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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 307 records · Page 17
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

Effects of irregularity anisotropy on Faraday polarization fluctuations

The previous model (Lee et al., 1982) of the Faraday polarization fluctuations (FPF) is extended after taking into account the anisotropic nature of the commonly observed, rodlike and sheetlike ionospheric irregularities. Striking effects of irregularity anisotropy are found in the longitudinal radio propagation. However, if the wave propagation angle is not small (say, greater than 5 deg), the effects of irregularity anisotropy on FPF introduced by rodlike irregularities weaken significantly, while those caused by sheetlike irregularities remain prominent. Therefore, under the same ionospheric propagation conditions, sheetlike ionospheric irregularities are more effective than rodlike ionospheric irregularities in causing the FPF of radio waves. It is expected that intense FPF of VHF radio signals can be observed not only near the equatorial anomaly but also in the auroral region.

Lee, M. C.↗
NASA NTRS1989

Mechanical model of an earthquake fault

The dynamic behavior of a simple mechanical model of an earthquake fault is studied. This model, introduced originally by Burridge and Knopoff (1967), consists of an elastically coupled chain of masses in contact with a moving rough surface. The present version of the model retains the full Newtonian dynamics with inertial effects and contains no externally imposed stochasticity or spatial inhomogeneity. The only nonlinear feature is a velocity-weakening stick-slip friction force between the masses and the moving surface. This system is being driven persistently toward a slipping instability and therefore exhibits noisy sequences of earthquakelike events. These events are observed in numerical simulations, and many of their features can be predicted analytically.

Carlson, J. M.↗
NASA NTRS1989

Post World War II trends in tropical Pacific surface trades

Multidecadal time series of surface winds from central tropical Pacific islands are used to compute trends in the trade winds between the end of WWII and 1985. Over this period, averaged over the whole region, there is no statistically significant trend in speed or zonal or meridional wind (or pseudostress). However, there is some tendency, within a few degrees of the equator, toward weakening of the easterlies and increased meridional flow toward the equator. Anomalous conditions subsequent to the 1972-73 ENSO event make a considerable contribution to the long-term trends. The period 1974-80 has been noted previously to have been anomalous, and trends over that period are sharply greater than those over the longer records.

Harrison, D. E.↗
NASA NTRS1989

Comparison of steady and unsteady secondary flows in a turbine stator cascade

The effect of periodic rotor wakes on the secondary flow structure in a turbine stator cascade was investigated. A mechanism simulated the wakes shed from rotor blades by passing cylindrical rods across the inlet to a linear cascade installed in a recirculating water flow loop. Velocity measurements showed a passage vortex, similar to that seen in steady flow, during the time associated with undisturbed fluid. However, as the rotor wake passed through the blade row, a large crossflow toward the suction surface was observed in the midspan region. This caused the development of two large areas of circulation between the midspan and endwall regions, significantly distorting and weakening the passage vortices.

Hebert, Gregory J.↗
NASA NTRS1990

Supersonic convection

Numerical simulations with high spatial resolution are used to study that the combined effects of stratification, pressure gradients, and nonadiabatic processes can lead to the formation of regions of supersonic motions near the upper thermal boundary layer. Within these regions, the dynamics is dominated by nonstationary shock structures. These form near the downflow sites and propagate upstream along the boundary layer to the upflow regions where they weaken and eventually disappear. The shock cycle, consisting of the formation, propagation, and disappearance of shock structures, has a time scale comparable to the sound crossing time over a portion of the convective cell, giving rise to vigorous time dependence in the convection.

Cattaneo, Fausto↗
NASA NTRS1990

The 2175-A feature from irradiated graphitic particles

In diffuse clouds, a subpopulation of the carbon grains is graphitized by UV starlight during cloud lifetimes of roughly 100 Myr. Graphitic dust created in this way will behave like monosize Rayleigh particles and produce a 2175-A absorption band showing a Lorentzian profile. A physical model is outlined in which the strength and width of the band correlate with the dust grain environment, but the central wavelength is fixed by the Froehlich frequency for surface plasmons in a homogeneous sphere. On this model, the band appears strong and narrow only in dust clouds where the flux of UV starlight is high and the H atom gas density is low. The width of the band increases from diffuse to dense clouds mainly as a result of gas-phase H atoms accreted on to the grain surface. It is shown that hydrogenation weakens the band in dense clouds, and thus explains the anticorrelation between band strength and high carbon depletion. The model as a whole uses only 30 percent of the cosmic carbon and gives good agreement with observations of the 2175-A feature and the average interstellar extinction law.

Sorrell, Wilfred H.↗
NASA NTRS1990

Information analysis of a spatial database for ecological land classification

An ecological land classification was developed for a complex region in southern California using geographic information system techniques of map overlay and contingency table analysis. Land classes were identified by mutual information analysis of vegetation pattern in relation to other mapped environmental variables. The analysis was weakened by map errors, especially errors in the digital elevation data. Nevertheless, the resulting land classification was ecologically reasonable and performed well when tested with higher quality data from the region.

Davis, Frank W.↗
NASA NTRS1990

The kink instability in infinite cylindrical flux tubes - Eigenvalues for power-law twist profiles

Simple, accurate methods of calculating ideal MHD instability eigenvalues for infinitely long cylindrical tubes with twist function T(r) are developed. The results show that the most rapidly growing and energetic instabilities occur in the Gold-Hoyle v = 0 field, with the instability progressively weakening with increasing v. However, the maximum force eigenvalue is always small, so that even in the Gold-Hoyle case only a small proportion of the available magnetic energy can be released in the linear phase. The results also confirm that the linear pinch is remarkably weak yet relatively resistant to line-tying. It is shown that the weakness of the force eigenvalue implies that the influence of uniform gas pressure on stability is negligible. Implications for the energy-release mechanism in solar flares are discussed.

Craig, I. J. D.↗
NASA NTRS1990

Ultraviolet fluxes for globular clusters in M31 - A rediscussion

Long-exposure observations of three bright globular clusters in M31 obtained with both the short- and long-wavelength low-resolution cameras of the International Ultraviolet Explorer satellite are discussed. All of the clusters are seen at the longer wavelengths, but only one of the clusters is seen at short wavelengths, and this detection is marginal. The ultraviolet fluxes are in fact known with only poor precision, and previous conclusions concerning the stellar population are weakened accordingly. Discrepancies between the ultraviolet fluxes obtained here and in other published work are described.

Crotts, Arlin P. S.↗
NASA NTRS1990

Substorm electrodynamics

The present one-dimensional model analysis of substorm electrodynamics proceeds from the standard scenario in which the plasma sheet collapses into a neutral sheet, and magnetic merging occurs between the two tail lobes; plasma flows into the neutral sheet from the lobes and the sides, undergoing acceleration in the dawn-dusk direction. The process is modified by the tendency of the accelerated plasma to unbalance charge neutrality, leading to an exchange of electrons with the ionosphere in order to maintain neutrality. The cross-tail current is weakened by the diversion: this reduces the adjacent lobe-field intensity, but without notable effects apart from a slight expansion of the tail boundary.

Stern, David P.↗
NASA NTRS1990

Tectonics of Lakshmi Planum, Venus - Tests for Magellan

The origin of Lakshmi Planum and its surrounding mountain belts remains an important unresolved element in the global tectonic framework of Venus. From the perspective of gravity signature and potential driving forces, the mantle upwelling model is the simplest, and its principal failure, that it cannot produce radial shortening on the uplift periphery, may be resolved if the lithosphere is laterally heterogeneous. The preferred model consists of a hot mantle plume rising beneath a preexisting block of tessera. The lithosphere is weakened at this hotter and presumably thicker crust, and the outward near-surface flow is attenuated at the peripheral discontinuity in lithospheric strength. Crustal thickening and mountain belt formation occur there. Several criteria are proposed to test this 'tessera-plume' model together with its competitors at the higher resolution in both imaging and gravity afforded by the Magellan mission.

Grimm, Robert E.↗
NASA NTRS1990

Catalytic carbide formation at aluminium-carbon interfaces

X-ray photoelectron spectroscopy investigations of the reaction of several monolayer-thick films of aluminum with glassy carbon substrates are presented. The influence of molecular oxygen and water vapor on the rate of reaction is examined. It is concluded that water vapor catalyzed the formation of aluminum carbide from aluminum and carbon by forming active sites which weakened carbon-carbon bonds at the glassy carbon surface, thus assisting their cleavage. The rate of carbide formation for undosed and molecular oxygen-dosed examples was less as neither metallic aluminum nor oxygen-formed alumina could bond to the carbon atom with sufficient strength to dissociate it quickly.

Maruyama, B.↗
NASA NTRS1990

Euler analysis comparison with LDV data for an advanced counter-rotation propfan at cruise

A fine mesh Euler solution of the F4/A4 unducted fan (UDF) model flowfield is compared with laser Doppler velocimeter (LDV) data taken in the NASA Lewis 8- by 6-Foot Supersonic Wind Tunnel. The comparison is made primarily at one axial plane downstream of the front rotor where the LDV particle lag errors are reduced. The agreement between measured and predicted velocities in this axial plane is good. The results show that a dense mesh is needed in the centerbody stagnation region to minimize entropy generation that weakens the aft row passage shock. The predicted radial location of the tip vortex downstream of the front rotor agrees well with the experimental results but the strength is overpredicted. With 40 points per chord line, the integrated performance quantities are nearly converged, but more points are needed to resolve passage shocks and flow field details.

Miller, Christopher J.↗
NASA NTRS1990

Mechanisms of nocturnal exchange between the rain forest and the atmosphere

A number of events that lead to nocturnal turbulent mixing between the Amazon rain forest and the atmosphere were observed. Two mechanisms that weaken the stable nocturnal thermal inversion at canopy top are identified with nocturnal release of carbon dioxide and concurrent heat flux into and moisture flux out of the Amazon forest canopy. In the first mechanism, abrupt onset of cloudiness interrupts nocturnal radiative cooling, leading to a period with lowered Richardson number and enhanced heat, moisture, and carbon dioxide fluxes. Significant exchange between the forest canopy and the atmosphere can also occur even on totally clear nights. On these nights, occasional nocturnal increases in the wind speed above the canopy stimulate vertical oscillations in the canopy layer that are both amplitude- and frequency-modulated during the event. A simple model is presented to support the postulate that these oscillations result from a resonant interaction, hypothesized to be a consequence of a vertically fluctuating pressure gradient force operating on a deepening stable layer near canopy top. Model results indicate that the recovery of the layer takes approximately twice as long as the period of enhanced wind that produced the event.

Fitzjarrald, David R.↗
NASA NTRS1989

The breakup of trailing-line vortices

It is now known that Batchelor's trailing-line vortex is extremely unstable to small amplitude disturbances for swirl numbers in the neighborhood of 0.83. The results of numerical calculations are presented that show the response of the vortex in this range of swirl numbers to finite amplitude, temporal, helical disturbances. Phenomena observed include: (1) ejection of axial vorticity and momentum from the core resulting in the creation of secondary, separate vortices; (2) a great intensification of core axial vorticity and a weakening of core momentum; and (3) the production of azimuthal vorticity in the form of a tightly wrapped spiral wave. The second phenomenon eventually stablizes the vortex, which then smooths and gradually returns to an axisymmetric state. The calculations are mixed spectral-finite-difference, fourth-order accurate, and have been carried out at Reynolds numbers of 1000 to 2000. Some linearized results are also discussed in an attempt to explain the process of vortex intensification.

Jacqmin, David↗
NASA NTRS1990

Flow-induced resonance of screen-covered cavities

An experimental study of screen-covered cavities exposed to airflow tangent to the screen is described. The term screen refers to a thin metal plate perforated with a repetitive pattern of round holes. The purpose was to find the detailed aerodynamic and acoustic mechanisms responsible for screen-covered cavity resonance and to find ways to control the pressure oscillations. Results indicate that strong cavity acoustic resonances are created by screen orifices that shed vortices which couple resonance by choosing hole spacings such that shed vortices do not arrive at a downstream orifice in synchronization with cavity pressure oscillations. The proper hole pattern is effective at all airspeeds. It was also discovered that a reduction of orifice size tended to weaken the screen/cavity interaction regardless of hole pattern, probably because of viscous flow losses at the orifices. The screened cavities that resonated did so at much higher frequencies than the equivalent open cavity. The classical large eddy phenomenon occurs at the relatively small scale of the orifices (the excitation is typically of high frequency). The wind tunnel study was made at airspeeds from 0 to 100m/sec. The 457-mm-long by 1.09-m-high rectangular cavities had length-to-depth ratios greater than one, which is indicative of shallow cavities. The cavity screens were perforated in straight rows and columns with hole diameters ranging from 1.59 to 6.35 mm and with porosities from 2.6 to 19.6 percent.

Soderman, Paul T.↗
NASA NTRS1990

Dynamics and configurations of galaxy triplets

The purpose is to infer the probable dynamical states of galaxy triplets by the observed data on their configurations. Two methods are proposed for describing the distributions of the triplet configuration parameters characterizing a tendency to alignment and hierarchy: (1) obtaining a representative sample of configurations and determining its statistical parameters (moments and percentages); and (2) dividing the region of possible configurations of triple systems (Agekian and Anosova, 1967) into a set of segments and finding the probabilities for the configurations to find themselves in each of them. Both these methods allow representation of the data by numerical simulations as well as observations. The effect of projection was studied. It rather overestimates the alignment and hierarchy of the triple systems. Among the parameters of interest there are found some parameters that are least sensitive to projection effects. The samples consist of simulated galaxy triplets (with hidden mass) as well as of 46 probably physical triple galaxies (Karachentseva et al., 1979). The observed triples as well as numerical models show a tendency to alignment. The triple galaxies do not show any tendency to hierarchy (formation of the temporary binaries), but this tendency may be present for simulated triplets without significant dark matter. The significant hidden mass (of order ten times the total mass of a triplet) decreases the probability of forming a binary and so weakens the hierarchy. Small galaxy groups consisting of 3 to 7 members are probably the most prevalent types of galaxy aggregate (Gorbatsky, 1987). Galaxy triplets are the simplest groups, but dynamically nontrivial ones.

Anosova, Joanna P.↗
NASA NTRS1990

A HOL theory for voting

Central to fault-tolerant computing is redundancy management, and common to proofs of fault-tolerance is a maximum fault assumption. Typically a maximum fault assumption is rather restrictive. Usually, this is necessary to avoid assumptions about the behavior of faulty channels. A maximum fault assumption is useful because it allows reasoning about fault tolerance in the presence of arbitrarily malicious fault behavior. However, analysis of the architecture may establish certain scenarios in which the assumption may be weakened. Proofs comparing majority and plurality and proofs of simple reconfiguration strategies are presented in viewgraph form.

Miner, Paul S.↗