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At least 1,477 records · Page 82

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NASA NTRS1989

On strongly nonlinear vortex/wave interactions in boundary-layer transition

The interactions between longitudinal vortices and accompanying waves considered are strongly nonlinear, in the sense that the mean-flow profile throughout the boundary layer is completely altered from its original undisturbed state. Nonlinear interactions between vortex flow and Tollmien-Schlichting waves are addressed first, and some analytical and computational properties are described. These include the possibility in the spatial-development case of a finite-distance break-up, inducing a singularity in the displacement thickness. Second, vortex/Rayleigh wave nonlinear interactions are considered for the compressible boundary-layer, along with certain special cases of interest and some possible solution properties. Both types, vortex/Tollmien-Schlichting and vortex/Rayleigh, are short-scale/long-scale interactions and they have potential applications to many flows at high Reynolds numbers. The strongly nonlinear nature is believed to make them very relevant to fully fledged transition to turbulence.

Hall, Philip↗

NASA NTRS1992

The solar wind structure that caused a large-scale disturbance of the plasma tail of comet Austin

The plasma tail of Comet Austin (1989c1) showed remarkable disturbances because of the solar maximum periods and its orbit. Figure 1 shows photographs of Comet Austin taken in Shibata, Japan, on 29 Apr. 1990 UT, during about 20 minutes with the exposure times of 90 to 120 s. There are two main features in the disturbance; one is many bowed structures, which seem to move tailwards; and the other is a large-scale wavy structure. The bowed structures can be interpreted as arcade structures brushing the surface of both sides of the cometary plasma surrounding the nucleus. We identified thirteen structures of the arcades from each of the five photographs and calculated the relation between the distance of each structure from the cometary nucleus, chi, and the velocity, upsilon. The result is shown. This indicates that the velocity of the structures increases with distance. This is consistent with the result obtained from the observation at the Kiso Observatory.

Kozuka, Yukio↗

NASA NTRS1993

Distribution and motions of atomic hydrogen in lenticular galaxies. X - The blue S0 galaxy NGC 5102

Results of the mapping of the blue gas-rich S0 galaxy NGC 5102 in the 21-cm H I line with a spatial resolution of 34 x 37 arcsec (delta(alpha) x Delta(delta)) and a velocity resolution of 12 km/s are presented. The H I distribution has a pronounced central depression of 1.9 kpc radius, and most of the H I is concentrated in a 3.6 kpc wide ring with an average radius of 3.7 kpc, assuming a distance of 4 Mpc for NGC 5102. The maximum azimuthally averaged H I surface density in the ring is 1.4 solar mass/sq pc, comparable to that found in other S0 galaxies. The HI velocity field is quite regular, showing no evidence for large-scale deviations from circular rotation, and the H I is found to rotate in the plane of the stellar disk. Both the H I mass/blue luminosity ratio and the radial H I distribution are similar to those in early-type spirals. The H I may be an old disk or it may have been acquired through capture of a gas-rich smaller galaxy. The recent starburst in the nuclear region, which gave the galaxy its blue color, may have been caused by partial radial collapse of the gas disk, or by infall of a gas-rich dwarf galaxy.

Van Woerden, H.↗

NASA NTRS1995

The latitudinal distribution of magnetic holes in the solar wind

A large number of magnetic holes have been found in the Ulysses data during its cruise in the ecliptic. They are interpreted as convecting structures, probably caused by the mirror instability which exists in high beta plasmas with anisotropic temperatures. The characteristics of the holes reflect the solar wind condition of the region in which the holes are formed, and the point of observation may be far removed from where the instability occurs. A preliminary survey appears to indicate that the number of holes has no significant radial dependence. However, the number of holes does appear to increase with increasing heliographic latitude. Yet the large scale solar wind structures with their compression regions disappeared at approximately 57 deg south latitude. Thus any causal relationship between the holes and large scale solar wind structures is questionable. The temperature anisotropy and high beta required by the mirror instability must be generated by other mechanisms. In order to tie the magnetic holes and the mirror instability to their cause, the evolution of their characteristics with heliocentric distance and latitude needs to be investigated. With the progression of Ulysses around the sun a survey will be conducted to ascertain the characteristics of the magnetic holes as a function of heliographic latitude and heliocentric distance. A comparison of the results with the solar wind conditions may lead to the identification of the magnetic hole generating mechanism(s).

Winterhalter, D.↗

NASA NTRS1999

The Heliosphere as Resonant Cavity

If a disturbance in the supersonic solar wind reaches the heliospheric shock, a number of events ensue. First, the shock itself responds with inward or outward motion. Secondly, the disturbance propagates outward through the heliosheath as a sound or magnetoacoustic wave; eventually it reaches the heliopause and is (partially) reflected back toward the termination shock. The reflected wave can return to the shock, affecting the shock's motion, and be reflected yet again. The repetition of these processes can produce a 'ringing' in the heliosheath. This suggests that it may be useful to regard the heliosheath as a resonant acoustic cavity with inner and outer boundaries at the termination shock and heliopause, respectively. To evaluate this concept we have developed a simple model of small-amplitude resonant oscillations in an outwardly flowing gas, with appropriate boundary conditions (shock on the interior, tangential discontinuity on the exterior boundary). The fundamental mode of oscillation has a period of order T approx. 2D/C, where C is the speed of sound in the heliosheath and D is the distance between the two boundaries. Typical numerical models of the heliosphere give C approx. 200-500 km/s and D approx. 20 - 100 AU, giving T approx. 0.5 - 2.5 years. Hence we suggest that motions of the heliosheath and termination shock will occur with time scales of the order of a year, and are the consequence of the resonant nature of the heliospheric cavity rather than the history of variation at the Sun and/or in the solar wind. In particular, we suggest that the motion of the termination shock may be unrelated to solar variations over the time scale of the sunspot cycle.

Bames, Aaron↗

NASA NTRS2001

A Model for the Breakup of Comet Linear (C/1999 S4)

We propose a mechanism based on the rubble-pile hypothesis of the cometary nucleus (Weissman 1986) to explain the catastrophic breakup of comet LINEAR (C/1999 S4) observed during July-August 2000. We suggest that a solid nucleus made up of 10-100 m "cometesimals" (Weidenschilling 1997) contains a network of inter-connected voids in the inter-cometesimal regions. The production of super-volatile (i.e., species more volatile than water) gases into these voids occurs due to the thermal wave propagating through the nucleus and associated phase transitions of water ice. The network of voids provides an efficient pathway for rapid propagation of these gases within the nucleus resulting in gas pressure caused stresses over a wide regime of the nucleus. This provides a mechanism for catastrophic breakups of small cometary nuclei such as comet LINEAR (C/1999 S4) as well as for some observed cometary outbursts including those that occur at large heliocentric distances (e.g., West et al. 1991). We emphasize the importance of techniques such as radar reflection tomography and radiowave transmission tomography (e.g., Kofman et al. 1998) aboard cometary missions to determine the three dimensional structure of the nucleus in particular the extent of large scale voids.

Samarasinha, Nalin H.↗

NASA NTRS2013

A Method for Localizing Energy Dissipation in Blazars Using Fermi Variability

The distance of the Fermi-detected blazar gamma-ray emission site from the supermassive black hole is a matter of active debate. Here we present a method for testing if the GeV emission of powerful blazars is produced within the sub-pc scale broad line region (BLR) or farther out in the pc-scale molecular torus (MT) environment. If the GeV emission takes place within the BLR, the inverse Compton (IC) scattering of the BLR ultraviolet (UV) seed photons that produces the gamma-rays takes place at the onset of the Klein-Nishina regime. This causes the electron cooling time to become practically energy independent and the variation of the gamma-ray emission to be almost achromatic. If on the other hand the -ray emission is produced farther out in the pc-scale MT, the IC scattering of the infrared (IR) MT seed photons that produces the gamma-rays takes place in the Thomson regime, resulting to energy-dependent electron cooling times, manifested as faster cooling times for higher Fermi energies. We demonstrate these characteristics and discuss the applicability and limitations of our method.

Dotson, Amanda↗

DOE OSTI2021

Optimizing Power Generation of a Bottom-Raised Oscillating Surge Wave Energy Converter Using a Theoretical Model: Preprint

Preliminary sizing of an oscillating surge wave energy converter (OSWEC) is an iterative process that relies on knowledge of the relevant hydrodynamic coefficients for a given geometry. Often, the complex definition of the device geometry requires coefficients to be obtained through experiments or numerical boundary element solvers such as WAMIT. These techniques demand significant user and computational effort, therefore inhibiting the fine scale parametrization of object dimensions. In this study, a theoretical model, originally presented in Michele et al. (2016), is developed and demonstrated to efficiently optimize the power production for an OSWEC device (subjected to certain environmental conditions) with variations in device widths, heights, and distances from the seabed. Assuming negligible device thickness, the OSWEC motions are modelled as a bottom-raised two-dimensional flap in regular waves using potential flow theory formulated in elliptical coordinates. The solutions to this diffraction-radiation problem are obtained using Mathieu functions with appropriate boundary conditions. The resulting potentials are then used to derive frequency-dependent expressions for the added mass and radiation damping coefficients, along with wave excitation magnitude in surge, pitch, and coupled surge-pitch motions. Good agreement in hydrodynamic coefficient curves is shown between the theoretical model and the numerical results obtained from the boundary element-based program WAMIT. The theoretical model is then employed to maximize the time-averaged output power while maintaining or reducing the hinge reaction force, with variations in device dimensions, wave frequency and amplitude.

ACE↗

DOE OSTI2021

Optimizing Power Generation of a Bottomraised Oscillating Surge Wave Energy Converter Using a Theoretical Model

ACE↗

DOE OSTI2022

Active galactic nuclei signatures in Red Geyser galaxies from Gemini GMOS-IFU observations

ABSTRACT Red Geysers are quiescent galaxies with galactic scale ionized outflows, likely due to low-luminosity active galactic nuclei (AGN). We used Gemini GMOS-IFU observations of the inner ∼1–3 kpc of nine Red Geysers selected from the MaNGA survey to study the gas ionization and kinematics. The emission-line ratios suggest the presence of Seyfert/LINER (Low Ionization Nuclear Emission Region) nuclei in all sources. Two galaxies show Hα equivalent width ( Hα EW) larger than 3 Å (indicative of AGN ionization) within an aperture 2 ${_{.}^{\prime\prime}}$5 of diameter (1.3–3.7 kpc at the distance of galaxies) for MaNGA data, while with the higher resolution GMOS data, four galaxies present Hα EW>3 Å within an aperture equal to the angular resolution (0.3–0.9 kpc). For two objects with GMOS-IFU data, the Hα EW is lower than 3 Å but larger than 1.5 Å, most probably due to a faint AGN. The spatially resolved electron density maps show values between 100 and 3000 cm −3 and are consistent with those determined in other studies. The large (MaNGA) and the nuclear scale (GMOS-IFU) gas velocity fields are misaligned, with a kinematic position angle difference between 12° and 60°. The [N ii] λ6583 emission-line profiles are asymmetrical, with blue wings on the redshifted side of the velocity field and red wings on the blueshifted side. Our results support previous indications that the gas in Red Geysers is ionized by an AGN, at least in their central region, with the presence of outflows, likely originating in a precessing accretion disc.

Ilha, Gabriele S. (ORCID:0000000191006106)↗

DOE OSTI2025

Path to a single-stage, 100-GeV electron beam via a flying-focus-driven laser-plasma accelerator

Dephasingless laser wakefield acceleration (DLWFA), a novel laser wakefield acceleration concept based on the recently demonstrated “flying focus” technology, offers a new paradigm in laser-plasma acceleration that could advance the progress toward a TeV linear accelerator using a single-stage system without guiding structures. The recently proposed NSF OPAL laser facility could be the transformative technology that enables this grand challenge in laser-plasma acceleration. We review the viable parameter space for DLWFA based on the scaling of its performance with laser and plasma parameters, and we compare that performance to traditional laser wakefield acceleration. These scalings indicate the necessity for ultrashort, high-energy laser architectures such as NSF OPAL to achieve groundbreaking electron energies using DLWFA. Initial results from MTW-OPAL, the platform for the 6-J DLWFA demonstration experiment, show a tight, round focal spot over a distance of 3.7 mm. New particle-in-cell simulations of that platform indicate that using hydrogen for DLWFA reduces the amount of laser light that is distorted due to refraction at ionization fronts. An experimental path, and the computational and technical design work along that path, from the current status of the field to a single-stage, 100-GeV electron beam via DLWFA on NSF OPAL is outlined. Progress along that path is presented.

Shaw, J. L. [University of Rochester, NY (United S↗

DOE OSTI2021

Sparse Approximate Multifrontal Factorization with Butterfly Compression for High-Frequency Wave Equations

In this work, we present a fast and approximate multifrontal solver for large-scale sparse linear systems arising from finite-difference, finite-volume or finite-element discretization of high-frequency wave equations. The proposed solver leverages the butterfly algorithm and its hierarchical matrix extension for compressing and factorizing large frontal matrices via graph-distance guided entry evaluation or randomized matrix-vector multiplication-based schemes. Complexity analysis and numerical experiments demonstrate $\mathcal{O}(N\log^2 N)$ computation and $\mathcal{O}(N)$ memory complexity when applied to an $N\times N$ sparse system arising from 3D high-frequency Helmholtz and Maxwell problems.

97 MATHEMATICS AND COMPUTING↗

DOE OSTI2022

TEAMER: FOSWEC Mooring Modeling and Analysis, Post Access Report and Data

Floating oscillating surge wave energy converters (FOSWECs) offer several advantages over bottom-hinged oscillating surge wave energy converters, including large wave potential at deep-water sites with fewer permitting and environmental concerns outside territorial waters. As a team, Stevens Institute of Technology, Virginia Tech and Resolute Marine Energy are designing a 100 kW FOSWEC with DOE support (2020-2021) for the PacWave test site "PacWave". The proposed FOSWEC consists of a floating platform, two pivoting flaps, and an innovative power-take-off (PTO). The distance between the two flaps is around half the typical wavelength, resulting in out-of-phase motion and a reduction in motion of the frame and mooring loads. The overall goal of the project is to design, build, deploy and analyze a 1:2 scale (100-kW annual averaged electrical power output) device with reduced levelized cost of energy (LCOE) and peak-to-average power ratio, through the co-design and control of the PTO, WEC, and floating platform. This submission includes a Post Access Report and data for the project of Mooring Modeling and Analysis for Floating Oscillating Surge Wave Energy Converter that Powers Marine Aquaculture of RFTS2 (request for technical support). The data are used to generate all figures in the Post Access Report. Project was a collaboration between Virginia Tech and the National Renewable Energy Lab, with funding from TEAMER.

16 TIDAL AND WAVE POWER↗

NASA NTRS1977

Evaluation of flight spoilers for vortex alleviation

The paper describes the facilities and test procedures used in a series of wind-tunnel and full-scale flight investigations of the effectiveness of flight spoilers currently existing on wide-bodied transport jet aircraft when used as trailing vortex hazard alleviation devices. Examples of the results of such studies include the variation of trailing wing rolling-moment coefficient with downstream distance behind a B-747 airplane model with various segments of its flight spoilers deflected 45 deg, and comparisons with models without spoilers deflected. It is concluded that the existing flight spoilers on the B-747 are effective as trailing vortex attenuators.

Croom, D. R.↗

NASA NTRS1978

Mercury's helium exosphere after Mariner 10's third encounter

From a comparison of the Mariner 10 third encounter UV spectrometer data with intensities generated from a newly constructed model exosphere, a new value of 4.5 x 10 to the -4th power for the fraction of the solar wind He(++) flux to be intercepted and captured by Mercury's magnetosphere, if the observed He atmosphere is maintained by the solar wind, has been derived. If an internal source for He prevails, the corresponding upper bound for the global outgassing rate is estimated to be 4.5 x 10 to the 22nd power/s. These values differ from those given earlier because of the present use of a surface temperature distribution satisfying the heat equation over Mercury's entire surface which employs Mariner-10-determined mean surface thermal characteristics. The mean standoff distance of Mercury's magnetopause averaged over Mercury's orbit is also used. Agreement between the observed and calculated intensities is found to be good. Minor discrepancies on the nightside of the terminator are explicable in terms of differences between actual and computed temperatures and scale height structure changes.

Curtis, S. A.↗

NASA NTRS1982

Coordinated observations of Markarian 501 at ultraviolet, X-ray, optical, and radio wavelengths

The results of multifrequency observations of MrK 501 = B2 1652+39 are presented here. It is a BL Lac object embedded in a giant elliptical galaxy obtained during the period August 1980 - March 1981. It is argued that MrK 501 varies sufficiently slowly at ultraviolet, optical and radio wavelengths on time scales of months that the data are representative of a spectrum obtained at a single epoch. The importance of MrK 501 derives mostly from its association of a quasar-like object with a galaxy whose absorption-line redshift (z = 0.034) unambiguously determines its distance. This implies that physical conditions within this local quasar can be discussed without the confusion introduced by distance uncertainties.

Hutter, D. J.↗

NASA NTRS1988

Buoyant surface jet analysis of the Yukon River

Thermal infrared satellite imagery of the discharge from the Yukon River obtained on July 5, 1985 was compared with hydraulic theory for the dilution of buoyant surface jets. In a crossflow, the theory predicts that the plume will follow an x exp 1/3 trajectory where x is distance alongshore, and that the plume temperature will decay according to x exp - 1/6 due to mixing with the receiving water. Measurements of the Yukon River discharge indicate very good agreement with the predicted trajectory, and less, but acceptable, agreement with the predicted dilution. Large scale thermal fronts are also observable in the thermal imagery; the fronts may be associated with excursions of the plume due to tidal currents.

Gosink, J. P.↗

NASA NTRS1992

Mars observer radio science (MORS) observations in polar regions

MORS observations will focus on two major areas of study: (1) the gravity field of Mars and its interpretation in terms of internal structure and history and (2) the structure of the atmosphere, with emphasis on both temperature-pressure profiles of the background atmosphere and small scale inhomogeneities resulting from turbulence. Scattering of cm wavelength radio signals from Mars' surface at highly oblique angles will also be studied during the primary mission; nongrazing scattering experiments may be possible during an extended mission. During the MORS primary mission, measurements of the spacecraft distance and velocity with respect to Earth based tracking stations will be used to develop models of the global gravity field. The improvement in knowledge of the gravity field will be especially evident in polar regions. The spatial and temporal coverage of atmospheric radio occultation measurements are determined by the geometry of the spacecraft orbit and the direction to the Earth. Profiles of atmospheric temperature and pressure will extend from the surface to altitudes of 50 to 70 km.

Simpson, Richard A.↗