVM, a highly efficient parallel SVM for x86 based multi-core and many core architectures, such as the Intel Ivy Bridge CPUs and Intel Xeon Phi coprocessor (MIC).

42 ENGINEERING↗
DOE OSTI2015

Allosteric transcriptional regulation via changes in the overall topology of the core promoter

Many transcriptional activators act at a distance from core promoter elements and work by recruiting RNA polymerase through protein-protein interactions. We show here how the prokaryotic regulatory protein CueR both represses and activates transcription by differentially modulating local DNA structure within the promoter. Structural studies reveal that the repressor state slightly bends the promoter DNA, precluding optimal RNA polymerase-promoter recognition. Upon binding a metal ion in the allosteric site, CueR switches into an activator conformation. It maintains all protein-DNA contacts but introduces torsional stresses that kink and undertwist the promoter, stabilizing an A-DNA-like conformation. Finally, these factors switch on and off transcription by exerting dynamic control of DNA stereochemistry, reshaping the core promoter and making it a better or worse substrate for polymerase.

59 BASIC BIOLOGICAL SCIENCES↗
DOE OSTI2022

Seven Percent Critical Experiment Core Analysis Approach on Fuel Rods – Core Configurations T155-NoMo, and T155-397Mo.

A series of experiments will be performed to test the integral effects of molybdenum on the reactivity of a critical system. These experiments will use the 7uPCX assembly with the 1.55 cm triangular pitch grid plates. Molybdenum sleeves, consisting of 19.6 inch long 0.5-inch nominal outside diameter molybdenum tubes with 0.031-inch nominal wall thickness and centering hardware, will be placed on some of the fuel rods in the array. The purpose of this analysis is to examine two configurations of the 7uPCX using the 1.55 cm triangular pitch grid plates in fully-reflected approach-to-critical experiments with the number of fuel rods in the array as the approach parameter. This document presents the results of the analysis that was done to allow completion of the 7uPCX Configuration Checklist from Appendix A of SPRF-AP-005 [SNL 2020] for the cores noted above. The checklists for these cores are shown in Appendix A.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2016

SHOULD ONE USE THE RAY-BY-RAY APPROXIMATION IN CORE-COLLAPSE SUPERNOVA SIMULATIONS?

We perform the first self-consistent, time-dependent, multi-group calculations in two dimensions (2D) to address the consequences of using the ray-by-ray+ transport simplification in core-collapse supernova simulations. Such a dimensional reduction is employed by many researchers to facilitate their resource-intensive calculations. Our new code (Fornax) implements multi-D transport, and can, by zeroing out transverse flux terms, emulate the ray-by-ray+ scheme. Using the same microphysics, initial models, resolution, and code, we compare the results of simulating 12, 15, 20, and 25 M {sub ⊙} progenitor models using these two transport methods. Our findings call into question the wisdom of the pervasive use of the ray-by-ray+ approach. Employing it leads to maximum post-bounce/pre-explosion shock radii that are almost universally larger by tens of kilometers than those derived using the more accurate scheme, typically leaving the post-bounce matter less bound and artificially more “explodable.” In fact, for our 25 M {sub ⊙} progenitor, the ray-by-ray+ model explodes, while the corresponding multi-D transport model does not. Therefore, in two dimensions, the combination of ray-by-ray+ with the axial sloshing hydrodynamics that is a feature of 2D supernova dynamics can result in quantitatively, and perhaps qualitatively, incorrect results.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Implications for Post-processing Nucleosynthesis of Core-collapse Supernova Models with Lagrangian Particles

We investigate core-collapse supernova (CCSN) nucleosynthesis with self-consistent, axisymmetric (2D) simulations performed using the neutrino hydrodynamics code Chimera. Computational costs have traditionally constrained the evolution of the nuclear composition within multidimensional CCSN models to, at best, a 14-species α-network capable of tracking only (α,γ) reactions from {sup 4}He to {sup 60}Zn. Such a simplified network limits the ability to accurately evolve detailed composition and neutronization or calculate the nuclear energy generation rate. Lagrangian tracer particles are commonly used to extend the nuclear network evolution by incorporating more realistic networks into post-processing nucleosynthesis calculations. However, limitations such as poor spatial resolution of the tracer particles; inconsistent thermodynamic evolution, including misestimation of expansion timescales; and uncertain determination of the multidimensional mass cut at the end of the simulation impose uncertainties inherent to this approach. We present a detailed analysis of the impact of such uncertainties for four self-consistent axisymmetric CCSN models initiated from solar-metallicity, nonrotating progenitors of 12, 15, 20, and 25 M{sub ⊙} and evolved with the smaller α-network to more than 1 s after the launch of an explosion.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Light Curves and Spectra from a Unimodal Core-collapse Supernova

To assess the effectiveness of optical emission as a probe of spatial asymmetry in core-collapse supernovae (CCSNe), we apply in this paper the radiative transfer software SuperNu to a unimodal CCSN model. The SNSPH radiation hydrodynamics software was used to simulate an asymmetric explosion of a 16 ${M}_{\odot }$ zero-age main-sequence binary star. The ejecta has 3.36 ${M}_{\odot }$ with 0.024 ${M}_{\odot }$ of radioactive 56 Ni, with unipolar asymmetry along the z-axis. For 96 discrete angular views, we find a ratio between maximum and minimum peak total luminosities of ~1.36. The brightest light curves emerge from views orthogonal to the z-axis. Multigroup spectra from UV to IR are obtained. We find a shift in wavelength with viewing angle in a near-IR Ca ii emission feature, consistent with Ca being mostly in the unimode. We compare emission from the gray gamma-ray transfer in SuperNu and from the detailed gamma-ray transfer code Maverick. Relative to the optical light curves, the brightness of the gamma-ray emission is more monotonic with respect to viewing angle. UBVRI broadband light curves are also calculated. Parallel with the unimode, the U and B bands have excess luminosity at $\gtrsim 10$ days post-explosion, due to 56 Ni on the unimode. Finally, we compare our CCSN model with SN 2002ap, which is thought to have a similar ejecta morphology.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

A Detailed Comparison of Multidimensional Boltzmann Neutrino Transport Methods in Core-collapse Supernovae

The mechanism driving core-collapse supernovae is sensitive to the interplay between matter and neutrino radiation. However, neutrino radiation transport is very difficult to simulate, and several radiation transport methods of varying levels of approximation are available. In this paper, we carefully compare for the first time in multiple spatial dimensions the discrete ordinates (DO) code of Nagakura, Yamada, and Sumiyoshi and the Monte Carlo (MC) code Sedonu, under the assumptions of a static fluid background, flat spacetime, elastic scattering, and full special relativity. We find remarkably good agreement in all spectral, angular, and fluid interaction quantities, lending confidence to both methods. The DO method excels in determining the heating and cooling rates in the optically thick region. The MC method predicts sharper angular features due to the effectively infinite angular resolution, but struggles to drive down noise in quantities where subtractive cancellation is prevalent, such as the net gain in the protoneutron star and off-diagonal components of the Eddington tensor. We also find that errors in the angular moments of the distribution functions induced by neglecting velocity dependence are subdominant to those from limited momentum-space resolution. We briefly compare directly computed second angular moments to those predicted by popular algebraic two-moment closures, and we find that the errors from the approximate closures are comparable to the difference between the DO and MC methods. Finally, included in this work is an improved Sedonu code, which now implements a fully special relativistic, time-independent version of the grid-agnostic MC random walk approximation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Reducing Uncertainties in the Production of the Gamma-emitting Nuclei {sup 26}Al, {sup 44}Ti, and {sup 60}Fe in Core-collapse Supernovae by Using Effective Helium Burning Rates

We have used effective reaction rates (ERRs) for the helium burning reactions to predict the yield of the gamma-emitting nuclei {sup 26}Al, {sup 44}Ti, and {sup 60}Fe in core-collapse supernovae (SNe). The variations in the predicted yields for values of the reaction rates allowed by the ERR are much smaller than obtained previously, and smaller than other uncertainties. A “filter” for SN nucleosynthesis yields based on pre-SN structure was used to estimate the effect of failed SNe on the initial mass function averaged yields; this substantially reduced the yields of all these isotopes, but the predicted yield ratio {sup 60}Fe/{sup 26}Al was little affected. The robustness of this ratio is promising for comparison with data, but it is larger than observed in nature; possible causes for this discrepancy are discussed.

79 ASTRONOMY AND ASTROPHYSICS↗