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At least 1,117 records · Page 62

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NASA NTRS1984

Why credible propeller noise measurements are possible in the acoustically untreated NASA Lewis 8 ft by 6 ft wind tunnel

An explanation is presented for the lack of acoustic reflections in noise studies of propfan models in the NASA-Lewis 8 x 6 ft wind tunnel, where trials were run at Mach numbers 0.5-0.85. The highly directional propeller noise, i.e., mainly in the plane of rotation, experiences a convective effect due to the high subsonic axial Mach number. Reflected sounds are carried downstream, out of range of the acoustic sensors in the tunnel. Furthermore, reflected noise is less audible, and therefore does not affect measurements near peak values. It is suggested that some data contamination may occur below Mach 0.6, and that measurements be performed on higher harmonics generated by low level reflected noise.

Dittmar, J. H.↗

NASA NTRS1984

Subducted slabs and the geoid - Constraints on mantle rheology and flow

The total geoid anomaly which is the result of a given density contrast in a convecting viscous earth is affected by the mass anomalies associated with the flow induced deformation of the upper surface and internal compositional boundaries, as well as by the density contrast itself is discussed. If the internal density contrasts can be estimated, the depth and variation of viscosity with depth of the convecting system can be constrained. The observed long wavelength geoid is highly correlated with that predicted by a density model for seismically active subducted slabs. The amplitude of the correlation is explained if the density contrasts associated with subduction extend into the lower mantle or if subducted slabs exceeding 350 km in thickness are piled up over horizontal distances of thousands of km at the base of the upper mantle. Mantle wide convection in a mantle that has a viscosity increasing with depth provides the explanation of the long-wavelength geoid anomalies over subduction zones. Previously announced in STAR as N83-22874

Hager, B. H.↗

NASA NTRS1984

Intensified silicon photodiode array detector linearity Application to coherent anti-Stokes Raman spectroscopy

The linearity of an intensified silicon photodiode array multichannel detector is studied with coherent anti-Stokes Raman and other similar signals. Studies with diffuse and focused (spherically and cylindrically) signals resolved apparent saturation problems which limit the dynamic range of the detector. In addition, it has been shown that there is no short-range wavelength (473-532-nm) dependence on this saturation. Theoretical explanations for these phenomena are also included.

Antcliff, R. R.↗

NASA NTRS1984

Search for 511 keV electron-positron annihilation radiation from mildly active galaxies using the HEAO 3 gamma-ray spectrometer

The HEAO 3 spacecraft's gamma-ray spectrometer has been used to obtain upper limits to the 511-keV electron-positron annihilation line flux from seven galaxies: M 81/M 82, NGC 4278, M 104, NGC 6500, NGC 2911, and NGC 262. These appear to contain scaled-up versions of the radio source in the galactic center, and their nonthermal activity appears to be confined to the central regions, implying that energetic particles produced in these nuclei do not escape into the low density regions of interstellar or intergalactic space. On the basis of the scaling law adopted, six of these seven galaxies should have been detected above the upper limits obtained for narrow 511 keV emission. Explanations are adduced which account for the scaling law's failure to predict the correct level of radiation annihilation.

Marscher, A. P.↗

NASA NTRS1984

Very slow classical Cepheids - Theoretical models with periods longer than 50 days

Systematics of the light curves of classical Cepheids with the longest known periods have been investigated with the help of full-amplitude models of pulsating stellar envelopes. For periods exceeding about 60 days, flat-topped light curves of the S Vul type are found to replace the smooth, asymmetric light curves characteristic of the slightly faster Cepheids. Predicted light and velocity amplitudes (although not the predicted radius amplitudes) agree well with observations. Variables with fluctuating light minima are observed to lie well off the mean period-luminosity relation, as are a few other (more stable?) variables with similarly long periods. The explanation for the long periods is probably low effective temperature rather than a low stellar mass. Because of the abnormal slowness of the classical Cepheids with periods longer than about 100 days, it is recommended that these variables not be used to calibrate the mean period-luminosity relation. Analogies between the slow classical Cepheids and the slow Population II Cepheids are drawn.

Carson, T. R.↗

NASA NTRS1984

Thermal ripples in a resistive and radiative instability

The development of the resistive tearing instability in the case of sheared magnetic fields is considered, taking into account also the occurrence of a radiatively driven thermal instability. It is pointed out that thermal conduction has generally been neglected in theories similar to those discussed. The present investigation is concerned with a consideration of both parallel and perpendicular thermal conduction, in addition to finite resistivity and radiative loss. Attention is given to the equations and the model, the spatial singularity which arises with consideration of only the parallel heat conduction, the removal of this singularity and the formation of temperature oscillations (thermal ripples) by inclusion of the perpendicular heat-flux component, and details regarding the numerical procedure. A brief explanation is provided of the conditions required for the oscillations, and potential implications of the results with respect to the solar flare are discussed.

Steinolfson, R. S.↗

NASA NTRS1984

The spectral evolution of low-frequency variable radio sources

The dynamic spectra of several low frequency extragalactic radio sources are presented. The observations were made at 318, 430, 606, 880, and 1400 MHz at several different radio observatories around the U.S. Two outbursts were observed in AO 0235 + 16 at 1.4 GHz, followed by a diminished variation at the lower frequencies. The dynamic frequencies of NRAO 140, PKS 1117 + 14, DA 406, CTA 102, and 3C 454.3 do not fit the same pattern. These radio sources displayed the following characteristics: (1) departure from straight or curved spectra at the frequencies of variation; (2) no obvious frequency drifting; and (3) negligible variation at 1.4 GHz. Possible explanations for this behavior are briefly discussed.

Dennison, B.↗

NASA NTRS1984

A tomographic image of mantle structure beneath southern California

The variations in seismic structure beneath southern California were determined by using a tomographic method of inversion on teleseismic P delays recorded with the Southern California Array. The inversion reveals two prominent features beneath the region. The first is a thin, vertical wedge directly beneath the Transverse Ranges that is 2-3 percent faster than the surrounding region. This feature deepens to the east, attaining a maximum depth of about 250 km beneath the San Bernardino Mountains. The second feature is a major zone of low velocity material that is 2-4 percent slow under the Salton Trough rift valley, extending to a depth of about 125 km. Two possible explanations for the spatial association of the Transverse Ranges with the velocity anomaly below are lithospheric subduction or small-scale sublithospheric convection in the region of the Big Bend of the San Andreas Fault. The low velocity anomaly beneath the Salton Trough is consistent with convective upwelling there.

Humphreys, E.↗

NASA NTRS1984

Mars - Wind streak production as related to obstacle type and size

The characteristics of wind streaks associated with Martian craters and hills in the size range of 100 m to 80 km (corresponding to obstacle heights of a few to several hundred meters) have been analyzed from Viking Orbiter images. Both dark erosional and bright depositional streaks form over the entire obstacle size range, but there are preferred obstacle sizes for producing streaks. Bright streaks form more readily in association with relatively smaller obstacles than do dark streaks. Small obstacles produce both types of streaks more effectively than do large ones. Hills produce streaks as effectively as do craters of comparable height. Alternative explanations of bright streak formation are evaluated in terms of their ability to account for these observations. The most satisfactory models invoke blocking of atmospheric flow downwind of an obstacle and consequent deposition of dust within the sheltered zone.

Lee, S. W.↗

NASA NTRS1984

Physics of gamma-ray bursts

Attention is given to the accumulating evidence for the view that gamma-ray bursts come from strongly magnetic neutron stars, discussing the physical properties of the emission region and the radiation processes expected in strong magnetic fields, and emphasizing that the observed burst spectra require that the emission region be optically thin. This entails that the energy of the emitting plasma and/or the plasma itself be continuously replenished during a burst, and that the cooling time scale of the emitting plasma be much shorter than the observed duration of the bursts. This characteristic of the cooling time scale implies that the burst intensity and spectrum can vary on extremely short time scales, and that the burst duration must have a separate explanation. It is emphasized that synchrotron emission is favored as the gamma-ray production mechanism; it is the only mechanism capable of satisfying the optical thinness constraint while producing the observed luminosity.

Lamb, D. Q.↗

NASA NTRS1984

Venus banded terrain - Tectonic models for band formation and their relationship to lithospheric thermal structure

Two classes of tectonic models for the formation of band structure on Venus mountain ranges are quantitatively evaluated: folding models and extensional models. The characteristics of banded terrain on Ishtar Terra are reviewed, particularly the evidence for a tectonic origin and such important details of band geometry as the spacing between adjacent bands. The tectonic models are tested against these observed features, and it it shown that both classes of models are possible explanations of banded terrain as long as the outer elastic-brittle layer of the Venus crust is at most a few kilometers thick. A general assessment is given of hypotheses for the origin of banded terrain and of the relationship between mechanical models for band formation and the thermal structure of the Venus lithosphere.

Solomon, S. C.↗

NASA NTRS1984

On the magnetic field in the white dwarf Grw + 70.8247 deg

Greenstein (1984) has estimated a 3.5 x 10 to the 8th gauss magnetic field strength for the case of a sharp absorption feature near 1345 A, from the white dwarf Grw + 70.8247 deg that is interpreted as a component of hydrogen Lyman-alpha in a strong magnetic field. A highly accurate multiparameter variational method is the basis of the present deduction of a maximum Lyman-alpha obtainable wavelength of 1342.6 A, with a magnetic field of 5.6 x 10 to the 8th gauss. Alternative explanations of the 1345 A line as H-alpha or H-beta components, or a combination of the two, are given.

Henry, R. J. W.↗

NASA NTRS1984

Kristian Birkeland - The man and the scientist

A review is presented of Birkeland's outstanding contributions to auroral theory and, in particular, to the foundation of modern magnetospheric physics. Birkeland's first years in research, after a study of mathematics and theoretical physics at the university, were concerned with Maxwell's theory, the investigation of electromagnetic waves in conductors, wave propagation in space, an energy transfer by means of electromagnetic waves, and a general expression for the Poynting vector. Experiments with cathode rays near a magnet in 1895, led Birkeland to the development of an auroral theory. This theory represented the first detailed, realistic explanation of the creation of an aurora. Attention is given to experiments conducted to verify the theory, the discovery of the polar elementary storm, and the deduction of auroral electric currents. Birkeland's background and education is also considered along with his personality.

Egeland, A.↗

NASA NTRS1984

Magnetospheric topology of fields and currents

The interaction of the solar wind with the magnetosphere is examined from the standpoint of fundamental processes in physics and, whenever possible, observational data. A conceptual model is developed which incorporates several features of the magnetosphere that are still open to debate, such as open magnetic field lines in the polar cap and a boundary layer just inside the magnetopause. It is shown that the length of the magnetotail, the existence of the polar rain, the bursts of particles at the edge of the plasma sheet, and the poleward motion of auroral forms during the recovery phase of a magnetospheric substorm may all have a natural explanation with the proposed model. The one exception, however, is the topology of the electric field. The problems associated with reversal of the electric field within the magnetotail are examined.

Heikkila, W. J.↗

NASA NTRS1985

Latchup in CMOS Integrated Circuits

Sensitivity to ion beams is studied. Latchup effect subject of paper presenting results of testing 19 types of complementary metal-oxide semiconductor (CMOS) chips from six manufacturers. Report gives details of sensitivity of chips to latchup caused by argon and krypton ion beams. Identifies parasitic npnp paths and proides latchup cross section and qualitative explanation of latchup sensitivity for each chip type.

Soliman, K. A.↗

NASA NTRS1986

Image-Processing Educator

Apple Image-Processing Educator (AIPE) explores ability of microcomputers to provide personalized computer-assisted instruction (CAI) in digital image processing of remotely sensed images. AIPE is "proof-of-concept" system, not polished production system. User-friendly prompts provide access to explanations of common features of digital image processing and of sample programs that implement these features.

Gunther, F. J.↗

NASA NTRS1984

Singular asymptotic expansions in nonlinear rotordynamics

During hot firing ground testing of the Space Shuttle's Main Engine, vibrations of the liquid oxygen pump occur at frequencies which cannot be explained by the linear Jeffcott model of the rotor. The model becomes nonlinear after accounting for deadband, side forces, and rubbing. Two phenomena present in the numerical solutions of the differential equations are unexpected periodic orbits of the rotor and tracking of the nonlinear frequency. A multiple scale asymptotic expansion of the differential equations is used to give an analytic explanation of these characteristics.

Day, W. B.↗

NASA NTRS1984

Sintered silicon nitride workpiece

A description is given of a sintering method for silicon nitride. A detailed explanation of the invention is presented. Four procedures for the sintering process are described.

Shinohara, N.↗