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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 1,243 records · Page 69
All sourcesNASA NTRSDOE OSTI
DOE OSTI2014

Probing the large and massive circumgalactic medium of a galaxy at z ∼ 0.2 using a pair of quasars

We present an analysis of two O VI absorbers at redshift z {sub abs} = 0.227, which were detected in the spectra of two closely spaced QSO sightlines (Q 0107–025A and B) and observed with the Cosmic Origins Spectrograph onboard the Hubble Space Telescope. At the same redshift, the presence of a single bright (∼1.2L {sub *}) galaxy at an impact parameter of ∼200 kpc (proper) from both the sightlines was reported by Crighton et al. Using detailed photoionization models, we show that the high ionization phases of both the O VI absorbers have similar ionization conditions (e.g., log U ∼ –1.1 to –0.9), chemical enrichment (e.g., log Z ∼ –1.4 to –1.0), total hydrogen column density (e.g., log N {sub H}(cm{sup –2}) ∼ 19.6 – 19.7), and line of sight thickness (e.g., l {sub los} ∼ 600-800 kpc). Therefore we speculate that the O VI absorbers are tracing different parts of same large-scale structure, presumably the circumgalactic medium (CGM) of the identified galaxy. Using sizes along and transverse to the line of sight, we estimate the size of the CGM to be R ∼ 330 kpc. The baryonic mass associated with this large CGM as traced by O VI absorption is ∼1.2 × 10{sup 11} M {sub ☉}. A low ionization phase is detected in one of the O VI systems with near-solar metallicity (log Z = 0.20 ± 0.20) and parsec scale size (l {sub los} ∼ 6 pc), possibly tracing the neutral phase of a high-velocity cloud embedded within the CGM.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2014

Possible evidence for metal accretion onto the surfaces of metal-poor main-sequence stars

The entire evolution of the Milky Way, including its mass-assembly and star-formation history, is imprinted onto the chemo-dynamical distribution function of its member stars, f(x, v, [X/H]), in the multi-dimensional phase space spanned by position, velocity, and elemental abundance ratios. In particular, the chemo-dynamical distribution functions for low-mass stars (e.g., G- or K-type dwarfs) are precious tracers of the earliest stages of the Milky Way's formation, since their main-sequence lifetimes approach or exceed the age of the universe. A basic tenet of essentially all previous analyses is that the stellar metallicity, usually parameterized as [Fe/H], is conserved over time for main-sequence stars (at least those that have not been polluted due to mass transfer from binary companions). If this holds true, any correlations between metallicity and kinematics for long-lived main-sequence stars of different masses, effective temperatures, or spectral types must strictly be the same, since they reflect the same mass-assembly and star-formation histories. By analyzing a sample of nearby metal-poor halo and thick-disk stars on the main sequence, taken from Data Release 8 of the Sloan Digital Sky Survey, we find that the median metallicity of G-type dwarfs is systematically higher (by about 0.2 dex) than that of K-type dwarfs having the same median rotational velocity about the Galactic center. If it can be confirmed, this finding may invalidate the long-accepted assumption that the atmospheric metallicities of long-lived stars are conserved over time.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2020

A low-luminosity core-collapse supernova very similar to SN 2005cs

ABSTRACT We present observations and analysis of PSN J17292918+7542390, a low-luminosity Type II-P supernova (LL SN IIP). The observed sample of such events is still low, and their nature is still under debate. Such SNe are similar to SN 2005cs, a well-observed LL Type II-P event, having low expansion velocities, and small ejected 56Ni mass. We have developed a robust and relatively fast Monte Carlo code that fits semi-analytic models to light curves of core-collapse SNe. This allows the estimation of the most important physical parameters, like the radius of the progenitor star, the mass of the ejected envelope, the mass of the radioactive nickel synthesized during the explosion, among others. PSN J17292918+7542390 has $R_0 = 91_{-70}^{+119} \times 10^{11} \, \text{cm}$, $M_\text{ej} = 9.89_{-1.00}^{+2.10} \, \mathrm{ M}_{\odot }$, $E_{\mbox{kin}} = 0.65_{-0.18}^{+0.19} \, \text{foe}$, and $v_{\mbox{exp}} = 3332_{-347}^{+216}$ km s−1, for its progenitor radius, ejecta mass, kinetic energy, and expansion velocity, respectively. The initial nickel mass of the PSN J17292918+7542390 turned out to be $1.55_{-0.70}^{+0.75} \times 10^{-3} \, \mathrm{M}_{\odot }$. The measured photospheric velocity at the earliest observed phase is 7000 km s−1. As far as we can tell based on the small population of observed LL SNe IIP, the determined values are typical for these events.

Jäger, Jr, Zoltán↗
DOE OSTI2016

Orientational ordering, buckling, and dynamic transitions for vortices interacting with a periodic quasi-one-dimensional substrate

In this study, we examine the statics and dynamics of vortices in the presence of a periodic quasi-one-dimensional substrate, focusing on the limit where the vortex lattice constant is smaller than the substrate lattice period. As a function of the substrate strength and filling factor, within the pinned state, we observe a series of order-disorder transitions associated with buckling phenomena in which the number of vortex rows that fit between neighboring substrate maxima increases. These transitions coincide with steps in the depinning threshold, jumps in the density of topological defects, and changes in the structure factor. At the buckling transition, the vortices are disordered, while between the buckling transitions the vortices form a variety of crystalline and partially ordered states. In the weak substrate limit, the buckling transitions are absent and the vortices form an ordered hexagonal lattice that undergoes changes in its orientation with respect to the substrate as a function of vortex density. At intermediate substrate strength, certain ordered states appear that are correlated with peaks in the depinning force. Under an applied drive, the system exhibits a rich variety of distinct dynamical phases, including plastic flow, a density-modulated moving crystal, and moving floating solid phases. We also find a dynamic smectic-to-smectic transition in which the smectic ordering changes from being aligned with the substrate to being aligned with the external drive. The different dynamical phases can be characterized using velocity histograms and the structure factor. We discuss how these results are related to recent experiments on vortex ordering in thin films with periodic thickness modulations. In conclusion, our results should also be relevant for other types of systems such as ions, colloids, or Wigner crystals interacting with periodic quasi-one-dimensional substrates.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Cross-Plane Acquisitions in Electrical Capacitance Volume Tomography

Electrical capacitance tomography (ECT) is a widely used imaging modality to image two-dimensional cross sections of multiphase flows. Recent developments in electrical capacitance volume tomography (ECVT) have made it possible to directly obtain volumetric images from measured data. An ECVT is instrumental for obtaining accurate phase hold up information and velocity information that are needed for the optimization of certain flow processes. However, compared to ECT, the high correlation between the measurements in ECVT exacerbates the ill-conditioning of the associated image reconstruction problem. Previous studies have suggested that neglecting mutual capacitance data between the ECVT electrodes located at cross-planes that are well separated along the sensor axis can be done without significantly affecting the reconstructed image. In addition, this may help constrain the ill-conditioning of the reconstruction problem. Here, we examine in detail and quantify the effect of reduced cross-plane acquisition strategies for optimizing the image reconstruction and constraining the ill-conditioning of typical ECVT settings.

20 FOSSIL-FUELED POWER PLANTS↗
DOE OSTI2024

Advanced Closures for Nonlinear Fluid Simulations of Plasmas (Final Report)

Advanced Closures for Nonlinear Fluid Simulations of Plasmas E. Held, Utah State University (Principal Investigator) J.-Y. Ji, Utah State University (Co-Investigator) Utah State University (USU) continued to develop and apply closures for hybrid fluid/kinetic simulations of magnetized fusion plasmas. The closures developed by USU's fusion theory and computation group use the drift ordering and are referred to as "continuum" and "general moment" approaches. They differ in their representation for the distribution functions which probabilistically describe the density of particles in 5 dimensional phase space. The two chosen velocity variables, needed in studies of kinetic physics for magnetized plasmas, are the speed and the component of a particle's velocity along the magnetic field. In magnetized plasmas, the charged particle trajectories are tied closely to the magnetic field lines. Both approaches account for binary interactions between particles using the Coulomb collision operator. Theoretical and computational development along both lines provided avenues for verification between the two approaches. This funding allowed USU to further develop physically accurate and numerically efficient models that advance our understanding of kinetic physics in experiments like the International Thermonuclear Experimental Reactor (ITER). The ITER device will likely prove the physical practicality of thermonuclear fusion by producing 500 megawatts of clean, abundant fusion power. This work helped to realize this goal by advancing our simulation capability using hybrid fluid/kinetic models.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

NUMERICAL SIMULATIONS OF KELVIN–HELMHOLTZ INSTABILITY: A TWO-DIMENSIONAL PARAMETRIC STUDY

Using two-dimensional simulations, we numerically explore the dependences of Kelvin–Helmholtz (KH) instability upon various physical parameters, including viscosity, the width of the sheared layer, flow speed, and magnetic field strength. In most cases, a multi-vortex phase exists between the initial growth phase and the final single-vortex phase. The parametric study shows that the evolutionary properties, such as phase duration and vortex dynamics, are generally sensitive to these parameters, except in certain regimes. An interesting result is that for supersonic flows, the phase durations and saturation of velocity growth approach constant values asymptotically as the sonic Mach number increases. We confirm that the linear coupling between magnetic field and KH modes is negligible if the magnetic field is weak enough. The morphological behavior suggests that the multi-vortex coalescence might be driven by the underlying wave–wave interaction. Based on these results, we present a preliminary discussion of several events observed in the solar corona. The numerical models need to be further improved to perform a practical diagnostic of the coronal plasma properties.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

A Multiwavelength View of IC 860: What Is in Action inside Quenching Galaxies

We present a multiwavelength study of IC 860, a nearby post-starburst galaxy at the early stage of transitioning from blue and star forming to red and quiescent. Optical images reveal a galaxy-wide, dusty outflow originating from a compact core. We find evidence for a multiphase outflow in the molecular and neutral gas phase from the CO position–velocity diagram and NaD absorption features. We constrain the neutral mass outflow rate to be ~0.5 M ⊙ yr -1 , and the total hydrogen mass outflow rate to be ~12 M ⊙ yr -1 . Neither outflow component seems able to escape the galaxy. We also find evidence for a recent merger in the optical images, CO spatial distribution, and kinematics, and evidence for a buried active galactic nucleus in the optical emission line ratios, mid-IR properties, and radio spectral shape. The depletion time of the molecular gas reservoir under the current star formation rate is ~7 Gyr, indicating that the galaxy could stay at the intermediate stage between the blue and red sequence for a long time. Thus the timescales for a significant decline in star formation rate (quenching) and gas depletion are not necessarily the same. Our analysis supports the quenching picture where outflows help suppress star formation by disturbing rather than expelling the gas and shed light on possible ongoing activities in similar quenching galaxies.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1970

Propagation of long waves in curved ducts

Mathematical model for two-dimensional, long-wave propagation in curved acoustic ducts determining vibrational velocity distribution and variation and phase of motion at any point in system

Rostafinski, W.↗
NASA NTRS1971

Nonlinear, nonlaminar-3D computation of electron motion through the output cavity of a klystron

The equations of motion used in the computation are discussed along with the space charge fields and the integration process. The following assumptions were used as a basis for the computation: (1) The beam is divided into N axisymmetric discs of equal charge and each disc into R rings of equal charge. (2) The velocity of each disc, its phase with respect to the gap voltage, and its radius at a specified position in the drift tunnel prior to the interaction gap is known from available large signal one dimensional programs. (3) The fringing rf fields are computed from exact analytical expressions derived from the wave equation assuming a known field shape between the tunnel tips at a radius a. (4) The beam is focused by an axisymmetric magnetic field. Both components of B, that is B sub z and B sub r, are taken into account. (5) Since this integration does not start at the cathode but rather further down the stream prior to entering the output cavity it is assumed that each electron moved along a laminar path from the cathode to the start of integration.

Albers, L. U.↗
NASA NTRS1975

Drag coefficient equations for small particles in high speed flows

The paper determines the effect of various available drag coefficient equations on particle velocity calculations for typical two phase flows encountered in supersonic and turbulent laser velocimeter applications. The predictions of the particle drag coefficient equations are compared with experimental sphere drag data. For the laser velocimeter applications, the relative Mach number less than 2 and the relative Reynolds number less than 200 are of particular importance.

Walsh, M. J.↗
NASA NTRS1977

A description of the University of Illinois meteor radar system and some first results

A recently established meteor radar system for observing motions in the upper atmosphere is described, which features a peak transmitter power of 5 megawatts. This large peak power should allow a much greater number of wind determination during a fixed period of time than other meteor radar stations. The essential features of the range algorithm, two velocity algorithms, and the interferometer phase difference algorithm in use are described. A typical measured variation in the radial southward wind as a function of time of day based on data from all altitudes is shown as an example, and some results of spectral analysis of the data, yielding amplitude and phase variation with altitude for significant waves, is discussed.

Geller, M. A.↗
NASA NTRS1983

The structure of a separating turbulent boundary layer. IV - Effects of periodic free-stream unsteadiness

Measurements were obtained of the sinusoidal unsteadiness of the free stream velocity during the separation of the turbulent boundary layer. Data were gathered by single wire and cross-wire, anemometry upstream of flow detachment, by laser Doppler velocimetry to detect the movement of the flow in small increments, and by a laser anemometer in the detached zone to measure turbulence and velocities. The study was restricted to a sinusoidal instability frequency of 0.61 and a ratio of oscillation amplitude to mean velocity of 0.3. Large amplitude and phase variations were found after the detachment, with unsteady effects producing hysteresis in the relationships between flow parameters. The detached shear layer decreased in thickness with increasing free-stream velocity and increases in the Reynolds shear stress. Deceleration of the free stream velocity caused thickening in the shear layer and upstream movement of the flow reversal location. The results are useful for studies of compressor blade and helicopter rotors in transition.

Simpson, R. L.↗
NASA NTRS1981

The preferred-mode coherent structure in the near field of an axisymmetric jet with and without excitation

Visualization and phase-locked hot-wire measurements have been used to investigate the preferred-mode coherent structure in the near field of an axisymmetric jet for different Reynolds numbers and for the two limiting asymptotic states of the exit boundary layer: laminar and fully developed turbulent. Distributions of phase-averaged longitudinal and lateral velocities, coherent vorticity, coherent and incoherent turbulence Reynolds stresses, and coherent strain rate and production have been obtained over the spatial extent of the coherent structure for a Strouhal number of 0.3. It is found that these properties are independent of the initial conditions, but weakly dependent on the Reynolds number up to about 100,000. The naturally occurring coherent structure in the unexcited jet, educed via an optimized conditional sampling scheme, shows no clear dependence on the jet Reynolds number up to 800,000.

Hussain, A. K. M. F.↗
NASA NTRS1988

Acoustic imaging of subtle porosity variations in ceramics

Acoustic images of silicon carbide ceramic disks were obtained using a precision scanning contact pulse-echo technique. Phase and cross-correlation velocity and attenuation maps were used to form color images of microstructural variations. These acoustic images reveal microstructural variations not observable with X-radiography.

Generazio, E. R.↗
NASA NTRS1989

Buoyancy effects on morphological instability during directional solidification

The onset of morphological instability during the directional solidification of a single-phase binary alloy at constant velocity vertically upwards is treated by a linear stability analysis. The case in which a heavier solute is rejected at the solidifying interface is considered, and the effect of natural convection on the critical concentration for the onset of instability is studied. For tin containing lead, a small destabilization of the system at low growth velocities, and a large increase in the wavelength of the instability at the onset are found. Calculations show that the destabilization is enhanced as the variation of density with solute concentration is reduced, and in the limit of neutrally-dense solute, there is a long wavelength instability for which the critical solute concentration is several orders of magnitude lower than that predicted by the Mullins and Sekerka (1964) analysis in the absence of convection. For the neutrally-dense solute, a simplified analysis indicates the roles played by the interface deformation and thermal convection in promoting the instability. In particular, the destabilization is very sensitive to the ratio of crystal and melt thermal conductivities.

Coriell, S. R.↗
NASA NTRS1990

Disintegration of liquid sheets

The development, stability, and disintegration of liquid sheets issuing from a two-dimensional air-assisted nozzle is studied. Detailed measurements of mean drop size and velocity are made using a phase Doppler particle analyzer. Without air flow the liquid sheet converges toward the axis as a result of surface tension forces. With airflow a quasi-two-dimensional expanding spray is formed. The air flow causes small variations in sheet thickness to develop into major disturbances with the result that disruption starts before the formation of the main break-up region. In the two-dimensional variable geometry air-blast atomizer, it is shown that the air flow is responsible for the formation of large, ordered, and small chaotic 'cell' structures.

Mansour, Adel↗
NASA NTRS1990

Low Mach number parallel and quasi-parallel shocks

The properties of low-Mach-number parallel and quasi-parallel shocks are studied using the results of one-dimensional hybrid simulations. It is shown that both the structure and ion dissipation at the shocks differ considerably. In the parallel limit, the shock remains coupled to the piston and consists of large-amplitude magnetosonic-whistler waves in the upstream, through the shock and into the downstream region, where the waves eventually damp out. These waves are generated by an ion beam instability due to the interaction between the incident and piston-reflected ions. The excited waves decelerate the plasma sufficiently that it becomes stable far into the downstream. The increase in ion temperature along the shock normal in the downstream region is due to superposition of incident and piston-rflected ions. These two populations of ions remain distinct through the downstream region. While they are both gyrophase-bunched, their counterstreaming nature results in a 180-deg phase shift in their perpendicular velocities.

Omidi, N.↗