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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 OSTI2015

Efficient small-scale dynamo in the solar convection zone

We investigate small-scale dynamo action in the solar convection zone through a series of high-resolution MHD simulations in a local Cartesian domain with 1 R{sub ⊙} (solar radius) of horizontal extent and a radial extent from 0.715 to 0.96 R{sub ⊙}. The dependence of the solution on resolution and diffusivity is studied. For a grid spacing of less than 350 km, the rms magnetic field strength near the base of the convection zone reaches 95% of the equipartition field strength (i.e., magnetic and kinetic energy are comparable). For these solutions the Lorentz force feedback on the convection velocity is found to be significant. The velocity near the base of the convection zone is reduced to 50% of the hydrodynamic one. In spite of the significant decrease of the convection velocity, the reduction in the enthalpy flux is relatively small, since the magnetic field also suppresses the horizontal mixing of the entropy between up- and downflow regions. This effect increases the amplitude of the entropy perturbation and makes convective energy transport more efficient. We discuss potential implications of these results for solar global convection and dynamo simulations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

CO-ORBITING PLANES OF SUB-HALOS ARE SIMILARLY UNLIKELY AROUND PAIRED AND ISOLATED HOSTS

Sub-halos in dark-matter-based cosmological simulations tend to be distributed approximately isotropically around their host. The existence of highly flattened, co-orbiting planes of satellite galaxies has therefore been identified as a possible problem for these cosmological models, but so far studies have not considered the hosts' environments. That satellite planes are now known around both major galaxies in the Local Group raises the question whether they are more likely to be found around paired hosts. In a first attempt to investigate this possibility, we focus on the flattening and orbital coherence of the 11 brightest satellite galaxies of the vast polar structure (VPOS) around the Milky Way (MW). We search for VPOS analogs in the ''Exploring the Local Volume in Simulations'' suite of cosmological simulations, which consist of 24 paired and 24 isolated host halos. We do not find significant differences between the properties of sub-halo distributions around paired and isolated hosts. The observed flattening and the observed orbital alignment are each reproduced by only 0.2%-2% of paired and isolated systems incorporating the obscuration of satellites by randomly oriented galactic disks. Only 1 of all 4800 analyzed realizations (0.02%) reproduces both parameters simultaneously, but the average orbital pole of this sub-halo system does not align as well with the normal to the plane fit as observed. That the MW is part of a galaxy pair thus does not help to explain the existence of the VPOS if the satellite galaxies are identified with sub-halos found in dissipationless simulations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2018

Optical Constants of an Apatite Single Crystal in the IR Range of 6–28 μm

On the basis of polarized IR-reflection spectra in the range of 5000–350 cm{sup –1} measured from the natural face of an optically transparent fluorapatite single crystal, components of a complex refractive index (optical constants) for radiation-vector orientations E ‖ c and E ⊥ c have been calculated by the Kramers–Kronig method. The fluorapatite single crystal has been chosen from several samples: it contains a minimum amount of impurities and has a high degree of crystallinity in accordance with the criteria of IR spectroscopy and Raman spectroscopy. Tabular data on optical constants for ordinary and extraordinary rays are given for the IR range of 6–28 μm. The obtained absorption spectra are compared with the results of quantum-chemical ab initio calculations within the B3LYP simulation.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

ANALYTIC H i-to-H{sub 2} PHOTODISSOCIATION TRANSITION PROFILES

We present a simple analytic procedure for generating atomic (H i) to molecular (H{sub 2}) density profiles for optically thick hydrogen gas clouds illuminated by far-ultraviolet radiation fields. Our procedure is based on the analytic theory for the structure of one-dimensional H i/H{sub 2} photon-dominated regions, presented by Sternberg et al. Depth-dependent atomic and molecular density fractions may be computed for arbitrary gas density, far-ultraviolet field intensity, and the metallicity-dependent H{sub 2} formation rate coefficient, and dust absorption cross section in the Lyman–Werner photodissociation band. We use our procedure to generate a set of H I-to-H{sub 2} transition profiles for a wide range of conditions, from the weak- to strong-field limits, and from super-solar down to low metallicities. We show that if presented as functions of dust optical depth, the H I and H{sub 2} density profiles depend primarily on the Sternberg “αG parameter” (dimensionless) that determines the dust optical depth associated with the total photodissociated H I column. We derive a universal analytic formula for the H I-to-H{sub 2} transition points as a function of just αG. Our formula will be useful for interpreting emission-line observations of H i/H{sub 2} interfaces, for estimating star formation thresholds, and for sub-grid components in hydrodynamics simulations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

Dynamical Instabilities in Systems of Multiple Short-period Planets Are Likely Driven by Secular Chaos: A Case Study of Kepler-102

We investigated the dynamical stability of high-multiplicity Kepler and K2 planetary systems. Our numerical simulations find instabilities in ∼20% of the cases on a wide range of timescales (up to 5 × 10{sup 9} orbits) and over an unexpectedly wide range of initial dynamical spacings. To identify the triggers of long-term instability in multiplanet systems, we investigated in detail the five-planet Kepler-102 system. Despite having several near-resonant period ratios, we find that mean-motion resonances are unlikely to directly cause instability for plausible planet masses in this system. Instead, we find strong evidence that slow inward transfer of angular momentum deficit (AMD) via secular chaos excites the eccentricity of the innermost planet, Kepler-102 b, eventually leading to planet–planet collisions in ∼80% of Kepler-102 simulations. Kepler-102 b likely needs a mass ≳0.1 M {sub ⊕}, hence a bulk density exceeding about half Earth’s, in order to avoid dynamical instability. To investigate the role of secular chaos in our wider set of simulations, we characterize each planetary system’s AMD evolution with a “spectral fraction” calculated from the power spectrum of short integrations (∼5 × 10{sup 6} orbits). We find that small spectral fractions (≲0.01) are strongly associated with dynamical stability on long timescales (5 × 10{sup 9} orbits) and that the median time to instability decreases with increasing spectral fraction. Our results support the hypothesis that secular chaos is the driver of instabilities in many nonresonant multiplanet systems and also demonstrate that the spectral analysis method is an efficient numerical tool to diagnose long-term (in)stability of multiplanet systems from short simulations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2018

Influence of Welding Strength Matching Coefficient and Cold Stretching on Welding Residual Stress in Austenitic Stainless Steel

Austenitic stainless steel is widely used in pressure vessels for the storage and transportation of liquid gases such as liquid nitrogen, liquid oxygen, and liquid hydrogen. Cryogenic pressure vessel manufacturing uses cold stretching technology, which relies heavily on welding joint performance, to construct lightweight and thin-walled vessels. Residual stress from welding is a primary factor in cases of austenitic stainless steel pressure vessel failure. In this paper, on the basis of Visual Environment 10.0 finite element simulation technology, the residual stress resulting from different welding strength matching coefficients (0.8, 1, 1.2, 1.4) for two S30408 plates welded with three-pass butt welds is calculated according to thermal elastoplastic theory. In addition, the stress field was calculated under a loading as high as 410 MPa and after the load was released. Path 1 was set to analyze stress along the welding line, and path 2 was set to analyze stress normal to the welding line. The welding strength matching coefficient strongly affected both the longitudinal residual stress (center of path 1) and the transverse residual stress (both ends of path 1) after the welding was completed. However, the coefficient had little effect on the longitudinal and transverse residual stress of path 2. Under the loading of 410 MPa, the longitudinal and transverse stress decreased and the stress distribution, with different welding strength matching coefficients, was less diverse. After the load was released, longitudinal and transverse stress distribution for both path 1 and path 2 decreased to a low level. Cold stretching could reduce the effect of residual stress to various degrees. Transverse strain along the stretching direction was also taken into consideration. The experimental results validated the reliability of the partial simulation.

36 MATERIALS SCIENCE↗
DOE OSTI2017

A novel partitioning method for block-structured adaptive meshes

We propose a novel partitioning method for block-structured adaptive meshes utilizing the meshless Lagrangian particle concept. With the observation that an optimum partitioning has high analogy to the relaxation of a multi-phase fluid to steady state, physically motivated model equations are developed to characterize the background mesh topology and are solved by multi-phase smoothed-particle hydrodynamics. In contrast to well established partitioning approaches, all optimization objectives are implicitly incorporated and achieved during the particle relaxation to stationary state. Distinct partitioning sub-domains are represented by colored particles and separated by a sharp interface with a surface tension model. In order to obtain the particle relaxation, special viscous and skin friction models, coupled with a tailored time integration algorithm are proposed. Numerical experiments show that the present method has several important properties: generation of approximately equal-sized partitions without dependence on the mesh-element type, optimized interface communication between distinct partitioning sub-domains, continuous domain decomposition which is physically localized and implicitly incremental. Therefore it is particularly suitable for load-balancing of high-performance CFD simulations.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2014

Numerical approach of the injection molding process of fiber-reinforced composite with considering fiber orientation

One of the most important challenges in the injection molding process of the short-glass fiber/thermoplastic composite parts is being able to predict the fiber orientation, since it controls the mechanical and the physical properties of the final parts. Folgar and Tucker included into the Jeffery equation a diffusive type of term, which introduces a phenomenological coefficient for modeling the randomizing effect of the mechanical interactions between the fibers, to predict the fiber orientation in concentrated suspensions. Their experiments indicated that this coefficient depends on the fiber volume fraction and aspect ratio. However, a definition of the fiber interaction coefficient, which is very necessary in the fiber orientation simulations, hasn't still been proven yet. Consequently, this study proposed a developed fiber interaction model that has been introduced a fiber dynamics simulation in order to obtain a global fiber interaction coefficient. This supposed that the coefficient is a sum function of the fiber concentration, aspect ratio, and angular velocity. The proposed model was incorporated into a computer aided engineering simulation package C-Mold. Short-glass fiber/polyamide-6 composites were produced in the injection molding with the fiber weight concentration of 30 wt.%, 50 wt.%, and 70 wt.%. The physical properties of these composites were examined, and their fiber orientation distributions were measured by micro-computed-tomography equipment μ-CT. The simulation results showed a good agreement with experiment results.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Design of 4-electrode optical device for application of vector electric fields to self-assembled quantum dot complexes

Self-assembled InAs quantum dots (QDs) are of great interest as components of optoelectronic devices that can operate at the quantum limit. The charge configuration, interdot coupling, and symmetry of complexes containing multiple QDs can all be tuned with applied electric fields, but the magnitude and angle of the electric field required to control each of these parameters depend on the orientation of the QD complex. We present a 4-electrode device compatible with optical excitation and emission that allows application of electric fields with arbitrary magnitudes and angles relative to isolated QD complexes. We demonstrate the electric field tunability of this device with numerical simulations.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Folding of small knotted proteins: Insights from a mean field coarse-grained model

A small but relevant number of proteins whose native structure is known features nontrivial topology, i.e., they are knotted. Understanding the process of folding from a swollen unknotted state to the biologically relevant native conformation is, for these proteins, particularly difficult, due to their rate-limiting topological entanglement. To shed some light into this conundrum, we introduced a structure-based coarse-grained model of the protein, where the information about the folded conformation is encoded in bonded angular interactions only, which do not favor the formation of native contacts. A stochastic search scheme in parameter space is employed to identify a set of interactions that maximizes the probability to attain the knotted state. The optimal knotting pathways of the two smallest knotted proteins, obtained through this approach, are consistent with the results derived by means of coarse-grained as well as full atomistic simulations.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2015

HELIOS-K: An ultrafast, open-source opacity calculator for radiative transfer

We present an ultrafast opacity calculator that we name HELIOS-K. It takes a line list as an input, computes the shape of each spectral line, and provides an option for grouping an enormous number of lines into a manageable number of bins. We implement a combination of Algorithm 916 and Gauss–Hermite quadrature to compute the Voigt profile, write the code in CUDA, and optimize the computation for graphics processing units (GPUs). We restate the theory of the k-distribution method and use it to reduce ∼10{sup 5}–10{sup 8} lines to ∼10–10{sup 4} wavenumber bins, which may then be used for radiative transfer, atmospheric retrieval and general circulation models. The choice of line-wing cutoff for the Voigt profile is a significant source of error and affects the value of the computed flux by ∼10%. This is an outstanding physical (rather than computational) problem, due to our incomplete knowledge of pressure broadening of spectral lines in the far line wings. We emphasize that this problem remains regardless of whether one performs line-by-line calculations or uses the k-distribution method and affects all calculations of exoplanetary atmospheres requiring the use of wavelength-dependent opacities. We elucidate the correlated-k approximation and demonstrate that it applies equally to inhomogeneous atmospheres with a single atomic/molecular species or homogeneous atmospheres with multiple species. Using a NVIDIA K20 GPU, HELIOS-K is capable of computing an opacity function with ∼10{sup 5} spectral lines in ∼1 s and is publicly available as part of the Exoclimes Simulation Platform (www.exoclime.org).

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2015

Testing ΛCDM cosmology at turnaround: where to look for violations of the bound?

In ΛCDM cosmology, structure formation is halted shortly after dark energy dominates the mass/energy budget of the Universe. A manifestation of this effect is that in such a cosmology the turnaround radius—the non-expanding mass shell furthest away from the center of a structure— has an upper bound. Recently, a new, local, test for the existence of dark energy in the form of a cosmological constant was proposed based on this turnaround bound. Before designing an experiment that, through high-precision determination of masses and —independently— turnaround radii, will challenge ΛCDM cosmology, we have to answer two important questions: first, when turnaround-scale structures are predicted to be close enough to their maximum size, so that a possible violation of the bound may be observable. Second, which is the best mass scale to target for possible violations of the bound. These are the questions we address in the present work. Using the Press-Schechter formalism, we find that turnaround structures have in practice already stopped forming, and consequently, the turnaround radius of structures must be very close to the maximum value today. We also find that the mass scale of ∼ 10{sup 13} M{sub ⊙} characterizes the turnaround structures that start to form in a statistically important number density today —and even at an infinite time in the future, since structure formation has almost stopped. This mass scale also separates turnaround structures with qualitatively different cosmological evolution: smaller structures are no longer readjusting their mass distribution inside the turnaround scale, they asymptotically approach their ultimate abundance from higher values, and they are common enough to have, at some epoch, experienced major mergers with structures of comparable mass; larger structures exhibit the opposite behavior. We call this mass scale the transitional mass scale and we argue that it is the optimal for the purpose outlined above. As a corollary, we explain the different accretion behavior of small and larger structures observed in already conducted numerical simulations.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

THE INNER DYNAMICAL MASS ACROSS GALAXY MORPHOLOGY: A WEAK SCALING WITH TOTAL STELLAR MASS

We compile a sample of dispersion-supported and rotation-supported galaxies for which the dynamical mass enclosed at 500 pc can be measured. We find that this dynamical quantity increases only slowly with stellar mass (M{sub total}(<500 pc)∝M{sub stars}{sup 0.16}) over ∼9 decades in baryonic mass and ∼3 in length scale, with a sudden upturn at the highest masses (M {sub stars} ≳ 10{sup 10} M {sub ☉}). This upturn occurs for the earliest types (S0 and Sa) of disk galaxies, and is consistent with the ''extra'' mass within 500 pc being predominantly stellar. This sudden change may be indicative of different bulge formation mechanisms between early- and late-type disks. We also compare the data to the results of recent galaxy formation simulations. These broadly track the observed trend, with some tendency to put too much mass in the inner 500 pc, depending on the details of each simulation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2019

Application of Grover’s Quantum Algorithm for SDES Key Searching

The problem of finding the key of Simplified-DES (SDES)—a model of a block cipher DES—by Grover’s quantum algorithm is considered. Examples of application of Grover’s algorithm are presented. A quantum system with the minimum number of qubits is constructed that implements SDES key searching by a single pair of plaintext and ciphertext and requires only 19 qubits. This quantum circuit is simulated by using a Quipper quantum simulator.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Comparison of Hydrocode Simulations with Measured Shock Wave Velocities

We have conducted detailed 1- and 2-dimensional hydrodynamics calculations to assess the quality of simulations commonly made to understand various shock processes in a sample and to design shock experiments. We began with relatively simple shock experiments, where we examined the effects of the equation of state and the viscoplastic strength models. Eventually we included spallation in copper and iron and a solid-solid phase transformation in iron to assess the quality of the damage and phase transformation simulations.

36 MATERIALS SCIENCE↗
DOE OSTI2016

X-ray simulation algorithms used in ISP

ISP is a simulation code which is sometimes used in the USNDS program. ISP is maintained by Sandia National Lab. However, the X-ray simulation algorithm used by ISP was written by scientists at LANL – mainly by Ed Fenimore with some contributions from John Sullivan and George Neuschaefer and probably others. In email to John Sullivan on July 25, 2016, Jill Rivera, ISP project lead, said “ISP uses the function xdosemeters_sim from the xgen library.” The is a fortran subroutine which is also used to simulate the X-ray response in consim (a descendant of xgen). Therefore, no separate documentation of the X-ray simulation algorithms in ISP have been written – the documentation for the consim simulation can be used.

97 MATHEMATICS AND COMPUTING↗
DOE OSTI2016

SNe Ia AND THEIR ENVIRONMENT: THEORY AND APPLICATIONS TO SN 2014J

We present theoretical semi-analytic models for the interaction of stellar winds with the interstellar medium (ISM) or prior mass loss implemented in our code SPICE, assuming spherical symmetry and power-law ambient density profiles and using the Π-theorem. This allows us to test a wide variety of configurations, their functional dependencies, and to find classes of solutions for given observations. Here, we study Type Ia Supernova (SN Ia) surroundings of single and double degenerate systems, and their observational signatures. Winds may originate from the progenitor prior to the white dwarf (WD) stage, the WD, a donor star, or an accretion disk (AD). For M{sub Ch} explosions, the AD wind dominates and produces a low-density void several light years across, surrounded by a dense shell. The bubble explains the lack of observed interaction in late time SN light curves for, at least, several years. The shell produces narrow ISM lines Doppler shifted by 10–100 km s{sup −1}, and equivalent widths of ≈100 mÅ and ≈1 mÅ in cases of ambient environments with constant density and produced by prior mass loss, respectively. For SN2014J, both mergers and M{sub Ch} mass explosions have been suggested based on radio and narrow lines. As a consistent and most likely solution, we find an AD wind running into an environment produced by the red giant wind of the progenitor during the pre-WD stage, and a short delay, 0.013–1.4 Myr, between the WD formation and the explosion. Our framework may be applied more generally to stellar winds and star formation feedback in large scale galactic evolution simulations.

79 ASTRONOMY AND ASTROPHYSICS↗