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At least 469 records · Page 26
All sourcesNASA NTRSDOE OSTI
NASA NTRS2004

Further Progress in Noise Source Identification in High Speed Jets via Causality Principle

To locate noise sources in high-speed jets, the sound pressure fluctuations p/, measured at far field locations, were correlated with each of density p, axial velocity u, radial velocity v, puu and pvv fluctuations measured from various points in fully expanded, unheated plumes of Mach number 0.95, 1.4 and 1.8. The velocity and density fluctuations were measured simultaneously using a recently developed, non-intrusive, point measurement technique based on molecular Rayleigh scattering (Seasholtz, Panda, and Elam, AIAA Paper 2002-0827). The technique uses a continuous wave, narrow line-width laser, Fabry-Perot interferometer and photon counting electronics. The far field sound pressure fluctuations at 30 to the jet axis provided the highest correlation coefficients with all flow variables. The correlation coefficients decreased sharply with increased microphone polar angle, and beyond about 60 all correlation mostly fell below the experimental noise floor. Among all correlations < puu; p/> showed the highest values. Interestingly, , in all respects, were very similar to . The and correlations with 90deg microphone fell below the noise floor. By moving the laser probe at various locations in the jet it was found that the strongest noise source lies downstream of the end of the potential core and extends many diameters beyond. Correlation measurement from the lip shear layer showed a Mach number dependency. While significant correlations were measured in Mach 1.8 jet, values were mostly below the noise floor for subsonic Mach 0.95 jet. Various additional analyses showed that fluctuations from large organized structures mostly contributed to the measured correlation, while that from small scale structures fell below the noise floor.

Panda, J.↗
NASA NTRS2010

Locating and Quantifying Broadband Fan Sources Using In-Duct Microphones

In-duct beamforming techniques have been developed for locating broadband noise sources on a low-speed fan and quantifying the acoustic power in the inlet and aft fan ducts. The NASA Glenn Research Center's Advanced Noise Control Fan was used as a test bed. Several of the blades were modified to provide a broadband source to evaluate the efficacy of the in-duct beamforming technique. Phased arrays consisting of rings and line arrays of microphones were employed. For the imaging, the data were mathematically resampled in the frame of reference of the rotating fan. For both the imaging and power measurement steps, array steering vectors were computed using annular duct modal expansions, selected subsets of the cross spectral matrix elements were used, and the DAMAS and CLEAN-SC deconvolution algorithms were applied.

Dougherty, Robert P.↗
NASA NTRS1998

Telemetry

The purpose of a telemetry system is to reliably and transparently convey measurement information from a remotely located data generating source to users located in space or on Earth. Typically, data generators are scientific sensors, science housekeeping sensors, engineering sensors and other subsystems on-board a spacecraft.

telemetry system measurement information data gene↗
NASA NTRS1982

Observations of continuum X-ray and gamma-ray emission from the galactic center

A review of observations of the galactic nucleus to date in the X ray and gamma ray regions is presented. Attention is given to data in the 1-10 keV, 10-100 keV, 100 keV to 10 MeV, and 10 MeV to 1 GeV ranges. It is shown that the galactic nucleus is as yet unresolvable from the other galactic center at energies above 4.5 keV, while large variations in the observed flux in the 100 keV-1 MeV range indicate a single source as the dominant emitter. An X ray source is concluded to be contained in the nucleus, which features a constant luminosity of 1.5 x 10 to the 35th erg/sec in the 0.5-4.5 keV band. A hard and bright source is located near the nucleus, whose emitter source exists at a temperature of about 10 billion K with a luminosity 30 times that of the Crab Nebula.

Matteson, J. L.↗
NASA NTRS1994

Galactic 0.511 MeV line emission

Observations over two decades with various balloon-borne instruments and instruments on HEAO 3, SMM, and Compton Gamma Ray Observatory (CGRO) have revealed the existence of strong 0.511 MeV line emission from the direction of the Galactic center. We have attempted to fit the spatial distribution of this emisssion with distributions of various Galactic constituents, but found that none of these distributions alone could fit the data. Of the assumed distributions, those of Galactic novae and hot (10(exp 8) K) plasma are the most strongly peaked at the Galactic center. But we found that the 0.511 MeV line emission is even more strongly peaked. This new result is the direct consequence of the analysis of recent data obtained with the OSSE instrument on CGRO. We have thus considered two-component models consisting of an enhanced central Galactic component superposed on a Galactic plane component given by any one of our previously assumend distributions. For the central Galactic component we take either a single point source at the location of the source of annihilation radiation 1E 1740.7-2942 or an extended spheroidal distribution of several hundred parsecs in size. We find acceptable fits in both cases. We suggest that this additional component of the emission from the Galactic center region is due to positrons produced near one or more black holes. Such a component was also suggested by the variability of the 0.511 MeV line flux observed with balloon-borne high-resolution detectors. To produce the very narrow line observed by these high-resolution detectors at almost precisely 0.511 MeV the positrons must escape from the vicinity of the holes and annihilate in the surrounding interstellar medium. We have reexamined the issure of the time variability of the 0.511 MeV line emission. Although the significance of the variability implied by all of the earlier observations is diminished by a large number of recent OSSE observations which show no variability, the latter measurements merely indicate that the flux remained relatively constant only over a period of about 1 year (1991-1992). Positron producation in the Galactic center region by a small number of variable sources would lead to variable 0.511 MeV line emission. The timescale of the variation depends on the density of the gas in which the positrons annihilate. It could be as short as several months if the annihilation is in dense moleduar clouds, but would be much longer and essentially undetectable if the positrons annihilate in lower density interstellar gas.

Ramaty, R.↗
NASA NTRS1974

Scorpius X-1 - Origin of the radio and hard X-ray emissions

The consequences of models for the central radio source and the hard X-ray emitting region in Sco X-1 are examined. The radio emission could result from noncoherent synchrotron radiation, and the X-rays may be produced by bremsstrahlung. It is shown that both these mechanisms require a mass outflow from Sci X-1. The radio source is located at about 30 million km from the center of the star, and its linear dimensions do not exceed 300 million km. The magnetic field in the radio source is on the order of 1 gauss. If the hard X-rays are produced by thermal bremsstrahlung, their source is located between 10,000 and 50,000 km from the center of the star, the temperature is 2 billion K, and the emission measure is 2 times 10 to the 56th power per cu cm. This hot plasma loses energy inward by conduction and outward by supersonic expansion. The rates of energy loss for both of these processes are about 10 to the 36th ergs per sec, comparable to the total luminosity of Sco X-1.

Ramaty, R.↗
NASA NTRS1973

SCO X-1: Origin of the radio and hard X-ray emissions

The consequences of models for the central radio source and the hard X-ray ( 30 keV) emitting region in Sco X-1 are examined. It was found that the radio emission could result from noncoherent synchrotron radiation and that the X-rays may be produced by bremsstrahlung. It is shown that both mechanisms require a mass outflow from Sco X-1. The radio source is located at r approximately 3x10 to the 12th power cm from the center of the star, and its linear dimensions do not exceed 3x10 to the 13th power cm. The magnetic field in the radio source is on the order of 1 gauss. If the hard X-rays are produced by thermal bremsstrahlung, their source is located at 10 to the 9th power approximately r approximately 5x10 to the 9th power cm, the temperature is 2x10 to the 9th power K, and the emission measure is 2x10 to the 56th power/cu cm. This hot plasma loses energy inward by conduction and outward by supersonic expansion. The rates of energy loss for both processes are about 10 to the 36th power erg/s, comparable to the total luminosity of Sco X-1.

Ramaty, R.↗
NASA NTRS1987

Constraints on the mass-radius relation for the neutron star in the X-ray burst source 4U/MXB 1820-30 located in the globular cluster NGC 6624

From the spectral evolution of seven X-ray bursts observed from 4U/MXB 1820-30 (Haberl et al., 1986) during the phases of photospheric expansion and cooling, constraints are derived on the mass-radius relation of the neutron star in this system. Taking into account the uncertainty in the source distance and the possible anisotropy of the burst emission. If during bursts with radius expansion the outer layers of the neutron star are of a hydrogen-poor composition, none of the current equations of state of high-density matter is excluded. For a hydrogen-rich composition, only very soft equations of state are allowed.

Van Paradijs, J.↗
NASA NTRS1983

TRISCAN Antenna-Positioning Algorithm

TRISCAN scanning algorithm improves alinement between boresight of radar antenna and target. Algorithm originally developed for digitally pointed antennas in deep-space network; however, is possibly used in locating radio sources on Earth. TRISCAN estimates coordinate errors in hour angle and declination between predicted and true locations of radio source.

Bunce, R. C.↗
NASA NTRS1992

Spectral infrared hemispherical reflectance measurements for LDEF tray clamps

This paper describes infrared hemispherical reflectance measurements (2-15 microns) that were made on 58 chromic acid anodized tray clamps retrieved from the LDEF spacecraft. These clamps were used for maintaining the experiments in place and were located at various locations about the spacecraft. Changes in reflectance of the tray clamps at these locations were compared with atomic oxygen fluxes at the same locations. A decrease in absorption band depth was seen for the surfaces exposed to space indicating that there was some surface layer erosion. In all of the surfaces measured, little evidence of contamination was observed and none of the samples showed evidence of the brown nicotine stain that was so prominent in other experiments. Total emissivity values were calculated for both exposed and unexposed tray clamp surfaces. Only small differences, usually less than 1 percent, were observed. The spectral reflectances were measured using a hemi-ellipsoidal mirror reflectometer matched with an interferometer spectrometer. The rapid scanning capability of the interferometer allowed the reflectance measurements to be made in a timely fashion. The ellipsoidal mirror has its two foci separated by 2 inches and located on the major axis. A blackbody source was located at one focus while the tray clamp samples were located at the conjugate focus. The blackbody radiation was modulated and then focused by the ellipsoid onto the tray clamps. Radiation reflected from the tray clamp was sampled by the interferometer by viewing through a hole in the ellipsoid. A gold mirror (reflectance approximately 98 percent) was used as the reference surface.

Wood, Bobby E.↗
NASA NTRS2003

North Alabama Lightning Mapping Array (LMA): VHF Source Retrieval Algorithm and Error Analyses

Two approaches are used to characterize how accurately the North Alabama Lightning Mapping Array (LMA) is able to locate lightning VHF sources in space and in time. The first method uses a Monte Carlo computer simulation to estimate source retrieval errors. The simulation applies a VHF source retrieval algorithm that was recently developed at the NASA Marshall Space Flight Center (MSFC) and that is similar, but not identical to, the standard New Mexico Tech retrieval algorithm. The second method uses a purely theoretical technique (i.e., chi-squared Curvature Matrix Theory) to estimate retrieval errors. Both methods assume that the LMA system has an overall rms timing error of 50 ns, but all other possible errors (e.g., multiple sources per retrieval attempt) are neglected. The detailed spatial distributions of retrieval errors are provided. Given that the two methods are completely independent of one another, it is shown that they provide remarkably similar results. However, for many source locations, the Curvature Matrix Theory produces larger altitude error estimates than the (more realistic) Monte Carlo simulation.

Koshak, W. J.↗
NASA NTRS2012

Interpretation of the Total Magnetic Field Anomalies Measured by the CHAMP Satellite Over a Part of Europe and the Pannonian Basin

In this study we interpret the magnetic anomalies at satellite altitude over a part of Europe and the Pannonian Basin. These anomalies are derived from the total magnetic measurements from the CHAMP satellite. The anomalies reduced to an elevation of 324 km. An inversion method is used to interpret the total magnetic anomalies over the Pannonian Basin. A three dimensional triangular model is used in the inversion. Two parameter distributions: Laplacian and Gaussian are investigated. The regularized inversion is numerically calculated with the Simplex and Simulated Annealing methods and the anomalous source is located in the upper crust. A probable source of the magnetization is due to the exsolution of the hematite-ilmenite minerals.

Kis, K. I.↗
NASA NTRS1984

Monopole excitation of vibrations in an infinite cylindrical elastic shell filled with fluid

The vibrations of a cylindrical elastic shell filled with fluid and excited by a monopole acoustic source are studied. The modal mobilities of the shell wall at the source plane are examined for different radial locations of the source. The distribution of vibrational energy between the shell and the fluid field is calculated at various axial locations along the shell and for different radial source locations. An interchange of energy of vibration between the shell and the fluid as the wave field propagates along the shell-fluid system is observed.

Fuller, C. R.↗
NASA NTRSDate not supplied

Supersonic Flight Testing to Assess Ground Recording Systems for NASA's Low Boom Flight Demonstrator

The National Aeronautics and Space Administration (NASA) has performed two supersonic flight test campaigns in preparation for the extensive ground measurements that will be taken during Quesst Phase 2 to verify the low boom from the X-59. The first test campaign was Carpet Determination In Entirety Measurements (CarpetDIEM) Phase II, with F/A-18B aircraft and a subscale ground sensor array and the second campaign was CarpetDIEM Phase III,with both F-15B and F/A-18D aircraft and a geographically large scale ground sensor array. CarpetDIEM Phase II served to evaluate prototypes of the state-of-the-art Ground Recording System (GRS) which NASA is developing in order to record the low boom of X-59. The prototype GRS measured both N-wave and evanescent wave sonic booms which were1.4±1.1PLdB from co-located truth sources during these flights. CarpetDIEM Phase II was also used to down select two NASA developed unattended trigger concepts, one based on Automatic Dependent Surveillance-Broadcast (ADS-B) and the other utilizing satellite communications. Both unattended trigger concepts performed well, with the ADS-B based concept being chosen at the conclusion of CarpetDIEM Phase II for integration with the GRS. CarpetDIEM Phase III was primarily a logistics risk-reduction effort for Quesst Phase 2, but also sought to evaluate the capabilities, including the newly implemented ADS-B unattended trigger, and ruggedness of the GRS during a long-term deployment. The F-15B stood in for the X-59, flying two different X-59flight profiles, including an accelerating climb maneuver. The GRS successfully self-triggered to capture 80% of sonic booms generated from both steady and dynamic flight maneuvers. Low booms, defined by the project as being≤80 PLdB, were recorded by the GRS near the end of CarpetDIEM Phase III. The GRS measured low booms were found to be within0.4±0.1PLdB of a co-located truth source.

Forrest L Carpenter IV↗
NASA NTRS1984

Methane concentrations and source strengths in urban locations

The results of a six year measurement of trace gases in 18 different urban and remote areas throughout the Northern and Southern Hemispheres are reported. It is found that the measurements are consistent with a yearly atmospheric release of 30 to 60 megatons of CH4 in urban environments. Concentrations of CCl3F, CCl2F2, and CH3CCl3 were also found to be higher in the same urban samples than in corresponding samples from remote areas. The 'urban excess' (urban concentration minus remote concentration) was generally 1000 to 2000 times larger on a molar basis for CH4 than for CCl3F. The fraction of world-wide methane release occurring in the urban environment is estimated from the concentration ratios for CH4 vs CCl3F. It is shown that about 8 to 15 percent of atmospheric methane release occurs in urban areas.

Blake, D. R.↗
NASA NTRS2002

Measurement of Correlation Between Flow Density, Velocity, and Density*velocity(sup 2) with Far Field Noise in High Speed Jets

To locate noise sources in high-speed jets, the sound pressure fluctuations p', measured at far field locations, were correlated with each of radial velocity v, density rho, and phov(exp 2) fluctuations measured from various points in jet plumes. The experiments follow the cause-and-effect method of sound source identification, where correlation is related to the first, and correlation to the second source terms of Lighthill's equation. Three fully expanded, unheated plumes of Mach number 0.95, 1.4 and 1.8 were studied for this purpose. The velocity and density fluctuations were measured simultaneously using a recently developed, non-intrusive, point measurement technique based on molecular Rayleigh scattering. It was observed that along the jet centerline the density fluctuation spectra S(sub rho) have different shapes than the radial velocity spectra S(sub v), while data obtained from the peripheral shear layer show similarity between the two spectra. Density fluctuations in the jet showed significantly higher correlation, than either rhov(sub 2) or v fluctuations. It is found that a single point correlation from the peak sound emitting region at the end of the potential core can account for nearly 10% of all noise at 30 to the jet axis. The correlation, representing the effectiveness of a longitudinal quadrupole in generating noise 90 to the jet axis, is found to be zero within experimental uncertainty. In contrast rhov(exp 2) fluctuations were better correlated with sound pressure fluctuation at the 30 location. The strongest source of sound is found to lie at the centerline and beyond the end of potential core.

Panda, Jayanta↗
NASA NTRS1996

Active Control of Fan Noise: Feasibility Study: Theoretical Analysis for Coupling of Active Noise Control Actuator Ring Sources to an Annular Duct with Flow - Volume 6

The objective of this effort is to develop an analytical model for the coupling of active noise control (ANC) piston-type actuators that are mounted flush to the inner and outer walls of an annular duct to the modes in the duct generated by the actuator motion. The analysis will be used to couple the ANC actuators to the modal analysis propagation computer program for the annular duct, to predict the effects of active suppression of fan-generated engine noise sources. This combined program will then be available to assist in the design or evaluation of ANC systems in fan engine annular exhaust ducts. An analysis has been developed to predict the modes generated in an annular duct due to the coupling of flush-mounted ring actuators on the inner and outer walls of the duct. The analysis has been combined with a previous analysis for the coupling of modes to a cylindrical duct in a FORTRAN computer program to perform the computations. The method includes the effects of uniform mean flow in the duct. The program can be used for design or evaluation purposes for active noise control hardware for turbofan engines. Predictions for some sample cases modeled after the geometry of the NASA Lewis ANC Fan indicate very efficient coupling in both the inlet and exhaust ducts for the m = 6 spinning mode at frequencies where only a single radial mode is cut-on. Radial mode content in higher order cut-off modes at the source plane and the required actuator displacement amplitude to achieve 110 dB SPL levels in the desired mode were predicted. Equivalent cases with and without flow were examined for the cylindrical and annular geometry, and little difference was found for a duct flow Mach number of 0.1. The actuator ring coupling program will be adapted as a subroutine to the cylindrical duct modal analysis and the exhaust duct modal analysis. This will allow the fan source to be defined in terms of characteristic modes at the fan source plane and predict the propagation to the arbitrarily-located ANC source plane. The actuator velocities can then be determined to generate the anti-phase mode. The resulting combined fan source/ANC pressure can then be calculated at any desired wall sensor position. The actuator velocities can be determined manually or using a simulation of a control system feedback loop. This will provide a very useful ANC system design and evaluation tool.

Kraft, R. E.↗
NASA NTRS1996

Characteristics of hard X-ray double sources in impulsive solar flares

Imaging observations of solar flare hard X-ray sources with the Hard X-ray Telescope (HXT) aboard the Yohkoh satellite have revealed that hard X-ray emissions (greater than 30 ke V) originate most frequently from double sources. The double sources are located on both sides of the magnetic neutral line, suggesting that the bulk of hard X-rays is emitted from footpoints of flaring magnetic loops. We also found that hard X-rays from the double sources are emitted simultaneously within a fraction of second and that the weaker source tends to be located in the stronger magnetic field region, showing a softer spectrum. Physcial implications on the observed characteristics of the hard X-ray double sources are discussed.

Sakao, T.↗