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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 235 records · Page 13
All sourcesNASA NTRSDOE OSTI
NASA NTRS1981

An analysis of short pulse and dual frequency radar techniques for measuring ocean wave spectra from satellites

A four frequency moment characterization of backscatter from the near-vertical is applied to an analysis of the short pulse and dual frequency microwave techniques. The range reflectivity modulation spectrum closely approximates the directional wave slope spectrum, while harmonic distortion is small and is a minimum near 10 deg incidence. The short pulse measurement signal-to-noise ratio (SNR) is typically greater than the narrowband dual frequency SNR, with the difference being the ratio of the range beam extent to pulse length, minus the ratio of beam-limited to pulse-limited Doppler spreads. It is concluded that dual frequency measurements are basically impractical, although short pulse measurements are useful and can employ existing space-qualified microwave hardware.

Jackson, F. C.↗
NASA NTRS1991

Alfven wave trapping, network microflaring, and heating in solar coronal holes

Fresh evidence that much of the heating in coronal holes is provided by Alfven waves is presented. This evidence comes from examining the reflection of Alfven waves in an isothermal hydrostatic model coronal hole with an open magnetic field. Reflection occurs if the wavelength is as long as the order of the scale height of the Alfven velocity. For Alfven waves with periods of about 5 min, and for realistic density, magnetic field strength, and magnetic field spreading in the model, the waves are reflected back down within the model hole if the coronal temperature is only slightly less than 1.0 x 10 to the 6th K, but are not reflected and escape out the top of the model if the coronal temperature is only slightly greater than 1.0 x 10 to the 6th K. Because the spectrum of Alfven waves in real coronal holes is expected to peak around 5 min and the temperature is observed to be close to 1.0 x 10 to the 6th K, the sensitive temperature dependence of the trapping suggests that the temperature in coronal holes is regulated by heating by the trapped Alfven waves.

Moore, R. L.↗
NASA NTRS1993

Observations of nonlinear interactions in directionally spread shoaling surface gravity waves

Shoaling wave fields generated in laboratory experiments were analyzed to determine the sensitivity of nonlinear interactions to the directional distributions of incident waves. Peaks in the directional spectra observed in shallow water were consistent with near resonating, quadratic interactions between two primary waves transferring energy to a third wave with the sum frequency and vector sum wavenumber of the primary waves. Directionally colinear waves forced a higher-frequency wave propagating in the same direction as the primary waves, while directionally spread (i.e., noncolinear) primary waves forced a higher-frequency wave that propagated in a direction between those of the interacting primary waves. Deepwater wave fields with similar frequency spectra but different directional spectra evolved to different shallow-water directional spectra, yet their shallow-water frequency spectra were remarkably similar. This result suggests that the shape of the directional spectrum of the incident wave field has only a small effect on the magnitudes of nonlinear energy transfer during shoaling. The principal effect of directionality in the incident wave field is on the directions, not the amplitudes, of the nonlinearly generated waves. The laboratory data demonstrate clearly the importance of triad interactions between noncolinear and colinear shoaling waves.

Elgar, Steve↗
NASA NTRS1995

Faint Object Camera imaging and spectroscopy of NGC 4151

We describe ultraviolet and optical imaging and spectroscopy within the central few arcseconds of the Seyfert galaxy NGC 4151, obtained with the Faint Object Camera on the Hubble Space Telescope. A narrowband image including (O III) lambda(5007) shows a bright nucleus centered on a complex biconical structure having apparent opening angle approximately 65 deg and axis at a position angle along 65 deg-245 deg; images in bands including Lyman-alpha and C IV lambda(1550) and in the optical continuum near 5500 A, show only the bright nucleus. In an off-nuclear optical long-slit spectrum we find a high and a low radial velocity component within the narrow emission lines. We identify the low-velocity component with the bright, extended, knotty structure within the cones, and the high-velocity component with more confined diffuse emission. Also present are strong continuum emission and broad Balmer emission line components, which we attribute to the extended point spread function arising from the intense nuclear emission. Adopting the geometry pointed out by Pedlar et al. (1993) to explain the observed misalignment of the radio jets and the main optical structure we model an ionizing radiation bicone, originating within a galactic disk, with apex at the active nucleus and axis centered on the extended radio jets. We confirm that through density bounding the gross spatial structure of the emission line region can be reproduced with a wide opening angle that includes the line of sight, consistent with the presence of a simple opaque torus allowing direct view of the nucleus. In particular, our modelling reproduces the observed decrease in position angle with distance from the nucleus, progressing initially from the direction of the extended radio jet, through our optical structure, and on to the extended narrow-line region. We explore the kinematics of the narrow-line low- and high-velocity components on the basis of our spectroscopy and adopted model structure.

Boksenberg, A.↗
NASA NTRS1987

Mean fields induced by local gravity-wave forcing in the middle atmosphere

The role of geostrophic adjustment in the middle atmosphere for given wave packet forcing by a three-dimensional hydrostatic model is examined. It is shown that the induced fields consist of two different kinds of modes. One, produced by forcing vorticity only, is steady quasi-geostrophic flow; this is restricted to the forcing region. The other, produced by both forcing vorticity and forcing divergence, is oscillatory, and is in the form of gravity waves propagating out of the forcing region plus inertial oscillations in the forcing region. The scales and amplitudes of the induced gravity waves are determined by the forcing. For a typical example, a 200 km x 200 km gravity wave packet of momentum flux 0.5 N/sq m absorbed in a layer of 5 km thickness centered near 18 km altitude, the gravity waves spread to a larger region about 1000 km x 1000 km at a level 7 km above the forcing region. At that level, the horizontal and vertical wavelengths of the induced waves are about 300 km and 7 km respectively, and the momentum flux is substantially reduced to 0.0004 N/sq m. The results suggest that geostrophic adjustment processes may play an important role in specifying the gravity wave spectrum in the middle atmosphere.

Zhu, Xun↗
NASA NTRS1999

Unusual Contact-Line Dynamics of Thick Films and Drops

We report several novel phenomena In contact-line and fingering dynamics of macroscopic spinning drops and gravity-driven films with dimensions larger than the capillary length. It is shown through experimental and theoretical analysis that such macroscopic films can exhibit various interfacial shapes, including multi valued ones, near the contact line due to a balance between the external body forces with capillarity. This rich variety of front shapes couples with the usual capillary, viscous, and intermolecular forces at the contact line to produce a rich and unexpected spectrum of contact-line dynamics. A single finger develops when part of the front becomes multivalued on a partially wetting macroscopic spinning drop in contrast to a different mechanism for microscopic drops of completely wetting fluids. Contrary to general expectation, we observe that, at high viscosity and low frequencies of rotation, the speed of a glycerine finger increases with increasing viscosity. Completely wetting Dow Corning 200 Fluid spreads faster over a dry inclined plane than a prewetted one. The presence of a thin prewetted film suppresses fingering both for gravity-driven flow and for spin coating. We analyze some of these unique phenomena in detail and offer qualitative physical explanations for the others.

Veretennikov, Igor↗
NASA NTRSDate not supplied

Key Observations and Discoveries from the Vector Electric Field Investigation on the C/NOFS Satellite

Launched in 2008 and operating for 7.5 years, the Air Force Communication /Navigation Outage Forecasting System (C/NOFS) satellite included the Vector Electric Field Investigation (VEFI) designed and built at NASA’s Goddard Space Flight Center. VEFI successfully met its objectives: 1) investigate the role of ambient electric fields in initiating nighttime density depletions and turbulence; 2) determine the quasi-DC electric fields associated with abrupt density depletions, and 3) quantify the spectrum of the irregularities associated with density depletions, providing many key observations and discoveries including: -- Global (low latitude) vector DC electric fields at 16 s/sec revealing variations with local time, longitude, and season between extremely low and moderate solar activity-- Reversed zonal E x B drifts below the F-region ledge at sunset and simultaneous observations of large scale Kelvin-Helmholtz instabilities as seeds of spread-F -- Large expanses of quasi-coherent kilometer-scale vector wave observations (electric field and density) below 450 km and their discovery as a source of scintillations using the C/NOFS GPS -- Reversed zonal DC electric fields and simultaneous observations of afternoon counter electrojet-- Enhanced zonal DC electric fields at sunrise and full vector plasma flow continuity at the terminator-- Vector electric field and density irregularities extending to meter-scales within equatorial plasma depletions -- Intense electric field structure within the equatorial ionosphere at night in the absence of density depletions -- First observations of the westward equatorial electrojet in post-midnight ionosphere and possible association with downward meridional drifts-- Spaceborne Dst observations and implications for ring current asymmetries-- Vector electric and magnetic field Poynting flux within depletions and TIDs-- Measurements of ionospheric reflectance and Alfvenic waves associated with TIDs-- Observations of Alfven resonators -- ULF magnetic field structure within the nightime equatorial ionosphere-- Discovery of Schumman resonances in space and implications of ELF radiowave propagation-- Vector observations of 50-60 Hz powerline radiation without harmonics-- Ion cyclotron resonance absorption lines associated with ELF hiss and the identification of ambient ions-- Parallel electric fields associated with lower hybrid waves driven by thunderstorm lightning-related sferics-- Discovery of Z-mode radiation in the equatorial ionosphere including its associated with density depletions-- Optical lightning detector waveforms and electric field sferics observed up to altitudes of 800 km

Robert F Pfaff↗
NASA NTRS1997

Simulations of the STIS CCD clear imaging mode PSF

Detailed understanding of the properties of the point-spread function (PSF) of Space Telescope Imaging Spectrograph (STIS) imaging modes is necessary to separate real effects of geometrically complex astronomical sources, such as protoplanetary disks surrounding bright stars, from instrumental effects, such as scattering and diffraction. In order to investigate STIS imaging properties we have numerically simulated broadband stellar PSFs generated by STIS in CCD clear imaging mode, including the effects of the Lyot stop and the coronagraphic wedges. The input spectrum is a stellar model atmosphere of the appropriate spectral type, convolved with the pre-flight STIS CCD response function. The PSF modeler generates broadband PSFs by co-adding weighted monochromatic PSFs across the waveband.

Cornett, R. H.↗
NASA NTRS2007

Preventing Raman Lasing in High-Q WGM Resonators

A generic design has been conceived to suppress the Raman effect in whispering- gallery-mode (WGM) optical resonators that have high values of the resonance quality factor (Q). Although it is possible to exploit the Raman effect (even striving to maximize the Raman gain to obtain Raman lasing), the present innovation is intended to satisfy a need that arises in applications in which the Raman effect inhibits the realization of the full potential of WGM resonators as frequency-selection components. Heretofore, in such applications, it has been necessary to operate high-Q WGM resonators at unattractively low power levels to prevent Raman lasing. (The Raman-lasing thresholds of WGM optical resonators are very low and are approximately proportional to Q(sup -2)). Heretofore, two ways of preventing Raman lasting at high power levels have been known, but both entail significant disadvantages: A resonator can be designed so that the optical field is spread over a relatively large mode volume to bring the power density below the threshold. For any given combination of Q and power level, there is certain mode volume wherein Raman lasing does not start. Unfortunately, a resonator that has a large mode volume also has a high spectral density, which is undesirable in a typical photonic application. A resonator can be cooled to the temperature of liquid helium, where the Raman spectrum is narrower and, therefore, the Raman gain is lower. However, liquid-helium cooling is inconvenient. The present design overcomes these disadvantages, making it possible to operate a low-spectral-density (even a single-mode) WGM resonator at a relatively high power level at room temperature, without risk of Raman lasing.

Savchenkov, Anatoliy↗
NASA NTRS1984

A further study of gravity wave induced drag and diffusion in the mesosphere

Lindzen's (1967) parameterization for the drag and eddy diffusion produced by breaking internal gravity waves in the mesosphere and lower thermosphere is applied to a modified version of the beta-plane channel model of Holton (1982) in which an isotropic source spectrum of waves is specified similar to that given in 1982 by Matsuno (1982). The transmission for each wave component is influenced by Newtonian cooling and by eddy diffusion induced by the breaking of other wave components. In general the waves with smallest Doppler-shifted phase speeds break first and produce sufficient eddy diffusion to significantly raise the breaking heights for the higher speed components. Thus, the wave drag and diffusion is spread through a deep layer and the resulting mean wind profiles for both summer and winter solstice conditions are more realistic than those computed previously by Holton.

Holton, J. R.↗
NASA NTRS1976

Limb brightening on Uranus - The visible spectrum

Results are presented for pinhole and slit narrowband photoelectric area-scanning photometry of the Uranus disk at eight central wavelengths ranging from 5600 to 7500 A. The observations were undertaken to detect limb brightening on the planet and to study the nature of its disk profile as a function of geometrical albedo. A coarse analysis of the results shows that the disk must exhibit limb brightening in the two CH4 bands at 6190 and 7300 A relative to all other wavebands investigated. Except for the two CH4 bands, the shapes and widths of the images are found to be essentially identical; point-spread-function data for all wavebands indicate no significant variation in the representative Gaussian l/e width with wavelength. A fine analysis of the data reveals absolute limb brightening in the 7300-A CH4 band and absolute limb darkening in the adjacent wavebands. These results are shown to be consistent with the hypothesis that clouds are absent from the visible atmosphere of Uranus.

Price, M. J.↗
NASA NTRS1985

Measurement of acoustic shielding by a turbulent jet

The acoustic shielding properties of a turbulent jet have been investigated experimentally. The experimental arrangement consisted of an acoustic point source and a turbulent shielding jet. The source and jet parameters investigated include the source frequency, source spectrum, jet velocity, jet heating by simulation and the lateral and longitudinal source positions with respect to the shielding jet. It is found that the maximum sound attenuation provided by the shielding jet depends on the balance between refraction and diffraction. Over the frequency range investigated, the redistribution of sound by the shielding air jet is power conserving. Comparison between measurement and prediction based on an idealized cylindrical uniform jet model indicates that the spreading and decay of the flow field in real jets are important. Comparisons between the present data trends and those reported for jet-by-jet shielding suggest that the major effects observed in the latter are acoustical rather than aerodynamical.

Yu, J. C.↗
NASA NTRS1971

Formation of giant H II regions following supernova explosions

The principal optical properties of type I supernovae are summarized. These include the light curve and the spectrum. The spectra consist of broad bands with very little continuum. According to the theory presented, the observed light is principally fluorescence, excited in the medium surrounding the supernova by ultraviolet radiation originating from the explosion. It is proposed that the spectrum that impinges on the fluorescent medium while emission is taking place must fall abruptly across the Lyman edge of He II. Such a filtering action is plausibly provided by a much denser internal region, rich in helium, immediately surrounding the exploding object. This will form a Stromgren sphere during the time the intense UV pulse is passing through it. The dense region also slows down the photons below the edge by Thomson scattering, thereby spreading out the UV pulse in time. Various proposed mechanisms for the production of ionization in the Gum nebula are discussed.

Sartori, L.↗
NASA NTRS1981

Multiple-scale turbulence modeling of boundary layer flows for scramjet applications

As part of an investigation into the application of turbulence models to the computation of flows in advanced scramjet combustors, the multiple-scale turbulence model was applied to a variety of flowfield predictions. The model appears to have a potential for improved predictions in a variety of areas relevant to combustor problems. This potential exists because of the partition of the turbulence energy spectrum that is the major feature of the model and which allows the turbulence energy dissipation rate to be out of phase with turbulent energy production. The computations were made using a consistent method of generating experimentally unavailable initial conditions. An appreciable overall improvement in the generality of the predictions is observed, as compared to those of the basic two-equation turbulence model. A Mach number-related correction is found to be necessary to satisfactorily predict the spreading rate of the supersonic jet and mixing layer.

Fabris, G.↗
NASA NTRS2019

Optical Design of the Origins Space Telescope

This paper discusses the optical design of the Origins Space Telescope. Origins is one of four large missions under study in preparation for the 2020 Decadal Survey in Astronomy and Astrophysics. Sensitive to the mid- and far-infrared spectrum (between 2.8 and 588 μm), Origins sets out to answer a number of important scientific questions by addressing NASA’s three key science goals in astrophysics. The Origins telescope has a 5.9 m diameter primary mirror and operates at f/14. The large on-axis primary consists of 18 ‘keystone’ segments of two different prescriptions arranged in two annuli (six inner and twelve outer segments) that together form a circular aperture in the goal of achieving a symmetric point spread function. To accommodate the 46 x 15 arcminute full field of view of the telescope at the design wavelength of λ = 30 μm, a three-mirror anastigmat configuration is used. The design is diffraction-limited across its instruments’ fields of view. A brief discussion of each of the three baselined instruments within the Instrument Accommodation Module (IAM) is presented: 1) Origins Survey Spectrometer (OSS), 2) Mid-infrared Spectrometer, Camera (MISC) transit spectrometer channel, and 3) Far-Infrared Polarimeter/Imager (FIP). In addition, the up scope options for the observatory are laid out as well including a fourth instrument: the Heterodyne Receiver for Origins (HERO).

Corsetti, James A.↗
NASA NTRS2019

Optical Design of the Origins Space Telescope

This paper discusses the optical design of the Origins Space Telescope. Origins is one of four large missions under study in preparation for the 2020 Decadal Survey in Astronomy and Astrophysics. Sensitive to the mid- and far-infrared spectrum (between 2.8 and 588 μm), Origins sets out to answer a number of important scientific questions by addressing NASA’s three key science goals in astrophysics. The Origins telescope has a 5.9 m diameter primary mirror and operates at f/14. The large on-axis primary consists of 18 ‘keystone’ segments of two different prescriptions arranged in two annuli (six inner and twelve outer segments) that together form a circular aperture in the goal of achieving a symmetric point spread function. To accommodate the 46 x 15 arcminute full field of view of the telescope at the design wavelength of λ = 30 μm, a three-mirror anastigmat configuration is used. The design is diffraction-limited across its instruments’ fields of view. A brief discussion of each of the three baselined instruments within the Instrument Accommodation Module (IAM) is presented: 1) Origins Survey Spectrometer (OSS), 2) Mid-infrared Spectrometer, Camera (MISC) transit spectrometer channel, and 3) Far-Infrared Polarimeter/Imager (FIP). In addition, the upscope options for the observatory are laid out as well including a fourth instrument: the Heterodyne Receiver for Origins (HERO).

James A Corsetti↗
NASA NTRS1980

Modelling of equatorial phase and amplitude scintillations from OGO-6 and AE irregularity data

The technique of modelling equatorial phase and amplitude scintillations from in-situ measurements of irregularity amplitude, ambient ionization density and irregularity power spectrum is outlined. The in-situ measurements do not directly provide information on irregularity layer thickness for which radar backscatter observations are utilized. The equatorial scintillation models developed from OGO-6 and AE-C in-situ measurements during the two solstices are presented. Considerable differences in the longitude variation of scintillations are predicted during the two solstices. The model estimates are in good agreement with the available ground based phase and amplitude measurements. Problems associated with the use of bottomside spread-F data for transionospheric propagation modelling at VHF/UHF are also discussed.

Basu, S.↗
NASA NTRS1981

Multiple-scale turbulence modeling of free turbulent flows

As part of an investigation into the application of turbulence models to the computation of flows in advanced scramjet combustors, the multiple-scale turbulence model has been applied to a variety of flowfield predictions. The model appears to have a potential for improved predictions in a variety of areas relevant to combustor problems. This potential exists because of the partition of the turbulence energy spectrum that is the major feature of the model and which allows the turbulence energy dissipation rate to be out of phase with turbulent energy production. To establish the general reliability of the approach, it has been tested through comparison of predictions with experimental data. An appreciable overall improvement in the generality of the predictions is observed, as compared to those of the basic two-equation turbulence model. A Mach number-related correction is found to be necessary to satisfactorily predict the spreading rate of the supersonic jet and mixing layer.

Fabris, G.↗