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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 523 records · Page 29
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

Comparative study of high-resolution shock-capturing schemes for a real gas

Recently developed second-order explicit shock-capturing methods, in conjunction with generalized flux-vector splittings, and a generalized approximate Riemann solver for a real gas are studied. The comparisons are made on different one-dimensional Riemann (shock-tube) problems for equilibrium air with various ranges of Mach numbers, densities and pressures. Six different Riemann problems are considered. These tests provide a check on the validity of the generalized formulas, since theoretical prediction of their properties appears to be difficult because of the non-analytical form of the state equation. The numerical results in the supersonic and low-hypersonic regimes indicate that these produce good shock-capturing capability and that the shock resolution is only slightly affected by the state equation of equilibrium air. The difference in shock resolution between the various methods varies slightly from one Riemann problem to the other, but the overall accuracy is very similar. For the one-dimensional case, the relative efficiency in terms of operation count for the different methods is within 30 percent. The main difference between the methods lies in their versatility in being extended to multidimensional problems with efficient implicit solution procedures.

Montagne, J.-L.↗
NASA NTRS1989

Scattering from nonspherical Chebyshev particles. III - Variability in angular scattering patterns

Shape-induced variability in the scattered intensity from randomly oriented nonspherical particles is studied. A comparison is made between bands of shape-induced variability for six size intervals within the size parameter range of 1 to 20 and corresponding spherical intensities. It is found that averaging spherical intensities over narrow size ranges produces effects that are qualitatively similar to a slightly distorted single sphere. The results also show that, for Chebyshev particles which are neither small nor large compared to the wavelength, shape-induced variability is often comparable to the mean.

Mugnai, Alberto↗
NASA NTRS1990

Micromechanics analysis of space simulated thermal deformations and stresses in continuous fiber reinforced composites

Space simulated thermally induced deformations and stresses in continuous fiber reinforced composites were investigated with a micromechanics analysis. The investigation focused on two primary areas. First, available explicit expressions for predicting the effective coefficients of thermal expansion (CTEs) for a composite were compared with each other, and with a finite element (FE) analysis, developed specifically for this study. Analytical comparisons were made for a wide range of fiber/matrix systems, and predicted values were compared with experimental data. The second area of investigation focused on the determination of thermally induced stress fields in the individual constituents. Stresses predicted from the FE analysis were compared to those predicted from a closed-form solution to the composite cylinder (CC) model, for two carbon fiber/epoxy composites. A global-local formulation, combining laminated plate theory and FE analysis, was used to determine the stresses in multidirectional laminates. Thermally induced damage initiation predictions were also made.

Bowles, David E.↗
NASA NTRS1990

The limits of direct satellite tracking with GPS

The expected accuracy of the direct user orbit solution and the magnitude of the principal error sources are evaluated using the techniques of covariance analysis. Utilization of advanced GPS receivers, measurement calibration methods, and geophysical models developed for high-precision GPS-based geodesy and differential satellite tracking are assumed in order to explore the limiting accuracy of the above technique. In addition, the dependence of user orbit accuracy on such factors as data arc length, the time interval between the end of the ground data arc used for GPS orbit solutions and the beginning of the user data arc, and data types used are studied. For comparison, results from the analysis of a full differential orbit solution are also presented. It is shown that submeter real-time accuracy can be readily achieved for a user above 700 km altitude, even when the user solution is based on a GPS ephemeris that is more than 12 h old.

Bertiger, Willy I.↗
NASA NTRS1990

Neutral beams in two-ribbon flares and in the geomagnetic tail

The current sheet created in the wake of an erupting filament during a two-ribbon flare is studied. A comparison with the geomagnetic tail shows that the physics of these systems is very similar, and therefore the existence of super Dreicer fields and the generation of netural beams traveling down the postflare loops with small pitch angles may be expected. The observational evidence for neutral beams in flares is reviewed and found to be generally supportive, while contracting the widely held hypothesis of electron beams. A dimensional analysis further demonstrates that the results for self-consistent numerical simulations of the current sheet in the geomagnetic tail can directly be scaled to the coronal current sheet, and the scaling parameters are derived.

Martens, P. C. H.↗
NASA NTRS1990

The 3 micron spectrum of NGC 4565

Researchers spectrum of NGC 4565 is essentially featureless. The absence of the 3.0 micron feature (Tau 3.0 less than 0.05) implies that the extinction to the nucleus does not arise to a significant degree in molecular clouds. Researchers deduce Tau 3.0/A sub V less than 0.01, compared with approx. 0.022 for GC-IRS7. These results support the conclusion (McFadzean et al. 1989) that the 3.0 micron absorption in the GC-IR sources is due to the presence of ice in a (probably single) foreground molecular cloud. The 3.4 micron feature is also weak or absent in the researchers spectrum of NGC 4565 (Tau 3.4 less than or equal to 0.07), hence, Tau 3.4/A sub V less than or equal to 0.016, compared with approx. 0.008 towards GC-IRS7. The absence of the feature in NGC 4565 at the signal-to-noise level of the current observations is consistent with a probable moderate degree of extinction towards the nucleus. The observations of NGC 4565 provide a useful comparison for studies of dust in the Galaxy. Limits have been set on the strengths of the 3.0 and 3.4 micron features in NGC 4565. The absence of 3.0 micron absorption is significant, and supports the view that the feature at this wavelength in the Galactic Centre is due to water-ice absorption in a foreground molecular cloud. The non-detection of the 3.4 micron absorption is less surprising and provides indirect support for the association between this feature and the diffuse interstellar medium. The current spectrum probably represents the best that can be achieved with a single-detector instrument within reasonable integration times. It will clearly be of interest in the future to obtain spectra of higher signal-to-noise, as a positive detection of the 3.4 micron feature in an external galaxy, even at a low level, would be of considerable astrophysical significance.

Adamson, A. J.↗
NASA NTRS1991

Isothermal fatigue behavior of a (90)(sub 8) SiC/Ti-15-3 composite at 426 C

The transverse fatigue behavior of a unidirectional, SiC/Ti-15-3 composite (35 v/o SiC, (90)(sub 8)) was evaluated at 426 C. The fatigue behavior of the composite along the fiber direction (0)(sub 8) and of unreinforced Ti-15-3 alloy were also studied for comparison purposes. The (90)(sub 8) composite fatigue life was much shorter then (0)(sub 8) life. Further, (90)(sub 8) fatigue life was also found to be far lower than that of the unreinforced Ti-15-3 alloys. A simple one-dimensional model for (90)(sub 8) fatigue behavior indicated that the short life of the composite in this orientation resulted, in large part, from weak fiber-matrix bond strength. This conclusion was supported by fractographic evidence showing numerous initiation sites along the fiber-matrix interfaces.

Gayda, John↗
NASA NTRS1990

Advanced rotorcraft transmission program

The Advanced Rotorcraft Transmission (ART) program is an Army-funded, joint Army/NASA program to develop and demonstrate lightweight, quiet, durable drivetrain systems for next generation rotorcraft. ART addresses the drivetrain requirements of two distinct next generation aircraft classes: Future Air Attack Vehicle, a 10,000 to 20,000 lb. aircraft capable of undertaking tactical support and air-to-air missions; and Advanced Cargo Aircraft, a 60,000 to 80,000 lb. aircraft capable of heavy life field support operations. Both tiltrotor and more conventional helicopter configurations are included in the ART program. Specific objectives of ART include reduction of drivetrain weight by 25 percent compared to baseline state-of-the-art drive systems configured and sized for the next generation aircraft, reduction of noise level at the transmission source by 10 dB relative to a suitably sized and configured baseline, and attainment of at least a 5000 hr mean-time-between-removal. The technical approach for achieving the ART goals includes application of the latest available component, material, and lubrication technology to advanced concept drivetrains that utilize new ideas in gear configuration, transmission layout, and airframe/drivetrain integration. To date, candidate drivetrain systems were carried to a conceptual design stage, and tradeoff studies were conducted resulting in selection of an ART transmission configuration for each of the four contractors. The final selection was based on comparative weight, noise, and reliability studies. A description of each of the selected ART designs is included. Preliminary design of each of the four selected ART transmission was completed, as have mission impact studies wherein comparisons of aircraft mission performance and life cycle costs are undertaken for the next generation aircraft with ART and with the baseline transmission.

Bill, Robert C.↗
NASA NTRS1991

Effect of tabs on the evolution of an axisymmetric jet

The effect of vortex generators, in the form of small tabs at the nozzle exit, on the evolution of an axisymmetric jet was investigated experimentally over a jet Mach number range of 0.34 to 1.81. The effects of one, two, and four tabs were studied in comparison with the corresponding case without a tab. Each tab introduced an indentation in the shear layer, apparently through the action of streamwise vortices which appeared to be of the trailing vortex type originating from the tips of the tab rather that of the necklace vortex type originating from the base of the tab. The resultant effect of two tabs, placed at diametrically opposite locations, was to essentially bifurcate the jet. The influence of the tabs was essentially the same at subsonic and supersonic conditions indicating that compressibility has little to do with the effect.

Zaman, K. B. M. Q.↗
NASA NTRS1990

Defect sensitivity of filament-wound structures in compression

Compression testing of 12-ply graphite/bismaleimide laminated plates with induced and natural defects is discussed, and emphasis is placed on resultant changes in strength and stiffness. Ply drops, ply cuts, and fiber crossovers are studied by comparison with a quasi-isotropic control group. It is observed that, as expected, failure initiates at the defects and progresses via delamination, sublaminate buckling, angle-ply cracking, and longitudinal ply failure. The ultimate stress is reduced by approximately 40 percent for all types of defects except the unsymmetrical cut-ply specimens which averages a 54-percent reduction. The average stiffness reduction of 10 percent is observed for specimens with induced defects. Fiber crossovers are found to reduce the laminate strength and stiffness significantly.

Jensen, David W.↗
NASA NTRS1989

A turbulent boundary layer at low Reynolds number with adverse pressure gradient

The evolution of a low Re(theta) turbulent boundary layer in an adverse pressure gradient (APG) is studied for comparison with CFD simulations by Spalart (1988). A short region of favorable pressure gradient is applied first to establish a self-preserving layer which is a suitable initial condition for the simulations. The APG is then applied rapidly such that Beta(x) of about 2 for Re(theta) of about 1500.

Watmuff, J. H.↗
NASA NTRS1989

The topology and vorticity dynamics of a three-dimensional plane compressible wake

The three-dimensional aspects of transition in a low Mach number plane compressible wake are studied numerically. Comparisons are made between the topology of the velocity field and the vorticity dynamics of the flow based on results from direct numerical simulations of the full compressible Navier-Stokes equations. The velocity field is integrated to obtain instantaneous streamlines at different stages in the evolution. A generalized three-dimensional critical point theory is applied to classify the critical points of the velocity field.

Chen, Jacqueline H.↗
NASA NTRS1990

An experimental investigation of a low Reynolds number turbulent boundary layer subject to an adverse pressure gradient

The evolution of a low Re(sub theta) turbulent boundary layer in an adverse pressure gradient (APG) is being studied for comparison with direct numerical simulations by Spalart. A short region of favorable pressure gradient (FPG) is applied first to establish a self preserving layer with Re(sub theta) equals approx. 600, which is a suitable initial condition for simulations. The APG is then applied rapidly such that beta equals approx. 2 at Re(sub theta) equals approx. 1500. The streamwise extent of the measurements exceeds the current capabilities of direct simulation so that the results should also serve as a useful data base for Reynolds averaged boundary layer prediction methods and in the future for direct simulation schemes as computer technology evolves.

Watmuff, J. H.↗
NASA NTRS1991

Void statistics of the CfA redshift survey

Clustering properties of two samples from the CfA redshift survey, each containing about 2500 galaxies, are studied. A comparison of the velocity distributions via a K-S test reveals structure on scales comparable with the extent of the survey. The void probability function (VPF) is employed for these samples to examine the structure and to test for scaling relations in the galaxy distribution. The galaxy correlation function is calculated via moments of galaxy counts. The shape and amplitude of the correlation function roughly agree with previous determinations. The VPFs for distance-limited samples of the CfA survey do not match the scaling relation predicted by the hierarchical clustering models. On scales not greater than 10/h Mpc, the VPFs for these samples roughly follow the hierarchical pattern. A variant of the VPF which uses nearly all the data in magnitude-limited samples is introduced; it accounts for the variation of the sampling density with velocity in a magnitude-limited survey.

Vogeley, Michael S.↗
NASA NTRS1991

Effect of tabs on the evolution of an axisymmetric jet

The effect of vortex generators, in the form of small tabs at the nozzle exit, on the evolution of an axisymmetric jet was investigated experimentally over a jet Mach number range of 0.34 to 1.81. The effects of one, two, and four tabs were studied in comparison with the corresponding case without a tab. Each tab introduced an indentation in the shear layer, apparently through the action of streamwise vortices which appeared to be of the trailing vortex type originating from the tips of the tab rather than of the necklace vortex type originating from the base of the tab. The resultant effect of two tabs, placed at diametrically opposite locations, was to essentially bifurcate the jet. The influence of the tabs was essentially the same at subsonic and supersonic conditions indicating that compressibility has little to do with the effect.

Zaman, K. B. M. Q.↗
NASA NTRS1992

An investigation of passive control methods for shock-induced separation at hypersonic speeds

The effectiveness of several passive control techniques on shock-induced boundary-layer separation at hypersonic speed was investigated. Two approaches for alleviating the turbulent separation losses were examined: porous surface mass transfer and surface grooving. A total of four perforated surfaces with varying porosities were evaluated, and three groove orientations with respect to the freestream direction were studied. A comparison of the results from passive control techniques with those from an 'uncontrolled' shock impingement showed that the porous surface with the greatest porosity provided the greatest reduction in the pressure rise across the oblique shock wave. The grooved surface tested were found to be not effective; each of the grooved configurations examined increased the peak pressure value.

Rallo, R.↗
NASA NTRS1993

Protective Coats For Zinc-Rich Primers

Report describes tests of topcoats for inorganic zinc-rich primers on carbon steel. Topcoats intended to provide additional protection against corrosion in acidic, salty seacoast-air/rocket-engine-exhaust environment of Space Shuttle launch site. Tests focused on polyurethane topcoats on epoxy tie coats on primers. Part of study involved comparison between "high-build" coating materials and thin-film coating materials.

Macdowell, Louis G, III↗
NASA NTRS1992

Modeling energy and mass fluxes from prairie canopies

The main emphasis of this research project is on partitioning of mass and energy fluxes between vegetation and soil at the FIFE site, preparation of data from the FIFE Information System for an international thermal data set comparison, and studying the relation between surface temperatures observed from satellites and in situ measurements of surface temperature.

Norman, John M.↗