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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 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
NASA NTRS1972

Theoretical models of photoionized intergalactic hydrogen.

The effects of ionizing radiation emitted by quasi-stellar objects on intergalactic hydrogen are studied. The hydrogen is assumed to expand with the Universe, and the amount of ionizing radiation is estimated from observations of the QSO luminosity function. The discreteness of the radiation sources is taken into account in computing the thermal history of the medium. The amount of ionizing radiation is shown to be sufficient to allow the existence of a universal medium with a temperature less than 10,000 K and a density several times the smeared-out density of luminous matter in galaxies. Such a medium would escape detection with presently available techniques.

Arons, J.↗
NASA NTRS1973

Theoretical model of vertical distributions of CO and CH4 in the mesosphere and upper stratosphere.

Carbon compounds (CO and CO2) and methane are added to an existing computer program (Shimazaki and Laird, 1970, 1972), which calculates diurnal variations in density distributions of pure oxygen compounds, hydrogen compounds, and nitrogen compounds. For the present model calculations, the original program was modified to calculate the steady-state profiles for all constituents, and then these solutions were used as the initial condition for solving the time-dependent equations. The effects of chemical reactions and vertical eddy diffusion transport on the CH4 and CO concentrations above 40 km, where one-dimensional treatment may not be too inappropriate, are discussed. Some of the 15 constituents included in the computer program interact with CO and CH4 chemically, and CH4 is more sensitive to the eddy diffusion coefficient than any other constituent. Therefore it is easier to determine the most appropriate profile of the eddy diffusion coefficient by comparing the model profile with the observations for CH4.

Shimazaki, T.↗
NASA NTRS1975

Theoretical models of magnetic field line merging. I

A review is presented of the models of magnetic field line merging defined as the process whereby plasma flows across a surface which separates regions including topologically different magnetic field lines. The models examined are characterized by uniform and antiparallel external magnetic fields. An attempt is made to simplify the presentation of the models, to clarify some doubtful mathematical points, or to extend the results to a different range of physical parameters. The models are described from a hydromagnetic point of view, with the configuration in any given case being determined by the boundary conditions. It is shown that the models developed by Sweet (1958), Parker (1957, 1963), Petschek (1964), Sonnerup (1970), and by Yeh and Axford (1970) are basically consistent, describing different aspects of the same problem; however, there is not a single model that would account for all the cases considered. The singular models and the compressible similarity models are physically not feasible.

Vasyliunas, V. M.↗
NASA NTRS1976

Theoretical models of Beta Cephei stars constructed with new radiative opacities

Equilibrium models for the main-sequence and early post-main-sequence phases of evolution are constructed for stars of 10.9 and 15 solar masses with two different initial hydrogen and metals abundances. Carson's (1976) radiative opacities are used in the calculations. It is found that the models which lie off the zero-age main sequence have cooler effective temperatures than corresponding models based on other opacities and cross the observed Beta Cephei strip in the H-R diagram only during the main phase of core hydrogen burning. The stability of radial and nonradial pulsations is studied by applying linear nonadiabatic perturbations to the models. The results show that all the modes examined are stable and that the margin of stability decreases with advancing evolution. It is suggested that the closeness of the periods of the two lowest nonradial quadrupole p-modes may be responsible for the 'beat' phenomenon observed in many Beta Cephei stars, if these modes are somehow excited. Possible instability mechanisms are evaluated, particularly the ultimate ionization of CNO elements.

Stothers, R.↗
NASA NTRS1977

Systems theoretic model of stratospheric pollution on ozone

Discrete state space analysis is applied to the development of a model for predicting the reduction in the amount of stratospheric ozone due to stratospheric pollution. The modeling approach is to segment the stratosphere into a number of altitudinal bands and to associate state variables with the state of ozone reduction in each band due to NO(x) injections. The ozone reductions in each band are then combined into an estimate of total global ozone reduction. The model proposed predicts changes in stratospheric ozone based on altitude of pollutant injection and volume of pollutant injected at that altitude. The present approach is not a detailed analysis of stratospheric constituents and their interactions, but rather an analysis procedure for modeling the results of other researchers in the field.

Wilkins, R. D.↗
NASA NTRS1977

A control theoretic model of driver steering behavior

A quantitative description of driver steering behavior such as a mathematical model is presented. The steering task is divided into two levels: (1) the guidance level involving the perception of the instantaneous and future course of the forcing function provided by the forward view of the road, and the response to it in an anticipatory open-loop control mode; (2) the stabilization level whereby any occuring deviations from the forcing function are compensated for in a closed-loop control mode. This concept of the duality of the driver's steering activity led to a newly developed two-level model of driver steering behavior. Its parameters are identified on the basis of data measured in driving simulator experiments. The parameter estimates of both levels of the model show significant dependence on the experimental situation which can be characterized by variables such as vehicle speed and desired path curvature.

Donges, E.↗
NASA NTRS1980

Theoretical model applicable to the experimental determination of surface anchoring energies of nematic liquid crystals

For a cell configuration consisting of a thin nematic layer bounded by two parallel plane surfaces, with opposing surfaces suitably treated to produce dissimilar molecular orientations, the elastic continuum theory for nematic liquid crystals was applied to derive an expression relating surface anchoring energies to elastic constants, director orientations at the substrate surfaces, and cell thickness. A numerical comparison with the elastically isotropic result over a range K sub 3 = 1.5 K sub 1 to K sub 3 = 10 K sub 1 showed the effect of elastic anisotropy could be quite significant. Surface anchoring energies calculated for anisotropic of K sub 3 = 2 K sub 1 and K sub 3 + 10 K sub 1 were approximately 50% and 500%, respectively, than the isotropic values.

Wintucky, E. G.↗
NASA NTRS1981

An assessment of theoretical models for viscous and transonic flow

Topics discussed include shear flow models, simplified models for treating separation, classical linear theory, a local linearization theory, a transonic linear theory, a transonic nonlinear theory, the experiment of Davis, and the experiment of Tijdeman. It is concluded that shear flow models, which have proven very accurate in taking into account boundary layer effects for panel flutter, are likely to be less so for lifting surface flutter. For many applications in transonic flow, transonic linear theory will be adequate.

Dowell, E. H.↗
NASA NTRS1981

Non-thermal O/1D/ produced by dissociative recombination of O2/+/ - A theoretical model and observational results

Thermal and nonthermal O(1D) number density profiles are calculated. The two populations are assumed to be coupled by a thermalization cross-section which determines the loss and production in the nonthermal and thermal populations, respectively. The sources, sinks and transport of the two populations are used to model volume emission rate profiles at 6300 A. The 6300 A brightness measured by the Visible Airglow Experiment is then used to establish the presence of the nonthermal population and to determine the thermalization cross-section.

Schmitt, G. A.↗
NASA NTRS1982

Observational constraints for a theoretical model describing the soft X-ray flare

High-resolution solar flare X-ray spectra have recently been obtained from X-ray spectrometer experiments flown on an Air Force spacecraft (P78-1) launched on 1979 February 24. Interpretation of the spectra has produced new results concerning the physical conditions and time behavior of the thermal soft X-ray emitting plasma at temperatures near 20,000,000 K. It is argued that soft and hard X-ray events are not causally related to each other, but are simply two different manifestations of flare energy release. They probably occur in different plasma volumes. The source of the preflare plasma appears to be in the cooler parts of the solar atmosphere, perhaps transition region loops with initial temperatures of 100,000 K and densities of 10 to the 11th per cu cm. Continuous energy input, rather than sequential activation of loops, is required to explain the observations. Compression coupled with chromospheric ablation may produce the high densities in coronal flare loops.

Feldman, U.↗
NASA NTRS1981

Theoretical models of X-ray emission from rich clusters of galaxies

After assessing the theory of cluster gas with polytropic equations of state, and discussing the prediction of local polytropic indices in the gas distribution, attention is given to the consequences of modifying the equation of state by thermal cooling in the central regions of the cluster. It is suggested that, if the equation of state can be understood locally, an attempt may be made at (1) predicting the X-ray luminosities of the clusters at distant cosmological epochs and (2) measuring the evolution of the cluster's potential, through observation of the evolution of X-ray luminosity and surface brightness.

Cowie, L. L.↗
NASA NTRS1982

BL Herculis stars - Theoretical models for field variables

Type II Cepheids with periods between 1 and 3 days, commonly designated as Bl Herculis stars, have been modeled here with the aim of interpreting the wide variety of light curves observed among the field variables. Previously modeled globular cluster members are used as standard calibration objects. The major finding is that only a small range of luminosities is capable of generating a large variety of light curve types at a given period. For a mass of approximately 0.60 solar mass, the models are able to reproduce the observed mean luminosities, dispersion of mean luminosities, periods, light amplitudes, light asymmetries, and phases of secondary features in the light curves of known BL Her stars. It is possible that the metal-rich variables (which are found only in the field) have luminosities lower than those of most metal-poor variables. The present revised mass for BL Her, a metal-rich object, is not significantly different from the mean mass of the metal-poor variables.

Carson, R.↗