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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 1,513 records · Page 84
All sourcesNASA NTRSDOE OSTI
NASA NTRS1982

Direct measurement of transmission loss of aircraft structures using the acoustic intensity approach

A measurement technique is developed in order to obtain the sound transmission loss of an aircraft fuselage which obviates the need for the two-room transmission suite. The sound transmission paths were determined in tests on a light aircraft fuselage using a two-microphone acoustic intensity method for measuring the acoustic intensity transmitted to the interior when the fuselage was exposed to an external random incidence sound-field. The intensity transmitted through different sections of the fuselage can be estimated accurately using this new technique. Results of these tests show that the plexiglass window is the major transmission path in the high frequency range. In addition, the transmission losses through a single and a double layer window were predicted theoretically by using the Statistical Energy Analysis Model. Very good agreement is found between the predictions and the measurements.

Wang, Y. S.↗
NASA NTRS1984

Sensitivity of Inferred Subphotospheric Velocity Field to Mode Selection, Analysis Technique and Noise

The horizontal velocity immediately below the photosphere was inferred from observations of high degree solar oscillations by an optimal averaging inversion technique. The sensitivity of the results to various details of both the inversion and the determination of the frequencies are investigated. The results are shown to be quite stable to the choice of most parameters, suggesting that this procedure produces reliable estimates of the subsurface velocity.

Hill, F.↗
NASA NTRS1988

Gravitational wave searches using the DSN (Deep Space Network)

The Deep Space Network Doppler spacecraft link is currently the only method available for broadband gravitational wave searches in the 0.01 to 0.001 Hz frequency range. The DSN's role in the worldwide search for gravitational waves is described by first summarizing from the literature current theoretical estimates of gravitational wave strengths and time scales from various astrophysical sources. Current and future detection schemes for ground based and space based detectors are then discussed. Past, present, and future planned or proposed gravitational wave experiments using DSN Doppler tracking are described. Lastly, some major technical challenges to improve gravitational wave sensitivities using the DSN are discussed.

Nelson, S. J.↗
NASA NTRS1988

Radiative transfer to space through a precipitating cloud at multiple microwave frequencies. I - Model description. II - Results and analysis

The impact of time-dependent cloud microphysical structure on the transfer to space of passive microwave radiation is studied at several frequencies across the EHF and lower SHF portions of the microwave spectrum. The feasibility of using multichannel passive-microwave retrieval techniques to estimate precipitation from space-based platforms is examined. The model is described, and the results are assessed in conjunction with a Nimbus-7 SMMR case study of precipitation in an intense tropical Pacific storm. It is concluded that the effects of cloud liquid water content must be considered to obtain a realistic estimation and distribution of rainrates.

Mugnai, Alberto↗
NASA NTRS1988

Planetary scaling laws and predictions for Neptune

This paper offers a prediction concerning Neptune's low-frequency radio emission based on the radiometric Bode's law in combination with a recent prediction for Neptune's global magnetic field strength. The latter is based on a dynamo scaling relation derived from the magnetospheric balance condition within planetary cores. The radio emission frequency range is predicted to extend from approximately 100 to 1000 kHz, with a spectral peak between 350 and 500 kHz. A crude estimate of the emission spectral shape, based on Saturn and earth-like models, is shown. If radiation is beamed approximately in the sunward direction, Neptune should be detectable by the Planetary Radio Astronomy experiment onboard the Voyager spacecraft sometime between 45 and 60 days before closest approach.

Desch, Michael D.↗
NASA NTRS1990

Doppler radar imaging of spherical planetary surfaces

This paper describes a technique for using echo power-spectra for making radar images of spherical planetary targets. In developing the Doppler-radar imaging system, the target's reflectivity distribution is expanded in a truncated spherical harmonic series; the distribution of echo power in rotational phase and the Doppler frequency are obtained in form of a system of linear equations which express the echo spectra as functions of the series coefficients. To estimate the coefficients from an observed phase-Doppler distribution, the inversion is cast as a least-squares problem, and is solved using singular value decompositions, yielding a linear imaging system which can be fully characterized by its impulse response as a function of latitude. Simulations were designed and used to explore the capabilities of Doppler radar imaging, and the sensitivity of imaging to such factors as subradar altitude coverage and signal-to-noise ratio was investigated.

Hudson, R. Scott↗
NASA NTRS1991

Bulk ice characterization using coupled cloud, multiparameter radar, and radiative transfer models

The study demonstrates the potential utility of employing scattering-based passive microwave channels (not less than 37 GHz) to estimate the amount of ice water path that exists above rain in precipitating clouds. This methodology can be used along with lower-frequency emission-based channels to infer the columnar melt water and ice water paths. Portions of the ice column that coexist with cloud water hide the usually steep brightness temperature cooling through this region and inhibit characterization of the total ice column.

Turk, J.↗
NASA NTRS1993

Biomass Burning

Biomass burning may be the overwhelming regional or continental-scale source of methane (CH4) as in tropical Africa and a significant global source of CH4. Our best estimate of present methane emissions from biomass burning is about 51.9 Tg/yr, or 10% of the annual methane emissions to the atmosphere. Increased frequency of fires that may result as the Earth warms up may result in increases in this source of atmospheric methane.

Levine, Joel S.↗
NASA NTRS1997

A Xylophone Detector of Gravitational Radiation

We discuss spacecraft Doppler tracking searches for gravitational waves in which Doppler data recorded on the ground are linearly combined with Doppler measurements made on board a spacecraft. By using the four-link radio system first proposed by Vessot and Levine, we describe a new method for removing from the combined data the frequency fluctuations due to the Earth troposphere, ionosphere, and mechanical vibrations of the antenna on the ground. This technique provides also a way for reducing by several orders of magnitude, at selected Fourier components, the frequency fluctuations due to other noise sources, such as the clock on board the spacecraft or the antenna and buffeting of the probe by nongravitational forces. In this respect spacecraft Doppler tracking can be regarded as a xylophone detector of gravitational radiation. In the assumption of calibrating the frequency fluctuations induced by the interplanetary plasma, a strain sensitivity equal to 4.7 x 10(exp -18) at 10(exp -3) Hz is estimated. This experimental technique could be extended to other tests of the theory of relativity, and to radio science experiments that rely on high-precision Doppler measurements.

Tinto, Massimo↗
NASA NTRS1994

Effects of Mistuning on the Forced Response of Turbomachinery Rotors

The basic purpose of this research has been to develop a fundamental understanding of the effects of blade-to-blade dissimilarities, or mistuning, on the dynamics of nearly cyclic bladed-disk assemblies. This topic is of importance as mistuning has been shown to increase the forced response amplitudes of some blades significantly, and even to lead to blade failure. Furthermore, the current trend toward high performance propulsion turbomachinery designed for finite service life demands an accurate prediction of system performance and dynamics at the design stage. This objective has been achieved by carrying out the following tasks. First, the investigation of the free and forced responses of representative, yet sufficiently simple blade assembly models that capture all the important characteristics of typical turbomachinery rotors. Second, the development of computational methods that predict the effects of mistuning in a systematic and reliable way, along with the development, of a systematic reduced-order modeling procedure for mistuned bladed disks. Third, the application of these findings and the tools developed to an industrial rotor, namely the first stage of turbine blades of the oxidizer turbopump in the space shuttle main rocket engine (SSME). The research supported by NASA has led to the development of a coherent theory for mistuned blade assemblies. The further implementation of these computational tools into the forced response prediction system currently under development in the Structural Dynamics Branch Forced Response Prediction System (FREPS) would enable the designer and the analyst (1) to identify types of blade assemblies highly sensitive to mistuning in various frequency and other parameter ranges and (2), to characterize mistuning effects on their forced response by predicting true response amplitudes and fatigue life estimates.

Stefko, George L.↗
NASA NTRS2003

Testing the Equivalence Principle in an Einstein Elevator: Detector Dynamics and Gravity Perturbations

We discuss specific, recent advances in the analysis of an experiment to test the Equivalence Principle (EP) in free fall. A differential accelerometer detector with two proof masses of different materials free falls inside an evacuated capsule previously released from a stratospheric balloon. The detector spins slowly about its horizontal axis during the fall. An EP violation signal (if present) will manifest itself at the rotational frequency of the detector. The detector operates in a quiet environment as it slowly moves with respect to the co-moving capsule. There are, however, gravitational and dynamical noise contributions that need to be evaluated in order to define key requirements for this experiment. Specifically, higher-order mass moments of the capsule contribute errors to the differential acceleration output with components at the spin frequency which need to be minimized. The dynamics of the free falling detector (in its present design) has been simulated in order to estimate the tolerable errors at release which, in turn, define the release mechanism requirements. Moreover, the study of the higher-order mass moments for a worst-case position of the detector package relative to the cryostat has led to the definition of requirements on the shape and size of the proof masses.

Hubbard, Dorthy↗
NASA NTRS2011

The Atacama Cosmology Telescope: Extragalactic Sources at 148 GHz in the 2008 Survey

We report on extragalactic sources detected in a 455 square-degree map of the southern sky made with data at a frequency of 148 GHz from the Atacama Cosmology Telescope 2008 observing season. We provide a catalog of 157 sources with flux densities spanning two orders of magnitude: from 15 mJy to 1500 mJy. Comparison to other catalogs shows that 98% of the ACT detections correspond to sources detected at lower radio frequencies. Three of the sources appear to be associated with the brightest cluster galaxies of low redshift X-ray selected galaxy clusters. Estimates of the radio to mm-wave spectral indices and differential counts of the sources further bolster the hypothesis that they are nearly all radio sources, and that their emission is not dominated by re-emission from warm dust. In a bright (> 50 mJy) 148 GHz-selected sample with complete cross-identifications from the Australia Telescope 20 GHz survey, we observe an average steepening of the spectra between .5, 20, and 148 GHz with median spectral indices of alp[ha (sub 5-20) = -0.07 +/- 0.06, alpha (sub 20-148) -0.39 +/- 0.04, and alpha (sub 5-148) = -0.20 +/- 0.03. When the measured spectral indices are taken into account, the 148 GHz differential source counts are consistent with previous measurements at 30 GHz in the context of a source count model dominated by radio sources. Extrapolating with an appropriately rescaled model for the radio source counts, the Poisson contribution to the spatial power spectrum from synchrotron-dominated sources with flux density less than 20 mJy is C(sup Sync) = (2.8 +/- 0.3) x 1O (exp-6) micro K(exp 2).

Marriage, T. A.↗
NASA NTRS2019

Determination of the Cosmic Infrared Background from COBE/FIRAS and Planck HFI Observations

New determinations are presented of the cosmic infrared background monopole brightness in thePlanckHFI bands from 100 to 857 GHz. Planck was not designed to measure the monopole component of sky brightness, so cross-correlation of the 2015 HFI maps with COBE/FIRAS data is used to recalibrate the zero level of the HFI maps. For the HFI 545 and 857 GHz maps, the brightness scale is also recalibrated. Correlation of the recalibrated HFI maps with a linear combination of Galactic HI and Hα data is used to separate the Galactic foreground emission and determine the cosmic infrared background brightness in each of the HFI bands. We obtain CIB values of 0.007, ±0.014, 0.010, ±0.019, 0.060, ±0.023, 0.149, ±0.017, 0.371, ±0.018, and 0.576, ±0.034 MJy sr−1at100, 143, 217, 353, 545, and 857 GHz, respectively. The estimated uncertainties for the 353–857 GHz bands are about 3–6 times smaller than those of previous direct CIB determinations at these frequencies. Our results are compared with integrated source brightness results from selected recent submillimeter and millimeter wavelength imaging surveys.

N. Odegard↗
NASA NTRS1984

Radio emission from the outer parts of stellar atmospheres (mantles) at centimeter wavelengths and the winds of early-type stars

It is noted that when magnetic lines of force are present in the mantle of an early-type star, gyroresonance radiation at radio wavelengths may occur at detectable levels in addition to bremsstrahlung. An expression is derived for the effective value of the ratio of the emissivity in gyroresonance radiation to that in bremsstrahlung, and some numerical values are estimated using values of the parameters which may be typical for conditions in the mantles of early-type stars. The volume emissivity in gyroresonance radiation may exceed that for bremsstrahlung by a large factor. The implications of this result for the interpretation of the radio fluxes from early-type stars are discussed. It is shown that the radio-frequency radiation emerging from a hot star probably consists of gyroresonance radiation from an array of magnetic loops as well as bremsstrahlung from a low-density wind. It is argued that it is probable that the estimates of rate of mass loss made ignoring the possibility of gyroresonance radiation may be too large by a factor of at least 10.

Underhill, A. B.↗
NASA NTRS1996

Ultralow Friction in a Superconducting Magnetic Bearing

Passive levitation by superconducting magnetic bearings can be utilized in flywheels for energy storage. Basic design criteria of such a bearing are high levitation force, sufficient vertical and horizontal stability and low friction. A test facility was built for the measurement and evaluation of friction in a superconducting magnetic bearing as a function of operating temperature and pressure in the vacuum vessel. The bearing consists of a commercial disk shaped magnet levitated above single grain, melt-textured YBCO high-temperature superconductor material. The superconductor was conduction cooled by an integrated AEG tactical cryocooler. The temperature could be varied from 50 K to 80 K. The pressure in the vacuum chamber was varied from 1 bar to 10(exp -5) mbar. At the lowest pressure setting, the drag torque shows a linear frequency dependence over the entire range investigated (0 less than f less than 40 Hz). Magnetic friction, the frequency independent contribution, is very low. The frequency dependent drag torque is generated by molecular friction from molecule-surface collisions and by eddy currents. Given the specific geometry of the set-up and gas pressure, the molecular drag torque can be estimated. At a speed of 40 Hz, the coefficient of friction (drag-to-lift ratio) was measured to be mu = 1.6 x 10(exp -7) at 10(exp -5) mbar and T = 60 K. This is equivalent to a drag torque of 7.6 x 10(exp -10) Nm. Magnetic friction causes approx. 1% of the total losses. Molecular friction accounts for about 13% of the frequency dependent drag torque, the remaining 87% being due to eddy currents and losses from rotor unbalance. The specific energy loss is only 0.3% per hour.

Bornemann, Hans J.↗
NASA NTRS1972

Research and development for a ground-based hydrogen-maser system

The results of a joint experiment aimed primarily at the determination of the frequency of the H(1) hyperfine transition are reported. The transition frequency value for Cs-133 hyperfine transition is found. The result is the mean of two independent evaluations against the cesium reference, which differ by 0.002 Hz. The one-sigma uncertainty of the value nu sub H is also estimated to be 0.002 Hz. One evaluation is based on wall shift experiments at Harvard University; the other is a result of new wall shift measurement using many storage bulbs of different sizes at the National Bureau of Standards. The experimental procedures and the applied corrections are described. Results for the wall shift and for the frequency of hydrogen are compared with previously published values, and error limits of the experiments are discussed.

Source record↗
NASA NTRS2023

Statistical Study of the Non-Thermal Continuum Radiation Beaming Angle Measured By the High Frequency Receiver on Van Allen Probes-A

The nonthermal continuum (NTC) radiation beaming angle is computed over the entire Van Allen Probes-A mission when the spacecraft was in the dawn sector. The conditions in the dawn sector are favorable for the wave vector to lie near/in the spacecraft's spin plane allowing a favorable estimate of the beaming angle, and the dawn sector is also advantageous in that previous studies show NTC occurrence to peak in this sector. We found that scatter plots, over the entire mission, of beaming angle versus magnetic latitude form a distinct inverted-V pattern, with the apex at/near the magnetic equator. This pattern was sharpest for frequencies (f) ≲ 100 kHz. Using the NTC beaming formula from the linear mode conversion theory (LMCT), we show that such an inverted-V pattern is expected due to the large variation in the plasmapause location over the entire mission. The theoretical derived pattern qualitatively reproduces the observed pattern but not quantitatively. The lack of quantitative agreement is discussed and is attributed to several factors, one factor is off-centered emissions from the radio window. The qualitative agreement strongly supports LMCT as being the dominant mechanism generating NTC for f ≲ 100 kHz. For f ≳ 100 kHz, the inverted-V pattern becomes less distinct, and strong near-equatorial beaming is observed. After considering contamination of our selections by left-handed polarized AKR, our study suggests that besides LMCT another unidentified NTC generation mechanism becomes important for f ≳ 100 kHz.

Linear Mode Conversion Theory↗
NASA NTRS2018

ULF Wave-Associated Density Irregularities and Scintillation at the Equator

This paper presents independent multi-instrument observations that address the physical mechanisms of how ultralow-frequency (ULF) wave-associated electric fields initiate ionospheric density fluctuation and scintillation at the equator. Since the magnetic field at the equator is entirely embedded in a relatively high-collision and high-conductivity medium, the condition may not be possible for the geomagnetic field to fluctuate due to ULF wave activity. This implies that the fluctuating electric field at the equator may not be produced through equatorial dynamo action due to fluctuating magnetic fields. Instead, the electric field penetrates from high latitudes and produces fluctuating magnetic field as well as modulates the vertical drift and hence causes the density to fluctuate at the equatorial region. We demonstrate this by estimating the ULF associated fluctuating electric field at high latitudes and at the equatorial region by applying the appropriate attenuation factor as it penetrates to lower latitudes. The periodicity of both electric field and density fluctuations appears to be between 6 and 9 min, which is a typical period of ULF waves in the Pc5 range. Because of its large amplitude and long periods compared to other ULF wave frequency bands, the Pc5 wave-associated electric field, which can even be estimated using magnetograms with low sensitivity and low sampling rate (e.g., 1 min), can easily penetrate to the lower latitude region and produce significant ionospheric density fluctuations that can be strong enough to create scintillation at the equatorial region.

Yizengaw, E.↗