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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 1,207 records · Page 67
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

Creep-fatigue behavior of NiCoCrAlY coated PWA 1480 superalloy single crystals

Single crystal specimens of a Ni base superalloy, PWA 1480, with a low pressure plasma sprayed NiCoCrAlY coating were tested in various 0.1 Hz fatigue and creep fatigue cycles both at 1015 and 1050 C. Creep fatigue tests of the cp, pc, and cc types were conducted with various constant total strain ranges employing creep dwells at various constant stresses. Considerable cyclic softening occurred as was evidenced particularly by rapidly increasing creep rates in the creep fatigue tests. The cycle time in the creep fatigue tests typically decreased by more than 80 percent at 0.5 N sub f. Though cyclic life did correlate with delta epsilon sub in a better correlation existed with sub f for both the fatigue and creep fatigue tests, and poor correlations were observed with either sigma sub max or the average cycle time. A model containing both delta sigma and delta sigma (sub in), N sub f = alpha delta sigma (sub in) beta delta sigma gamma, with best fit values of sigma for each cycle type, but the same values of beta and gamam, was found to provide good correlations. Life lines were not greatly different among the cycle types, differing only by a factor of about three. The cp cycle life line was lowest for both test temperatures, however among the other three cycle types there was no consistent ranking. For all test types failure occurred predominately by multiple internal cracking originating at pores. The strong correlation of life with delta sigma may reflect a significant crack growth period in the life of the specimens.

Miner, R. V.↗
NASA NTRS1988

Comparison of SAGE II solar extinction data with airborne measurements of atmospheric backscattering in the troposphere and lower stratosphere

In April 1986, during passage of the SAGE I satellite, the first simultaneous measurements of the atmospheric backscattering coefficient beta (pi, 10.6 microns) were made with an airborne CO2 lidar. Individual ratios of beta and the SAGE II extinction sigma (1.02 micron) are in reasonable accord with previously calculated values. The trend with height shows a distinctly nonlinear relation, which is probably attributable to steadily changing size distributions of aerosols.

Vaughn, J. M.↗
NASA NTRS1987

Silicon carbide - Progress in crystal growth

Recent progress in the development of two processes for producing large-area high-quality single crystals of SiC is described: (1) a modified Lely process for the growth of the alpha polytypes (e.g., 6H SiC) initially developed by Tairov and Tsvetkov (1978, 1981) and Ziegler et al. (1983), and (2) a process for the epitaxial growth of the beta polytype on single-crystal silicon or other substrates. Growth of large-area cubic SiC on Si is described together with growth of defect-free beta-SiC films on alpha-6H SiC crystals and TiC lattice. Semiconducting qualities of silicon carbide crystals grown by various techniques are discussed.

Powell, J. Anthony↗
NASA NTRS1988

Critical exponents and scaling relations for self-organized critical phenomena

Critical indices beta, gamma delta, nv, etc. are defined and calculated for self-organized critical phenomena. Scaling relations are derived and checked numerically. The order-parameter exponent beta describes the spontaneous current and the relaxation to the criticl point. The power spectrum has 'l/f' behavior with the exponent phi = nv x z, where z is the dynamical critical exponent.

Tang, Chao↗
NASA NTRS1988

A spectral dissection of the ultraviolet emissions of Capella

Phase-resolved observations of Capella were conducted with the IUE to separate the spectrum of the core-helium-burning primary from that of the fast-rotating, chromospherically active secondary. Characteristics of the primary and secondary spectra are presented. The UV emissions of the primary are compared to those of the four K0 giants of the Hyades, and the field K giants Beta Cet and Beta Gem. The Capella primary is found to be similar to the two active Hyads, Gamma Tauri and Theta Tauri, suggesting that if Capella is coeval with the Hyades nucleus, the age of the cluster might be less than half that commonly assumed.

Ayres, Thomas R.↗
NASA NTRS1988

Plasma waves in the range of the lower hybrid frequency - ISEE 1 and 2 observations at the earth's bow shock

This report presents a characterization of plasma wave noise in the range of the lower hybrid frequency associated with 65 crossings of earth's bow shock observed by the ISEE 1 and 2 satellites. Wave growth generally becomes detectable at the upstream edge of the shock foot, increases at the upstream edge of the shock ramp, peaks within the ramp, and then quickly decays to steady downstream values. The upstream extent of the noise is on the same order as that of specularly reflected gyrating ions. Similar profiles were observed in subcritical and supercritical shocks, and no special behavior was associated with the first critical Mach number. Spectra in the foot and ramp were similar in shape, although the noise was 1 to 2 orders of magnitude more intense in the shock ramps than in the feet. Electric field intensities are positively correlated with solar wind speed and inversely related to electron beta and Mach number. Magnetic components are positively correlated with Mach number and beta. The results are generally consistent with suggestions that the noise consists of lower hybrid waves driven by reflected gyrating ions in the foot, and by additional instabilities, such as the cross-field current, in the shock ramp.

Mellott, M. M.↗
NASA NTRS1988

Ion equation of state in quasi-parallel shocks - A simulation result

Ion equation of state in the quasi-parallel collisionless shock is deduced from simulation results. The simulations were performed for theta(bn) = 10 deg, beta = 0.5 and M sub A in the range from 1.2 to 8, where M sub A is the Alfven Mach number, beta is the upstream ratio of plasma pressure to magnetic pressure, and theta(bn) is the angle between the shock normal and the upstream magnetic field. The equation of state can be approximated by a power law with different exponents in the upstream and downstream sides of the shock transition region. The exponent in the upstream side of the transition region is much greater than the adiabatic value of 5/3 and increases with M sub A. The exponent in the downstream side of the transition region is slightly less than 5/3. The results show that ion heating in the quasi-parallel shock is highly nonadiabatic with a large increase in entropy and in temperature ratio in the upstream side of the transition region, while the heating is highly isentropic with a large increase in temperature difference across the principal density jump in the downstream side of the transition region.

Mandt, M. E.↗
NASA NTRS1986

Improved silicon carbide for advanced heat engines

Work performed to develop silicon carbide materials of high strength and to form components of complex shape and high reliability is described. A beta-SiC powder and binder system was adapted to the injection molding process and procedures and process parameters developed capable of providing a sintered silicon carbide material with improved properties. The initial effort has been to characterize the baseline precursor materials (beta silicon carbide powder and boron and carbon sintering aids), develop mixing and injection molding procedures for fabricating test bars, and characterize the properties of the sintered materials. Parallel studies of various mixing, dewaxing, and sintering procedures have been carried out in order to distinguish process routes for improving material properties. A total of 276 MOR bars of the baseline material have been molded, and 122 bars have been fully processed to a sinter density of approximately 95 percent. The material has a mean MOR room temperature strength of 43.31 ksi (299 MPa), a Weibull characteristic strength of 45.8 ksi (315 MPa), and a Weibull modulus of 8.0. Mean values of the MOR strengths at 1000, 1200, and 14000 C are 41.4, 43.2, and 47.2 ksi, respectively. Strength controlling flaws in this material were found to consist of regions of high porosity and were attributed to agglomerates originating in the initial mixing procedures. The mean stress rupture lift at 1400 C of five samples tested at 172 MPa (25 ksi) stress was 62 hours and at 207 MPa (30 ksi) stress was 14 hours. New fluid mixing techniques have been developed which significantly reduce flaw size and improve the strength of the material. Initial MOR tests indicate the strength of the fluid-mixed material exceeds the baseline property by more than 33 percent.

Whalen, T. J.↗
NASA NTRS1988

The crack problem in bonded nonhomogeneous materials

The plane elasticity problem for two bonded half planes containing a crack perpendicular to the interface was considered. The effect of very steep variations in the material properties near the diffusion plane on the singular behavior of the stresses and stress intensity factors were studied. The two materials were thus, assumed to have the shear moduli mu(o) and mu(o) exp (Beta x), x=0 being the diffusion plane. Of particular interest was the examination of the nature of stress singularity near a crack tip terminating at the interface where the shear modulus has a discontinuous derivative. The results show that, unlike the crack problem in piecewise homogeneous materials for which the singularity is of the form r/alpha, 0 less than alpha less than 1, in this problem the stresses have a standard square-root singularity regardless of the location of the crack tip. The nonhomogeneity constant Beta has, however, considerable influence on the stress intensity factors.

Erdogan, Fazil↗
NASA NTRS1989

Redistribution in astrophysically important hydrogen lines

Under typical solar chromospheric conditions for hydrogen radiators, strong collisions due to both electrons and ions are well separated in time, so that a binary collision theory for collisional redistribution is applicable. However, a simple impact approximation may not be used, but rather a unified type theory is required in which frequency dependent line shape parameters are used to describe both impact and quasi-static regions of the spectrum. In addition, correlated terms which describe absorption and emission during a collision are important, and, in fact, without correlated terms describing both transfer of excitation and emission during the same collision unphysical predictions (such as negative intensities) would be obtained. In this paper theory is specifically developed for the coupled Lyman-alpha, Lyman-beta, Hydrogen-alpha system, and equations of statistical equilibrium and absorption and emission coefficients are given. All correlated events are examined and emission during a collision is found to be important in the line wings. Stimulated emission and absorption is also included within a broadband approximation. The major approximation is to ignore lower state interaction. It is found that for Lyman-beta Raman-coupling with Hydrogen-alpha occurs and the overall scattering of radiation in the line wings is mostly coherent. In contrast, for Hydrogen-alpha, incoherent redistribution due to lower state radiative decay (which occurs even in the absence of collisions) is found to dominate the coherent scattering. Finally, in the Lyman series the dominant incoherent contribution is associated with cascade transitions and inelastic collisions between different principal quantum states.

Cooper, J.↗
NASA NTRS1988

A numerical study of bifurcations in a barotropic shear flow

In the last few years, more and more evidence has emerged suggesting that transition to turbulence may be viewed as a succession of bifurcations to deterministic chaos. Most experimental and numerical observations have been restricted to Rayleigh-Benard convection and Taylor-Couette flow between concentric cylinders. An attempt is made to accurately describe the bifurcation sequence leading to chaos in a 2-D temporal free shear layer on the beta-plane. The beta-plane is a locally Cartesian reduction of the equations describing the dynamicss of a shallow layer of fluid on a rotating spherical planet. It is a valid model for large scale flows of interest in meteorology and oceanography.

Huerre, P.↗
NASA NTRS1988

On the number of digital straight line segments

A closed-form expression has been reported in the literature for L(N), the number of digital line segments of length N that correspond to lines of the form y = (alpha)x + beta, (0 less than or equal to alpha, beta less than 1). An asymptotic estimate for L(N) that might prove useful for many applications is proved; namely, L(N) = (N cubed)/(pi squared) + O(N squared log N). An application to an image registration problem is given.

Berenstein, Carlos A.↗
NASA NTRS1989

Comet ejection and dynamics of nonspherical dust particles and meteoroids

This paper generalizes the formalism for calculating the ejection velocity of meteoroids and dust from comets and the forces to which such objects are subject in interplanetary space, including the dust tail of comets. It is found that spheres have the smallest cross section of any geometrical figures of the same valume averaged over random orientations, so for a fixed volume and mass, both the ejection velocity and beta reaches a minimum for bodies of spherical shapes. Flakes in random orientation are ejected near 70 percent of the highest ejection velocity for any orientation. Needles in random orientation escape a comet at nearly 90 percent of their maximum velocities. Randomly oriented cylinders of finite thickness escape at lower velocities that are slightly closer to their maximum velocities. The average beta acting on spin-aligned, perfectly absorbing needles is more than half that acting on a sphere of the same material and radius.

Gustafson, Bo A. S.↗
NASA NTRS1989

Star-formation rates and forbidden O II emission in blue galaxies

Observations of forbidden O II 3727 A and H-beta emission-line strengths are presented from large-aperture spectrophotometric observations of nearby (z less than 0.1) optically blue galaxies covering a wide range in surface brightness and luminosity. The forbidden O II emission-line equivalent widths in these galaxies primarily lie between 30 and 100 A. Emission-line strengths in nearby blue galaxies thus resemble those found in faint galaxies at moderate-to-high redshifts. The most actively star-forming nearby galaxies are characterized by surface brightness in the forbidden O II line of about 10 to the -15th to 10 to the -14th ergs/s per sq cm per sq arcsec over spatial distances of more than 1 kpc. The fluxes of the forbidden O II and H-beta emission lines are well correlated, and thus forbidden O II luminosities can be used to derive approximate star-formation rates in blue galaxies.

Gallagher, J. S.↗
NASA NTRS1989

Mystery spot in supernova 1987A - Reflection or fluorescence by an interstellar cloud?

This paper explores fluorescence and reflection models of the companion to SN 1987A obseved by speckle interferometry, recalling a 1901 precedent. The apparent small angular size of the companion is a severe constraint. A fluorescence model cannot reach the observed brightness unless the ultraviolet burst from the supernova contained as many as 2 x 10 to the 58th ionizing photons. This is about 25 times stronger than generous current models. Even then, the expected line ratios and widths do not fit the observations. The absence of narrow H-alpha and H-beta lines in the supernova spectrum, the ratio of fluxes of the companion in H-alpha and forbidden N II line filters, the invisibility of the companion at 4861 (H-beta), and its detection at 5330 fail to agree with theory. A dust-reflection model is more promising, and the color can be reddened by the evaporation of small grains, but the model still falls more than about 1 mag short in brightness. Furthermore, a dust reflection should have increased in relative brightness in May-June 1987, rather than disappearing as the mystery spot did. If all the observations are correct, neither model is likely to work.

Felten, James E.↗
NASA NTRS1989

Problems with the Baade-Wesselink method

The discrepancy noted in radii obtained by the Baade-Wesselink method when different colors are used to determine the effective temperatures is explored. The discrepancy is found to be due to an inconsistency in the applied temperature-color calibrations. The assumption of the maximum likelihood method that beta (the effective temperature + 0.1 times the bolometric correction) is a linear function of the color is valid for the B-V and V-I colors, but not for the V-R colors. It is suggested that the errors introduced by the nonlinearity in the relation between beta and the V-R colors will produce radii which are too large. The radii derived from the V-B colors appear to be too small.

Bohm-Vitense, E.↗
NASA NTRS1989

Observations of wind-forced deep ocean currents in the North Pacific

The role of the quasi-steady Sverdrup balance in the midlatitude atmospheric response in the North Pacific was investigated by examining 200 instrument-years of moored current measurements in the deep ocean across the breadth of the midlatitude North Pacific. These data provide evidence of a seasonal modulation in the ocean eddy kinetic energy beneath the thermocline at several locations north of 35 deg N, which was often found to be in phase with the local atmospheric forcing function. Results of the measurements indicate that, throughout the midlatitude North Pacific, topography plays a crucial role in the wind forced response; the bottom slope tends to enhance the beta effect in this basin. Topography narrows the available bandwidth for forced barotropic Rossby waves, facilitating a quasi-steady topographic Sverdrup response over most of the basin. However, the enhanced beta effect reduces the magnitude of the ocean's response to wind forcing.

Koblinsky, C. J.↗
NASA NTRS1989

Detection of potential space station control/structure interaction with CO-ST-IN

The NASA Lewis Research Center is concerned with the potential of interaction between space station controllers and the solar PV array structures. The models required to handle this problem are very large, and automated methods were developed for the transfer of data between structural dynamic and control system analysis software. These methods emphasize the need to achieve accurate coupled analysis results while using as small a model as possible. Specific tools which help the analyst in this regard include modal order techniques, the use of mode acceleration to calculate internal loads and stresses and the transfer of Craig-Bampton components to reduce problems associated with modal sufficiency. These techniques were applied to a space station model with 366 modes below 1 Hz. Attitude control, and alpha and beta joint control were simulated. The inclusion of alpha and beta joint controllers is important when examining overall space station dynamics. An initial choice of control parameters does indicate a potential for control/structure interaction during reboost. As expected this is exacerbated by increasing the rate gain and decreasing the hysteresis of the reaction control system (RCS) in order to improve rigid body performance.

Carney, Kelly↗