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At least 955 records · Page 53
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NASA NTRS1989

Inception length to a fully-developed fin-generated shock wave boundary-layer interaction

An experimental study of fin-generated shock wave turbulent boundary-layer interactions confirmed previous observations that, sufficiently far from the fin apex, such interactions become conical. The inception length to conical symmetry was found to increase weakly with Mach number for Mach numbers from 2.5 to 4 and fin angles from 4 to 22 deg. For the range of interactions examined, the inception length was found to depend primarily upon the inviscid shock angle, this angle ranging from 21 to 40 deg. The behavior of the inception length with shock angle can be broadly divided into two categories. For 'weak' interactions with shock angles less than about 35 deg, the inception length decreased as the shock angle increased. For 'strong' interactions with shock angles greater than about 35 deg, the inception region was small and was approximately constant at three boundary-layer thicknesses in length. In the latter, strong interaction case, the inception length was an order of magnitude smaller than that found in the weakest interactions examined, to the extent that strong interactions were practically fully-developed from the apex.

Lu, Frank K.↗
NASA NTRS1989

Surface structure of neutron stars with high magnetic fields

The equation of state of cold dense matter in strong magnetic fields is calculated in the Thomas-Fermi and Thomas-Fermi-Dirac approximations. For use in the latter calculation, a new expression is derived for the exchange energy of the uniform electron gas in a strong magnetic field. Detailed calculations of the density profile in the surface region of a neutron star are described for a variety of equations of state, and these show that the surface density profile is strongly affected by the magnetic field, irrespective of whether or not matter in a magnetic field has a condensed state bound with respect to isolated atoms. It is also shown that, as a consequence of the field dependence of the screening potential, magnetic fields can significantly increase nuclear reaction rates.

Fushiki, I.↗
NASA NTRS1990

Electron energy spectrum and magnetic interactions in high-T(sub c) superconductors

The character of magnetic interactions in La-Sr-Cu-O and Y-Ba-Cu-O systems is of primary importance for analysis of high-T(sub c) superconductivity in these compounds. Neutron diffraction experiments showed the antiferromagnetic ground state for nonsuperconducting La2CuO4 and YBa2Cu3O6 with the strongest antiferromagnetic superexchange being in the ab plane. The nonsuperconducting '1-2-3' system has two Neel temperatures T sub N1 and T sub N2. The first one corresponds to the ordering of Cu atoms in the CuO2 planes; T sub N2 reflects the antiferromagnetic ordering of magnetic moments in CuO chains relatively to the moments in the planes T sub N1 and T sub N2 depend strongly on the oxygen content. Researchers describe magnetic interactions in high-T superconductors based on the Linear Muffin-Tin Orbitals (LMTO) band structure calculations. Exchange interaction parameters can be defined from the effective Heisenberg hamiltonian. When the magnetic moments are not too large, as copper magnetic moments in superconducting oxides, J sub ij parameters can be defined through the non-local magnetic susceptibility of spin restricted solution for the crystal. The results of nonlocal magnetic susceptibility calculations and the values of exchange interaction parameters for La CuO and YBa2Cu3O7 systems are given in tabular form. Strong anisotropy of exchange interactions in the ab plane and along the c axis in La2CuO4 is obviously seen. The value of Neel temperature found agrees well with the experimental data available. In the planes of '1-2-3' system there are quite strong antiferromagnetic Cu-O and O-O interaction which appear due to holes in oxygen subbands. These results are in line with the magnetic model of oxygen holes pairing in high-T(sub c) superconductors.

Turshevski, S. A.↗
NASA NTRS1990

Impulsive signals in the night ionosphere of Venus - Comparison of results obtained below the local electron gyro frequency with those above

Impulsive VLF signals at low altitudes in the night ionosphere of Venus occur both above and below the electron gyro frequency. The strength of the magnetic field has a very strong influence on the occurrence rates of these impulsive emissions at all frequencies. Above about one-quarter of the local electron gyro frequency the waves occur most frequently for strong magnetic fields and much less frequently for weak fields. However, below about one-quarter of the electron gyro frequency, the occurrence rate is much less sensitive to field strength. At all frequencies the occurrence rate depends little on the direction of the magnetic field. The occurrence rate is strongly dependent on local time especially above the electron gyro frequency. Here, the occurrence rate peaks sharply at 2100 LT. Below the local electron gyro frequency the occurrence rate also shows a maximum near 2100 LT but decreases much more slowly with increasing local time. The rate of occurrence of low frequency signals varies little with altitude but the occurrence of the higher frequency signals decreases rapidly. These properties are consistent with a broadband source of VLF waves in the Venus atmosphere such as would be provided by intracloud lightning.

Russell, C. T.↗
NASA NTRS1990

High Tc composite silver/oxide superconductors

Factors involved in the strong flux pinning effect of high-Tc YBa2Cu3O7/AgO (Y-123/AgO) composite and other REE-123/AgO composites were investigated. Samples of superconducting REE-123 and REE-123/AgO (where REE was Nd, Sm, Eu, Gd, Dy, Ho, Er, or Y) were prepared and used to obtain magnetic moments, critical field, and microstructure data. The optimum heat treatment conditions for the formation of strong flux-pinning REE-123/AgO composites were found to be different for different REE-123 compounds. It was found that the annealing temperature depends on the ionic size of the REE, with larger rare-earth ions requiring higher temperature. It was also found that strong flux-pinning REE-123/AgO composites form only in a narrow annealing temperature range.

Curreri, P. A.↗
NASA NTRS1990

Ginga observations of quasi-periodic oscillations in type II bursts from the Rapid Burster

During Ginga observations of the 'Rapid Burster' in August 1988, strong quasi-periodic oscillations (QPOs) were detected in its X-ray intensity. The QPOs had centroid frequencies of 5 and 2 Hz during type II X-ray bursts which lasted for 10 and 30 s, respectively. The presence of the QPOs is correlated with the time scale-invariant burst profile. They are very strong during the initial peak in the burst, absent in the second peak, and strong again at the onset of the third peak. From an analysis of the X-ray spectrum as observed during the maxima and minima of the oscillations, it is found that the oscillations can be described by changes of the temperature of a blackbody emitter of constant apparent area.

Dotani, T.↗
NASA NTRS1989

The nonlinear breakup of the sun's toroidal field

There are good reasons for believing that the sun has a strong toroidal magnetic field in the stably stratified region of convective overshoot sandwiched between the radiative zone and convective zone proper. The magnetic field in this region is modeled by studying the behavior of a layer of uniform field embedded in a subadiabatic atmosphere. Since the field can support extra mass, such a configuration is top-heavy, and instabilities of the Rayleigh-Taylor type can occur. Numerical integration of the two-dimensional compressible MHD equations makes it possible to follow the evolution of this instability into the nonlinear regime. The initial buoyancy-driven instability of the magnetic field gives rise to strong shearing motions, thereby exciting secondary Kelvin-Helmholtz instabilities which wrap the gas into regions of intense vorticity. The somewhat surprising subsequent motions are determined primarily by the strong interactions between vortices.

Hughes, D. W.↗
NASA NTRS1990

Energetic particle observations at the Helios 1 spacecraft of shocks associated with coronal mass ejections

Energetic particle data from the Helios 1 spacecraft have been used to study the particles associated with 26 coronal mass ejection (CME)/shock pairs. The particle events are divided into four classes on the basis of the relative intensities of their prompt and shock-associated components. Events in group 1 have strong prompt and shock components and are associated with energetic CMEs originating, most commonly, with central meridian flares. Group 2 events characteristically have strong prompt components and weaker shock components and are associated with western hemisphere (i.e., favorably located) flares. Group 3 consists of events with absent prompt components and strong shock components; the CMEs probably originate in disappearing filament events near central meridian. Group 4 events show no particle enhancements and are associated with slow CMEs and slow shocks.

Cane, H. V.↗
NASA NTRS1990

Turbulent supersonic convection in three dimensions

Previous numerical calculations of two-dimensional, compressible convection are extended to three dimensions, using a higher order Godunov scheme. The results show that the flow readily becomes supersonic in the upper boundary layer, where shock structures form intermittently in the vicinity of the strong downflow lanes. The convection as a whole is strongly time-dependent and evolves on a time scale comparable to the sound crossing time. The motions in the upper layers are characterized by the rapid expansion of the upward-moving fluid elements. In the interior, most of the heat is carried by a small fraction of the fluid residing in strong, highly coherent downflows. The remaining fluid is dominated by small-scale, disorganized turbulent motions.

Malagoli, Andrea↗
NASA NTRS1989

Airborne observations of the infrared emission bands

Earlier airborne studies of the infrared bands between 5 and 8 microns have now been extended to a sample of southern sources selected from the IRAS Low Resolution Spectra (LRS) atlas. The correlation between the strongest bands at 6.2 and 7.7 microns is now based on a total sample of 40 sources and is very strong. A new emission band at 5.2 microns, previously predicted for polycyclic aromatic hydrocarbons (PAHs), is recognized in 27 sources; it too correlates with the dominant 7.7 micron band, showing that the 5.2 micron feature also belongs to the generic spectrum of PAH features at 3.3, 5.6, 6.2, 6.2, 7.7, 8.7, 11.3, and 12.7 microns. Sufficient sources are had now to define the relative strengths of most of these bands in three separate nebular environments: planetaries, H II regions, and reflection nebulae. Significant variations are detected in the generic spectra of PAHs in these different environments which are echoed by variations in the exact wavelength of the strong 7.7 micron peak. The earlier suggestion that, in planetaries, the fraction of total emission observed by IRAS that is carried by the PAH emissions is correlated with nebular gas-phase C/O ratio is supported by the addition of newly-observed southern planetaries, including the unusually carbon-rich (WC10) nebular nuclei. These (WC10) nuclei also exhibit a strong plateau of emission linking the 6.2 and 7.7 micron features.

Cohen, M.↗
NASA NTRS1990

Observations of multiple nucleus galaxies

Disturbed galaxies with two nuclei display the final state of the interaction process of two galaxies (Kollatschny et al., 1986; Fricke and Kollatschny, 1989). A few of these double nucleus galaxies contain Seyfert nuclei. Making the assumption that the Seyfert galaxies Mkn 231 and Mkn 273 are galaxies in the final state of merging, having strong tidal arms but unresolved nuclei, one can estimate that 4 percent of all Seyfert galaxies are in the merging process. The luminosities of multiple nucleus Seyfert galaxies are extremely high in comparison to morphologically undisturbed Seyfert galaxies. In a table, mean values of the visual and blue luminosities and of the far-infrared and radio (6 cm) luminosities as well as the H alpha fluxes are listed for both classes are shown. In addition, the authors have separated Seyfert 1 and Seyfert 2 galaxies. In all cases the luminosities of double nucleus Seyfert galaxies are higher by a factor of more than two with respect to undisturbed Seyfert galaxies. This result might be explained by higher luminosities in the early phases of a Seyfert's life-under the assumption that the nonthermal activity is triggered by tidal interaction-and/or additional strong starburst phenomena. Due to strong nuclear absorption, the UV spectra of these Seyfert nuclei are unusually weak. Corresponding to the Seyfert survey, the authors obtained the H alpha and far infrared radiation (FIR) luminosities as well as the (0III)5007/H beta line ratios of a small sample of non-Seyfert nuclei in double nucleus galaxies. The direct image and the velocity field of the double starburst galaxy Mkn 788 (Kollatschny et al., 1986) are shown. The authors compared their measurements with those of normal interacting galaxies of Keel et al. (1985) and Bushouse (1987). The mean FIR luminosity per nucleus in multiple systems is the same as that of interacting galaxies. But the mean H alpha luminosities as well as the (OIII)/H beta line ratios are higher by a factor of 1.5-2 than those of normal interacting galaxies.

Kollatschny, W.↗
NASA NTRS1990

The role of shocks in NGC 6240

The case of the infrared-bright system NGC 6240 encapsulates in many ways the major issues in understanding processes in infrared-bright systems in general. Its morphology strongly suggests that this is a merger in progress, with energetic phenomena being triggered as evidenced by strong radio emission and optical emission lines as well as the strong far-infrared output. Evidence has been produced supporting various schemes for producing the energy being radiated by this object. Early work on the optical spectrum suggested a Seyfert 2 classification, but the ionization level and spatial extent of the emission implied a more distributed source of energy. IR line studies found evidence for young supergiants, and for immense amounts of H2 excited by low-velocity shocks, which might contribute to a large enhancement in star-formation rate. Finally, the radio-continuum morphology shows compact sources possibly associated with active nuclei in both remnants of the merger. This is one of the nearest IR-bright galaxies objects at high luminosity (Cz = 7500 km s(exp -1), so that spatially resolved studies can yield further insight into which of these processes are most important - star formation, nuclear activity, or shock excitation. The velocity field, linewidth distribution, and ionization structure of the gas in NGC 6240 was mapped with the Dense-Pak fiber-optic array on the Kitt Peak 4-m telescope. In all, 135 spectra were obtained through an array of 2 inch apertures, with considerable overlap between many of the adjacent apertures produced by offsetting the telescope between the exposures. As the strongest features with lambda less than 1.1 ma, H alpha and the adjacent (NII) lines were observed, along with the neighboring (O I) and (S II) features. Maps of velocity and linewidth were constructed by interpolating measurements from these spectra onto a regular grid, and show irregular patterns with large velocity gradients and, particularly, an extended region of large local (shear?) velocity width.

Keel, William C.↗
NASA NTRS1991

Numerical investigation of an internal layer in turbulent flow over a curved hill

A numerical investigation of incompressible and compressible turbulent flows over strongly curved surfaces is presented. The turbulent flow equations are solved by a pressure based Navier-Stokes equations solver. In the method, the conservation of mass equation is replaced by a pressure correction equation applicable for both compressible and incompressible flows. The turbulence is described by a multiple time scale turbulence model supplemented with a near-wall turbulence model. The numerical results show that the internal layer is a strong turbulence field which is developed beneath the external boundary layer and is located very close to the wall. The development of the internal layer is attributed to the enormous mean flow strain rate caused by the streamline curvature. The external boundary layer flow responds rather slowly to the streamline curvature. Thus, the turbulence field of the forward corner of the curved hill is characterized by two turbulence fields interacting with each other. The turbulence intensity of the internal layer is much stronger than that of the external boundary layer, so that the development of a new boundary layer in the downstream region of the curved hill depends mostly on the internal layer. These numerical results are in good agreement with the measured data, and show that the turbulence model can resolve the turbulence field subjected to the strong streamline curvature.

Kim, S.-W.↗
NASA NTRS1990

Scanning electron acoustic microscopy of indentation-induced cracks and residual stresses in ceramics

The ability of scanning electron acoustic microscopy (SEAM) to characterize ceramic materials is assessed. SEAM images of Vickers indentations in SiC whisker-reinforced alumina clearly reveal not only the radial cracks, the length of which can be used to estimate the fracture toughness of the material, but also reveal strong contrast, interpreted as arising from the combined effects of lateral cracks and the residual stress field left in the SiC whisker-reinforced alumina by the indenter. The strong contrast is removed after the material is heat treated at 1000 C to relieve the residual stresses around the indentations. A comparison of these observations with SEAM and reflected polarized light observations of Vickers indentations in soda-lime glass both before and after heat treatment confirms the interpretation of the strong contrast.

Cantrell, John H.↗
NASA NTRS1991

Simulated optical images of galaxies at z about 1 using ultraviolet images of nearby galaxies

Ultraviolet sounding rocket images of several nearby galaxies are used to simulate the appearance in optical bandpasses of similar systems at redshifts z = 0.5-2.67, as observed by the WFPC on HST and by ground-based instruments. Since the morphology of galaxies is a strong function of the wavelength, the appearance of galaxies at large redshifts is subjected to a large k-correction effect. The strong dependence of monochromatic surface brightness on redshift also implies that observed morphology of distant systems will be crucially dependent on the limiting surface brightness set by the sky background. Although the angle subtended by sources depends only weakly on z, the fraction of a galaxy whose surface brightness is above the detection threshold varies strongly with z. Morphological distinctions of spiral from elliptical, spiral from irregular, and barred from unbarred types become more difficult, as does accurate evaluation of interactions or of the local environment.

Bohlin, Ralph C.↗
NASA NTRS1991

Ultraviolet and optical spectra of high-ionization Seyfert galaxies with narrow lines

Ultraviolet and optical spectra are presented for three unusual Seyfert galaxies (Mrk 1239, Mrk 42, and Mrk 493) that resemble Seyfert 1 galaxies in that they have strong high-ionization lines and strong nonstellar continua, but resemble Seyfert 2 galaxies in that the widths of their permitted lines are as narrow as the widths of their forbidden lines. The He II lambda 1640 and He II lambda 4686 lines are used to determine an upper limit to the reddening experienced by the emission lines. Published optical data show that these particular high-ionization narrow-line (HINL) Seyferts have low lambda 5007 H beta ratios and strong Fe II emission, which suggest the presence of high-density regions. The low Lalpha/H-beta ratios in these objects indicate that high-density clouds are indeed present, and, like the broad-line region clouds in Seyfert 1 galaxies, these clouds have large optical depths with partially ionized zones. Overall, the line ratios and continuum fluxes of these particular HINL Seyferts are indistinguishable from those of broad-lined Seyfert 1 galaxies.

Crenshaw, D. Michael↗
NASA NTRS1990

Some theoretical aspects of boundary layer stability theory

Increased understanding in recent years of boundary layer transition has been made possible by the development of strongly nonlinear stability theories. After some twenty or so years when nonlinear stability theory was restricted to the application of the Stuart-Watson method (or less formal amplitude expansion procedures), there now exist strongly nonlinear theories which can describe processes which have an 0(1) effect on the basic state. These strongly nonlinear theories and their possible role in pushing theoretical understanding of transition ever further into the nonlinear regime are discussed.

Hall, Philip↗
NASA NTRS1991

Optical polarization of a complete sample of radio sources

The radio and optical properties of a complete 5 GHz sample of strong radio sources are studied. High polarization is strongly correlated with the fraction of the total 5 GHz flux density found in a milliarcsec core. The detection rate of radio galaxies containing optical cores with high polarization is consistent with the hypothesis that all radio sources contain optically polarized cores, with strength proportional to the core radio flux density. High optical polarization, optical power-law fraction, line-to-continuum ratio, emission lines of small equivalent widths, and large amplitude flux variability are all strongly correlated with the fraction of the 5 GHz radio flux density that is unresolved on VLBI scales. The distributions of these properties are well matched by a model where radio and optical radiation have the same beaming geometry and with Doppler boosting of a factor of 10-100 in the optical. A striking alignment of the position angle of the VLBI structure axis and the position angle of optical polarization in highly polarized sources is confirmed.

Impey, C. D.↗