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At least 73 records · Page 4

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NASA NTRS1974

Study of an instrument for sensing errors in a telescope wavefront

Focal plane sensors for determining the error in a telescope wavefront were investigated. The construction of three candidate test instruments and their evaluation in terms of small wavefront error aberration measurements are described. A laboratory wavefront simulator was designed and fabricated to evaluate the test instruments. The laboratory wavefront error simulator was used to evaluate three tests; a Hartmann test, a polarization shearing interferometer test, and an interferometric Zernike test.

Golden, L. J.↗

NASA NTRS1974

Magneto-fluid-mechanics free convection turbulent flow

The present work is an experimental study of the influence of a uniform magnetic field on the structure of a free convection turbulent boundary layer in a conducting fluid. The boundary layer was formed along the heated vertical wall of a cell. The applied magnetic field was normal to the wall. The measured mean temperature profiles, temperature turbulent intensity distributions, and temperature spectra along the wall, indicated that transition from turbulent to laminar flow occurs at a constant value of the ratio (Rayleigh number)/(Hartmann number). The study of the recorded spectra indicated that the presence of the magnetic field enhances the mechanism of turbulent suppression due to the buoyancy forces. Finally, a possible mechanism by which turbulence is suppressed by the presence of a magnetic field is discussed.

Papailiou, D. D.↗

NASA NTRS1975

Planetary atmospheres

A few of the advances and theoretical issues gained or raised by recent missions of unmanned spacecraft, notably Mariner 9, Pioneer 10, and Mariner 10, are discussed. Emphasis is on dynamics. Workers reviewed include Murray et al, Young, Marov et al, Hansen (Venus), Leovy, Sagan, Masursky, Leighton and Murray, Hartmann, Goody, Conrath (Mars), Gehrels et al, Williams and Robinson, Westphal, Kliore et al (Jupiter), Trafton, Hunten, Gillette et al, Pollack, Danielson et al (Titan). Planned future spacecraft missions are mentioned.

Gierasch, P. J.↗

NASA NTRS1976

Aeroacoustics: Acoustic wave propagation; Aircraft noise prediction; Aeroacoustic instrumentation

The papers in this volume deal with recent research into acoustic-wave propagation through the atmosphere and progress in aeroacoustic instrumentation, facilities, and test techniques. Topics include the propagation of aircraft noise over long distances in the lower atmosphere, measured effects of turbulence on the rise time of a weak shock, sound scattering from atmospheric turbulence, saturation effects associated with sound propagation in a turbulent medium, and a computer model of the lightning-thunder process. Other papers discuss the development of a computer system for aircraft noise prediction; aircraft flyover noise measurements; and theories and methods for the prediction of ground effects on aircraft noise propagation, for the prediction of airframe aerodynamic noise, for turbine noise prediction, and for combustion noise prediction. Attention is also given to the use of Hartmann generators as sources of high-intensity sound in a large absorption flow-duct facility, an outdoor jet noise facility, factors in the design and performance of free-jet acoustic wind tunnels, and the use of a laser shadowgraph for jet noise diagnosis.

Schwartz, I. R.↗

NASA NTRS1977

Plasma rotation by electric and magnetic fields in a discharge cylinder

A theoretical model for an electric discharge consisting of a spatially diverging plasma sustained electrically between a small ring cathode and a larger ring anode in a cylindrical chamber with an axial magnetic field is developed to study the rotation of the discharge plasma in the crossed electric and magnetic fields. The associated boundary-value problem for the coupled partial differential equations which describe the electric potential and the plasma velocity fields is solved in closed form. The electric field, current density, and velocity distributions are discussed in terms of the Hartmann number and the Hall coefficient. As a result of Lorentz forces, the plasma rotates with speeds as high as 1 million cm/sec around its axis of symmetry at typical conditions. As an application, it is noted that rotating discharges of this type could be used to develop a high-density plasma-ultracentrifuge driven by j x B forces, in which the lighter (heavier) ion and atom components would be enriched in (off) the center of the discharge cylinder.

Wilhelm, H. E.↗

NASA NTRS1977

Boundary-value problem for plasma centrifuge at arbitrary magnetic Reynolds numbers

We solve in closed form the boundary-value problem for the partial differential equations which describe the (azimuthal) rotation velocity and induced magnetic fields in a cylindrical plasma centrifuge with ring electrodes of different radii and an external, axial magnetic field. The electric field, current density, and velocity distributions are discussed in terms of the Hartmann number H and the magnetic Reynolds number R. For small Hall coefficients, the induced magnetic field does not affect the plasma rotation. As a result of the Lorentz forces, the plasma rotates with speeds as high as 100,000 cm/sec around its axis of symmetry at typical conditions, so that the lighter (heavier) ion and atom components are enriched at (off) the center of the discharge cylinder.

Wilhelm, H. E.↗

NASA NTRS1978

Boundary-value problem for a counterrotating electrical discharge in an axial magnetic field

An electrical discharge between two ring electrodes embedded in the mantle of a cylindrical chamber is considered, in which the plasma in the anode and cathode regions rotates in opposite directions under the influence of an external axial magnetic field. The associated boundary-value problem for the coupled partial differential equations describing the azimuthal velocity and radial current-density fields is solved in closed form. The velocity, current density, induced magnetic induction, and electric fields are presented for typical Hartmann numbers, magnetic Reynolds numbers, and geometry parameters. The discharge is shown to produce anodic and cathodic plasma sections rotating at speeds of the order 1,000,000 cm/sec for conventional magnetic field intensities. Possible application of the magnetoactive discharge as a plasma centrifuge for isotope separation is discussed.

Hong, S. H.↗

NASA NTRS1978

Line shifts due to blending

The reported investigation is concerned with the case in which the measured position of a strong line of the spectrum is blended with a weaker one, taking into account the traditional solution proposed by Hartmann (1901) concerning the mean of the solar lines. An arbitrary element regarding the use of the centroid is related to the decision at which separation the components pass from being 'blended' to being 'resolved'. At this separation, the adopted wavelength suddenly jumps from the centroid of the blend to the position of the stronger line alone. It is pointed out that this discontinuous behavior is physically implausible. Attention is given to a solution involving a gradual diminution of the effect of the weaker line with increasing separation. Thus the weight of each component should depend on its position as well as its intensity. When partially blended lines cannot be decomposed into their components, and an effective line-position for the hole blend is needed, the line peak is the criterion least affected by blending. A method is presented for calculating such peak positions.

Young, A. T.↗

NASA NTRS1981

Theoretical studies of chromospheres and winds in cool stars

Calculated radiative losses from H, H-, Ca II, and Mg II show that cooling for the chromosphere of the supergiant epsilon Gem do not differ greatly from the solar law, although there are differences at approximately 6000K due to ionization effects. With a rough standard law for computation of stellar winds using the Hartmann-MacGregor theory and standard stellar evolutionary calculations, the wind velocities and temperatures in the HR diagram were systematically explored. Results show that cool winds with tempratures 1,000,00K are not possible for log g or = 2. Predicted wind velocities are approximately 1.5 to 2 x larger than observed, particularly for the most luminous cool stars. The ionization balance for the wind of alpha ORI and the hydrogen profile lines for T Tauri stars were computed using the PANDORA computer program.

Dupree, A. K.↗

NASA NTRS1981

Observations of the interaction of ozone and dynamics

Data on the global distribution of temperature and ozone obtained with the Limb Radiance Inversion Radiometer which flew on the Nimbus 6 spacecraft are analyzed within the conceptual framework of Hartmann and Garcia (1979). The analysis supports the conclusion that ozone perturbations are dynamically controlled when the photochemical time constant is much greater than the dynamical time constant; when the photochemical constant is much less than the dynamical constant, ozone perturbations are photochemically controlled. It is also shown that the transports of planetary waves, or eddy transports, appear to be strongly localized in the region between the regimes in which dynamical and photochemical controls operate.

Gille, J. C.↗

NASA NTRS1984

Midstratospheric circulations in the southern hemisphere from super pressure balloon trajectories

Three instrumented super pressure balloons were flown throughout the southern hemisphere at 18 mbar, over a period of five months. The balloon trajectories, reconstructed from NIMBUS 6 satellite location data, have proved to be valuable indicators of midstratospheric structure and dynamics in the Lagrangian frame of reference. Several examples of high zonal and meridional winds are reported. Also shown in the analysis is a feature encountered between 30 S-40 S during a two-week period. The feature appears to be a breakdown of large-scale zonal flow and its replacement by weak cyclonic eddies. During the observational period the balloon motions document the spring reversal. Mean zonal wind speed analysis from balloon positions were obtained covering the area 15 S-65 S. The results presented here are in substantial agreement with those of Hartmann (1977).

Olivero, J. J.↗

NASA NTRS1984

Ultraviolet photometry from the Orbiting Astronomical Observatory. XXXIX - The structure of the eclipsing Wolf-Rayet binary V444 Cygni as derived from light curves between 2460 A and 3. 5 microns

The WN5 component of V444 Cyg is characterized on the basis of light curves constructed using 1971 OAO observations at 425, 332, 298, and 246 nm and the broadband data at 2.2 and 3.5 microns of Hartmann (1978). Data and calculation results are presented in tables and graphs and discussed. Parameters estimated include core radius 2.9 solar radii, surface temperature 90,000 K, surface electron density 9 x 10 to the 12th/cu cm, surface outflow velocity 400 km/s, and terminal velocity (at about 10 solar radii) 2500 km/s, suggesting that the flow is He. These results are found to agree with the model of Wolf-Rayet atmospheres proposed by Beals (1929, 1944).

Cherepashchuk, A. M.↗

NASA NTRS1986

Models of transition regions in hybrid stars

Models for the transition regions of six hybrid stars, four bright giants and two supergiants, are calculated. The models include mass loss and prescribe Alfven waves as the source of mechanical energy. The momentum and energy deposition rates required at each level of the atmosphere are evaluated. The final models for all six stars have mass loss rates lying below the current VLA upper limits by factors of two to ten, and have densities which agree with those derived by density-sensitive line ratios. The density vs. temperature structure in Alpha TrA agree well with that derived by Hartmann et al. (1985). Wave amplitudes and magnetic field strengths are derived as functions of height, and the amplitudes are found to agree well with the observed line widths in Alpha TrA.

Brosius, J. W.↗

NASA NTRS1986

The temperature structure in accretion flows onto massive protostars

Radiation transfer problems involved in the infall of dust and gas during star formation are studied. Dust properties are discussed, and modifications of spherical radiative transfer equations are presented that permit forward scattering by dust to be treated for the small size of the star relative to the inner radius of the shell. A procedure for deriving the stellar radiation field incident on the inner edge of the shell is developed. The temperature correction procedure of Cassinelli and Hartmann (1975) for extended stellar atmospheres is modified so that the multitemperature nature of the grains in the cloud may be derived. Temperature distributions for three schematic models in which the density is prespecified are discussed. Radiative acceleration of grains is addressed, showing that the proper mean opacity differs by a large factor from the Rosseland mean opacity that is commonly used. Emergent fluxes for the models are given.

Wolfire, Mark G.↗

NASA NTRS1987

Magnetic field twist driven by remote convective motions: Characteristics and twist rates

It is generally believed that convective motions below the solar photosphere induce a twist in the coronal magnetic field as a result of frozen-in physics. A question of interest is how much twist can one expect from a persistent convective motion, given the fact that dissipative effects will eventually figure. This question is examined by considering a model problem: two conducting plates, with finite resistivity, are set in sheared motion and forced at constant relative speed. A resistive plasma is between the plates and an initially vertical magnetic field connects the plates. The time rate of tilt experienced by the field is obtained as a function of Hartmann number and the resistivity ratio. Both analytical and numerical approaches are considered.

Wang, Zheng-Zhi↗

NASA NTRS1986

Wavefront error sensing

A two-step approach to wavefront sensing for the Large Deployable Reflector (LDR) was examined as part of an effort to define wavefront-sensing requirements and to determine particular areas for more detailed study. A Hartmann test for coarse alignment, particularly segment tilt, seems feasible if LDR can operate at 5 microns or less. The direct measurement of the point spread function in the diffraction limited region may be a way to determine piston error, but this can only be answered by a detailed software model of the optical system. The question of suitable astronomical sources for either test must also be addressed.

Tubbs, Eldred F.↗

NASA NTRS1987

Barotropic instability of midlatitude zonal jets on Mars, earth and Venus

A linearized, nondivergent, barotropic vorticity model on a sphere is used to intercompare the fastest growing, barotropically unstable wave modes computed for zonal jets at high latitudes in the middle atmospheres of Venus, earth, and Mars. The model is briefly described, and the choice of a range of zonal jet parameters - primarily Rossby numbers and the jet width - appropriate to Venus and Mars is discussed. The results are presented and compared with those found by Elson (1982) and Hartmann (1983) for nondivergent, barotropically unstable modes in relatively broad, midlatitude zonal jets in planetary atmospheres. Some specific examples appropriate to Venus and Mars are presented.

Michelangeli, Diane V.↗

NASA NTRS1988

Cyclic unequal error protection codes constructed from cyclic codes of composite length

The unequal error correction capabilities of binary cyclic codes of composite length are investigated. Under certain conditions, direct sums of concatenated codes have unequal error correction capabilities. By a modified Hartmann and Tzeng algorithm, it is shown that a binary cyclic code of composite length is equivalent to the direct sum of concatenated codes. With this, some binary cyclic unequal error protection (UEP) codes are constructed. Finally, two-level UEP cyclic direct-sum codes are presented which provide error correction capabilities higher than those guaranteed by the Blokh-Zyablov constructions.

Lin, Mao-Chao↗