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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 433 records · Page 24
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NASA NTRS1977

The vibrational spectrum and lattice dynamics of polycrystalline ammonium hydrosulfide

The infrared spectra of alkali metal and ammonium hydrosulfides have been recorded from wavenumbers of 200 to 4000 at liquid nitrogen temperatures, and the Raman spectra of these substances have been recorded from wavenumbers of 0 to 4000 over the temperature range 83-390 K. No evidence of a second solid phase was obtained. Internal and external fundamentals were assigned, in detail, based on selection rules, isotopic frequency shifts, and analogy with structurally similar salts. Barriers to anion and cation reorientation of 3.8 and 1.9 kcal/mole, respectively, have been calculated from librational assignments. The implications of the infrared spectrum of ammonium hydrosulfide for the possible spectroscopic detection of this substance in the atmosphere of the planet Jupiter are discussed.

Bragin, J.↗
NASA NTRS1977

Lower hybrid emission diagnostics on the NASA Lewis Bumpy Torus

The feasibility of using RF emission near the lower-hybrid frequency of the NASA Lewis Bumpy Torus plasma for diagnostic purposes is examined. The emission is detected using a spectrum analyzer and a 50-ohm miniature coaxial antenna that is sensitive to the polarization of the incoming signal. The frequency shift of the lower-hybrid emission peak is monitored as a function of the background pressure, electrode voltage, electrode ring configuration, and the strength of the toroidal dc magnetic field. Simultaneous measurements of the average plasma density are made with a polarization diplexing microwave interferometer. The experimental results extend previous work to include negative electrode voltages and plasma densities up to 1 trillion per cu cm. The information derived from the experiment is discussed with reference to: (1) the strength of the dc magnetic field in the emitting region, (2) a comparison of the lower-hybrid plasma density with the average plasma density, and (3) the validity of the cold-plasma lower-hybrid resonance formula in the high-density operating regime of the bumpy-torus plasma.

Mallavarpu, R.↗
NASA NTRS1979

Coherent anti-Stokes Raman spectroscopy - Spectra of water vapor in flames

The results of experimental measurements of the coherent anti-Stokes Raman spectra of water vapor in flames are reported. A pulsed, frequency-doubled neodymium laser was used to supply the pump beam and to pump a dye laser to provide a broadband Stokes beam at 6600 A. Spectra were obtained in the postflame region of a premixed methane-air flame in the Raman frequency shift region of the symmetric stretch mode (3651.7 kaysers) at an approximate temperature of 1675 K. A theoretical calculation of the coherent anti-Stokes Raman spectrum of water vapor at this temperature was made, taking into account only isotropic Q-branch transitions, and using the energy level data of Floud et al. (1976). The theoretical prediction is shown essentially to reproduce all qualitative features of the experimental spectrum, and to exhibit a strong temperature dependence.

Hall, R. J.↗
NASA NTRS1979

A method for predicting the noise levels of coannular jets with inverted velocity profiles

A coannular jet was equated with a single stream equivalent jet with the same mass flow, energy, and thrust. The acoustic characteristics of the coannular jet were then related to the acoustic characteristics of the single jet. Forward flight effects were included by incorporating a forward exponent, a Doppler amplification factor, and a Strouhal frequency shift. Model test data, including 48 static cases and 22 wind tunnel cases, were used to evaluate the prediction method. For the static cases and the low forward velocity wind tunnel cases, the spectral mean square pressure correlation coefficients were generally greater than 90 percent, and the spectral sound pressure level standard deviation were generally less than 3 decibels. The correlation coefficient and the standard deviation were not affected by changes in equivalent jet velocity. Limitations of the prediction method are also presented.

Russell, J. W.↗
NASA NTRS1979

Laser Doppler velocity simulator

A method and apparatus for inducing a Doppler frequency shift in a reference beam laser velocimeter light beam to simulate target velocity are described. The light beam is passed through a rotating refractive transparent block positioned between two reference points along the beam which results in a continuous change in the light beam optical path length between the reference points according to a known function. The velocity indicated by the laser velocimeter is compared to the known simulated velocity function for velocimeter testing and calibration.

Franke, J. M.↗
NASA NTRS1980

Prediction of unsuppressed jet engine exhaust noise in flight from static data

A methodology developed for predicting in-flight exhaust noise from static data is presented and compared with experimental data for several unsuppressed turbojet engines. For each engine, static data over a range of jet velocities are compared with the predicted jet mixing noise and shock-cell noise. The static engine noise over and above the jet and shock noises is identified as excess noise. The excess noise data are then empirically correlated to smooth the spectral and directivity relations and account for variations in test conditions. This excess noise is then projected to flight based on the assumption that the only effects of flight are a Doppler frequency shift and a level change given by 40 log (1 - m sub 0 cos theta), where M sub 0 is the flight Mach number and theta is the observer angle relative to the jet axis.

Stone, J. R.↗
NASA NTRS1980

Heterodyne signal-to-noise ratios in acoustic mode scattering experiments

The relation between the signal to noise ratio (SNR) obtained in heterodyne detection of radiation scattered from acoustic modes in crystalline solids and the scattered spectral density function is studied. It is shown that in addition to the information provided by the measured frequency shifts and line widths, measurement of the SNR provides a determination of the absolute elasto-optical (Pockel's) constants. Examples are given for cubic crystals, and acceptable SNR values are obtained for scattering from thermally excited phonons at 10.6 microns, with no external perturbation of the sample necessary. The results indicate the special advantages of the method for the study of semiconductors.

Cochran, W. R.↗
NASA NTRS1979

Particle size and velocity measurement in flames by laser anemometer

Simultaneous droplet size and velocity measurements by a particle counting Laser Doppler Anemometer (LDA) in kerosene fuel sprays under burning and non-burning conditions are presented. Particle sizes are derived from pulse height analysis of the mean LDA signals and velocities are simultaneously determined by measuring Doppler shift frequencies. The measurements show that droplet velocity is a function of droplet diameter for burning and non-burning conditions, and spatially averaged size distributions are derived from velocity data. A comparison of results obtained under burning and non-burning conditions show changes in size distribution due to preferential vaporization of small droplets, acceleration due to thermal expansion of gases, and corresponding changes in droplet momentum.

Chigier, N. A.↗
NASA NTRS1980

On the detection of a stochastic background of gravitational radiation by the Doppler tracking of spacecraft

Consideration is given to the possibility of detection of an isotropic background gravitational radiation of a stochastic nature by the method of Doppler tracking of spacecraft. Attention is given in the geometrical optics limit, to the general formula for the frequency shift of an electromagnetic signal in the gravitational radiation field, and it is shown to be gauge independent. The propagation of a free electromagnetic wave in a gravitational radiation field is examined with the conclusion that no resonance phenomena can be expected. Finally, the 'Doppler noise' due to a stochastic background is evaluated, and it is shown to depend on the total energy density of the background and a parameter that is a characteristic of the radiation spectrum and the detection system used.

Mashhoon, B.↗
NASA NTRS1980

Prediction of unsuppressed jet engine exhaust noise in flight from static data

In order to assess the impact of aircraft noise on the environment in the vicinity of an airport, it is essential that a methodology be developed for predicting in-flight exhaust noise from static data. Such a methodology is presented in this paper and is compared with experimental data for several unsuppressed turbojet engines. For each engine, static data over a range of jet velocities are compared with the predicted jet mixing noise and shock-cell noise. The static engine noise over and above the jet and shock noises is identified as 'excess' noise. The excess noise data are then empirically correlated to smooth the spectral and directivity relations and account for variations in test conditions. This excess noise is then projected to flight based on the assumption that the only effects of flight are a Doppler frequency shift and a level change. The effects of flight on jet mixing noise and shock noise are computed by published NASA methods.

Stone, J. R.↗
NASA NTRS1980

Cosmic gamma-ray lines - Theory

The various processes that lead to gamma-ray line emission and the possible astrophysical sources of such emission are reviewed. The processes of nuclear excitation, radiative capture, positron annihilation, and cyclotron radiation, which may produce gamma-ray line emission from such diverse sources as the interstellar medium, novas, supernovas, pulsars, accreting compact objects, the galactic nucleus and the nuclei of active galaxies are considered. The significance of the relative intensities, widths, and frequency shifts of the lines are also discussed. Particular emphasis is placed on understanding those gamma-ray lines that have already been observed from astrophysical sources.

Lingenfelter, R. E.↗
NASA NTRS1981

Stochastic analysis of spectral broadening by a free turbulent shear layer

The effect of the time-varying shear layer between a harmonic acoustic source and an observer on the frequency content of the observed sound is considered. Experimental data show that the spectral content of the acoustic signal is considerably broadened upon passing through such a shear layer. Theoretical analysis is presented which shows that such spectral broadening is entirely consistent with amplitude modulation of the acoustic signal by the time-varying shear layer. Thus, no actual frequency shift need be hypothesized to explain the spectral phenomenon. Experimental tests were conducted at 2, 4, and 6 kHz and at free jet flow velocities of 10, 20, and 30 m/s. Analysis of acoustic pressure time histories obtained from these tests confirms the above conclusion, at least for the low Mach numbers considered.

Hardin, J. C.↗
NASA NTRS1981

Doppler-cancelled response to VLF gravitational waves

The interaction of long periodic gravitational waves with a three link microwave system known as the Doppler Cancelling System is discussed. This system, which was developed for a gravitational redshift experiment, uses one-way and two-way Doppler informatin to construct the beat signal of two reference oscillators moving with respect to each other. The geometric optics approximation is used to derive the frequency shift produced on a light signal propagating in a gravitational wave space-time. The signature left on the Doppler-cancelled beat by burst and continuous gravitational waves is analyzed. A comparison is made between the response to gravitational waves of the Doppler Cancelling System and that of a Doppler tracking system which employs two-way, round-trip radio waves. A three-fold repetition of the gravitational wave form is found to be a common feature of the response functions of both systems. These two functions otherwise exhibit interesting differences.

Caporali, A.↗
NASA NTRS1980

Determination of spacecraft attitude using synthetic aperture radar data

A method has been developed for precise sensor platform attitude determination which exploits the availability of synthetic aperture radar (SAR) sensors on future planned NASA earth orbiting spacecraft. Using the ephemeris along with data on planet shape and rotation, the relative motion between sensor and target area and therefore the Doppler frequency shift in the returned echo data is determined. Equations have been developed that relate the Doppler shift to the antenna attitude. The method is tested by comparing attitude predictions from SEASAT SAR digital echo data with available conventionally determined SEASAT attitude information.

Wu, C.↗
NASA NTRS1980

Test of relativistic gravitation with a space-borne hydrogen maser

The results of a test of general relativity with use of a hydrogen-maser frequency standard in a spacecraft launched nearly vertically upward to 10,000 km are reported. The agreement of the observed relativistic frequency shift with prediction is at the 70 x 10 to the -6th level.

Vessot, R. F. C.↗
NASA NTRS1982

Cryogenic masers

Various factors affecting the frequency stability of hydrogen masers are described and related to maser design parameters. The long-term frequency stability of a hydrogen maser is limited by the mechanical stability of the cavity, and the magnitudes of the wall relaxation, spin exchange, and recombination rates which affect the Q of the line. Magnetic resonance studies of hydrogen atoms at temperatures below 1 K and in containers coated with liquid helium films demonstrated that cryogenic masers may allow substantial improvements in all of these parameters. In particular the thermal expansion coefficients of most materials are negligible at 1 K. Spin exchange broadening is three orders of magnitude smaller at 1 K than at room temperature, and the recombination and wall relaxation rates are negligible at 0.52 K where the frequency shift due to the 4 He-coated walls of the container has a broad minimum as a function of temperature. Other advantages of the helium-cooled maser result from the high purity, homogeneity, and resilence of helium-film-coated walls and the natural compatibility of the apparatus with helium-cooled amplifiers.

Berlinsky, A. J.↗
NASA NTRS1982

Design of helicopter rotor blades for optimum dynamic characteristics

The possibilities and the limitations of tailoring blade mass and stiffness distributions to give an optimum blade design in terms of weight, inertia, and dynamic characteristics are investigated. Changes in mass or stiffness distribution used to place rotor frequencies at desired locations are determined. Theoretical limits to the amount of frequency shift are established. Realistic constraints on blade properties based on weight, mass moment of inertia size, strength, and stability are formulated. The extent hub loads can be minimized by proper choice of EL distribution is determined. Configurations that are simple enough to yield clear, fundamental insights into the structural mechanisms but which are sufficiently complex to result in a realistic result for an optimum rotor blade are emphasized.

Peters, D. A.↗
NASA NTRS1981

Observation and measurement of molecular flow using stimulated Raman gain spectroscopy

The observation and measurement of frequency shifts in stimulated Raman gain spectroscopy resulting from subsonic molecular nitrogen flows emerging from a simple nozzle are reported. The flow velocity in the region of the measurement was determined to be 145 m/s with an accuracy of plus or minus 30 m/s. It is expected that significantly better flow-velocity resolution will be obtained in future experiments using inverse Raman scattering.

Herring, G. C.↗