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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 NTRS1988

Damage initiation and growth in fiber-reinforced MMCs

On the basis of the simplifying assumption of equivalent constituent strain rates in the absence of damage, a strain-based failure criterion can be applied to the phenomena of damage-initiation and growth in fiber-reinforced MMCs to ascertain when and where the initial damage will occur. The failure modes fall into four categories, depending on the relative fatigue behavior of the reinforcing fibers and their matrix. Attention is given to experimental results illustrating current understanding of fiber-dominated damage, self-similar fatigue damage growth, and fiber/matrix interface failures, for such MMCs as B/Al alloy, SiC/Al alloy, and SiC/Ti-15-3 alloy.

Saff, C. R.↗
NASA NTRS1989

Intercomparison and interpretation of satellite-derived directional albedos over deserts

Issues related to the dependence of planetary albedo upon solar zenith angle are studied using Nimbus-7, GOES, and Meteosat data over deserts. Geographical variations of the planetary albedo are isolated from the albedo's solar zenith angle dependence. An atmospheric solar radiation model is coupled with desert surface bidirectional reflectance measurements to test the consistency of satellite-derived directional planetary albedos. Consideration is given to the use of narrowband versus broadband instruments, the impact of desert aerosols on the directional planetary albedo, and potential differences in the directional planetary albedo associated with different types of deserts. The results show that the directional planetary albedo is dominated by the directional surface albedo, although surface brightness influences the atmospheric limb brightening and limb darkening processes.

Cess, Robert D.↗
NASA NTRS1989

Unsteady stagnation-point heat transfer during passage of a concentrated vortex

The unsteady boundary layer due to a single rectilinear vortex filament approaching a two-dimensional stagnation point is investigated. The unsteady boundary-layer equations are solved by an alternating-direction-implicit finite-difference method. Two mechanisms which cause fluctuations in heat transfer are the unsteady velocity field in the boundary layer and secondly, the unsteady total temperature at the edge of the boundary layer. The relative importance of these mechanisms is dependent upon the total temperature fluctuations relative to the imposed temperature difference.

Rigby, David L.↗
NASA NTRS1989

JPS heater and sensor lightning qualification

Simulated lightning strike testing of the Redesigned Solid Rocket Motor (RSRM) field joint protection system heater assembly was performed at Thiokol Corp., Wendover Lightning Facility. Testing consisted of subjecting the lightning evaluation test article to simulated lightning strikes and evaluating the effects of heater cable transients on cables within the systems tunnel. The maximum short circuit current coupled onto a United Space Boosters, Inc. operational flight cable within the systems tunnel, induced by transients from all cables external to the systems tunnel, was 92 amperes. The maximum open-circuit voltage coupled was 316 volts. The maximum short circuit current coupled onto a United Space Boosters, Inc. operational flight cable within the systems tunnel, induced by heater power cable transients only, was 2.7 amperes; the maximum open-circuit voltage coupled was 39 volts. All heater power cable induced coupling was due to simulated lightning discharges only, no heater operating power was applied during the test. The results showed that, for a worst-case lightning discharge, the heater power cable is responsible for a 3.9 decibel increase in voltage coupling to operational flight cables within the systems tunnel. Testing also showed that current and voltage levels coupled onto cables within the systems tunnel are partially dependant on the relative locations of the cables within the systems tunnel.

Cook, M.↗
NASA NTRS1990

Mode-medium instability and its correction with a Gaussian reflectivity mirror

A high power CO2 laser beam is known to deteriorate after a few microseconds due to a mode-medium instability (MMI) which results from an intensity dependent heating rate related to the vibrational-to-translational decay of the upper and lower CO2 lasing levels. An iterative numerical technique is developed to model the time evolution of the beam as it is affected by the MMI. The technique is used to study the MMI in an unstable CO2 resonator with a hard-edge output mirror for different parameters like the Fresnel number and the gas density. The results show that the mode of the hard edge unstable resonator deteriorates because of the diffraction ripples in the mode. A Gaussian-reflectivity mirror was used to correct the MMI. This mirror produces a smoother intensity profile which significantly reduces the effects of the MMI. Quantitative results on peak density variation and beam quality are presented.

Webster, K. L.↗
NASA NTRS1989

The shape of the mass distribution in M31 from its globular cluster system

The velocity dispersion and rotation velocity of the M31 globular cluster system depend on the relative division of mass between the flat disk and a spherically symmetric halo. Using the tensor virial theorem, it is shown in detail how the mass ratio can be constrained. Radial velocities have been collected for 149 globular clusters in M31. With no assumptions about the isotropy of the velocity distribution, the globular cluster kinematics are consistent with the mass distribution inferred by the rotation curve but otherwise place no strong constraints on the relative division of the mass. If the velocity distribution is isotropic, models with the disk mass ranging between 1/2 and 1 times its maximum possible value are marginally favored.

Kent, Stephen M.↗
NASA NTRS1990

Propeller tip vortex interactions

Propeller wakes interacting with aircraft aerodynamic surfaces are a source of noise and vibration. For this reason, flow visualization work on the motion of the helical tip vortex over a wing and through the second stage of a counterrotation propeller (CRP) has been pursued. Initially, work was done on the motion of a propeller helix as it passes over the center of a 9.0 aspect ratio wing. The propeller tip vortex experiences significant spanwise displacements when passing across a lifting wing. A stationary propeller blade or stator was installed behind the rotating propeller to model the blade vortex interaction in a CRP. The resulting vortex interaction was found to depend on the relative vortex strengths and vortex sign.

Johnston, Robert T.↗
NASA NTRS1989

Machine characterization based on an abstract high-level language machine

Measurements are presented for a large number of machines ranging from small workstations to supercomputers. The authors combine these measurements into groups of parameters which relate to specific aspects of the machine implementation, and use these groups to provide overall machine characterizations. The authors also define the concept of pershapes, which represent the level of performance of a machine for different types of computation. A metric based on pershapes is introduced that provides a quantitative way of measuring how similar two machines are in terms of their performance distributions. The metric is related to the extent to which pairs of machines have varying relative performance levels depending on which benchmark is used.

Saavedra-Barrera, Rafael H.↗
NASA NTRS1989

Fatigue testing and damage development in continuous fiber reinforced metal matrix composites

A general overview of the fatigue behavior of metal matrix composites (MMC) is presented. The first objective is to present experimental procedures and techniques for conducting a meaningful fatigue test to detect and quantify fatigue damage in MMC. These techniques include interpretation of stress-strain responses, acid etching of the matrix, edge replicas of the specimen under load, radiography, and micrographs of the failure surfaces. In addition, the paper will show how stiffness loss in continuous fiber reinforced metal matrix composites can be a useful parameter for detecting fatigue damage initiation and accumulation. Second, numerous examples of how fatigue damage can initiate and grow in various MMC are given. Depending on the relative fatigue behavior of the fiber and matrix, and the interface properties, the failure modes of MMC can be grouped into four categories: (1) matrix dominated, (2) fiber dominated, (3) self-similar damage growth, and (4) fiber/matrix interfacial failures. These four types of damage will be discussed and illustrated by examples with the emphasis on the fatigue of unnotched laminates.

Johnson, W. S.↗
NASA NTRS1990

Multilayer sea ice in the Arctic - Model- and satellite-derived

The behavior of sea ice is a strong function of ice strength which, in the central Arctic, depends on the relative fractions of multiyear and first-year ice coverage. A dynamic-thermodynamic sea ice model has been configured to include both multiyear and first-year ice. The model has been run for a 7-year period, 1979-1985, for which a satellite microwave record of multiyear sea ice coverage is available. The simulated concentrations of multiyear ice are slightly higher than the satellite-derived concentrations, although the total coverage of simulated multiyear ice agrees well with the satellite-derived coverage. Both the simulated and the satellite-derived coverages in the early 1980s show large interannual variations that represent potentially large perturbations in the mass balance of Arctic pack ice. These fluctuations are generally consistent in the model and satellite data, and they are generally consistent with year-to-year differences in the large-scale wind forcing. In at least some cases the wintertime distribution of multiyear ice is influenced by the pattern of ice drift as far back as the previous summer. The late December coverage of multiyear ice in the Alaskan region appears to foreshadow summer ice conditions in the same region. The short-term response to an intense synoptic system is considerably larger in the satellite data than in the model results.

Walsh, John E.↗
NASA NTRS1991

Supercritical baroclinic disturbances under the influence of topography

An Eady model modified to include Ekman dissipation and sloping horizontal boundaries is used to investigate the nonlinear evolution of baroclinic waves in the presence of surface topography. The effectiveness of the topography to phase lock and equilibrate a given mode vs that of baroclinity to propagate and vacillate that mode is found to depend on the topographic height, its zonal structure, and the level of supercriticality. When topography is sinusoidal and of the same wavelength as the baroclinically most unstable mode, it induces a short-period amplitude modulation whose envelope represents the baroclinic evolution. The dynamics of this modulation is explored analytically. When the sinusoidal topography is of a longer zonal wavelength, a 'mixed' wave state persists with amplitude dominance shifting between the topographic mode and the baroclinic mode, depending on the relative strength of form drag and baroclinicity. Model results are also compared, in appropriate parameter limits, with near-resonance asymptotic results and annulus experiments.

Chou, Shih-Hung↗
NASA NTRS1992

Measurement of the passive attitude control performance of a recovered spacecraft

A novel silver detector incorporated in the Long Duration Exposure Facility (LDEF) has been used to measure the attitude stability of the vehicle. It is thereby established that the LDEF spacecraft maintained a very stable attitude during its nearly 6-year-long flight, notwithstanding a small offset yaw of 8.0 +/- 0.4 deg clockwise from nominal attitude (as viewed from space) and a +/- 0.2-deg oscillation about this offset yaw. LDEF experiments which depend on orientation relative to the forward direction may need to be corrected for the angular offset.

Gregory, J. C.↗
NASA NTRS1992

Mode-medium instability and its correction with a Gaussian-reflectivity mirror

A high-power CO2 laser beam is known to deteriorate after a few microseconds due to a mode-medium instability (MMI) which results from an intensity-dependent heating rate related to the vibrational-to-translational decay of the upper and lower CO2 lasing levels. An iterative numerical technique is developed to model the time evolution of the beam as it is affected by the MMI. The technique is used to study the MMI in an unstable CO2 resonator with a hard-edge output mirror for different parameters like the Fresnel number and the gas density. The results show that the mode of the hard edge unstable resonator deteriorates because of the diffraction ripples in the mode. A Gaussian-reflectivity mirror was used to correct the MMI. This mirror produces a smoother intensity profile which significantly reduces the effects of the MMI. Quantitative results on peak density variation and beam quality are presented.

Webster, K. L.↗
NASA NTRS1992

Structure/load dependent vectors for linear structural dynamic analysis

The dynamic solution vectors yielded by the present structure/load dependent-vectors method for large-scale linear structural dynamic analyses involving complex loadings can be used as starting vectors, so that both structure and load characteristics are encompassed by the basis vectors. The method is shown to entail fewer vectors than current alternatives for a given level of accuracy, especially in the cases of structures that have external concentrated masses. Numerical results are presented which illustrate the advantages of this dependent-vectors method relative to other reduction methods.

Qin, Jiangning↗
NASA NTRS1991

Diurnal changes in the dielectric properties and water status of eastern hemlock and red spruce from Howland, ME

The diurnal characteristics of microwave dielectric properties and water potential of two conifer species were investigated in July and September, 1990. P-band and C-band radial dielectric profiles of hemlock and red spruce, as well as hemlock diurnal water potential and dielectric profiles, are presented. The resulting radial dielectric profiles matched the regions of the functional sapwood (water transport component of the active xylem) in both species such that the sapwood was characterized by a higher dielectric than the bark and heartwood tissues. This is probably due to characteristic differences in the water content of each tissue. As the hemlocks progressed through their diurnal water potential pattern, the dielectric profile remained static until mid-afternoon. As the tension in the water column relaxed (2 to 3 bars) the dielectric constant decreased by 30 to 40 percent. There are several possible explanations for this phenomenon, and these may relate to the dependency of the dielectric measurements on temperature, salinity, and volumetric water content.

Salas, W. A.↗
NASA NTRS1992

Remote sensing applications in water resources - An opportunity for research in developing countries

A review is presented of first-hand experience with remote sensing research in developing countries to illustrate the inherent semiempirical basis of remote sensing applications. This task is accomplished by means of examples drawn from actual research work. Results of case studies in different farming systems and countries are summarized to exemplify the relative, application-dependent, weight of satellite versus ground information.

Menenti, M.↗
NASA NTRS1992

Scenarios for optimizing potato productivity in a lunar CELSS

The use of controlled ecological life support system (CELSS) in the development and growth of large-scale bases on the Moon will reduce the expense of supplying life support materials from Earth. Such systems would use plants to produce food and oxygen, remove carbon dioxide, and recycle water and minerals. In a lunar CELSS, several factors are likely to be limiting to plant productivity, including the availability of growing area, electrical power, and lamp/ballast weight for lighting systems. Several management scenarios are outlined in this discussion for the production of potatoes based on their response to irradiance, photoperiod, and carbon dioxide concentration. Management scenarios that use 12-hr photoperiods, high carbon dioxide concentrations, and movable lamp banks to alternately irradiate halves of the growing area appear to be the most efficient in terms of growing area, electrical power, and lamp weights. However, the optimal scenario will be dependent upon the relative 'costs' of each factor.

Wheeler, R. M.↗
NASA NTRS1992

Knowledge of coronal heating and solar-wind acceleration obtained from observations of the solar wind near 1 AU

Clues to the nature of the mechanisms responsible for heating the corona and accelerating the solar wind can be obtained by contrasting the properties of the quasi-stationary and transient states of the solar wind. Substantial differences exist in the proton temperatures and anisotropies, the entropy, the field strength, the Alfvenicity of fluctuations in the field, the distribution of MHD discontinuities, and the helium abundance of the two types of flow. Those differences are displayed as a function of the solar wind speed. Several signals of wave acceleration can be found in the data for quasi-stationary flows. The relatively smooth velocity dependences of proton temperature, helium abundance, and frequency of occurrence of rotational discontinuities suggest that the acceleration mechanisms for flow from coronal holes, coronal streamers, and the quasi-stationary low-speed flows between them may be basically the same, differing only in degree.

Neugebauer, M.↗