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At least 451 records · Page 25
All sourcesNASA NTRSDOE OSTI
NASA NTRS1999

A Method for Calculating Strain Energy Release Rates in Preliminary Design of Composite Skin/Stringer Debonding Under Multi-Axial Loading

Three simple procedures were developed to determine strain energy release rates, G, in composite skin/stringer specimens for various combinations of unaxial and biaxial (in-plane/out-of-plane) loading conditions. These procedures may be used for parametric design studies in such a way that only a few finite element computations will be necessary for a study of many load combinations. The results were compared with mixed mode strain energy release rates calculated directly from nonlinear two-dimensional plane-strain finite element analyses using the virtual crack closure technique. The first procedure involved solving three unknown parameters needed to determine the energy release rates. Good agreement was obtained when the external loads were used in the expression derived. This superposition technique was only applicable if the structure exhibits a linear load/deflection behavior. Consequently, a second technique was derived which was applicable in the case of nonlinear load/deformation behavior. The technique involved calculating six unknown parameters from a set of six simultaneous linear equations with data from six nonlinear analyses to determine the energy release rates. This procedure was not time efficient, and hence, less appealing. A third procedure was developed to calculate mixed mode energy release rates as a function of delamination lengths. This procedure required only one nonlinear finite element analysis of the specimen with a single delamination length to obtain a reference solution for the energy release rates and the scale factors. The delamination was extended in three separate linear models of the local area in the vicinity of the delamination subjected to unit loads to obtain the distribution of G with delamination lengths. This set of sub-problems was Although additional modeling effort is required to create the sub- models, this local technique is efficient for parametric studies.

Krueger, Ronald↗
NASA NTRS1972

Research on combustion instability and application to solid propellant rocket motors. II.

Review of the current state of analyses of combustion instability in solid-propellant rocket motors, citing appropriate measurements and observations. The work discussed has become increasingly important, both for the interpretation of laboratory data and for predicting the transient behavior of disturbances in full-scale motors. Two central questions are considered - namely, linear stability and nonlinear behavior. Several classes of problems are discussed as special cases of a general approach to the analysis of combustion instability. Application to motors, and particularly the limitations presently understood, are stressed.

Culick, F. E. C.↗
NASA NTRS1974

Applications of Skylab EREP photographs to mapping of landforms and environmental geology in the Great Plains and Midwest

The author has identified the following significant results. The utility of Skylab 2 and 3 S-190A multispectral photos for environmental-geologic/geomorphic applications is being tested by using them to prepare 1:250,000-scale maps of geomorphic features, surficial geology, geologic linear features, and soil associations of large, representative parts of the Great Plains and Midwest. Parts of Nebraska, Iowa, Missouri, and South Dakota were mapped. The maps were prepared primarily by interpretation of the S-190A photos, supplemented by information from topographic, geologic, and soil maps and reports. The color band provides the greatest information on geology, soils, and geomorphology; its resolution also is the best of all the multispectral bands and permits maximum detail of mapping. The color-IR band shows well the differences in soil drainage and moisture, and vegetative types, but has only moderate resolution. The B/W-red band is superior for topographic detail and stream alinements. The B/W-infrared bands best show differences in soil moisture and drainage but have poor resolution, especially those from SL 2. The B/W-green band generally is so low contrast and degraded by haze as to be nearly useless. Where stereoscopic coverage is provided, interpretation and mapping are done most efficiently using a Kern PG-2 stereoplotter.

Morrison, R. B.↗
NASA NTRS1981

Investigation of the effects of external current systems on the MAGSAT data utilizing grid cell modeling techniques

Efforts devoted to reading MAGSAT data tapes in preparation for further analysis of the MAGSAT data are discussed. A modeling procedure developed to compute the magnetic fields at satellite orbit due to hypothesized current distributions in the ionosphere and magnetosphere is described. This technique utilizes a linear current element representation of the large-scale space-current system. Several examples of the model field perturbations computed along hypothetical satellite orbits are shown.

Klumpar, D. M.↗
NASA NTRS1983

Analysis and testing of large space structures

Considerations and approaches to the ground testing of large space structures are discussed. The large size combined with the loading due to gravity makes testing of the complete structure difficult. Gravitational stiffening, suspension effects, virtual air mass, preloads, and air damping alter the dynamic characteristics. Low resonant frequencies and high modal densities within the frequency range of interest combine with small motions and accelerations to make testing difficult. Mechanism complexities and nonlinearities associated with space-erected/assembled structures cause structural complexity regardless of other considerations. Ground test approaches include scale models, element and substructure tests, and structural linearization. Analytical approaches are also discussed.

Stahle, C. V.↗
NASA NTRS1982

Planetary-scale characteristics of the atmospheric circulation during January and February 1979

The global scale mass and wind fields observed during FGGE special observing period (SOP) are studied, with attention given to the time variation and vertical structure of the ultralong waves. A spherical harmonic representation is developed for the global fields. The tropical circulation is found to imprint more amplitude on the lower-order components than the midlatitude circulation does. High amplitude oscillations are determined to be present in the divergent fields on all time scales, i.e., diurnal to monthly. The global scale wind was found to change on scales approaching a week. Good agreement with teleconnection predictions was demonstrated for the cross-equatorial divergent and zonal rotational flow changes. The global scale was concluded to resemble force modes of linear tidal theory, with important contributions from longitudinal heating gradients.

Paegle, J.↗
NASA NTRS1984

Linear baroclinic instability in the Martian atmosphere

A spherical quasi-geostrophic model is used in an examination of linear baroclinic instability in such zonal-mean flows as those of the Martian atmosphere in winter, under both relatively nondusty and very dusty conditions. These zonal flows, which possess both vertical and meridional shear, are characterized by baroclinically unstable modes whose growth rates and phase speeds are generally consistent with available observations. The structures of the spherical modes are similar to those obtained for terrestrial zonal flows, if similar zonal wavelengths are compared. Zonally symmetric topography, like that of Mars' northern hemisphere, reduces linear growth rates without changing the most unstable scale. It also increases phase speeds.

Barnes, J. R.↗
NASA NTRS1984

Nonlinear evolution of the lower-hybrid drift instability

The results of simulations of the lower-hybrid drift instability in a neutral sheet configuration are described. The simulations use an implict formulation to relax the usual time step limitations and thus extend previous explicit calculations to weaker gradients, larger mass ratios, and long times compared with the linear growth time. The numerical results give the scaling of the saturation level, heating rates, resistivity, and cross-field diffusion and a demonstration by comparison with a fluid electron model that dissipation in the lower-hybrid drift instability is caused by electron kinetic effects.

Brackbill, J. U.↗
NASA NTRS1987

Evaluation of a scale-model experiment to investigate long-range acoustic propagation

Tests were conducted to evaluate the feasibility of using a scale-model experiment situated in an anechoic facility to investigate long-range sound propagation over ground terrain. For a nominal scale factor of 100:1, attenuations along a linear array of six microphones colinear with a continuous-wave type of sound source were measured over a wavelength range from 10 to 160 for a nominal test frequency of 10 kHz. Most tests were made for a hard model surface (plywood), but limited tests were also made for a soft model surface (plywood with felt). For grazing-incidence propagation over the hard surface, measured and predicted attenuation trends were consistent for microphone locations out to between 40 and 80 wavelengths. Beyond 80 wavelengths, significant variability was observed that was caused by disturbances in the propagation medium. Also, there was evidence of extraneous propagation-path contributions to data irregularities at more remote microphones. Sensitivity studies for the hard-surface and microphone indicated a 2.5 dB change in the relative excess attenuation for a systematic error in source and microphone elevations on the order of 1 mm. For the soft-surface model, no comparable sensitivity was found.

Parrott, Tony L.↗
NASA NTRS1988

Testing for periodicity of extinction

The statistical techniques used by Raup and Sepkoski (1984 and 1986) to identify a 26-Myr periodicity in the biological extinction record for the past 250 Myr are reexamined, responding in detail to the criticisms of Stigler and Wagner (1987). It is argued that evaluation of a much larger set of extinction data using a time scale with 51 sampling intervals supports the finding of periodicity. In a reply by Sigler and Wagner, the preference for a 26-Myr period is attributed to a numerical quirk in the Harland et al. (1982) time scale, in which the subinterval boundaries are not linear interpolations between the stage boundaries but have 25-Myr periodicity. It is stressed that the results of the stringent statistical tests imposed do not disprove periodicity but rather indicate that the evidence and analyses presented so far are inadequate.

Raup, David M.↗
NASA NTRS1989

The effects of magnetic fields on the growth of thermal instabilities in cooling flows

The effects of heat conduction and magnetic fields on the growth of thermal instabilities in cooling flows are examined using a time-dependent hydrodynamics code. It is found that, for magnetic field strengths of roughly 1 micro-Gauss, magnetic pressure forces can completely suppress shocks from forming in thermally unstable entropy perturbations with initial length scales as large as 20 kpc, even for initial amplitudes as great as 60 percent. Perturbations with initial amplitudes of 50 percent and initial magnetic field strengths of 1 micro-Gauss cool to 10,000 K on a time scale which is only 22 percent of the initial instantaneous cooling time. Nonlinear perturbations can thus condense out of cooling flows on a time scale substantially less than the time required for linear perturbations and produce significant mass deposition of cold gas while the accreting intracluster gas is still at large radii.

David, Laurence P.↗
NASA NTRS1989

Conceptual design for a Mercury relativity satellite

It was shown earlier that 1 x 10 to the -14th Doppler data and 3 cm accuracy range measurements to a small Mercury Relativity Satellite in a polar orbit with four-hour period can give high-accuracy tests of gravitational theory. A particular conceptual design has been developed for such a satellite, which would take less than 10 percent of the approach mass for a possible future Mercury Orbiter Mission. The spacecraft is similar to the Pioneer Venus Orbiter, but scaled down by about a factor four in linear dimensions. A despun antenna 30 cm in diameter is used for tracking. The transmitted power is roughly 0.2 watts at K-band and 0.5 watts at X-band. The orbit parameters for individual eight-hour arcs and the gravity field of Mercury through degree and order 10 are determined mainly from the Doppler data. A 50 MHz K-band sidetone system provides the basic ranging accuracy. The spacecraft mass is 50 kg or less.

Bender, P. L.↗
NASA NTRS1991

Rugged Direct-Current Transducer

Direct-current transducer withstands thermal and mechanical shocks. Operates from single, nominally 15-V power supply and nearly insensitive to both large variations in temperature and variations of as much as plus or minus 5 V in supply voltage. Its output voltage highly linear function of sensed current, with full-scale value of about 3 Vdc and offset of about 0.1 Vdc at 0 sensed current. Ruggedness of transducer due in large part to novel magnetic core, machined from solid block of nickel/iron high-permeability, low-hysteresis alloy.

Mclyman, W. T.↗
NASA NTRS1991

An experimental analysis of critical factors involved in the breakdown process of leading edge vortex flows

Experimental crosswire measurements of the flowfield above a 70 and 75 degree flat plate delta wing were performed at a Reynolds number of 250,000. Survey grids were taken normal to the platform at a series of chordwise locations for angles of attack of 20 and 30 degrees. Axial and azimuthal vorticity distributions were derived from the velocity fields. The dependence of circulation on distance from the vortex core as well as on chordwise location was examined. The effects of nondimensionalization in comparison with other experimental data was made. The circulation distribution scales with the local semispan and grows approximately linearly in the chordwise direction. For regions of the flow outside of the vortex subcore, the circulation at any chordwise station was observed to vary logarithmically with distance from the vortex axis. The circulation was also found to increase linearly with angle of incidence at a given chordwise station. A reduction in the local circulation about the vortex axis occurred at breakdown. The spanwise distribution of axial vorticity was severely altered through the breakdown region and the spanwise distribution of axial vorticity present appeared to reach a maximum immediately preceding breakdown. The local concentration of axial vorticity about the vortex axis was reduced while the magnitude of the azimuthal vorticity decreased throughout the breakdown zone. The axial vorticity components with a negative sense, found in the secondary vortex, remained unaffected by changes in wing sweep or angle of attack, in direct contrast to the positive components. The inclusion of the local wing geometry into a previously derived correlation parameter indicated that the circulation of growing leading edge vortex flows were similar at corresponding radii from the vortex axis. It was concluded that the flow over a delta wing, upstream of the breakdown regions and away from the apex and trailing edge regions, is conical. In addition, the dominating factors leading to the onset of breakdown are felt to be the local circulation of the vortex and the accompanying pressure field.

Visser, Kenneth D.↗
NASA NTRS1991

Efficient multitasking of Choleski matrix factorization on CRAY supercomputers

A Choleski method is described and used to solve linear systems of equations that arise in large scale structural analysis. The method uses a novel variable-band storage scheme and is structured to exploit fast local memory caches while minimizing data access delays between main memory and vector registers. Several parallel implementations of this method are described for the CRAY-2 and CRAY Y-MP computers demonstrating the use of microtasking and autotasking directives. A portable parallel language, FORCE, is used for comparison with the microtasked and autotasked implementations. Results are presented comparing the matrix factorization times for three representative structural analysis problems from runs made in both dedicated and multi-user modes on both computers. CPU and wall clock timings are given for the parallel implementations and are compared to single processor timings of the same algorithm.

Overman, Andrea L.↗
NASA NTRS1991

Parallel variable-band Choleski solvers for computational structural analysis applications on vector multiprocessor supercomputers

A Choleski method used to solve linear systems of equations that arise in large scale structural analyses is described. The method uses a novel variable-band storage scheme and is structured to exploit fast local memory caches while minimizing data access delays between main memory and vector registers. Several parallel implementations of this method are described for the CRAY-2 and CRAY Y-MP computers demonstrating the use of microtasking and autotasking directives. A portable parallel language, FORCE, is also used for two different parallel implementations, demonstrating the use of CRAY macrotasking. Results are presented comparing the matrix factorization times for three representative structural analysis problems from runs made in both dedicated and multi-user modes on both the CRAY-2 and CRAY Y-MP computers. CPU and wall clock timings are given for the various parallel methods and are compared to single processor timings of the same algorithm. Computation rates over 1 GIGAFLOP (1 billion floating point operations per second) on a four processor CRAY-2 and over 2 GIGAFLOPS on an eight processor CRAY Y-MP are demonstrated as measured by wall clock time in a dedicated environment. Reduced wall clock times for the parallel methods relative to the single processor implementation of the same Choleski algorithm are also demonstrated for runs made in multi-user mode.

Poole, E. L.↗
NASA NTRS1993

Low-frequency mobility response functions for the central plasma sheet with application to tearing modes

Consideration is given to the effect of constant cross-tail magnetic field By on the collisionless conductivity produced by chaotic scattering and stochastic diffusion of particles in the current sheet for a parabolic geometry. It is shown that the correlation time scales as (By/Bz)-squared, and from this strong By scaling a strong tendency toward stabilization of the linear tearing modes with increasing values of By is inferred. This effect of increased dawn-dusk mobility is particularly dramatic when electrons are introduced in the calculation, and is in agreement with the results of kinetic particle simulations. The collisionless conductivity is expressed in terms of the ensemble-averaged power spectrum of the single particle trajectories, which makes it possible to calculate directly the linear conductivity instead of deriving it from the calculation of the irreversible heating rates.

Hernandez, J.↗