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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 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
NASA NTRS1978

Wall region of a relaxing three-dimensional incompressible turbulent boundary layer

A three-dimensional turbulent boundary layer relaxing behind a transverse hump (shaped as a 30-deg swept 5-ft chord wing-type model) was studied in a low-speed wind tunnel. Data obtained with hot-wire probes showed that the apparent dimensionless velocity profiles in the viscous sublayer prevail universally; evidence for wall similarity in the relaxing flow field was confirmed in the form of a log law. An unusual region of slightly decreasing cross-flow angle was found in collateral regions, and a near-wall noncollateral flow was posited. Streamwise relaxation of the mean flow field was also investigated.

Hebbar, K. S.↗
NASA NTRS1979

Molecular collision processes in the presence of picosecond laser pulses

Radiative transitions in molecular collision processes taking place in the presence of picosecond pulses are studied within a semiclassical formalism. An expression for adiabatic potential surfaces in the electronic-field representation is obtained, which directly leads to the evaluation of transition probabilities. Calculations with a Landau-Zener-type model indicate that picosecond pulses can be much more effective in inducing transitions than a single long pulse of the same intensity and the same total energy, if the intensity is sufficiently high that the perturbation treatment is not valid.

Lee, H. W.↗
NASA NTRS1980

An assessment of the risk arising from electrical effects associated with the release of carbon fibers from general aviation aircraft fires

A Poisson type model was developed and exercised to estimate the risk of economic losses through 1993 due to potential electric effects of carbon fibers released from United States general aviation aircraft in the aftermath of a fire. Of the expected 354 annual general aviation aircraft accidents with fire projected for 1993, approximately 88 could involve carbon fibers. The average annual loss was estimated to be about $250 (1977 dollars) and the likelihood of exceeding $107,000 (1977 dollars) in annual loss in any one year was estimated to be at most one in ten thousand.

Rosenfield, D.↗
NASA NTRS1982

Numerical analysis of confined turbulent flow

The considered investigation is concerned with the development of an efficient computational method for obtaining a physical understanding of an internal turbulent field. The employed approach makes use of a 'two equation' type model for the turbulence to obtain the numerical solution of a two-dimensional confined turbulent flow. The mean flow governing equations are considered along with the governing equation of the mean temperature and concentrations, and the boundary conditions. The numerical procedure for solving the turbulent flow is discussed, taking into account an approximation to the nonlinear terms, and the inner and outer coupling. Attention is given to a stability convergence analysis, the stability characteristics, and computational examples.

Lin, A.↗
NASA NTRS1983

Particle size distributions in Saturn's rings from Voyager 1 radio occultation

Information on Saturn ring particle sizes obtained with the Voyager 1 ring occultation experiment is discussed. The theory underlying the determination of the particle size distribution is presented, including differential extinction and inversion of the scattered signal. Experimental observations and results for the observed spectra, differential cross sections, suprameter and sub- to suprameter size distributions are presented. The size and mass distributions both cut off sharply at about 4-5 m; the mass distribution peaks over the 3-4 m size range for four ring system features at 1.35, 1.51, 2.01, and 2.12 Saturn radii. A power-law type model is consistent with the data over a limited size range of 0.01 to 1 m. The fractional contribution of the suprameter particles to the microwave opacity for the four features appears to be about 1/3, 1/3, 2/3, and 1, respectively, and their cumulative surface mass per unit area are about 11, 16, 41, and 132 g/sq cm if the particles are solid water ice.

Marouf, E. A.↗
NASA NTRS1984

The mesoscale stability of entrainment into cloud-topped mixed layers

The Lilly-type models for stratocumulus-capped mixed layers are shown to allow for a mesoscale instability in which mesoscale fluctuations of buoyancy and humidity are reinforced in phase by entrainment. In a model of an AMTEX mixed layer, this mesoscale instability has a maximum growth rate of about 0.00002 per sec at wavelengths 30 times the depth of the mixed layer. The instability is able to account for the existence and broad scale of the stratiform cloud patterns known as mesoscale cellular convection.

Fiedler, B. H.↗
NASA NTRS1985

Ion beam sputter deposited zinc telluride films

Zinc telluride is of interest as a potential electronic device material, particularly as one component in an amorphous superlattice, which is a new class of interesting and potentially useful materials. Some structural and electronic properties of ZnTe films deposited by argon ion beam sputter depoairion are described. Films (up to 3000 angstroms thick) were deposited from a ZnTe target. A beam energy of 1000 eV and a current density of 4 mA/sq. cm. resulted in deposition rates of approximately 70 angstroms/min. The optical band gap was found to be approximately 1.1 eV, indicating an amorphous structure, as compared to a literature value of 2.26 eV for crystalline material. Intrinsic stress measurements showed a thickness dependence, varying from tensile for thicknesses below 850 angstroms to compressive for larger thicknesses. Room temperature conductivity measurement also showed a thickness dependence, with values ranging from 1.86 x to to the -6/ohm. cm. for 300 angstrom film to 2.56 x 10 to the -1/ohm. cm. for a 2600 angstrom film. Measurement of the temperature dependence of the conductivity for these films showed complicated behavior which was thickness dependent. Thinner films showed at least two distinct temperature dependent conductivity mechanisms, as described by a Mott-type model. Thicker films showed only one principal conductivity mechanism, similar to what might be expected for a material with more crystalline character.

Gulino, D. A.↗
NASA NTRS1986

The outer atmosphere of Procyon (Alpha CMi F5IV-V) - Evidence of supergranulation or active regions

Observations made with the Einstein X-ray observatory and Exosat have shown Procyon (Alpha CMi) to have a measurable X-ray flux. The flux observed is similar to the upper limits previously reported. The interpretation of the X-ray data in the context of models made previously by Brown and Jordan (1981) based on spectra obtained with the IUE satellite. The X-ray data lead to a higher electron pressure than does the UV spectrum. These results support the conclusion by Brown and Jordan that the UV data cannot be reconciled with a hot corona in a spherically symmetric uniform model. Two types of models are explored; one in which the X-ray emission originates from an isothermal corona but the UV emission is restricted to limited regions of the disk, analogous to solar supergranulation structure; the other in which hot loop structures are embedded in a cool (T(e) of about 300,000 K) corona. Monitoring of Procyon to search for variability is urgently required to distinguish between these models. Some comparisons with the predictions of scaling laws are made. This hot isothermal coronal model agrees quite well with the predictions of Hearn's (1977) minimum flux model.

Jordan, C.↗
NASA NTRS1986

Ion beam sputter deposited zinc telluride films

Zinc telluride is of interest as a potential electronic device material, particularly as one component in an amorphous superlattice, which is a new class of interesting and potentially useful materials. Some structural and electronic properties of ZnTe films deposited by argon ion beam sputter deposition are described. Films (up to 3000 angstroms thick) were deposited from a ZnTe target. A beam energy of 1000 eV and a current density of 4 mA/sq cm resulted in deposition rates of approximately 70 angstroms/min. The optical band gap was found to be approximately 1.1 eV, indicating an amorphous structure, as compared to a literature value of 2.26 eV for crystalline material. Intrinsic stress measurements showed a thickness dependence, varying from tensile for thicknesses below 850 angstroms to compressive for larger thicknesses. Room temperature conductivity measurement also showed a thickness dependence, with values ranging from 1.86 x 10 to the -6th/ohm cm for 300 angstrom film to 2.56 x 10 to the -1/ohm cm for a 2600 angstrom film. Measurement of the temperature dependence of the conductivity for these films showed complicated behavior which was thickness dependent. Thinner films showed at least two distinct temperature dependent conductivity mechanisms, as described by a Mott-type model. Thicker films showed only one principal conductivity mechanism, similar to what might be expected for a material with more crystalline character.

Gulino, D. A.↗
NASA NTRS1987

The temperature dependence of inelastic light scattering from small particles for use in combustion diagnostic instrumentation

A computer calculation of the expected angular distribution of coherent anti-Stokes Raman scattering (CARS) from micrometer size polystyrene spheres based on a Mie-type model, and a pilot experiment to test the feasibility of measuring CARS angular distributions from micrometer size polystyrene spheres by simply suspending them in water are discussed. The computer calculations predict a very interesting structure in the angular distributions that depends strongly on the size and relative refractive index of the spheres.

Cloud, Stanley D.↗
NASA NTRS1987

A subsonic to Mach 5.5 subscale engine test facility

The NASA-Langley Mach 4 facility, in which air-hydrogen combustion is used to heat the test gas, was modified to make the facility suitable for tests of ramjet-type models at subsonic and transonic, as well as supersonic, Mach numbers. The construction details and the instrumentation of this facility, designated the Combustion-Heated Scramjet Test Facility, are described, and modifications to enhance the test capability range from the subsonic/transonic and Mach 3.5 to 5.5 are discussed. Design diagrams and test result graphs are included.

Andrews, Earl H., Jr.↗
NASA NTRS1988

A high fidelity real-time simulation of a small turboshaft engine

A high-fidelity component-type model and real-time digital simulation of the General Electric T700-GE-700 turboshaft engine were developed for use with current generation real-time blade-element rotor helicopter simulations. A control system model based on the specification fuel control system used in the UH-60A Black Hawk helicopter is also presented. The modeling assumptions and real-time digital implementation methods particular to the simulation of small turboshaft engines are described. The validity of the simulation is demonstrated by comparison with analysis-oriented simulations developed by the manufacturer, available test data, and flight-test time histories.

Ballin, Mark G.↗
NASA NTRS1988

Wavenumber selection for single-wave steady states in a nonlinear baroclinic system

The principles involved in the selection of a wavenumber for single-wave steady states are examined numerically and analytically in the framework of an Eady-type model with uneven Ekman dissipation. The process of selection involves the determination of the preferred wavenumber for a given parameter setting in the nonlinear system by testing the stability of steady single-wave states in a triad with wavenumbers n(0) - 1, n(0), and n(0) + 1, where n(0) may change successively in the selection procedure. In a given triad for a given parameter setting, the preferred wave is the wave with the nonlinear Eady angle that is last to vanish.

Weng, H.-Y.↗
NASA NTRS1988

Gaseous cometary coronal

Ground-based observations of coma emission line profiles permit direct insight into the composition and kinetic state of a cometary atmosphere and in turn provide much needed constraints in reconstructing a comet's history; the profiles themselves yield information on the mechanisms generating the observed species, while their intensities correspond to overall production rates by the nucleus. Intensities and line profiles at wavelength 6562.1A (H-alpha) and 6300.3A (O(1D) deday) were obtained for Comet Halley in the period March - April 1986 and for Comet Wilson in the period March 1987 using the Fabry-Perot optical facility at Arecibo; the Comet Halley observations have been undergoing careful analysis as the main effort of this project. Attention was centered on O(1D) 6300A observations obtained over four consecutive nights in April 1986. Surprisingly, the sorts of profiles to be expected in coma observations have not been theoretically developed in the past, necessitating a reappraisal of this question. It turns out that coma profiles vary noticeably according to excitation mechanism, and the proper interpretation of a profile requires the use of the appropriate model type. A paper cataloging the basic types and their interpretation will be submitted for publication in a few months. The intensities of the 6300A emission can be used independently to estimate the production rate of H2O by the comet nucleus; indeed, this may be the most reliable way of determining this basic quantity using ground-based facilities. The H2O production rates implied by our observations throughout the period March-April 1986 have been derived and are to be presented at the upcoming COSPAR special session Aeronomy of Comets and Outer Planets.

Atreya, Sushil K.↗
NASA NTRS1988

Wavenumber transition and wavenumber vacillation in Eady-type baroclinic flows

Wavenumber transition and wavenumber vacillation are studied numerically in a maximum simplification Eady-type model with uneven Ekman dissipation. The spectral wave solution for the model contains three waves with adjacent zonal wavenumbers and the gravest meridional mode. The model only allows for wave-mean-flow interaction. The model is used to obtain the main characteristics of the process of wavenumber transition at various stratifications and the flow regime of wave-number vacillation at specific parameter settings. It is found that wavenumber vacillation is a flow regime observed at some parameter settings when the flow has at least two spatial and three temporal degrees of freedom. It is suggested that the balance between baroclinic forcing and damping adjusted by wave-mean-flow interaction is a basic mechanism for wavenumber transition and wavenumber vacillation.

Weng, H.-Y.↗
NASA NTRS1991

A new spectrum of Triton near the time of the Voyager encounter

A 5200-10,000 A spectrum of Triton that was telescopically obtained during the summer of 1989, just before the Voyager II encounter with the Neptune system, exhibits a measurable 8900 A CH4 ice absorption band. A combination of these data with those of Voyager indicates that the absorption is caused solely by Triton surface CH4 ice. A Hapke-type model for the Triton spectrum (1) sets a 20-micron lower limit on the CH4 ice's mean grain size (although it is suspected that actual grain size is closer to 100 microns), and (2) indicates that CH4 ice is widely distributed on the southern-hemisphere surface of Triton.

Grundy, William M.↗
NASA NTRS1992

Simulation studies for surfaces and materials strength

The progress report for the period May 1, 1992 to October 31, 1992 is presented. During this period investigations were carried out in three areas. Binding energies and high energy binding sites were calculated for carbon atoms deposited on a (2 x 1) dimerized Si(100) surface. Structure- and energy-related properties of defects formed on relaxed diamond surfaces were investigated. Tersoff-type model functions for Si were employed to investigate the energetics of the following processes: (1) neutral monovacancy and divacancy formation and migration energies; (2) neutral bond-centered, site sharing, tetrahedral, and hexagonal self-interstitial formation and migration energies; and (3) the variation of these energies with distance from a bulk site of Frenkel defect formation and from (100) and (111) surfaces. Calculations are included in the appendices.

Halicioglu, Timur↗