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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 757 records · Page 42
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

Theory of droplet. Part 1: Renormalized laws of droplet vaporization in non-dilute sprays

The vaporization of a droplet, interacting with its neighbors in a non-dilute spray environment is examined as well as a vaporization scaling law established on the basis of a recently developed theory of renormalized droplet. The interacting droplet consists of a centrally located droplet and its vapor bubble which is surrounded by a cloud of droplets. The distribution of the droplets and the size of the cloud are characterized by a pair-distribution function. The vaporization of a droplet is retarded by the collective thermal quenching, the vapor concentration accumulated in the outer sphere, and by the limited percolative passages for mass, momentum and energy fluxes. The retardation is scaled by the local collective interaction parameters (group combustion number of renormalized droplet, droplet spacing, renormalization number and local ambient conditions). The numerical results of a selected case study reveal that the vaporization correction factor falls from unity monotonically as the group combustion number increases, and saturation is likely to occur when the group combustion number reaches 35 to 40 with interdroplet spacing of 7.5 diameters and an environment temperature of 500 K. The scaling law suggests that dense sprays can be classified into: (1) a diffusively dense cloud characterized by uniform thermal quenching in the cloud; (2) a stratified dense cloud characterized by a radial stratification in temperature by the differential thermal quenching of the cloud; or (3) a sharply dense cloud marked by fine structure in the quasi-droplet cloud and the corresponding variation in the correction factor due to the variation in the topological structure of the cloud characterized by a pair-distribution function of quasi-droplets.

Chiu, H. H.↗
NASA NTRS1993

Radial transport of storm time ring current ions

Radial transport of energetic ions for the development of the main phase of geomagnetic storms is investigated with data from the medium energy particle analyzer (MEPA) on the Charge Composition Explorer spacecraft, which monitored protons, helium ions, and the carbon-nitrogen-oxygen group, which is mostly dominated by oxygen ions. From a study of four geomagnetic storms, we show that the flux increase of these ions in the inner ring current region can be accounted for by an inward displacement of the ring current population by 0.5 to 3.5 R(E). There is a general trend that a larger inward displacement occurs at higher L shells than at lower ones. These results are in agreement with previous findings. The radially injected population consists of the prestorm population modified by substorm injections which occur on a much shorter time scale than that for a storm main phase. It is also found that the inward displacement is relatively independent of ion mass and energy, suggesting that the radial transport of these energetic ions is effected primarily by convective motion from a large electric field or by diffusion resulting from magnetic field fluctuations.

Lui, A. T. Y.↗
NASA NTRS1993

Geomagnetic field models incorporating frozen-flux constraints

An algorithm is developed for constructing plausible field models satisfying the frozen-flux hypothesis of Roberts and Scott (1965), which supposes that, for short time intervals, diffusion can be neglected. The algorithm is based on a new parameterization of the field in terms of its radial component B(r) at the core-mantle boundary (CMB). The model consists of values of B(r) at a finite set of points on the CMB, together with a rule for interpolating the values to other points. The parameterization of the B(r) is used to construct field models satisfying the frozen-flux hypothesis for the epochs 1945.5 and 1980.

Constable, Catherine G.↗
NASA NTRS1991

Computations of the three-dimensional flow and heat transfer within a coolant passage of a radial turbine blade

A numerical code is developed for computing three-dimensional, turbulent, compressible flow within coolant passages of turbine blades. The code is based on a formulation of the compressible Navier-Stokes equations in a rotating frame of reference in which the velocity dependent variable is specified with respect to the rotating frame instead of the inertial frame. The algorithm employed to obtain solutions to the governing equation is a finite-volume LU algorithm that allows convection, source, as well as diffusion terms to be treated implicitly. In this study, all convection terms are upwind differenced by using flux-vector splitting, and all diffusion terms are centrally differenced. This paper describes the formulation and algorithm employed in the code. Some computed solutions for the flow within a coolant passage of a radial turbine are also presented.

Shih, T. I.-P.↗
DOE OSTI2015

Measurements and modeling of the impact of weak magnetic fields on the plasma properties of a planar slot antenna driven plasma source

The radial line slot antenna plasma source is a type of surface wave plasma source driven by a planar slot antenna. Microwave power is transmitted through a slot antenna structure and dielectric window to a plasma characterized by a generation zone adjacent to the window and a diffusion zone that contacts a substrate. The diffusion zone is characterized by a very low electron temperature. This renders the source useful for soft etch applications and thin film deposition processes requiring low ion energy. Another property of the diffusion zone is that the plasma density tends to decrease from the axis to the walls under the action of ambipolar diffusion at distances far from where the plasma is generated. A previous simulation study [Yoshikawa and. Ventzek, J. Vac. Sci. Technol. A 31, 031306 (2013)] predicted that the anisotropy in transport parameters due to weak static magnetic fields less than 50 G could be leveraged to manipulate the plasma profile in the radial direction. These simulations motivated experimental tests in which weak magnetic fields were applied to a radial line slot antenna source. Plasma absorption probe measurements of electron density and etch rate showed that the magnetic fields remote from the wafer were able to manipulate both parameters. A summary of these results is presented in this paper. Argon plasma simulation trends are compared with experimental plasma and etch rate measurements. A test of the impact of magnetic fields on charge up damage showed no perceptible negative effect.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Suppression of fiber modal noise induced radial velocity errors for bright emission-line calibration sources

Modal noise in optical fibers imposes limits on the signal-to-noise ratio (S/N) and velocity precision achievable with the next generation of astronomical spectrographs. This is an increasingly pressing problem for precision radial velocity spectrographs in the near-infrared (NIR) and optical that require both high stability of the observed line profiles and high S/N. Many of these spectrographs plan to use highly coherent emission-line calibration sources like laser frequency combs and Fabry-Perot etalons to achieve precision sufficient to detect terrestrial-mass planets. These high-precision calibration sources often use single-mode fibers or highly coherent sources. Coupling light from single-mode fibers to multi-mode fibers leads to only a very low number of modes being excited, thereby exacerbating the modal noise measured by the spectrograph. We present a commercial off-the-shelf solution that significantly mitigates modal noise at all optical and NIR wavelengths, and which can be applied to spectrograph calibration systems. Our solution uses an integrating sphere in conjunction with a diffuser that is moved rapidly using electrostrictive polymers, and is generally superior to most tested forms of mechanical fiber agitation. We demonstrate a high level of modal noise reduction with a narrow bandwidth 1550 nm laser. Our relatively inexpensive solution immediately enables spectrographs to take advantage of the innate precision of bright state-of-the art calibration sources by removing a major source of systematic noise.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2016

Thermal Diffusivity Measurement of Irradiated SiC-SiC Composite Material

Silicon Carbide fiber and matrix composite (SiC-SiC) is being considered as a fuel cladding candidate because of its high melting point, low susceptibility to corrosion, good thermal and mechanical shock resistance, and low degradation of mechanical properties under irradiation. The thermal properties of SiC-SiC composite depend heavily on the fabrication process, thus cannot be simply referenced from available database. Similarly, the influence of irradiation on SiC-SiC composite thermal properties needs to be measured to provide general trends of property degradation. For ion irradiated samples, the damaged layer is only a few to a few tens of micrometers. It is impossible to directly measure the bulk property change of a SiC-SiC composite because of the thin, damaged layer. This paper focuses on a methodology that is based on microscale measurement of fiber and matrix properties to determine the overall bulk property change due to ion irradiation. Laser-based photothermal reflectance technique is one of the ideal methods to measure local thermal conductivity and diffusivity with high spatial resolution. The main principle of this technique is to heat the solid sample using an intensity-modulated laser, and scan the sample with a constant intensity continuous-wave (CW) laser to detect the phase lag of the thermal wave propagation. On bulk, isotropic material, if the heating laser intensity is modulated as a sine wave, thermal diffusivity of the sample can be derived from the phase lag, scan distance and modulation frequency in a straight forward manner. By using more specific analytical models, thermal conductivity and diffusivity in complicated cases, such as layered structure materials, composite materials, and/or anisotropic materials, are measurable as well. In this work, a new measurement approach of SiC-SiC composite thermal diffusivity is presented. Due to the micrometer spatial resolution of the photothermal reflectance technique, the measurements are performed locally on fibers along both radial and axial directions, and on the matrix. The influence of Silicon ion irradiation (75 MeV, 10 dpa) is then examined by comparing the experimental results of samples before and after irradiation. (authors)

36 MATERIALS SCIENCE↗
DOE OSTI2012

COSMIC-RAY DIFFUSION IN A SECTORED MAGNETIC FIELD IN THE DISTANT HELIOSHEATH

Very high intensities of galactic cosmic rays measured by Voyager 1 in the heliosheath appear to be incompatible with the presence of a modulation 'wall' near the heliopause produced by a pile up of the heliospheric magnetic field. We propose that the modulation wall is a structure permeable to cosmic rays as a result of a sectored magnetic field topology compressed by plasma slowdown on approach to the heliopause and stretched to high latitudes by latitudinal flows in the heliosheath. The tightly folded warped current sheet permits efficient cosmic-ray transport in the radial direction via a drift-like mechanism. We show that when stochastic variations in the sector widths are taken into account, particle transport becomes predominantly diffusive both along and across the magnetic sectors. Using a test-particle model for cosmic rays in the heliosheath we investigate the dependence of the diffusion coefficients on the properties of the sector structure and on particle energy.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2022

BEBOP II: sensitivity to sub-Saturn circumbinary planets using radial-velocities

ABSTRACT BEBOP is a radial-velocity survey that monitors a sample of single-lined eclipsing binaries, in search of circumbinary planets by using high-resolution spectrographs. Here, we describe and test the methods we use to identify planetary signals within the BEBOP data and establish how we quantify our sensitivity to circumbinary planets by producing detection limits. This process is made easier and more robust by using a diffusive nested sampler. In the process of testing our methods, we notice that contrary to popular wisdom, assuming circular orbits in calculating detection limits for a radial-velocity survey provides overoptimistic detection limits by up to $40{{\ \rm per\ cent}}$ in semi-amplitude with implications for all radial-velocity surveys. We perform example analyses using three BEBOP targets from our Southern HARPS survey. We demonstrate for the first time a repeated ability to reach a residual root mean squared scatter of $3\, \rm m\, s^{-1}$ (after removing the binary signal), and find that we are sensitive to circumbinary planets with masses down to that of Neptune and Saturn, for orbital periods up to $1000\, \rm d$.

Standing, Matthew R. (ORCID:0000000276088905)↗
NASA NTRS1981

The role of fluctuation-induced transport in a toroidal plasma with strong radial electric fields

Previous work employing digitally implemented spectral analysis techniques is extended to demonstrate that radial fluctuation-induced transport is the dominant ion transport mechanism in an electric field dominated toroidal plasma. Such transport can be made to occur against a density gradient, and hence may have a very beneficial effect on confinement in toroidal plasmas of fusion interest. It is shown that Bohm or classical diffusion down a density gradient, the collisional Pedersen-current mechanism, and the collisionless electric field gradient mechanism described by Cole (1976) all played a minor role, if any, in the radial transport of this plasma.

Roth, J. R.↗
DOE OSTI2014

Effect of interwire separation on growth kinetics and properties of site-selective GaAs nanowires

We report tuning of the growth kinetics, geometry, and properties of autocatalytic GaAs nanowires (NW) by precisely controlling their density on SiO{sub 2}-mask patterned Si (111) substrates using selective area molecular beam epitaxy. Using patterned substrates with different mask opening size (40–120 nm) and pitch (0.25–3 μm), we find that the NW geometry (length, diameter) is independent of the opening size, in contrast to non-catalytic GaAs NWs, whereas the NW geometry strongly depends on pitch, i.e., interwire separation and NW density. In particular, two distinct growth regimes are identified: a diffusion-limited regime for large pitches (low NW density) and a competitive growth regime for smaller pitches (high NW density), where axial and radial NW growth rates are reduced. The transition between these two regimes is significantly influenced by the growth conditions and shifts to smaller pitches with increasing As/Ga flux ratio. Ultimately, the pitch-dependent changes in growth kinetics lead to distinctly different photoluminescence properties, highlighting that mask template design is a very critical parameter for tuning intrinsic NW properties.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
NASA NTRS1999

Modelling of Spherical Gas Bubble Oscillations and Sonoluminescence

The discovery of single-bubble sonoluminescence has led to a renewed interest in the forced radial oscillations of gas bubbles. Many of the more recent studies devoted to this topic have used several simplifications in the modelling, and in particular in accounting for liquid compressibility and thermal processes in the bubble. In this paper the significance of these simplifications is explored by contrasting the results of Lohse and co-workers with those of a more detailed model. It is found that, even though there may be little apparent difference between the radius-versus time behaviour of the bubble as predicted by the two models, quantities such as the spherical stability boundary and the threshold for rectified diffusion are affected in a quantitatively significant way. These effects are a manifestation of the subtle dependence upon dissipative processes of the phase of radial motion with respect to the driving sound field. The parameter space region, where according to the theory of Lohse and co-workers, sonoluminescence should be observable, is recalculated with the new model and is found to be enlarged with respect to the earlier estimate. The dependence of this parameter region on sound frequency is also illustrated.

Prosperetti, A.↗
DOE OSTI2011

THE LARGE-SCALE MAGNETIC FIELDS OF ADVECTION-DOMINATED ACCRETION FLOWS

We calculate the advection/diffusion of the large-scale magnetic field threading an advection-dominated accretion flow (ADAF) and find that the magnetic field can be dragged inward by the accretion flow efficiently if the magnetic Prandtl number P{sub m}={eta}/{nu}{approx}1. This is due to the large radial velocity of the ADAF. It is found that the magnetic pressure can be as high as {approx}50% of the gas pressure in the inner region of the ADAF close to the black hole horizon, even if the external imposed homogeneous vertical field strength is {approx}< 5% of the gas pressure at the outer radius of the ADAF, which is caused by the gas in the ADAF plunging rapidly to the black hole within the marginal stable circular orbit. In the inner region of the ADAF, the accretion flow is significantly pressured in the vertical direction by the magnetic fields, and therefore its gas pressure can be two orders of magnitude higher than that in the ADAF without magnetic fields. This means that the magnetic field strength near the black hole is underestimated by assuming equipartition between magnetic and gas pressure with the conventional ADAF model. Our results show that the magnetic field strength of the flow near the black hole horizon can be more than one order of magnitude higher than that in the ADAF at {approx}3R{sub g} (R{sub g} = 2GM/c{sup 2}), which implies that the Blandford-Znajek mechanism could be more important than the Blandford-Payne mechanism for ADAFs. We find that the accretion flow is decelerated near the black hole by the magnetic field when the external imposed field is strong enough or the gas pressure of the flow is low at the outer radius, or both. This corresponds to a critical accretion rate, below which the accretion flow will be arrested by the magnetic field near the black hole for a given external imposed field. In this case, the gas may accrete as magnetically confined blobs diffusing through field lines in the region very close to the black hole horizon, similar to those in compact stars. Our calculations are also valid for the case that the inner ADAF connects to the outer cold thin disk at a certain radius. In this case, the advection of the external fields is quite inefficient in the outer thin disk due to its low radial velocity, and the field lines thread the disk almost vertically, while these field lines can be efficiently dragged inward by the radial motion of the inner ADAF.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

Microstructure characterization of electric field assisted sintering (EFAS) sintered metallic and ceramic materials using local thermal diffusivity measurement

Electric Field Assisted Sintering (EFAS, also referred to as spark plasma sintering) is a powerful technology for the consolidation of powder materials. The high heating rate during the sintering process is critical for minimizing energy consumption, but it can also cause microstructure heterogeneities in sintered parts, such as spatially varied porosity. The examination of localized porosity usually requires the use of a scanning electron microscope with a carefully prepared surface. In this paper, photothermal radiometry is used to measure local thermal diffusivity and extract localized porosity of EFAS-sintered parts by using a percolation-threshold model. Applying this approach, we identified the radial position-dependent porosity variation in EFAS parts, which is likely formed due to the large temperature gradient during the sintering process. This approach has a unique advantage because it can measure samples with minimal or no surface preparation, enabling the possibility of in situ characterization in EFAS with proper system modification. Necessary modifications on the measurement approach for EFAS deployment and in situ characterization are also discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2015

One-dimensional time-dependent fluid model of a very high density low-pressure inductively coupled plasma

A time-dependent two-fluid model has been developed to understand axial variations in the plasma parameters in a very high density (peak n e~ > 5x10 19 m –3 ) argon inductively coupled discharge in a long 1.1 cm radius tube. The model equations are written in 1D, with radial losses to the tube walls accounted for by the inclusion of effective particle and energy sink terms. The ambipolar diffusion equation and electron energy equation are solved to find the electron density n e (z,t) and temperature T e (z,t), and the populations of the neutral argon 4s metastable, 4s resonant, and 4p excited state manifolds are calculated in order to determine the stepwise ionization rate and calculate radiative energy losses. The model has been validated through comparisons with Langmuir probe ion saturation current measurements; close agreement between the simulated and measured axial plasma density profiles and the initial density rise rate at each location was obtained at p Ar = 30-60 mTorr. Lastly, we present detailed results from calculations at 60 mTorr, including the time-dependent electron temperature, excited state populations, and energy budget within and downstream of the radiofrequency (RF) antenna.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2013

GALACTIC COSMIC-RAY MODULATION IN A REALISTIC GLOBAL MAGNETOHYDRODYNAMIC HELIOSPHERE

To understand the behavior of cosmic-ray modulation seen by the two Voyager spacecraft in the region near the termination shock (TS) and in the heliosheath at a distance of >100 AU, a realistic magnetohydrodynamic global heliosphere model is incorporated into our cosmic-ray transport code, so that the detailed effects of the heliospheric boundaries and their plasma/magnetic geometry can be revealed. A number of simulations of cosmic-ray modulation performed with this code result in the following conclusions. (1) Diffusive shock acceleration by the TS can significantly affect the level of cosmic-ray flux and, in particular, its radial gradient profile in the region near the TS and in the inner heliosheath. (2) The radial profile of cosmic-ray flux strongly depends on longitude. There is a slight north-south asymmetry due to an asymmetric TS, but the larger difference in the radial profiles comes from longitudinal variation. Voyager 1 and 2 are separated by {approx} 40 Degree-Sign in longitude. Simulations in these two directions show a large difference in the radial profile of cosmic-ray flux. Thus, it is not appropriate to determine the cosmic-ray radial gradient by directly using the two-point Voyager measurements. Various other simulations are also performed to show how sensitively the modulation level depends on latitude, cosmic-ray energy, and interstellar spectrum.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2026

Understanding Large-Scale Dynamos in Unstratified Rotating Shear Flows

For this work, we combine simulations with new analyses that overcome previous pitfalls to explicate how nonhelical mean-field dynamos grow and saturate in unstratified, magnetorotationally driven turbulence. Shear of the mean radial magnetic field amplifies the azimuthal component. Radial fields are regenerated by velocity fluctuations that induce shear of radial magnetic fluctuations, followed by Lorentz and Coriolis forces that source a negative off-diagonal component in the turbulent diffusivity tensor. We present a simple schematic to illustrate this dynamo growth. A different part of the Lorentz force forms a third-order correlator in the mean electromotive force that saturates the dynamo.

accretion disk & black-hole plasma↗
NASA NTRS1982

Diffusive-convective physical vapor transport of PbTe from a Te-rich solid source

Crystal growth of PbTe by physical vapor transport (sublimation) in a closed ampoule is governed by the vapor species in thermal equilibrium with the solid compound. Deviations from stoichiometry in the source material cause diffusion limitation of the transport rate, which can be modified by natural (gravity-driven) convection. Mass-transport experiments have been performed using Te-rich material wherein sublimation rates have been measured in order to study the effects of natural convection in diffusion-limited vapor transport. Linear velocities for both crystal growth and evaporation (back sublimation) have been measured for transport in the direction of gravity, horizontally, and opposite to gravity. The experimental results are discussed in terms of both the one-dimensional diffusive-advective model and current, more sophisticated theory which includes natural convection. There is some evidence that convection effects from radial temperature gradients and solutal density gradients have been observed.

Zoutendyk, J.↗