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At least 469 records · Page 26
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NASA NTRS1975

On upstream blocking in a viscous diffusive stratified flow

The effect of diffusion of specie upon the flow about a transverse flat plate moving horizontally in a viscous stratified medium is considered. Asymptotic expansions are used to define a parameter regime where a viscous-diffusive-buoyancy balance is dominant. The solution, expressed in terms of an inverse Fourier transform, is numerically integrated. The results show that, as in the non-diffusive problem, a region of closed streamlines exists ahead of the body. However, unlike the case where diffusion is neglected, the density field within this recirculating region is uniquely determined and found to be statically stable. It is also found that varying the relative amount of diffusion affects not only the density distribution, but the velocity profile as well, indicating a strong coupling between the vorticity and specie equation.

Koop, C. G.↗
NASA NTRS1975

Mercury's helium exosphere

A model He exosphere of Mercury is developed whose source derives from accretion of the small fraction of solar-wind He(++) that is captured by the magnetosphere and absorbed at the planetary surface. The model is based on a lunar analogy whereby the surface is saturated with He. Surface and radial density distributions are derived, and the dayside concentration is determined by requiring that the limb intensity at 584 A equals the observed intensity. To maintain the He exosphere, it is found that only a fraction of the solar-wind He(++) flux intercepted by the magnetosphere need be captured. This corresponds to a total accretion rate of 6.8 by 10 to the 22nd power He(++) ions per sec at the surface, which also represents the upper limit to the total outgassing rate of He from the interior.

Hartle, R. E.↗
NASA NTRS1976

Narrow band photometry of comet Kohoutek

Photometric observations of the coma of comet Kohoutek were made at the Cassegrain focus of a 36-inch telescope. The observations consisted of one wide (visual, 5454 A) and six narrow (CN, 3879 A; C3, 4057 A; C2, 4732 A, 5165 A, 5634 A; continuum, 5200 A) band interference filters. In addition each filter was used with six diaphragms. Good quality data were obtained on 13 days between November 1973 and February 1974. A small flare was observed on 1 December for all filters, a CN flare on 13 January, and a visual flare on 28 January. The data were reduced to absolute narrow band magnitudes of the comet for the 13 days. The radial dependence of the surface brightness was derived from the set of diaphragms and future work will be directed toward using these results for modeling density distributions for the coma.

Brown, L. W.↗
NASA NTRS1976

Defining constants, equations, and abbreviated tables of the 1975 US Standard Atmosphere

The U.S. Standard Atmosphere, 1975 (COESA, 1975) is an idealized, steady-state representation of the earth's atmosphere from the surface of the earth to 1000-km altitude, as it is assumed to exist in a period of moderate solar activity. From 0 to 86 km, the atmospheric model is specified in terms of the hydrostatic equilibrium of a perfect gas, with that portion of the model from 0 to 51 geopotential kilometers being identical with that of the U.S. Standard Atmosphere, 1962 (COESA, 1962). Between 51 and 86 km, the defining temperature-height profile has been modified from that of the 1962 Standard to lower temperatures between 51 and 69.33 km, and to greater values between 69.33 and 86 km. Above 86 km, the model is defined in terms of quasi-dynamic considerations involving the vertical component of the flux of molecules of individual gas species. These conditions lead to the generation of independent number-density distributions of the major species, N2, O2, O, Ar, Ne, and H, consistent with observations. The detailed definitions of the model are presented along with graphs and abbreviated tables of the atmospheric properties of the 1975 Standard.

Minzner, R. A.↗
NASA NTRS1975

Two dimensional thermal and charge mapping of power thyristors

The two dimensional static and dynamic current density distributions within the junction of semiconductor power switching devices and in particular the thyristors were obtained. A method for mapping the thermal profile of the device junctions with fine resolution using an infrared beam and measuring the attenuation through the device as a function of temperature were developed. The results obtained are useful in the design and quality control of high power semiconductor switching devices.

Hu, S. P.↗
NASA NTRS1975

Electron impact ionization of Io's sodium emission cloud

The geometry of the sodium cloud associated with Io (Jupiter I) indicates that the lifetime of the neutral sodium atoms is an order of magnitude less than the photoionization lifetime. We suggest that ionization by thermal plasma electrons in the Jovian magnetosphere is the dominant Na loss process. Using plasma densities deduced from Pioneer 10 measurements, the lifetime and density distributions are calculated for Na and other species which may be present in the cloud around Io. Electron ionization of Na is found to be an order of magnitude faster than photoionization, in agreement with the lifetime deduced from Na cloud observations.

Carlson, R. W.↗
NASA NTRS1975

EUV analysis of an active region

A sequence of extreme-ultraviolet (EUV) spectroheliograms of McMath region No. 10283 were obtained by OSO-6. The lines O VI (1032 A) Mg X (625 A), Si XII (499 A), and Fe XVI (335 A) were used to determine coronal temperatures and densities above the active region. A comparison of theoretical and observed line ratios yielded coronal temperatures of 2.2 to 2.3 million K above the active region and 2.0 to 2.1 million K in the surrounding area. The temperatures derived from ratios involving the O VI intensities are systematically higher than the others. This is attributed to an error in the theoretical O VI intensities. The intensities observed above the limb are compared with intensities predicted by a simple model based on cylindrical geometry. The overall agreement shows that the assumption of an isothermal corona in hydrostatic equilibrium above the active region is a resonable working hypothesis and that the adopted geometrical model for the electron density distribution is adequate.

Raghavan, N.↗
NASA NTRS1976

Langley facility for tests at Mach 7 of subscale, hydrogen-burning, airframe-integratable, scramjet models

Modifications to a 20-megawatt arc-heated facility for testing a hydrogen-burning, airframe-integratable, subscale, scramjet model are described. Arc-heated flow is mixed with unheated air to furnish a test flow duplicating Mach 7 flight. (Stagnation temperature is 2220 K.) Modifications to the commercially available heater to improve survivability and smoothness are described. Pitot profiles show uniform flow and a slightly thinner nozzle boundary layer than predicted. Comparison of the tunnel boundary layer, which will be ingested by the engine model, with the boundary layer that a flight engine might ingest from its vehicle forebody shows a difference in the density distribution through the boundary layer. Calculations of wall heating and transient wall temperatures of the engine model show that for a 30-sec burn, the heat sink model requires cooling at selected locations to avoid thermal-stress, cycle-life problems. Model performance predictions show that fuel equivalence ratio and nozzle exit area both have large effects on thrust. Average inlet entrance Mach number (as affected by boundary-layer ingestion) has little effect on thrust.

Boatright, W. B.↗
NASA NTRS1975

Digital image centering. I

A series of parallax plates have been measured on a PDS microdensitometer to assess the possibility of using the PDS for precision relative astrometry and to investigate centering algorithms that might be used to analyze digital images obtained with the Large Space Telescope. The basic repeatability of the PDS is found to be plus or minus 0.6 micron, with the potential for reaching plus or minus 0.2 micron. A very efficient centering algorithm has been developed which fits the marginal density distributions of the image with a Gaussian profile and a sloping background. The accuracy is comparable with the best results obtained with a photoelectric image bisector.

Van Altena, W. F.↗
NASA NTRS1976

A comparison of one-dimensional theoretical models of stratospheric minor constituents

The density profiles of stratospheric minor constituents are compiled from several one-dimensional theoretical models that have appeared in the literature. The models are compared with each other and also with observations if data are available. The similarity and disparity of the models are clarified and discussed. If a large difference exists among the models, attempts are made to interpret it as much as possible in terms of the physical and chemical data employed in each model. The inadequacy of one-dimensional models in calculating the realistic ozone density distributions and the problems related to it are briefly discussed.

Shimazaki, T.↗
NASA NTRS1976

Effects on the plasmasphere of irregular electric fields

A conservative convection electric field model developed by Volland (1973) to describe the solar wind induced plasma flow within the inner magnetosphere is modified to include a noisy spatial component. Under steady state conditions such a random component will result in spatial irregularities in the thermal plasma density distribution in the vicinity of the plasmapause. Spatial irregularities in the convection can produce longitudinally restricted perturbations near the plasmapause, some of which are detached from the main body of the plasmasphere. Temporal variations in the midnight to noon flow intensity produce elongated extensions of the plasmasphere, but even short period variations of the overall magnitude of the convection cannot produce longitudinally localized perturbations in the thermal plasma distribution. Convection models based on the 3 hr magnetic index K sub p yield plasmasphere structures which are qualitatively similar to those based on shorter period variations, but the exact location at any given time of the plasmapause is dependent upon the characteristic time scale employed.

Grebowsky, J. M.↗
NASA NTRS1976

Computer simulations of planetary accretion dynamics: Sensitivity to initial conditions

The implications and limitations of program ACRETE were tested. The program is a scheme based on Newtonian physics and accretion with unit sticking efficiency, devised to simulate the origin of the planets. The dependence of the results on a variety of radial and vertical density distribution laws, the ratio of gas to dust in the solar nebula, the total nebular mass, and the orbital eccentricity of the accreting grains was explored. Only for a small subset of conceivable cases are planetary systems closely like our own generated. Many models have tendencies towards one of two preferred configurations: multiple star systems, or planetary systems in which Jovian planets either have substantially smaller masses than in our system or are absent altogether. But for a wide range of cases recognizable planetary systems are generated - ranging from multiple star systems with accompanying planets, to systems with Jovian planets at several hundred AU, to single stars surrounded only by asteroids.

Isaacman, R.↗
NASA NTRS1976

Radiative transfer in circumstellar dust shells

An iterative numerical procedure is developed to calculate the radiation field as a function of position, direction, and wavelength within a spherically symmetric circumstellar dust shell. The dust shell is assumed to consist of gray, isotropically scattering dust particles in thermal equilibrium with the radiation field and to be characterized by seven parameters: radius of the central star, inner radius of the shell, outer radius of the shell, total optical depth of the shell, an index which specifies the density distribution, albedo of the dust particles, and temperature of the central star. When the shell is close to the star, the angular distribution of the stellar component of the radiation field is determined by direct integration along lines of sight to the stellar surface, and the back-warming effect of the shell is included in the calculations to determine the net stellar flux. Using this procedure, the temperature distributions and radiation fields within several model dust shells are determined and used to calculate for each shell the spectral-energy and spatial-energy distributions of the radiation emitted at 11 wavelengths from 0.4 to 20 microns. For shells of moderate optical depth, it is found that the radiation field is quite anisotropic, especially at shorter wavelengths, and that the shape of the spectral-energy distribution is strongly dependent on all seven model parameters. The Eddington factor as a function of wavelength and optical depth is calculated for each model.

Harvel, C. A.↗
NASA NTRS1977

Physical properties of a polar coronal hole from 2 to 5 solar radii

Observations with a white-light coronagraph aboard Skylab are used to determine the boundaries of a coronal hole in the northern polar region and the three-dimensional density structure within the hole between heights of 2 and 5 solar radii. The boundary of the hole is found to be essentially axisymmetric about the polar axis, nearly radial from 3 to 6 solar radii, and located near 25 deg latitude at these heights. The radiances arising from the hole are interpreted as resulting from an axisymmetric density distribution whose logarithmic radial gradient is independent of position within the hole and whose magnitude increases with angular distance away from the hole's axis. The velocity distribution within the hole is obtained from the continuity equation by assuming that the particle flux flowing outward in the hole is similar to that measured for high-speed solar-wind streams at 1 AU, and it is shown that the transition from subsonic to supersonic flow occurs between 2.2 and 3 solar radii.

Munro, R. H.↗
NASA NTRS1977

Computer simulations of planetary accretion dynamics - Sensitivity to initial conditions

A computer simulation program which generates mature planetary systems using only Newtonian physics and accretion with unit sticking efficiency is examined. The dependence of the simulation on a variety of radial and vertical density distribution laws, on the ratio of gas to dust in the solar nebula, on the total nebular mass, and on the orbital eccentricity of the accreting grains is explored. To approach physically realistic models, these parameters are altered according to various empirically- and theoretically-suggested criteria. The program is found to generate many recognizable planetary systems, including multiple-star systems with accompanying planets; single stars surrounded only by asteroids; and systems with large Jovian planets, planets similar to Pluto, or objects of asteroidal mass, as well as the usual terrestrial planets. Planetary spacing in the simulation obeys a relation of the Titius Bode variety; in general, the number of planets per system is inversely proportional to the orbital eccentricity. Some reservations about the simulation program are expressed; however, it does support the idea of abundant and diverse planetary systems throughout the galaxy.

Isaacman, R.↗
NASA NTRS1977

Solar polar coronal hole - A mathematical simulation

The northern polar region of the sun was studied during July 1973 by Munro and Jackson through use of the white-light coronagraph and the X-ray photographs produced by the Skylab mission. They described the northern polar hole as nearly axisymmetric and gave the geometry and density distribution under this approximation. The present work gives quasi-radial approximation to the full magnetohydrodynamic equations for axisymmetric, polytropic solar wind flow to simulate this polar hole, with the benefit that model temperature and magnetic field intensities and distributions in this particular polar hole can be deduced. It is concluded that from 2 out to 5 solar radii the temperature varies only slightly with radius, but is larger near the center of the polar hole than at the edge. It is also found that the magnetic field intensity at 2 solar radii could be about 1 gauss at the center of the hole, decreasing toward the edge of the hole. If this is extrapolated to the surface, a field as high as 20 gauss is suggested.

Suess, S. T.↗
NASA NTRS1978

Transonic flow past an airfoil with condensation

In connection with investigations conducted to determine the influence of water vapor on experiments in wind tunnels, the question arose as to what changes due to vapor condensation might be expected in airfoil measurements. Density measurements on circular-arc airfoils aided by an interferometer in choked tunnels with parallel walls show that increasing humidity produces increasing changes in the flow field. The flow becomes nonstationary at high humidity. At the airfoil, however, the influence of the condensation is only felt, inasmuch as the shock bounding the local supersonic region moves upstream with increasing humidity while its intensity decreases. The density distribution upstream of the shock remains unchanged. Even if the flow becomes nonstationary in the vicinity of the airfoil, no changes occur at the airfoil.

Schmidt, B.↗