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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 217 records · Page 12

All sources NASA NTRS DOE OSTI

NASA NTRS1984

The spectral evolution of low-frequency variable radio sources

The dynamic spectra of several low frequency extragalactic radio sources are presented. The observations were made at 318, 430, 606, 880, and 1400 MHz at several different radio observatories around the U.S. Two outbursts were observed in AO 0235 + 16 at 1.4 GHz, followed by a diminished variation at the lower frequencies. The dynamic frequencies of NRAO 140, PKS 1117 + 14, DA 406, CTA 102, and 3C 454.3 do not fit the same pattern. These radio sources displayed the following characteristics: (1) departure from straight or curved spectra at the frequencies of variation; (2) no obvious frequency drifting; and (3) negligible variation at 1.4 GHz. Possible explanations for this behavior are briefly discussed.

Dennison, B.↗

NASA NTRS1984

Multifrequency light curves of low-frequency variable radio sources

Light curves for the low-frequency variable sources AO 0235 + 16, NRAO 140, PKS 1117 + 14, DA 406, CTA 102, and 3C 454.3, obtained in monthly observations at 318, 430, and 606 MHz using the 305-m telescope at Arecibo and in bimonthly observations at 880 MHz and 1.4 GHz using the 91-m Green Bank transit telescope during 1980-1983, are presented and analyzed. AO 0235 + 16 is found to have basically canonical variability which is attributed to relativistically moving evolving synchrotron components; but in the other sources, strong simultaneous variations at 318, 430, and 606 MHz are observed to be greatly diminished in amplitude at 880 MHz and 1.4 GHz, confirming the existence of the intermediate-frequency gap at about 1 GHz proposed by Spangler and Cotton (1981). The possibility that a second variability mechanism is active in these sources is explored.

Altschuler, D. R.↗

NASA NTRS1988

Loads and low frequency dynamics - An ENVIRONET data base

The loads and low frequency dynamics data base, part of Environet, is described with particular attention given to its development and contents. The objective of the data base is to provide the payload designer with design approaches and design data to meet STS safety requirements. Currently the data base consists of the following sections: abstract, scope, glossary, requirements, interaction with other environments, summary of the loads analysis process, design considerations, guidelines for payload design loads, information data base, and references.

Garba, John A.↗

NASA NTRS1992

Theory for low-frequency modulated Langmuir wave packets

Langmuir wave packets with low frequency modulations (or beats) observed in the Jovian foreshock are argued to be direct evidence for the Langmuir wave decay L yields L-prime + S. In this decay, 'pump' Langmuir waves L, driven by an electron beam, produce backscattered product Langmuir waves L-prime and ion sound waves S. The L and L-prime waves beat at the frequency and wavevector of the S waves, thereby modulating the wave packets. Beam speeds calculated using the modulated Jovian wave packets (1) are reasonable, at 4-10 times the electron thermal speed, (2) are consistent with theoretical limits on the decay process, and (3) decrease with increasing foreshock depth, as expected theoretically. These results strongly support the theory. The modulation depth of some wave packets suggests saturation by the decay L yields L-prime + S. Applications to modulated Langmuir packets in the Venusian and terrestrial foreshocks and in a type III radio source are proposed.

Cairns, Iver H.↗

NASA NTRS1976

Low-frequency noise reduction of lightweight airframe structures

The results of an experimental study to determine the noise attenuation characteristics of aircraft type fuselage structural panels were presented. Of particular interest was noise attenuation at low frequencies, below the fundamental resonances of the panels. All panels were flightweight structures for transport type aircraft in the 34,050 to 45,400 kg (75,000 to 100,000 pounds) gross weight range. Test data include the results of vibration and acoustic transmission loss tests on seven types of isotropic and orthotropically stiffened, flat and curved panels. The results show that stiffness controlled acoustically integrated structures can provide very high noise reductions at low frequencies without significantly affecting their high frequency noise reduction capabilities.

Getline, G. L.↗

NASA NTRS2005

Impact of LISA's Low Frequency Sensitivity on Observations of Massive Black Hole Mergers

LISA will be able to detect gravitational waves from inspiralling massive black hole (MBH) binaries out to redshifts z > 10. If the binary masses and luminosity distances can be extracted from the Laser Interferometer Space Antenna (LISA) data stream, this information can be used to reveal the merger history of MBH binaries and their host galaxies in the evolving universe. Since this parameter extraction generally requires that LISA observe the inspiral for a significant fraction of its yearly orbit, carrying out this program requires adequate sensitivity at low frequencies, f < 10(exp -4) Hz. Using several candidate low frequency sensitivities, we examine LISA's potential for characterizing MBH binary coalescences at redshifts z > 1.

Baker, J.↗

NASA NTRS1981

The AKR emission cone at low frequencies

It is noted that certain of the ISEE-1 observations between the plasmasphere and the auroral zone have revealed the emission cone of auroral kilometric radiation (AKR) unaffected by plasmaspheric refraction. At some distance from the source, the cone produced a sharp low-frequency boundary in the AKR signals, which was displaced above the cyclotron frequency. The variation of this boundary, together with other aspects of the AKR signals, suggested that the AKR emission cone closed toward a hollow, roughly 45 deg limit cone with decreasing frequency, duplicating the behavior previously found with ISIS-1 at the opposite end of the AKR spectrum. It is pointed out that the hollow limit cone at low frequencies is a new feature, not previously reported.

Calvert, W.↗

DOE OSTI2025

Quantum metrology of low-frequency electromagnetic modes with frequency upconverters

We present the RF Quantum Upconverter (RQU) and describe its application to quantum metrology of electromagnetic modes between dc and the very high frequency band (VHF) ( ≲ 300 MHz). The RQU uses a Josephson interferometer made up of superconducting loops and Josephson junctions to implement a parametric interaction between a low-frequency electromagnetic mode (between dc and VHF) and a mode in the microwave C Band ( ∼ 5 GHz), analogous to the radiation pressure interaction between electromagnetic and mechanical modes in cavity optomechanics. We analyze RQU performance with quantum amplifier theory and show that the RQU can operate as a quantum-limited op-amp in this frequency range. It can also use nonclassical measurement protocols equivalent to those used in cavity optomechanics, including back-action evading (BAE) measurements, sideband cooling, and two-mode squeezing. These protocols enable experiments using dc VHF electromagnetic modes as quantum sensors with sensitivity better than the standard quantum limit (SQL). We demonstrate signal upconversion from low frequencies to the microwave C band using an RQU and show a phase-sensitive gain (extinction ratio) of 46.9 dB , which is a necessary step towards the realization of full BAE. Published by the American Physical Society 2025

Kuenstner, Stephen E. (ORCID:0000000346128846)↗

NASA NTRS1999

Fine Structure of the Low-Frequency Raman Phonon Bands of Single-Wall Carbon Nanotubes

The Raman spectra of singled-wall carbon nanotubes (SWNT) produced by laser and are process were studied between 5 and 500 kappa. The line width vs. temperature dependence of the low-frequency Raman bands between 150 and 200/ cm deviates from that expected for phonon decay through phonon-phonon scattering mechanism. The experimental results and their analysis provided convincing evidence that each of the low-frequency Raman lines is a superposition of several narrower Raman lines corresponding to tubes of nearly the same diameter. The application of Raman spectroscopy to probe the distribution of SWNT by both diameter and chirality is discussed.

Iliev, M. N.↗

DOE OSTI2022

The Regulatory Treatment of Low Frequency External Events as Part of a Risk-Informed, Performance-Based Approach

To assist the developing advanced reactor industry in future licensing efforts, the U.S. Department of Energy Advanced Reactor Demonstration Program Regulatory Development area initiated a project at Argonne National Laboratory to examine the regulatory treatment of external hazards as part of a risk-informed performance-based (RIPB) licensing framework. A RIPB licensing framework for advanced reactors built on establishing an affirmative safety case offers the benefits of increased flexibility regarding key design and licensing decisions based on a detailed assessment and understanding of plant risk. Historically, reactor licensing addressed events of very low frequency primarily through the application of design margin and defense-in-depth philosophy. In contrast, RIPB approaches attempt to evaluate these scenarios at a level of detail commensurate with their risk, which often necessitates an explicit treatment of their frequency and associated consequence. While the detailed analysis of low frequency events provides insights that can help justify alternative treatments to past conservatism, the findings are dependent on the quality and confidence associated with the analyses. The assessment of external hazards presents a unique challenge, as their potential frequency of occurrence, especially of large magnitude events, is inherently uncertain given the long return periods in question. This project aims to identify the benefits and challenges of such approaches for advanced reactor vendors and aid in the development of consistent and appropriate analysis methodologies. The paper summarizes project findings and explores the application of various approaches for different external hazards. In addition, the current work also evaluates the application of the quantitative health objectives as a limit on external event risk, as they are a potential regulatory requirement under the current draft 10 CFR Part 53, which is a new technology-neutral reactor licensing pathway in the U.S.

Grabaskas, David↗

NASA NTRS1977

Attenuation of upstream-generated low frequency noise by gas turbines

The acoustic transfer functions of low frequency (below 3500 Hz) noise through aircraft turbines were investigated. Model test results were compared with theoretical predictions in order to assess the validity of the theory. Component tests were conducted on both high pressure and low pressure model turbines. The influence of inlet temperature and turbine speed attenuation was evaluated, while the effects of turbine pressure ratio, blade-row choking, and additional downstream stages were determined. Preliminary identification of pertinent aeroacoustic correlating parameters was made.

Doyle, V. L.↗

NASA NTRS1972

Low frequency creep in CoNiFe films

The results of an investigation of domain wall motion excited by slow rise-time, bipolar, hard-axis pulses in vacuum deposited CoNiFe films 1500A to 2000A thick are presented. The results are consistent with those of comparable NiFe films in spite of large differences in film properties. The present low frequency creep data together with previously published results in this and other laboratories can be accounted for by a model which requires that the wall structure change usually associated with low frequency creep be predominately a gyromagnetic process. The correctness of this model is reinforced by the observation that the wall coercive force, the planar wall mobility, and the occurrence of an abrupt wall structure change are the only properties closely correlated to the creep displacement characteristics of a planar wall in low dispersion films.

Bartran, D. S.↗

NASA NTRS2012

A Review of Low Frequency Electromagnetic Wave Phenomena Related to Tropospheric-Ionospheric Coupling Mechanisms

Investigation of coupling mechanisms between the troposphere and the ionosphere requires a multidisciplinary approach involving several branches of atmospheric sciences, from meteorology, atmospheric chemistry, and fulminology to aeronomy, plasma physics, and space weather. In this work, we review low frequency electromagnetic wave propagation in the Earth-ionosphere cavity from a troposphere-ionosphere coupling perspective. We discuss electromagnetic wave generation, propagation, and resonance phenomena, considering atmospheric, ionospheric and magnetospheric sources, from lightning and transient luminous events at low altitude to Alfven waves and particle precipitation related to solar and magnetospheric processes. We review in situ ionospheric processes as well as surface and space weather phenomena that drive troposphere-ionosphere dynamics. Effects of aerosols, water vapor distribution, thermodynamic parameters, and cloud charge separation and electrification processes on atmospheric electricity and electromagnetic waves are reviewed. We also briefly revisit ionospheric irregularities such as spread-F and explosive spread-F, sporadic-E, traveling ionospheric disturbances, Trimpi effect, and hiss and plasma turbulence. Regarding the role of the lower boundary of the cavity, we review transient surface phenomena, including seismic activity, earthquakes, volcanic processes and dust electrification. The role of surface and atmospheric gravity waves in ionospheric dynamics is also briefly addressed. We summarize analytical and numerical tools and techniques to model low frequency electromagnetic wave propagation and solving inverse problems and summarize in a final section a few challenging subjects that are important for a better understanding of tropospheric-ionospheric coupling mechanisms.

Simoes, Fernando↗

DOE OSTI2023

BEYONDPLANCK X. Planck Low Frequency Instrument frequency maps with sample-based error propagation

We present Planck Low Frequency Instrument (LFI) frequency sky maps derived within the BEYONDPLANCK framework. This framework draws samples from a global posterior distribution that includes instrumental, astrophysical, and cosmological parameters, and the main product is an entire ensemble of frequency sky map samples, each of which corresponds to one possible realization of the various modeled instrumental systematic corrections, including correlated noise, time-variable gain, as well as far sidelobe and bandpass corrections. This ensemble allows for computationally convenient end-to-end propagation of low-level instrumental uncertainties into higher-level science products, including astrophysical component maps, angular power spectra, and cosmological parameters. We show that the two dominant sources of LFI instrumental systematic uncertainties are correlated noise and gain fluctuations, and the products presented here support – for the first time – full Bayesian error propagation for these effects at full angular resolution. We compared our posterior mean maps with traditional frequency maps delivered by the Planck Collaboration, and find generally good agreement. The most important quality improvement is due to significantly lower calibration uncertainties in the new processing, as we find a fractional absolute calibration uncertainty at 70 GHz of Δg 0 /g 0 = 5 x 10 -5 , which is nominally 40 times smaller than that reported by Planck 2018. However, we also note that the original Planck 2018 estimate has a nontrivial statistical interpretation, and this further illustrates the advantage of the new framework in terms of producing self-consistent and well-defined error estimates of all involved quantities without the need of ad hoc uncertainty contributions. We describe how low-resolution data products, including dense pixel-pixel covariance matrices, may be produced from the posterior samples directly, without the need for computationally expensive analytic calculations or simulations. We conclude that posterior-based frequency map sampling provides unique capabilities in terms of low-level systematics modeling and error propagation, and may play an important role for future Cosmic Microwave Background (CMB) B-mode experiments aiming at nanokelvin precision.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS1987

Pseudomorphic InGaAs/AlGaAs modulation-doped FET's with reduced low-frequency noise and thermally stable performance

A high-performance MODFET structure grown by MBE with the incorporation of a single quantum well In(0.15)Ga(0.85)As layer for the transport of two-dimensional electron gas has been critically examined for its thermal stability at 80 K and low-frequency noise form 0.01 to 10 to the 8th Hz. Experimental results indicate that the behavior of this device in both these respects is much superior when compared with the same behavior of conventional MODFETs. A maximum low-field carrier mobility of 29,000 sq cm/s at 80 K and an average carrier saturation velocity of 2 x 10 to the 7th cm/s at 300 K in a 1-micron gate device clearly indicate that the quality of the pseudomorphic quantum well (InGaAs) layer is either comparable or better than that of the usual GaAs buffer layer. The deep level spectra, obtained through photo-FET measurements, and the low-frequency noise spectra at different temperatures obtained for the new pseudomorphic and conventional MODFET's have clearly indicated that contributions from various deep levels present in the new structure are significantly reduced.

Liu, Shih-Ming↗

NASA NTRS1982

Three dimensional ray tracing Jovian magnetosphere in the low frequency range

Ray tracing of the Jovian magnetosphere in the low frequency range (1+40 MHz) has resulted in a new understanding of the source mechanism for Io dependent decametric radiation (DAM). Our three dimensional ray tracing computer code has provided model DAM arcs at 10 deg. intervals of Io longitude source positions for the full 360 deg of Jovian system III longitude. In addition, particularly interesting arcs were singled out for detailed study and modelling. Dependent decametric radiation arcs are categorized according to curvature--the higher curvature arcs are apparently due to wave stimulation at a nonconstant wave normal angle, psi. The psi(f) relationship has a signature that is common to most of the higher curvature arcs. The low curvature arcs, on the other hand, are adequately modelled with a constant wave normal angle of close to 90 deg. These results imply that for higher curvature arcs observed for from Jupiter (to diminish spacecraft motion effects) the electrons providing the gyroemission are relativistically beamed.

Menietti, J. D.↗

NASA NTRS2000

The Quasar Fraction in Low-Frequency Selected Complete Samples and Implications for Unified Schemes

Low-frequency radio surveys are ideal for selecting orientation-independent samples of extragalactic sources because the sample members are selected by virtue of their isotropic steep-spectrum extended emission. We use the new 7C Redshift Survey along with the brighter 3CRR and 6C samples to investigate the fraction of objects with observed broad emission lines - the 'quasar fraction' - as a function of redshift and of radio and narrow emission line luminosity. We find that the quasar fraction is more strongly dependent upon luminosity (both narrow line and radio) than it is on redshift. Above a narrow [OII] emission line luminosity of log(base 10) (L(sub [OII])/W) approximately > 35 [or radio luminosity log(base 10) (L(sub 151)/ W/Hz.sr) approximately > 26.5], the quasar fraction is virtually independent of redshift and luminosity; this is consistent with a simple unified scheme with an obscuring torus with a half-opening angle theta(sub trans) approximately equal 53 deg. For objects with less luminous narrow lines, the quasar fraction is lower. We show that this is not due to the difficulty of detecting lower-luminosity broad emission lines in a less luminous, but otherwise similar, quasar population. We discuss evidence which supports at least two probable physical causes for the drop in quasar fraction at low luminosity: (i) a gradual decrease in theta(sub trans) and/or a gradual increase in the fraction of lightly-reddened (0 approximately < A(sub V) approximately < 5) lines-of-sight with decreasing quasar luminosity; and (ii) the emergence of a distinct second population of low luminosity radio sources which, like M8T, lack a well-fed quasar nucleus and may well lack a thick obscuring torus.

Willott, Chris J.↗

NASA NTRS1984

Low frequency noise in a quiet, clean, general aviation turbofan engine

A quiet, clean, general aviation, turbofan engine was instrumented to measure the fluctuating pressures in the combustor, turbine exit duct, engine nozzle and the far field. Both a separate flow nozzle and an internal mixer nozzle were tested. The fluctuating pressure data are presented in overall pressure and power levels and in spectral plots. The combustor data are compared to recent theory and found to be in excellent agreement. The results indicate that microphone correction procedures for elevated mean pressures are questionable. Ordinary coherence function analysis suggests the presence of an additional low frequency noise source downstream of the turbine that is due to the turbine itself. Low frequency narrowband data and coherence function analysis are presented.

Huff, R. G.↗