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At least 1,405 records · Page 78
All sourcesNASA NTRSDOE OSTI
NASA NTRS1974

Experimental and analytical determination of vibration characteristics of corrugated, flexibly supported, heat-shield panels

Experimental and analytical natural frequencies, nodal patterns, and typical modal displacements for a corrugated, flexibly supported, heat-shield panel are discussed. Good correlation was found between the experimental data and NASTRAN analytical results for the corrugated panel over a relatively wide frequency spectrum covered in the investigation. Of the two experimental techniques used for mode shape and displacement measurements (a noncontacting displacement sensor system and a holographic technique using a helium-neon, continuous-wave laser), the holographic technique was found, in the present investigation, to be faster and better suited for determining a large number of complex nodal patterns of the corrugated panel.

Carden, H. D.↗
NASA NTRS1978

Frequency analysis of 4- to 6-keV electrons associated with an auroral arc

The high time resolution (HTR) experiment considered consists of four sections, including electrostatic deflection plates, electron multiplier, preamplifier-integrator, and frequency analyzer. Electrons in the energy range from 4 to 6 keV electrons are selected by the deflection system, after which they are detected, multiplied, and passed to the charge sensitive preamplifier. The charge pulses are amplified and integrated, and the resulting voltage pulse series is fed to the frequency analyzer. The experiment is essentially a flux detector, so that estimates of the input flux can be made from the mean value of the output spectrum. The results obtained with the HTR in a rocket flight on March 14, 1974, are discussed. The lack of modulation noticed in the high-frequency range indicates that there is insufficient wave energy available in the vicinity of the rocket to produce a wave-particle interaction which the HTR is able to observe. This finding is consistent with the stabilization of the plasma by the low flux of energetic electrons.

Murphree, J. S.↗
NASA NTRS1980

Measurements of potential of a cylindrical monopole antenna on a rotating spacecraft

Measurements of antenna potential on the Helios 1 spacecraft are presented and discussed. The behavior of the potential indicates that Lyman-alpha and shorter wavelengths play an important role in determining antenna and spacecraft potential, and that conversely, longer wavelengths are unimportant. As expected, a voltage spike usually occurs when the antenna is shadowed. However, when the spike is not present, the potential variations on an antenna which is not shadowed can be estimated. For this case, the spectrum is about (Fourier coefficient) 2 V x (2fo/f)2 where fo is the spacecraft spin frequency, and f is an even harmonic of the spin frequency. Suggestions for measuring low-frequency waves are briefly discussed in relation to the experimental results, and slight tipping of the spacecraft is recommended to avoid shadowing.

Kellogg, P. J.↗
NASA NTRS1985

Relativistic cosmic ray spectra in the full non-linear theory of shock acceleration

The non-linear theory of shock acceleration was generalized to include wave dynamics. In the limit of rapid wave damping, it is found that a finite ave velocity tempers the acceleration of high Mach number shocks and limits the maximum compression ratio even when energy loss is important. For a given spectrum, the efficiency of relativistic particle production is essentially independent of v sub Ph. For the three families shown, the percentage of kinetic energy flux going into relativistic particles is (1) 72%, 2) 44%, and (3) 26% (this includes the energy loss at the upper energy cuttoff). Even small v sub ph, typical of the HISM, produce quasi-universal spectra that depend only weakly on the acoustic Mach number. These spectra should be close enough to e(-2) to satisfy cosmic ray source requirements.

Eichler, D.↗
NASA NTRS1986

Strong scintillations during atmospheric occultations Theoretical intensity spectra

Each of the two Voyager spacecraft launched in 1977 has completed a reconnaissance of the Jovian and Saturnian systems. In connection with occultation experiments, strong scintillations were observed. Further theoretical work is required before these scintillations can be interpreted. The present study is, therefore, concerned with the derivation of a theory for strong scattering during atmospheric occultation experiments, taking into account as fundamental quantity of interest the spatial spectrum (or spectral density) of intensity fluctuations. Attention is given to a theory for intensity spectra, and numerical calculations. The new formula derived for Phi-i accounts for strong scattering of electromagnetic waves during atmospheric occultations.

Hinson, D. P.↗
NASA NTRS1986

Recent observations of the very low frequency interplanetary radio emission

Observations of radio emissions in the frequency range of 2 to 3 kHz have been made in the distant heliosphere by the Voyager 1 and 2 plasma wave instruments. Based primarily on wideband observations made periodically throughout the cruise phases of the missions the radio emission, first observed in 1982, appears to have been present almost continuously since 1983. The spectrum is complex, usually showing two peaks, one near 2 and another near 3 kHz. Occasionally, only one of the peaks is observed. A possible source for the radio emissions is the terminal shock in the outer heliosphere.

Kurth, W. S.↗
NASA NTRS1992

Ionospheric gravity wave measurements with the USU dynasonde

A method for the measurement of ionospheric Gravity Wave (GW) using the USU Dynasonde is outlined. This method consists of a series of individual procedures, which includes functions for data acquisition, adaptive scaling, polarization discrimination, interpolation and extrapolation, digital filtering, windowing, spectrum analysis, GW detection, and graphics display. Concepts of system theory are applied to treat the ionosphere as a system. An adaptive ionogram scaling method was developed for automatically extracting ionogram echo traces from noisy raw sounding data. The method uses the well known Least Mean Square (LMS) algorithm to form a stochastic optimal estimate of the echo trace which is then used to control a moving window. The window tracks the echo trace, simultaneously eliminating the noise and interference. Experimental results show that the proposed method functions as designed. Case studies which extract GW from ionosonde measurements were carried out using the techniques described. Geophysically significant events were detected and the resultant processed results are illustrated graphically. This method was also developed for real time implementation in mind.

Berkey, Frank T.↗
NASA NTRS1993

Interaction of turbulence with a detonation wave

This paper addresses a specific reactive-flow configuration, namely, the interaction of a detonation wave with convected homogeneous isotropic weak turbulence (which can be constructed by a Fourier synthesis of small-amplitude vorticity waves). The effect of chemical heat release on the rms fluctuations downstream of the detonation is presented as a function of Mach number. In addition, for the particular case of the von Karman spectrum, the one-dimensional power spectra of these flow quantities are given.

Jackson, T. L.↗
NASA NTRS1994

Boundary-layer transition detection with infrared imaging emphasizing cryogenic applications

This paper reviews the technique of boundary-layer transition detection using infrared (IR) imaging, emphasizing cryogenic wind-tunnel testing. With the exception of the low-temperature effects on the IR radiation, the discussion is relevant to conventional wind-tunnel and flight testing as well. At low temperatures, IR imaging encounters a reduction in the radiated energy throughout the IR spectrum, combined with a shift to longer wavelengths of the bulk of the radiation. This radiation behavior affects the minimum resolvable temperature difference (MRTD) of the IR imaging system because of its fixed wave band sensitivity. In the absence of commercial long wavelength IR imaging systems, operating at wavelengths longer than 13 micron, some measures can be taken to alleviate the problem caused by the MRTD limitation. The thermal signature of transition can be enhanced by allowing a small and controlled temperature increase of the wind-tunnel flow that induces a transient heat transfer to the model. This action temporarily reveals the model area under the turbulent regime through its higher heating rate compared with the laminar regime. The contrast between the areas exposed to the two regimes can be enhanced by subtraction of thermograms (the equilibrium thermogram from the transient thermogram). Further visual improvement can be obtained through shade stretching or binary shading.

Gartenberg, Ehud↗
NASA NTRS2002

Evidence of an Intrinsic Intraseasonal Oscillation over Tropical South America During Austral Summer

The intraseasonal variation (ISV) in the 30-60 day band, also known as Madden-Julian oscillation (MJO), has been studied for decades. Madden and Julian showed that the oscillation originated from the western Indian Ocean, propagated eastward, got enhanced over the maritime continent and weakened after passing over the dateline. Composite studies showed evidences of a signal in upper and lower level zonal wind propagating around the globe during an oscillation. Theoretical studies pointed out that the interaction with the warm ocean surface and the coupling with the convective and radiative processes in the atmosphere could manifest the oscillation, which propagates eastward via mutual feedbacks between the wave motions and the cumulus heating. Over tropical South America, no independent 30-60 day oscillation has been reported so far, despite that Amazon is the most distinct tropical convection center over the western hemisphere and the fluxes from its surface of tropical rainforests are close to that from the warm tropical ocean. Liebmann et al. showed a distinct spectral peak of 40-50 day oscillation in outgoing longwave radiation (OLR) over tropical South America and considered that was manifested by the MJO propagation. Nogues-Paegle et al. (2000) focused on a dipole pattern of the OLR anomaly with centers of action over the South Atlantic Convergence Zone (SACZ) and the subtropical plain. They used the regional 10-90 day filtered data and demonstrated this pattern could be represented by the fifth mode of the rotated empirical orthogonal function. Its principal component was further analyzed using the singular spectrum analysis. Their result showed two oscillatory modes with periods of 36-40 days and 22-28 days, of which the former was related to the MJO influence and the latter linked to the remote forcing over southwest of Australia, which produced a wave train propagating southeastward, rounding the southern tip of South America and returning back toward the northeast. The 22-28 day mode has distinct impact on SACZ, responsible for the regional seesaw pattern of alternating dry and wet conditions. In this study we will focus on the 30-60-day spectral band and investigate whether the independent oscillation source over tropical South America is existed. First, we will show the seasonal dependence of the tropical South American ISV in Section 3. Then, the leading principal modes of 30-60 day bandpass filtered 850-hPa velocity potential (VP850) will be computed to distinguish the stationary ISV over tropical South America (SISA) from the propagating MJO in the austral summertime in Section 4. The importance of SISA in representing the regional ISV over South America will be discussed. In Section 5, we will demonstrate the mass oscillation regime of SISA, which is well separated from that of MJO by the Andes, and the convective coupling with rainfall. The dynamical response of SISA and the impact on the South American summer monsoon (SASM) will be presented. Finally, we will give the concluding remarks.

Zhou, Jiayu↗
NASA NTRS1982

Measurements of ocean surface spectrum from an aircraft using the two-frequency microwave resonance technique

The present investigation is concerned with the results of a two-frequency (Delta k) microwave radar experiment conducted from an aircraft and aimed primarily at the development of remote sensing techniques to measure ocean surface wave spectral characteristics. The experiment was conducted as part of the Maritime Remote Sensing (MARSEN) project in the North Sea during the autumn of 1979. The objective was to demonstrate the feasibility of and study the performance of the Delta k technique from a higher altitude platform, at shallower incidence angles, and at higher Doppler velocities than earlier stationary platform experiments allowed. A quantitative engineering evaluation of the results of two comprehensive flights is provided, and the qualitative significance of the results is discussed from a geophysical point of view in terms of the existing theory.

Johnson, J. W.↗
NASA NTRS1984

The nature of AFGL 2591 and its associated molecular outflow: Infrared and millimeter-wave observations

The results of infrared photometry from 2 to 160 microns of AFGL and CO(12) observations of its associated molecular cloud and high velocity molecular outflow are presented and discussed. The observed solar luminosity is 6.7 x 10(4) at a distance of 2 kpc. The spectrum of AFGL 2591 is interpreted in the context of a model in which a single embedded object is the dominant source of the infrared luminosity. This object is determined to be surrounded by a compact, optically thick dust shell with a temperature in excess of several hundred degrees kelvin. The extinction to this source is estimated to be between 26 and 50 visual magnitudes. The absolute position of the infrared sources at 10 microns was determined to an accuracy of + or in. This indicates for the first time that the IR source and H2O source are not coincident. The CO(12) observations show the high-velocity molecular flow near AFGL 2591 to be extended, bipolar and roughly centered on the infrared emission. The observations suggest that the red-shifted flow component extends beyond the boundary of the ambient cloud within which AFGL 2591 is embedded. The CO(12) observations also show that AFGL 2591 is embedded in a molecular cloud with an LSR velocity of -5 km/s.

Lada, C. J.↗
NASA NTRS1984

The nature of AFGL 2591 and its associated molecular outflow Infrared and millimeter-wave observations

The results of infrared photometry from 2 to 160 microns of AFGL and CO(12) observations of its associated molecular cloud and high velocity molecular outflow are presented and discussed. The observed solar luminosity is 6.7 x 10(4) at a distance of 2 kpc. The spectrum of AFGL 2591 is interpreted in the context of a model in which a single embedded object is the dominant source of the infrared luminosity. This object is determined to be surrounded by a compact, optically thick dust shell with a temperature in excess of several hundred degrees kelvin. The extinction to this source is estimated to be between 26 and 50 visual magnitudes. The absolute position of the infrared sources at 10 microns was determined to an accuracy of + or - 1 in. This indicates for the first time that the IR source and H2O source are not coincident. The CO(12) observations show the high-velocity molecular flow near AFGL 2591 to be extended, bipolar and roughly centered on the infrared emission. The observations suggest that the red-shifted flow component extends beyond the boundary of the ambient cloud within which AFGL 2591 is embedded. The CO(12) observations also show that AFGL 2591 is embedded in a molecular cloud with an LSR velocity of -5 km/s.

Lada, C. J.↗
NASA NTRS1987

A radiative bow shock wave (?) driven by nuclear ejecta in a Seyfert galaxy

New VLA maps at 2 cm of the 13-arcsec-scale linear radio source in the center of NGC 1068 are described. The northeast lobe shows a limb-brightened conical morphology, very sharp 'leading' edges, and a magnetic field running parallel to these edges. The spectral index between 2 and 6 cm in these line-brightened regions is near 1.0. The northeast subpeak has a very steep radio spectrum between 18 and 2 cm which is attributed to inverse Compton losses of the relativistic electrons on the infrared photons. The spectral indices in the southwest lobe lie in the range 0.9-1.5 except in its northern parts, where a much larger index is found. The northeast lobe radio emission could arise in either the cocoon of old jet material which has passed through the internal shock in the ejecta and blown out to either side, or in interstellar material compressed by a bow shock wave driven into the galactic ISM.

Wilson, Andrew S.↗
DOE OSTI2025

$\mathrm{SageNet}$: Fast Neural Network Emulation of the Stiff-amplified Gravitational Waves from Inflation

Accurate modeling of the inflationary gravitational waves (GWs) requires time-consuming, iterative numerical integrations of differential equations to take into account their backreaction on the expansion history. To improve computational efficiency while preserving accuracy, we present the Stiff-amplified Gravitational-wave Emulator Network (SageNet), a deep learning framework designed to replace conventional numerical solvers (code available at https://github.com/YifangLuo/SageNet). SageNet employs a long short-term memory architecture to emulate the present-day energy density spectrum of the inflationary GWs with possible stiff amplification, Ω GW (f). Trained on a data set of 25,689 numerically generated solutions, SageNet allows accurate reconstructions of Ω GW (f) and generalizes well to a wide range of cosmological parameters; 90.9% of the test emulations with randomly distributed parameters exhibit errors of under 4%. In addition, SageNet demonstrates its ability to learn and reproduce the artificial, adaptive sampling patterns in numerical calculations, which implement denser sampling of frequencies around changes in spectral indices in Ω GW (f). The dual capability of learning both physical and artificial features of the numerical GW spectra establishes SageNet as a robust alternative to exact numerical methods. Finally, our benchmark tests show that SageNet reduces the computation time from tens of seconds to milliseconds, achieving a speedup of ∼10 4 times over standard CPU-based numerical solvers with the potential for further acceleration on GPU hardware. These capabilities make SageNet a powerful tool for accelerating Bayesian inference procedures for extended cosmological models. In a broad sense, the SageNet framework offers a fast, accurate, and generalizable solution to modeling cosmological observables whose theoretical predictions demand costly differential equation solvers.

Astronomy data modeling↗
DOE OSTI2025

High-energy $η^{(')}π$ photoproduction and the nature of exotic waves

The observation of hybrid mesons in photoproduction experiments can provide essential insight into the inner workings of quantum chromodynamics in the strong coupling regime. In particular, the study of final $η^{(')}π$ states is of great interest due to the presence of the lowest lying hybrid candidate with manifestly exotic quantum numbers, the $π_1$(1600). In this work, a double-vector exchange model with Reggeized $ρ$ and $ω$ trajectories is developed to describe the photoproduction of $η^{(')}π$ in the high-mass region. Results are presented for the differential cross sections and forward-backward asymmetries in the energy region of interest to the GlueX experiment. The model contains no free parameters, and reproduces the magnitude and t-dependence of existing CLAS data at $E_γ$ = 5 GeV. The model predicts a stronger asymmetry in the $η'π$ channel than in the $ηπ$ channel, consistent with what has previously been observed in pion beam experiments. This suggests that the sizeable production of exotic odd waves in $η'π$ is not necessarily related to the presence of gluon-rich environments. Confirmation of these predicted asymmetries from forthcoming GlueX data would enable further predictions of the low-energy spectrum.

Montaña, Glòria [Thomas Jefferson National Acceler↗
NASA NTRS1975

Measurements of the electronic transition moments of C2-band systems

Electronic transition moments of seven C2 singlet and triplet band systems, which are in the 0.2 to 1.2 micron spectral region, have been measured. The measurements were made in emission behind incident shock waves in C2H2-argon mixtures. Narrow band-pass radiometers were used to obtain absolute measurements of shock-excited C2 radiation from which absolute electronic transition moments are derived by a synthetic spectrum analysis. New results are reported for the Ballik-Ramsay, Phillips, Swan, Deslandres-d'Azambuja, Fox-Herzberg, Mulliken, and Freymark systems.

Cooper, D. M.↗
NASA NTRS1977

Monte Carlo simulation of wave sensing with a short pulse radar

A Monte Carlo simulation is used to study the ocean wave sensing potential of a radar which scatters short pulses at small off-nadir angles. In the simulation, realizations of a random surface are created commensurate with an assigned probability density and power spectrum. Then the signal scattered back to the radar is computed for each realization using a physical optics analysis which takes wavefront curvature and finite radar-to-surface distance into account. In the case of a Pierson-Moskowitz spectrum and a normally distributed surface, reasonable assumptions for a fully developed sea, it has been found that the cumulative distribution of time intervals between peaks in the scattered power provides a measure of surface roughness. This observation is supported by experiments.

Levine, D. M.↗