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At least 1,333 records · Page 74

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NASA NTRS1980

Radius and brightness temperature observations of Titan at centimeter wavelengths by the Very Large Array

Brightness and radius measurements of the surface of Titan at 6, 2, and 1.3 cm wavelengths obtained with the Very Large Array radio interferometer are presented. Combined results for the three wavelengths indicate that the radius is 2400 + or - 250 km, implying a density of 2.4 + or - 0.7 g/cu cm, and that the brightness temperature is 87 + or - 9 K. The surface temperature may be somewhat higher if the emissivity is less than unity. The new data do not permit a choice between an inversion model for the atmosphere of Titan that predicts a surface temperature of 78 K and a model with both a stratospheric temperature inversion and a modest greenhouse effect that would increase the surface temperature by 10-40 K.

Jaffe, W.↗

NASA NTRS1980

The adhesion, friction, and wear of binary alloys in contact with single-crystal silicon carbide

Sliding friction experiments were conducted with various iron-base alloys (alloying elements were Ti, Cr, Ni, Rh, and W) in contact with a single-crystal silicon carbide (0001) surface in vacuum. Results indicate atomic size misfit and concentration of alloying elements play a dominant role in controlling adhesion, friction, and wear properties of iron-base binary alloys. The controlling mechanism of the alloy properties is an intrinsic effect involving the resistance to shear fracture of cohesive bonding in the alloy. The coefficient of friction generally increases with an increase in solute concentration. The coefficient of friction increases as the solute-to-iron atomic radius ratio increases or decreases from unity. Alloys having higher solute concentration produce more transfer to silicon carbide than do alloys having low solute concentrations. The chemical activity of the alloying element is also an important parameter in controlling adhesion and friction of alloys.

Miyoshi, K.↗

NASA NTRS1980

On the skewness of the temperature derivative in turbulent flows

An attempt is made to explain the fact that, contrary to the requirements of local isotropy, the skewness of the streamwise temperature derivative has been observed to be a nonzero constant of magnitude of about unity in high-Reynolds-number and high-Peclet-number turbulent shear flows. Measurements in slightly heated homogeneous shear flows and in unsheared grid turbulence suggest that the skewness is nonzero only when the mean shear and the mean temperature gradient are both nonzero.

Sreenivasan, K. R.↗

NASA NTRS1981

A thermochemical calculation of the pyroxene saturation surface in the system diopside-albite-anorthite

The pyroxene saturation surface in the system diopside-albite-anorthite may be calculated to + or - 10 C from thermochemical data over most of its composition range. The thermochemical data used are the experimentally determined enthalpies of mixing of the ternary liquids and the enthalpy of fusion of diopside. These are combined with a mixing model for the configurational entropy in the melt and the activity of CaMgSi2O6 in the clinopyroxene, which is less than unity due to departures from CaMgSi2O6 stoichiometry. The two-lattice melt model appears to work satisfactorily throughout the pyroxene primary phase field but probably needs modification at more anorthite-rich compositions.

Hon, R.↗

NASA NTRS1981

Ultrarelativistic electromagnetic pulses in plasmas

The physical processes of a linearly polarized electromagnetic pulse of highly relativistic amplitude in an underdense plasma accelerating particles to very high energies are studied through computer simulation. An electron-positron plasma is considered first. The maximum momenta achieved scale as the square of the wave amplitude. This acceleration stops when the bulk of the wave energy is converted to particle energy. The pulse leaves behind as a wake a vacuum region whose length scales as the amplitude of the wave. The results can be explained in terms of a snow plow or piston-like action of the radiation on the plasma. When a mass ratio other than unity is chosen and electrostatic effects begin to play a role, first the ion energy increases faster than the electron energy and then the electron energy catches up later, eventually reaching the same value.

Ashour-Abdalla, M.↗

NASA NTRS1981

Results of a new approach to determining the density function in the galactic plane - The local system

Star counts in the directions of 15 opaque CO clouds of known distance are used to determine the density function in the galactic plane of the local system of apparently enhanced stellar density. Counts were made on Palomar Sky Survey blue prints in the directions of the highest opacity portions of 15 molecular clouds. Average values of the density functions obtained are found to be significantly less than unity for clouds more distant than 800 pc regardless of galactic longitude, confirming the presence of the local mass density enhancement. Data suggest that the local system represents a density enhancement of a factor of 2 over a region of approximately 500 pc. An empirical relationship is also determined between the distance to a molecular cloud and the number of foreground stars projected on it, which may be useful in determining distances to clouds nearer than 3 kpc. The potential of the Space Telescope for complete stellar mass determinations out to a distance of 1 kpc is pointed out.

Herbst, W.↗

NASA NTRS1980

Radar observations of the icy Galilean satellites

It is shown that when combined with previous observations, 12.6 cm radar observations of Europa, Ganymede and Callisto made at the Arecibo Observatory in November 1977 and February 1979 firmly establish the distinguishing radar properties of the satellites: (1) high geometric albedos, (2) circular polarization ratios which anomalously exceed unity, (3) linear polarization ratios of approximately 0.5, and (4) diffuse scattering, which varies with the cosine of the angle of incidence. The weighted-mean values of these properties are tabulated, and it is found that although significant albedo and/or polarization features are common in the radar spectra, the fractional root mean square fluctuations in disk-integrated properties is only about 10%.

Ostro, S. J.↗

NASA NTRS1981

Generalization of low pressure, gas-liquid, metastable sound speed to high pressures

A theory is developed for isentropic metastable sound propagation in high pressure gas-liquid mixtures. Without simplification, it also correctly predicts the minimum speed for low pressure air-water measurements where other authors are forced to postulate isothermal propagation. This is accomplished by a mixture heat capacity ratio which automatically adjusts from its single phase values to approximately the isothermal value of unity needed for the minimum speed. Computations are made for the pure components parahydrogen and nitrogen, with emphasis on the latter. With simplifying assumptions, the theory reduces to a well known approximate formula limited to low pressure.

Bursik, J. W.↗

NASA NTRS1981

Molecular beam facility for studying mass spectrometer performance

An apparatus which produces neutral gas beams in the velocity range from thermal to 6 km/s makes possible studies simulating the motion of instruments through tenuous atmospheres. Use of the apparatus in studying the response of an open-source mass spectrometer is examined. Experiments performed include responses when the instrument is in a normal mode of operation, when the ion source potentials are adjusted to reject striking gas particles, when the beam is restricted so that portions of the ion source are struck, and when the beam strikes at angles other than normal incidence. Results indicate specular radiation of particles out of the source and stagnation ratios close to unity for normal operation. Retarding potential studies dropped the sensitivity by a factor of about 10, and as angles of attack are varied, the effect was found to depend upon beam velocity and the way the ions are initially accelerated.

Ballenthin, J. O.↗

NASA NTRS1981

Outer atmospheres of cool stars. IX - A survey of ultraviolet emission from F-K dwarfs and giants with IUE

Low-dispersion ultraviolet spectra (1150-2000 A) of a representative sample of cool stars, including dwarfs and giants of spectral types F-K, obtained with the IUE, are examined. The observation and the absolute calibration procedures are described. Correlation diagrams are constructed that compare chromospheric and transition-region emission line strengths and broadband coronal soft X-ray fluxes. The transition-region and coronal emission in the G-K dwarfs and G giants is well correlated with the Mg II (wavelength 2800) doublet emission strength, which is symptomatic of chromospheric energy losses. The power-law slopes are steeper than unity, particularly for soft X-rays. The implications of the correlations are discussed with respect to the weakening or disappearance of transition regions and hot coronae in the cool half of the red-giant branch and possible chromospheric and coronal heating mechanisms. It is proposed that the weakness of outer atmospheres in the red giants compared with the yellow giants can be understood as a consequence of stellar evolution, since it is possible that stars of slightly different spectral type in the giant branch have very different main-sequence progenitors.

Ayres, T. R.↗

NASA NTRS1981

Spectral energy distributions of hot stars with circumstellar dust

Combined spectral energy distribution curves of six Ae/Be stars (BD+61 deg 154, AB Aur, V380 Ori, HD 259431, ZCMa, and BD+40 deg 4124), five peculiar shell stars (HD 31648, HD 45677, HD 50138, HD 163296, and HD 190073), and the peculiar central object in the Red Rectangle nebula (HD 44179), obtained with the IUE and a variety of ground-based optical and infrared telescope systems, are presented, covering the wavelength range of approximately 1300 A to 12.5 microns. The observations are reviewed, and include ultraviolet observations, low-resolution absolute visual spectrophotometry, moderate-resolution relative visual spectrophotometry, and infrared photometry. For most of the objects the ratio of the flux deficiency for lambda equals less than 1 micron to the flux excess for lambda equals greater than 1 micron is approximately unity. Only two of the objects (AB Aur and HD 163296) possess a strong emission feature at 9.7 microns, while one other (HD 31648) may have a weak 9.7 micron emission feature. Most of the objects have a broad, smooth, infrared emission curve resembling those seen in some WC stars. It is concluded that the dust surrounding these stars may be different from that seen in the diffuse interstellar medium.

Sitko, M. L.↗

NASA NTRS1980

Sound attenuation by liners in a blown flap environment

Sound propagation through a hot wall-jet flow over an absorbing wall is studied. The radiated sound field subject to the influence of flow convection and refraction is evaluated, and the nature of acoustic attenuation attributable to a sound absorbing liner is determined. Using a two-dimensional model, the noise field under the aircraft is also determined, and a slug-flow model is used to describe the influence of flow, density, and temperature on acoustic sources in jets. Results show significant changes in the radiated source due to the interference phenomenon, and a good absorber has the potential of changing the sound pressure range of variation to unity. A liner is also found to increase or decrease sound pressure, depending on the frequency.

Parthasarathy, S. P.↗

NASA NTRS1981

Shock-associated noise reduction from inverted-velocity-profile coannular jets

The reduction of shock-associated noise in inverted velocity profile coannular jets is investigated and quantified by making optical and acoustic measurements as a function of outer and inner stream pressure ratio combinations and interpreting noise results with the aid of new theoretical models. Schlieren photographs are obtained in three series of test conditions with a varying primary stream pressure ratio, and acoustic experiments are conducted on a polar arc radius of 3.05 m over a frequency range of 200 Hz to 80 kHz. A constant-thrust comparison is made, and data are plotted and discussed with emphasis on the variation of jet noise levels with primary stream Mach number and equivalent single jets on an absolute basis. It is shown that shock noise from the outer stream is almost eliminated when the inner stream is operated at a Mach number above unity at inverted and normal velocity profile conditions, and optical measurements indicate that for a fixed supercritical fan stream pressure ratio, the repetitive shock structure in the outer stream is minimized when the primary stream ratio becomes supercritical.

Tanna, H. K.↗

NASA NTRS1953

Analogy Between Mass and Heat Transfer with Turbulent Flow

An analysis of combined heat and mass transfer from a flat plate has been made in terms of Prandtl t s simplified physical concept of the turbulent boundary layer. The results of the analysis show that for conditions of reasonably small heat and mass transfer, the ratio of the mass-and heat-transfer coefficients is dependent on the Reynolds number of the boundary layer, the Prandtl number of the medium of diffusion, and the Schmidt number of the diffusing fluid in the medium of diffusion. For the particular case of water evaporating into air, the ratio of mass-transfer coefficient to heat-transfer coefficient is found to be slightly greater than unity.

Callaghan, Edmund E.↗

NASA NTRS1981

A study of the viscous dissipation and surface loading on a vibrating surface

The energy dissipated by viscosity at the edge of a vibrating flat plate is calculated and compared to the radiated acoustic energy. A correction to the Kirchhoff integral estimate of the noise is derived. For Helmholtz number of order unity and smaller the dissipation can be comparable to or greater than the acoustic energy. A viscous compressible theory of the load distribution on a vibrating two dimensional body is developed. First it is shown that load calculations based on potential theory and the Newmann uniqueness condition (continuity of potential or pressure on the surface) are not in agreement with experiment or the more correct viscous theory. For a flat plate airfoil the eigensolution of potential theory is indeterminant while viscous theory yields a unique solution that has square root singularities at the edges. It is also shown that for compact surfaces the far field acoustics depend only on the magnitude of the eigensolutions of potential theoy and so will be uniquely determined by the viscous theory. It is suggested that the general viscous theory of vibrating surfaces with cross sectional geometry will lead to results in agreement with expermentally measured load distributions.

Yates, J. E.↗

NASA NTRS1981

Energy normalization of TV viewed optical correlation (automated correlation plane analyzer for an optical processor)

An automatic correlation plane processor that can rapidly acquire, identify, and locate the autocorrelation outputs of a bank of multiple optical matched filters is described. The read-only memory (ROM) stored digital silhouette of each image associated with each matched filter allows TV video to be used to collect image energy to provide accurate normalization of autocorrelations. The resulting normalized autocorrelations are independent of the illumination of the matched input. Deviation from unity of a normalized correlation can be used as a confidence measure of correct image identification. Analog preprocessing circuits permit digital conversion and random access memory (RAM) storage of those video signals with the correct amplitude, pulse width, rising slope, and falling slope. TV synchronized addressing of 3 RAMs permits on-line storage of: (1) the maximum unnormalized amplitude, (2) the image x location, and (3) the image y location of the output of each of up to 99 matched filters. A fourth RAM stores all normalized correlations. A normalization approach, normalization for cross correlations, a system's description with block diagrams, and system's applications are discussed.

Grumet, A.↗

NASA NTRS1982

Coupled bending-bending-torsion flutter of a mistuned cascade with nonuniform blades

A set of aeroelastic equations describing the motion of an arbitrarily mistuned cascade with flexible, pretwisted, nonuniform blades is developed using an extended Hamilton's principle. The derivation of the equations has its basis in the geometric nonlinear theory of elasticity in which the elongations and shears are negligible compared to unity. A general expression for foreshortening of a blade is derived and is explicity used in the formulation. The blade aerodynamic loading in the subsonic and supersonic flow regimes is obtained from two dimensional, unsteady, cascade theories. The aerodynamic, inertial and structural coupling between the bending (in two planes) and torsional motions of the blade is included. The equations are used to investigate the aeroelastic stability and to quantify the effect of frequency mistuning on flutter in turbofans. Results indicate that a moderate amount of intentional mistuning has enough potential to alleviate flutter problems in unshrouded, high aspect ratio turbofans.

Kaza, K. R. V.↗

NASA NTRS1982

Spectra of cosmic X-ray sources

X-ray measurements provide the most direct probes of astrophysical environments with temperatures exceeding one million K. Progress in experimental research utilizing dispersive techniques (e.g., Bragg and grating spectroscopy) is considerably slower than that in areas utilizing photometric techniques, because of the relative inefficiency of the former for the weak X-ray signals from celestial sources. As a result, the term "spectroscopy" as applied to X-ray astronomy has traditionally satisfied a much less restrictive definition (in terms of resolving power) than it has in other wavebands. Until quite recently, resolving powers of order unity were perfectly respectable, and still provide (in most cases) the most useful spectroscopic data. In the broadest sense, X-ray photometric measurements are spectroscopic, insofar as they represent samples of the overall electromagnetic continua of celestial objects.

Holt, S. S.↗