SEARCH · Engineering Papers

Results for “theoretical models”

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.

SourceResearch connectionFrom yearThrough year

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 487 records · Page 27

All sources NASA NTRS DOE OSTI

NASA NTRS1997

UVCS/SOHO empirical models of solar coronal holes

A self-consistent empirical model for the major plasma parameters in a solar-minimum coronal hole was developed using the ultraviolet coronagraph spectrometer (UVCS) operating onboard the Solar and Heliospheric Observatory (SOHO). The radial and latitudinal distribution of density, velocity, and kinetic temperature for electrons, neutral hydrogen, and ionized oxygen was obtained. The data were acquired during November 1996 and April 1997. The model provides experimental values which can be used to constrain theoretical models of the fast solar wind. The implications on various models of coronal heating and acceleration are discussed.

Cranmer, S. R.↗

NASA NTRS2018

Constraints on the chemical enrichment history of the Perseus Cluster of galaxies from high-resolution X-ray spectroscopy

High-resolution spectroscopy of the core of the Perseus Cluster of galaxies, using the Hitomi satellite above 2 keV and the XMM–Newton Reflection Grating Spectrometer at lower energies, provides reliable constraints on the abundances of O, Ne, Mg, Si, S, Ar, Ca, Cr, Mn, Fe, and Ni. Accounting for all known systematic uncertainties, the Ar/Fe, Ca/Fe, and Ni/Fe ratios are determined with a remarkable precision of less than 10 per cent, while the constraints on Si/Fe, S/Fe, and Cr/Fe are at the 15 per cent level, and Mn/Fe is measured with a 20 percent uncertainty. The average biases in determining the chemical composition using archival CCD spectra from XMM–Newton and Suzaku typically range from 15 to 40 per cent. A simple model in which the enrichment pattern in the Perseus Cluster core and the protosolar nebula are identical gives a surprisingly good description of the high-resolution X-ray spectroscopy results, with X^2=10.7 for 10 degrees of freedom. However, this pattern is challenging to reproduce with linear combinations of existing supernova nucleosynthesis calculations, particularly given the precise measurements of intermediateα-elements enabled by Hitomi. We discuss in detail the degeneracies between various supernova progenitor models and explosion mechanisms, and the remaining uncertainties in these theoretical models. We suggest that including neutrino physics in the core-collapse supernova yield calculations may improve the agreement with the observed pattern ofα-elements in the Perseus Cluster core. Our results provide a complementary benchmark for testing future nucleosynthesis calculations required to understand the origin of chemical elements.

A Simionescu↗

NASA NTRS1991

Long-term prediction test procedure for most ICs, based on linear response theory

Experimentally, thermal annealing is known to be a factor which enables a number of different integrated circuits (IC's) to recover their operating characteristics after suffering radiation damage in the space radiation environment; thus, decreasing and limiting long term cumulative total-dose effects. This annealing is also known to be accelerated at elevated temperatures both during and after irradiation. Linear response theory (LRT) was applied, and a linear response function (LRF) to predict the radiation/annealing response of sensitive parameters of IC's for long term (several months or years) exposure to the space radiation environment were constructed. Compressing the annealing process from several years in orbit to just a few hours or days in the laboratory is achieved by subjecting the IC to elevated temperatures or by increasing the typical spaceflight dose rate by several orders of magnitude for simultaneous radiation/annealing only. The accomplishments are as follows: (1) the test procedure to make predictions of the radiation response was developed; (2) the calculation of the shift in the threshold potential due to the charge distribution in the oxide was written; (3) electron tunneling processes from the bulk Si to the oxide region in an MOS IC were estimated; (4) in order to connect the experimental annealing data to the theoretical model, constants of the model of the basic annealing process were established; (5) experimental data obtained at elevated temperatures were analyzed; (6) time compression and reliability of predictions for the long term region were shown; (7) a method to compress test time and to make predictions of response for the nonlinear region was proposed; and (8) nonlinearity of the LRF with respect to log(t) was calculated theoretically from a model.

Litovchenko, V.↗

NASA NTRS1975

Analytical simulation of the far-field jet noise and the unsteady jet flow-field by a model of periodic shedding of vortex ring from the jet exit

The construction of a theoretical flow field due to shedding of vortex rings, the identification of the controlling parameters, and the determination of whether the theoretical model successfully simulated the unsteady pressure field near jet (and consequently the far field noise) was studied. The basic parameters contained in the analytic solutions were the epoch at which a vortex ring was shed near the jet exit and the eddy viscosity coefficient. These parameters were identified from the experimental data for the real-time pressure and from the spread of the mixing layer of the jet. Results of the theoretical analysis show good qualitative agreement with the experimental data.

Liu, C. H.↗

NASA NTRS1993

The ultraviolet absorption spectrum of CO - Applications to planetary atmospheres

Laboratory gas-phase photoabsorption cross sections of the CO Cameron 0-0 band and the underlying pseudocontinuum have been measured at a temperature of 147 K and pressures of about 200 mbar, conditions similar to ambient in various planetary and satellite stratospheres in the solar system. A theoretical modeling program has also been used to calculate the band's spectrum. Agreement between the theoretical and the experimental spectra is very good. Models suggest that the observations of the CO Cameron band using the Hubble Space Telescope will be straightforward for Mars, but marginal for Titan.

White, H. P.↗

NASA NTRS1980

Exploratory models of the earth's thermal regime during segregation of the core

Some simple exploratory theoretical models of the thermal effects of core segregation have been investigated, assuming an initially homogeneous earth and including convective heat transport through a 'parameterized convection' approximation. The results indicate that either (1) mantle temperatures 30% or more above present values may have resulted from the gravitational energy released during core segregation, (2) the earth retained very little of its accretional energy, (3) core segregation lasted for one billion years or more, or (4) the earth accreted heterogeneously. Option 3 seems to be precluded by terrestrial lead isotope data, and the alternatives each raise substantial questions concerning the mechanics, chemistry, and petrology of the earth's early history. There is no recognized evidence for the early hot phase of option 1, and option 4 implies, among other things, an analogous early hot phase. Although it has not been favored, option 2 may be viable.

Davies, G. F.↗

NASA NTRS1995

Ultraviolet extinction to 10.8 inverse microns

UV extinction curves that represent observed variations i diffuse clouds (Zeta Oph, Xi Per, and Sigma Sco) have been derived by combining IUE and Voyager data and have been modeled by a multicomponent size distribution of silicate and carbonaceous giants. We have used theoretical model atmospheres as our reddening-free standards, and, in the case of Zeta Oph, also compare with the use of a stellar standard. An attempt was made to classify the UV spectra via theoretical spectral indices, but still missing line opacity appears to limit the use of this method at the present. The modeling includes five possible grain ingrediants, largge (0.015-0.25 micrometers) and small (0.005 micrometers) silicate grains, similarly large and small carbonaceous grains, and polycyclic aromatic hydrocarbons (PAHs). The observed extinction curves are fitted by the model via nonlinear Chi-square-minimization, varying the fractional amount of cosmic Si and C that are locked up in each igredient. The observed variation from 'steep' to 'flat' UV extinction curves is found to correspond to a modest model variation in the lower limit of the power-law size distribution of the large grains. Graphite is found to be necessary grain ingredient that cannot be replaced by the laboratory amorphous carbon materials. Carbonaceous mantles on the larger silicate grains provide no good fits to the observed curves. Using absorption properties of neutral laboratory PAHs limits PAHs to a few percent of the cosmic C abundance for acceptable fits that also give values for R(sub V) close to the observed values.

Aannestad, Per A.↗

NASA NTRS1975

Flame stability in combusting turbulent jets

A simplified theoretical model for turbulent methane/air flames from a subscale power plant burner is developed and applied to predict the stability of flames as a function of (1) ambient atmosphere temperature and composition, (2) initial preheat temperature, and (3) percentage of external product gas recirculation. The flame model equates the rate at which mass is supplied per unit volume to the rate it is chemically reacted per unit volume using a one-step reaction and a perfectly stirred reactor (PSR). The model contains a free parameter which is evaluated by comparing the theoretical results to experimental results available in the literature. The stability results show that premixed, turbulent flames are stable over a wide equivalence ratio range if the ambient gas is air; however, if combustion products are entrained, the stability is sharply reduced.

Nelson, H. F.↗

NASA NTRS1976

Scanning laser-velocimeter surveys and analysis of multiple vortex wakes of an aircraft

A laser velocimeter capable of rapidly scanning a flow field while simultaneously sensing two components of the velocity was used to measure the vertical and streamwise velocity structure 1.5 spans downstream in the wake of a model typical of a large subsonic transport (Boeing 747). This flow field was modeled by a superposition of axisymmetric vortices with finite cores. This theoretical model was found to agree with the measured velocities everywhere except where two vortices were in close proximity. Vortex strengths derived from the span loading on the wing as predicted by vortex-lattice theory also agree with the present measurements. The axisymmetric vortex model used herein is a useful tool for analytically investigating the vortex wakes of aircraft.

Corsiglia, V. R.↗

NASA NTRS1980

Semiempirical models of chromospheric flare regions

Homogeneous plane-parallel semiempirical flare model atmospheres which reproduce observations in lines and continua of H I, Si I, C I, Ca II, and Mg II have a thin transition zone at the top of the enhanced chromosphere, indicating a significant amount of heating from the zone to the temperature minimum level. The minimum temperature is located deeper and is higher than in the quiet-sun and active-region models. The results do not agree with the particle-heated theoretical models, and it is suggested that the models of Brown (1973) and Henoux and Nakagawa (1977, 1978) do not include an essential term for heat conduction in their energy balance equations. It is concluded that substantial Ly-alpha radiative heating occurs in the upper chromosphere resulting from the conductive energy flux in the transition zone where the Ly-alpha line cools the gas.

Machado, M. E.↗

NASA NTRS1984

IUE observations of DQ Herculis and its nebula, and the nature of the cold nova shells

The nebula ejected in the 1934 outburst of the classical nova DQ Her is remarkable for its unprecedentedly low temperature of Te 500 K as measured by Williams et al. (1978). In this paper, IUE observations are combined with Steward optical spectra. It is confirmed that the gas is quite cold. It is further shown that the gas is ionized by the radiation field of the central object. X-ray, ultraviolet, optical and infrared observations of the underlying binary are combined with the extreme-ultraviolet continuum deduced from the level of ionization of the nebula to obtain a composite energy distribution for the central object. This energy distribution bears no resemblance to that predicted by theoretical models of accretion disks. Photoionization models of the nebula using the deduced continuum, as well as theoretical accretion disk continua, are presented to show that the low electron temperature is the result of the very high metal abundances which characterize nova shells. Infrared fine-structure lines are efficient coolants, and low temperatures are achieved for a wide variety of radiation fields. The implications of these results for nebulae surrounding other old novae are discussed.

Ferland, G. J.↗

NASA NTRS1984

Ionosphere-magnetosphere coupling and convection

The following international Magnetospheric Study quantitative models of observed ionosphere-magnetosphere events are reviewed: (1) a theoretical model of convection; (2) algorithms for deducing ionospheric current and electric-field patterns from sets of ground magnetograms and ionospheric conductivity information; and (3) empirical models of ionospheric conductances and polar cap potential drop. Research into magnetic-field-aligned electric fields is reviewed, particularly magnetic-mirror effects and double layers.

Wolf, R. A.↗

NASA NTRS1984

NGC 7213 - A key to the nature of liners?

New optical spectra of the nearby S0 galaxy NGC 7213 are analyzed. The observations are described and a brief overview of the spectrum is given. The subtraction of the stellar component is presented, and the measurement of various emission lines is discussed in detail. Important physical parameters are derived, including those pertaining to atomic data, line widths, physical conditions, geometry and kinematics, the Balmer decrement, and ultraviolet emission lines. The Seyfert characteristics of NGC 7213 are reviewed, and the galaxy's shock features are reconciled with photoionization models. The presented arguments may apply to classical liners as well. Theoretical models that can help explain the relationships discovered among the emission lines are examined.

Filippenko, A. V.↗

NASA NTRS1985

Extension of ground-based testing for large space structures

A major concern for future large space structures is the ability to verify their dynamic characteristics by ground test. This article presents the results of the multiple boundary conditions test (MBCT) approach, which provides a complete ground test of a large structure that will provide, in turn, the data necessary to construct a test-verified final mathematical model. Theoretical studies indicate that this approach can provide a better final model than a ground test of the full-scale very flexible structure in a 1-g field. The approach is demonstrated by a mathematical simulation.

Wada, B. K.↗

NASA NTRS1986

Direct solar-pumped iodine laser amplifier

In order to evaluate the feasibility of the solar pumped dye laser, the parametric study of a dye laser amplifier pumped by a solar simulator and flashlamp was carried out, and the amplifier gains were measured at various pump beam irradiances on the dye cell. Rhodamine 6G was considered as a candidate for the solar pumped laser because of its good utilization of the solar spectrum and high quantum efficiency. The measurement shows that a solar concentration of 20,000 is required to reach the threshold of the dye. The work to construct a kinetic model algorithm which predicts the output parameter of laser was progressed. The kinetic model was improved such that there is good agreement between the theoretical model and experimental data for the systems defined previously as flashlamp pumped laser oscillator, and the long path length solar pumped laser.

Han, Kwang S.↗

NASA NTRS1989

An observational study of stratocumulus entrainment and thermodynamics

The stratocumulus (SC) entrainment and thermodynamics are studied using data from the Dynamics and Chemistry of Marine Stratocumulus (DYCOMS) experiment. The rate of entrainment of air from the free troposphere into the cloud-topped PBL is estimated using a technique based on the measurement of ozone flux and mean distribution. The average measured value of entrainment rate was found to be 3.0 mm/s with a range of 1.0 to 5.0 mm/s for cloudy cases. Thermodynamic budgets are constructed for eight DYCOMS cases. It was found that the divergence of the solar radiative flux is an important component of the boundary-layer energetics during midday, the factor which must be accounted for in mixed-layer models. The net longwave radiative flux profiles show good agreement with theoretical models, but an unambiguous partitioning of the flux divergence between inversion and mixed layers could not be established.

Kawa, S. R.↗

NASA NTRS1999

The Physics of Flaring Atmospheres

The objective of this project is to study the structure of heated and flaring atmospheric models, including the corona, chromosphere, and photosphere, by means of both theoretical modeling and data analysis.

Fisher, George H.↗

NASA NTRS2000

Optical Performance Modeling of FUSE Telescope Mirror

We describe the Metrology Data Processor (METDAT), the Optical Surface Analysis Code (OSAC), and their application to the image evaluation of the Far Ultraviolet Spectroscopic Explorer (FUSE) mirrors. The FUSE instrument - designed and developed by the Johns Hopkins University and launched in June 1999 is an astrophysics satellite which provides high resolution spectra (lambda/Delta(lambda) = 20,000 - 25,000) in the wavelength region from 90.5 to 118.7 nm The FUSE instrument is comprised of four co-aligned, normal incidence, off-axis parabolic mirrors, four Rowland circle spectrograph channels with holographic gratings, and delay line microchannel plate detectors. The OSAC code provides a comprehensive analysis of optical system performance, including the effects of optical surface misalignments, low spatial frequency deformations described by discrete polynomial terms, mid- and high-spatial frequency deformations (surface roughness), and diffraction due to the finite size of the aperture. Both normal incidence (traditionally infrared, visible, and near ultraviolet mirror systems) and grazing incidence (x-ray mirror systems) systems can be analyzed. The code also properly accounts for reflectance losses on the mirror surfaces. Low frequency surface errors are described in OSAC by using Zernike polynomials for normal incidence mirrors and Legendre-Fourier polynomials for grazing incidence mirrors. The scatter analysis of the mirror is based on scalar scatter theory. The program accepts simple autocovariance (ACV) function models or power spectral density (PSD) models derived from mirror surface metrology data as input to the scatter calculation. The end product of the program is a user-defined pixel array containing the system Point Spread Function (PSF). The METDAT routine is used in conjunction with the OSAC program. This code reads in laboratory metrology data in a normalized format. The code then fits the data using Zernike polynomials for normal incidence systems or Legendre-Fourier polynomials for grazing incidence systems. It removes low order terms from the metrology data, calculates statistical ACV or PSD functions, and fits these data to OSAC models for the scatter analysis. In this paper we briefly describe the laboratory image testing of FUSE spare mirror performed in the near and vacuum ultraviolet at John Hopkins University and OSAC modeling of the test setup performed at NASA/GSFC. The test setup is a double-pass configuration consisting of a Hg discharge source, the FUSE off-axis parabolic mirror under test, an autocollimating flat mirror, and a tomographic imaging detector. Two additional, small fold flats are used in the optical train to accommodate the light source and the detector. The modeling is based on Zernike fitting and PSD analysis of surface metrology data measured by both the mirror vendor (Tinsley) and JHU. The results of our models agree well with the laboratory imaging data, thus validating our theoretical model. Finally, we predict the imaging performance of FUSE mirrors in their flight configuration at far-ultraviolet wavelengths.

Saha, Timo T.↗