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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 487 records · Page 27
All sourcesNASA NTRSDOE OSTI
NASA NTRS1977

Coherent microwave backscatter of natural snowpacks

The backscatter of natural snowpacks was measured using a swept-frequency system operating from 5.8 to 8.0 GHz. Snow layering produced sequences of maxima and minima in backscatter intensity, with typical peak-to-valley ratios of 15 db. Wetness produced in the upper portion of the snowpack by solar heat input enhanced the effect of layering. The layer response persisted for incidence exhibits predominantly coherent properties. Frequency modulation of the incident signal masked the layer response by averaging the unmodulated response over the bandwidth represented by the modulation. Further changes in backscatter were attributed to changes in wetness in the surface regions of the snowpack; for a fixed frequency of 13.5 GHz and incidence angle of 39 deg, the backscatter decreased typically 15 db between 11 A.M. and noon, and returned to approximately its initial level of overnight.

Linlor, W. I.↗
NASA NTRS1978

On the period of the coherent structure in boundary layers at large Reynolds numbers

The period of the large coherent structure in a subsonic, compressible, turbulent boundary layer was determined using the autocorrelation of the velocity and pressure fluctuations for Reynolds numbers between 5,000 and 35,000. In low Reynolds number flows the overall correlation period scaled with the outer variables - namely, the free stream velocity and the boundary layer thickness.

Narayanan, M. A. B.↗
NASA NTRS1977

Coherent optical processing of synthetic aperture radar data

Consideration is given to the data processing algorithm used in synthetic aperture radar (SAR) systems. Examples are presented of radar imagery obtained with the ERIM X-band SAR system and processed with a precision optical processor. An optical recording of the data is the input to the coherent optical processing channel. The encoding technique, spatial format, and quality of the stored data are discussed. Several optical processor configurations are outlined, noting particularly the tilted-plane system. A scanning photodetector serves to convert the optical processor output image data into an electronic signal stream. Solid-state two-dimensional photodetector arrays may provide this function at increased speeds and with greater accuracy.

Cindrich, I.↗
NASA NTRS1979

Investigations of coherent anti-Stokes Raman spectroscopy /CARS/ for combustion diagnostics

Investigations of coherent anti-Stokes Raman spectroscopy (CARS) in a variety of flames are presented. Thermometry has received the primary emphasis in these studies, but species spectral and sensitivity studies will also be described. CARS is generated by mixing a 10 pps, frequency-doubled neodymium 'pump' laser with a spectrally broadband, laser-pumped, Stokes-shifted dye laser. This approach obviates the requirement to frequency scan the dye laser and generates the entire CARS spectrum with each pulse permitting, in principle, instantaneous measurements of medium properties. CARS spectra of N2, CO, O2, H2O, CO2 and CH4 in flames will be presented. In general these spectra exhibit very good agreement with computer synthesized spectra and permit measurements of temperature and species concentration. To illustrate the applicability of CARS to practical combustion diagnostics, CARS signatures from N2 have been employed to map the temperature field throughout a small, luminous, highly sooting propane diffusion flame

Eckbreth, A. C.↗
NASA NTRS1979

Sound production due to large-scale coherent structures

The sound due to the large-scale (wavelike) structure in an infinite free turbulent shear flow is examined. Specifically, a computational study of a plane shear layer is presented, which accounts, by way of triple decomposition of the flow field variables, for three distinct component scales of motion (mean, wave, turbulent), and from which the sound - due to the large-scale wavelike structure - in the acoustic field can be isolated by a simple phase average. The computational approach has allowed for the identification of a specific noise production mechanism, viz the wave-induced stress, and has indicated the effect of coherent structure amplitude and growth and decay characteristics on noise levels produced in the acoustic far field.

Gatski, T. B.↗
NASA NTRS1978

Effect of noisy phase reference on coherent detection of unbalanced QPSK signals

A phase error in the reference signal used in coherent detection of PSK signals causes a degradation in performance; this effect on bit error probability has been much discussed in the literature for BPSK and balanced QPSK systems. For unbalanced QPSK (UQPSK) systems, where the power and data rates in the two channels are unequal, and where the channel data symbols might also be different, the problem of computing the error probability is more complex, and only approximate solutions have been previously given. In this paper, exact expressions are derived for the conditional probability of error for the I and Q channels as a function of the channel power and data rate ratio. This is done for all combinations of Manchester and NRZ signalling schemes, and it is shown that in some cases the low data rate channel can be more seriously affected by a phase offset. Numerical results are also given for the average probability of error, assuming a biphase Costas loop is used for carrier synchronization.

Osborne, H. C.↗
NASA NTRS1979

Convergent perturbation analysis of intense coherent multiphoton interactions

Use has been made of flow graphs to deduce Feenberg perturbation expansions for radiative interactions. It is demonstrated that these expansions can in certain cases be summed to provide closed form expressions for the molecular response. In particular, it is shown that the coherent state response can be obtained by the summation of a continued fraction perturbation expansion for the harmonic oscillator. Anharmonicity in the lower levels is treated and its shown to introduce Rabi flopping identifiable with multiphoton transitions among isolated tightly coupled subsystems of levels. Relevance to laser induced multiphoton excitation and energy level shift calculations in the presence of a strong field are also discussed.

Gower, M. C.↗
NASA NTRS1980

Optical communication with two-photon coherent states. III - Quantum measurements realizable with photoemissive detectors

Homodyne detection is shown to achieve the same signal-to-noise ratio as the quantum field quadrature measurement, thus providing a receiver which realizes linear modulation TCS performance gain. The full equivalence of homodyne detection and single-quadrature field measurement is established. A heterodyne configuration which uses a TCS image-band oscillator in addition to the usual coherent state local oscillator is studied. Results are obtained by means of a representation theorem which shows that photoemissive detection realizes the photon flux density measurement.

Yuen, H. P.↗
NASA NTRS1980

Coherent Anti-stokes Raman Spectroscopy (CARS) of gun propellant flames

Temperature measurements were made in a slightly fuel rich, premixed propane/air reference flame and nitrate ester propellant flames burning in air at atmospheric pressure using coherent anti-stokes raman scattering (CARS). Both single and multiple pulse VARS spectra of nitrogen in the reference flame were in good agreement with calculated and reported values. Single pulse CARS nitrogen spectra obtained in the propellant flames were analyzed to give temperatures consistent with values calculated using the NASA-Lewis thermochemical calculation. Comparison of a 0.1 second separated sequence of single pulse CARS spectra indicate turbulent air mixing in these propellant flames. The CARS spectral results demonstrate that temporal and spatially resolved temperature measurements could be determined in transient, turbulent flames.

Mcilwain, M. E.↗
NASA NTRS1981

Coherent reference generator phase stability

Approximate phase stability estimates for the coherent reference generator (CRG) unit in the DSN's Frequency and Timing Subsystem (FTS) are calculated. The method used involves estimating the phase noise introduced by CRG components based upon measurements made in the past on similar components in other parts of the FTS and obtaining the CRG phase noise from the component phase noises. Three estimates of phase stability are calculated: the fractional frequency change for a 5 C step in temperature, the phase noise spectral density, and the Allan standard deviation. It is found from these estimates that the CRG phase stability is better than that of the H-maser physics unit + receiver. Thus, the first step in improving FTS phase stability would be to make improvements in the H-maser physics unit + receiver. These results are corroborated by indirect clock stability estimates calculated from Doppler data.

Korwar, V. N.↗
NASA NTRS1981

Coherent- and incoherent-scatter radar

The ionized regions of the atmosphere were studied using high power backscatter radars at VHF and UHF frequencies were studied. The first year of data from the Urbana coherent-scatter radar were analyzed. It is suggested that the VHF scattered power profile from the mesosphere over Urbana depends on the existence of short vertical wave length tidal components to generate turbulence. Short period altitude and amplitude variations are believed to be due to gravity wave interacting with the tidal components.

Source record↗
NASA NTRS1981

A study of the interactions between large-scale coherent structures and fine-grained turbulence in a round jet

An approximate energy integral description is used to study the development of large-scale coherent structures in the technologically important problem of the round turbulent jet. The analysis begins from the radially integrated form of the kinetic energy equations of the mean flow, the large-scale structure and the fine-grained turbulence, which are obtained by using the usual Reynolds time average and a conditional average with reference to the frequency of the idealized monochromatic component of the large-scale wavelike structure. This is the basis for obtaining the amplitude equations for the three components of the flow in terms of the mean flow momentum thickness, the large-scale structure kinetic energy and the fine-grained turbulence kinetic energy across the jet.

Mankbadi, R.↗
NASA NTRS1981

ISEE-1 observations in the magnetosphere of VLF emissions triggered by nonducted coherent VLF waves during VLF wave-injection experiments

It is noted that recent data from the ISEE-1 spacecraft indicate that VLF emissions triggered by conducted coherent VLF waves often possess spectral characteristics markedly different from those of emissions triggered by ducted waves. Emission triggering by nonducted waves is found to involve a strong path selectiveness. It is also established that emission triggering by nonducted waves can take place over a wide L-range and may involve inner radiation belt particles of energy up to 200 keV. It is concluded that the VLF emission generation process can occur under much more general conditions than previously believed on the basis of the results of ground-to-ground wave-injection experiments.

Bell, T. F.↗
NASA NTRS1981

Premixed turbulent flame blowoff velocity correlation based on coherent structures in turbulent flows

A correlation for the blowoff velocity of premixed turbulent flames stabilized by bluff bodies is devloped using a simple model for coherent structures in turbulent flows. The correlation shows the correct trends for variations in equivalence ratio, free stream pressure and temperature, flameholder characteristic size, and turbulent Reynolds number, and good agreement with published experimental data is obtained.

Radhakrishnan, K.↗
NASA NTRS1981

The effects of focusing on the efficiency of coherent anti-Stokes Raman scattering

The efficiency of coherent anti-Stokes Raman scattering (CARS) is calculated as a function of the input beam sizes of the Stokes and pump laser beams. A Green's function formalism is used to show that maximum efficiency occurs when the confocal parameters of the two input beams are unequal and when the phase mismatch between the anti-Stokes field and the nonlinear polarization is nonzero.

Guha, S.↗
NASA NTRS1980

Coherent multiple tone technique for ground based SPS phase control

The ground based phase control concept was studied as an alternative approach to the reference SPS phase control system. The details of the ground based phase control system study are documented. The coherent multiple tone technique used for the ground based phase measurement waveform design and phase control system is summarized.

Chie, C. M.↗