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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 325 records · Page 18
All sourcesNASA NTRSDOE OSTI
NASA NTRS1976

An experimental study of the structure and acoustic field of a jet in a cross stream

The plane of symmetry of a high speed circular jet was surveyed to measure the mean and turbulent velocity fields by using constant temperature hot wire anemometry. The intensity of the noise radiated from the jet was determined in the tunnel test section by utilizing the cross-correlation at a particular time delay between the signals of two microphones suitably located along a given direction. Experimental results indicate that the turbulent intensity inside the crossflow jet increases by a factor of (1 + 1/2) as compared to the turbulent intensity of the same jet under free conditions, with r indicating the ratio of the jet velocity by the cross stream velocity. The peak observed in the turbulence spectra obtained inside the potential core of the jet has a frequency that increases by the same factor with respect to the corresponding frequency measured in the case of the free jet. The noise radiated by the jet becomes more intense as the crossflow velocity increases. The measured acoustic intensity of the crossflow jet is higher than the value which would be expected from the increase of the turbulent intensity only.

Camelier, I.↗
NASA NTRS1994

The morningside low-latitude boundary layer as determined from electric and magnetic field measurements on Geotail

On October 17, 1992, the Geotail satellite crossed the dawnside magnetopause approximately 41 times. At the majority of these crossings, the magnetic field and the normal component of the electric field were larger and the plasma density was smaller in a low-latitude boundary layer than they were in either the nearby magnetosheath or in the magnetosphere. These results are intepreted in terms of the Kelvin-Helmholtz instability associated with velocity shear at the magnetopause. Consistent with this interpretation, it is shown that the low-latitude boundary layer was the region where the flow decreased from its magnetosheath to magnetospheric value. Evidence is presented that the magnetopause was locally oriented within less than 20 deg of its nominal geometry on these crossings and that it moved with an amplitude of 1500-6000 kilometers. The thickness of the low-latitude boundary layer averaged 800 kilometers and the electric potential across it averaged 4 kilovolts with a spread of a factor of 2. Thus, the viscous interaction and similar processes are not significant contributors to magnetospheric convection.

Mozer, F. S.↗
NASA NTRS1990

A planar near-field scanning technique for bistatic radar cross section measurements

A progress report on the development of a bistatic radar cross section (RCS) measurement range is presented. A technique using one parabolic reflector and a planar scanning probe antenna is analyzed. The field pattern in the test zone is computed using a spatial array of signal sources. It achieved an illumination pattern with 1 dB amplitude and 15 degree phase ripple over the target zone. The required scan plane size is found to be proportional to the size of the desired test target. Scan plane probe sample spacing can be increased beyond the Nyquist lambda/2 limit permitting constant probe sample spacing over a range of frequencies.

Tuhela-Reuning, S.↗
NASA NTRS2014

The FIELDS Instrument Suite on MMS: Scientific Objectives, Measurements, and Data Products

The FIELDS instrumentation suite on the Magnetospheric Multiscale (MMS) mission provides comprehensive measurements of the full vector magnetic and electric fields in the reconnection regions investigated by MMS, including the dayside magnetopause and the night-side magnetotail acceleration regions out to 25 Re. Six sensors on each of the four MMS spacecraft provide overlapping measurements of these fields with sensitive cross-calibrations both before and after launch. The FIELDS magnetic sensors consist of redundant flux-gate magnetometers (AFG and DFG) over the frequency range from DC to 64 Hz, a search coil magnetometer (SCM) providing AC measurements over the full whistler mode spectrum expected to be seen on MMS, and an Electron Drift Instrument (EDI) that calibrates offsets for the magnetometers. The FIELDS three-axis electric field measurements are provided by two sets of biased double-probe sensors (SDP and ADP) operating in a highly symmetric spacecraft environment to reduce significantly electrostatic errors. These sensors are complemented with the EDI electric measurements that are free from all local spacecraft perturbations. Cross-calibrated vector electric field measurements are thus produced from DC to 100 kHz, well beyond the upper hybrid resonance whose frequency provides an accurate determination of the local electron density. Due to its very large geometric factor, EDI also provides very high time resolution (∼1 ms) ambient electron flux measurements at a few selected energies near 1 keV. This paper provides an overview of the FIELDS suite, its science objectives and measurement requirements, and its performance as verified in calibration and cross-calibration procedures that result in anticipated errors less than 0.1 nT in B and 0.5 mV/m in E. Summaries of data products that result from FIELDS are also described, as well as algorithms for cross-calibration. Details of the design and performance characteristics of AFG/DFG, SCM, ADP, SDP, and EDI are provided in five companion papers.

Electromagnetic field measurements↗
NASA NTRS1989

Trochoidal analysis of scattered electrons in a merged electron-ion beam geometry

The method and apparatus of this invention provides a plurality of measurements indicative of the absolute cross section for excitation of an ion beam. The ion beam is merged for excitation by specific energies of electrons in an electron beam. Both beams are merged in an evacuated enclosure having a longitudinal magnetic field and a crossed uniform electric field. The ions and electrons interact over a known merged longitudinal length in a merged beam area. After collision, the electron and ion beams are demerged. Forward and backward-scattered electrons are collected and position-detected by a pair of microchannel plate arrays located at opposite ends of said longitudinal beam-merging area. A series of electron and ion primary current measurements are taken at full ion and electron beam strength. Measurements are also taken at greatly reduced beam strength to obtain a beam overlap profile.

Ara Chutjian↗
NASA NTRS1992

System III variations in apparent distance of Io plasma torus from Jupiter

System III variations in apparent distance of the Io plasma torus from Jupiter are examined on the basis of data obtained from UVS scans across Jupiter's satellite system. The displacement of the dawn and dusk ansae are found to be unexpectedly complex. The displacements are unequal and both ansae are in motion with the motion of the approaching ansa being the lesser of the two. The radial motions, as measured from either the center of Jupiter or the offset-tilted dipole, are of unequal magnitude and have the System III periodicity. It is concluded that the cross-tail electric field that causes these torus motions is concentrated on the dusk ansa, varied with the System III period, and shows magnetic-anomaly phase control. It is found that the dawn-dust asymmetry in brightness is not explained simply by the cross-tail electric field. It is concluded that there is a heating mechanism that causes the dusk side of the Io plasma torus to be brighter than the dawn side.

Dessler, A. J.↗
NASA NTRS1975

An experimental study of the structure and acoustic field of a jet in a cross stream

The investigation reported is related to studies of the noise generated by a lifting jet in forward motion such as in the case of a V/STOL aircraft. The investigation involved experiments in a subsonic wind tunnel. The two-microphone cross-correlation method used for measuring the acoustic intensity provides data concerning the noise directly radiated from the source, practically free from any reflection effects. The theoretical basis of the two-microphone method is discussed along with the experimental apparatus, the characteristics of the flow inside the jet, and the features of the acoustic field.

Camelier, I.↗
NASA NTRS1974

Cross-correlation methods for studying near- and far-field noise characteristics of several flow-surface interaction problems

Systematic methods based on cross-correlation techniques are presented for experimental studies of near- and far-field noise characteristics of airflow-surface interaction problems. By cross-correlating any two of three-kinds of microphone measurements: one flush-mounted on the surface, one located in the near field, and one placed in the far field, far-field noise intensities and near-field acoustic energy fluxes can be determined. In both near and far fields, the noise characteristics due to the flow turbulence and surface fluctuating pressure contributions can be evaluated separately. Specific applications to the noise field generated by impinging jets, by surface blowing jets, and by turbulent flow over trailing edges are discussed.

Pan, Y. S.↗
NASA NTRS2001

Electron Dynamics in the Magnetotail

The goal of this research has been to study the effects of electrons on magnetotail dynamics and current sheet structure. The approach is to follow ion trajectories in a global model of the magnetotail, use a Boltzmann approximation to include electrons, and then to update the field model according to the currents that are generated by the cross-tail electric field and/or induced fields. Parallel (and perpendicular) electric fields that form are included through the Boltzmann relation. Transverse electron currents are to be included through adiabatic drift equations.

Schriver, David↗
NASA NTRS2014

Reconstruction of a Broadband Spectrum of Alfvenic Fluctuations

Alfvenic fluctuations in the solar wind exhibit a high degree of velocities and magnetic field correlations consistent with Alfven waves propagating away and toward the Sun. Two remarkable properties of these fluctuations are the tendencies to have either positive or negative magnetic helicity (-1 less than or equal to sigma(sub m) less than or equal to +1) associated with either left- or right- topological handedness of the fluctuations and to have a constant magnetic field magnitude. This paper provides, for the first time, a theoretical framework for reconstructing both the magnetic and velocity field fluctuations with a divergence-free magnetic field, with any specified power spectral index and normalized magnetic- and cross-helicity spectrum field fluctuations for any plasma species. The spectrum is constructed in the Fourier domain by imposing two conditions-a divergence-free magnetic field and the preservation of the sense of magnetic helicity in both spaces-as well as using Parseval's theorem for the conservation of energy between configuration and Fourier spaces. Applications to the one-dimensional spatial Alfvenic propagation are presented. The theoretical construction is in agreement with typical time series and power spectra properties observed in the solar wind. The theoretical ideas presented in this spectral reconstruction provide a foundation for more realistic simulations of plasma waves, solar wind turbulence, and the propagation of energetic particles in such fluctuating fields.

waves↗
NASA NTRS2016

The Effects of Magnetic-Field Geometry on Longitudinal Oscillations of Solar Prominences: Cross-Sectional Area Variation for Thin Tubes

Solar prominences are subject to both field-aligned (longitudinal) and transverse oscillatory motions, as evidenced by an increasing number of observations. Large-amplitude longitudinal motions provide valuable information on the geometry of the filament channel magnetic structure that supports the cool prominence plasma against gravity. Our pendulum model, in which the restoring force is the gravity projected along the dipped field lines of the magnetic structure, best explains these oscillations. However, several factors can influence the longitudinal oscillations, potentially invalidating the pendulum model. Aims. The aim of this work is to study the influence of large-scale variations in the magnetic field strength along the field lines, i.e., variations of the cross-sectional area along the flux tubes supporting prominence threads. Methods. We studied the normal modes of several flux tube configurations, using linear perturbation analysis, to assess the influence of different geometrical parameters on the oscillation properties. Results. We found that the influence of the symmetric and asymmetric expansion factors on longitudinal oscillations is small.Conclusions. We conclude that the longitudinal oscillations are not significantly influenced by variations of the cross-section of the flux tubes, validating the pendulum model in this context.

Luna, M.↗
NASA NTRS1977

Reactions between NO/+/ and metal atoms using magnetically confined afterglows

A new method of studying thermal energy ion-neutral collision processes involving nongaseous neutral atoms is described. A long magnetic field produced by a solenoid in a vacuum chamber confines a thermal-energy plasma generated by photoionization of gas at very low pressure. As the plasma moves toward the end of the field, it is crossed by a metal atom beam. Ionic products of ion-atom reactions are trapped by the field and both the reactant and product ions move to the end of the magnetic field where they are detected by a quadrupole mass filter. The cross sections for charge transfer between NO(+) and Na, Mg, Ca, and Sr and that for rearrangement between NO(+) and Ca have been obtained. The charge-transfer reaction is found strongly dominant over the rearrangement reaction that forms metallic oxide ions.

Lo, H. H.↗
NASA NTRS1974

Photon opacity in surfaces of magnetic neutron stars

Approximate expressions are derived for free-free, bound-free, and Thomson cross-sections of photons by gaseous matter in the presence of superstrong magnetic fields. For photons in modes whose electric field polarization is perpendicular to this magnetic field, the cross-section is reduced by approximately the squared ratio of the photon frequency to the electron cyclotron frequency if this ratio is small.

Lodenquai, J.↗
NASA NTRS1981

Auroral arc electrodynamic parameters measured by AE-C and the Chatanika radar

Auroral arc electrodynamic parameters are studied using coordinated measurements between the AE-C satellite and the Chatanika radar. On January 4, 1978, the spacecraft twice crossed an east-west aligned auroral arc at widely spaced longitudes, spanning more than 3 hours in local time. The Chatanika radar was scanning in elevation at a magnetic longitude equidistant from the two AE-C crossings. The electric field pattern around this arc was remarkably similar at the three longitudes. Equatorward of the arc the north-south field was very intense (greater than approximately 60 mV/m), whereas poleward of the arc it was small (approximately 10 mV/m). The east-west field was small and almost constant across the arc. The same arc was observed by the radar for about 1 hour, and this electric field pattern did not change, even though the arc location, intensity, and width changed substantially. For a given electric field the AE-C measured ion temperature was substantially different during each of the arc crossings. This dissimilarity is attributed to differences in the F region neutral wind at the two longitudes. It is shown that the electric field variations are consistent with the existence of polarization charges within the arc, even though the electric field remained small poleward of the arc.

Beaujardiere, O. D. L.↗
NASA NTRS1976

A new look at decomposition of turbulence forcing field and the structural response

Measured cross-spectrum of a turbulence field usually shows some decay in the statistical correlation in addition to convection at a characteristic velocity. It is shown that a decaying turbulence can be decomposed into frozen-pattern components thus permitting a simpler way to calculate the structural response. This procedure also provides a relationship whereby the measured input spectra can be incorporated. The theory is applied to an infinite beam which is backed on one side by a fluid filled cavity and is exposed on the other side by the turbulence excitation. The effect of the free stream velocity is also taken into consideration.

Lin, Y. K.↗
NASA NTRS1983

A comparison of coronal and interplanetary current sheet inclinations

The HAO white light K-coronameter observations show that the inclination of the heliospheric current sheet at the base of the corona can be both large (nearly vertical with respect to the solar equator) or small during Cararington rotations 1660 - 1666 and even on a single solar rotation. Voyager 1 and 2 magnetic field observations of crossing of the heliospheric current sheet at distances from the Sun of 1.4 and 2.8 AU. Two cases are considered, one in which the corresponding coronameter data indicate a nearly vertical (north-south) current sheet and another in which a nearly horizontal, near equatorial current sheet is indicated. For the crossings of the vertical current sheet, a variance analysis based on hour averages of the magnetic field data gave a minimum variance direction consistent with a steep inclination. The horizontal current sheet was observed by Voyager as a region of mixed polarity and low speeds lasting several days, consistent with multiple crossings of a horizontal but irregular and fluctuating current sheet at 1.4 AU. However, variance analysis of individual current sheet crossings in this interval using 1.92 see averages did not give minimum variance directions consistent with a horizontal current sheet.

Behannon, K. W.↗