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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 271 records · Page 15
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NASA NTRS1977

Different approaches of spectral analysis

Several approaches to the problem of the calculation of spectral power density of a random function from an estimate of the autocorrelation function were studied. A comparative study was presented of these different methods. The principles on which they are based and the hypothesis implied were pointed out. Some indications on the optimization of the length of the estimated correlation function was given. An example of application of the different methods discussed in this paper was included.

Lacoume, J. L.↗
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 NTRS1976

Investigations of scaling laws for jet impingement

The statistical properties of tangential flows over surfaces were investigated by two techniques. In one, a laser-Doppler velocimeter was used in a smoke-laden jet to measure one-point statistical properties, including mean velocities, turbulent intensities, intermittencies, autocorrelations, and power spectral densities. In the other technique, free stream and surface pressure probes connected to 1/8 inch microphones were used to obtain single point rms and 1/3 octave pressures, as well as two point cross correlations, the latter being converted to auto spectra, amplitude ratios, phase lags, and coherences. The results of these studies support the vortex model of jets, give some insights into the effects of surface impingement, and confirm that jet diameter and velocity are the scaling parameters for circular jets, while Reynolds number is relatively unimportant.

Morton, J. B.↗
NASA NTRS1978

Joint observations of 4U1223-62 by the SAS-3 satellite and Columbia University proportional counter experiment on NASA rocket 26.054 UH

The pulsating X-ray binary 4U1223-62 and Vela X-1 were observed by Aerobee rocket-borne proportional counters. Valid X-ray events were telemetered and analyzed for possible flaring, quasiperiodic, and periodic pulsations, and for other nonstatistical behavior in the source. Both fast Fourier transform and autocorrelation programs were used. For several hours four days before and after the rocket flight, the SAS-3 satellite scanned the galactic plane in order to identify X-ray sources in the vicinity of 4U1223-62 and their intensities, and to provide positional accuracy of 0.25 for sources with intensity greater than 10% of the target. Observations of the source near the main peak of its pulsating period as defined by SAS-3 are discussed. There is no evidence of a spectral feature although twice as many photons were received as than from Vela X-1.

Novick, R.↗
NASA NTRS1977

The theory and measurement of noncoherent microwave scattering parameters

The radar equation for incoherent scenes is derived and scattering coefficients are introduced in a systematic way to account for the complete interaction between the incident wave and the random scene. Intensity (power) and correlation techniques similar to that for coherent targets are proposed to measure all the scattering parameters. The sensitivity of the intensity technique to various practical realizations of the antenna polarization requirements is evaluated by means of computer simulated measurements, conducted with a scattering characteristic similar to that of the sea. It was shown that for scenes satisfying reciprocity one must admit three new cross-correlation scattering coefficients in addition to the commonly measured autocorrelation coefficients.

Claassen, J. P.↗
NASA NTRS1978

Short-term time variability of Cygnus X-1. II

A complete theory for shot-noise analysis is presented. The theory takes into account the effects of counting statistics, time resolution, and finite observing times on the autocorrelation function and on the moments of data assumed to consist of shot noise plus a steady component. When applied to X-ray observations of Cyg X-1 in the high and low states, the results provide evidence that both the typical correlation time (about 0.5 s) and the fraction of flux in shots (about 30%) are constant, independent of the state. The rate of occurrence of the shots may also be constant at approximately one shot per second, a possibility which, if verified, would imply that the principal difference in time variability in the two states is the amplitude of the shots. We also suggest that the occasionally observed large flares may possibly be just the rare superposition of several shots.

Sutherland, P. G.↗
NASA NTRS1977

A hybrid technique for spacecraft attitude interpolation with arbitrary attitude data gaps

An algorithm for obtaining sufficiently accurate estimates of a system parameter Y, given a data record of the parameter in which at arbitrary times no direct observations of Y exist, and describing the variance of these estimates, is proposed. The method provides estimates at all requested times, and the variance of the estimates is maintained as close to the variance of directly measured data as the quality of input information will allow. Three classes of information are used: (1) the measured data record of Y and the statistical extension of this record into time intervals of no direct measurement, obtained by a modification of standard autocorrelation techniques; (2) an a priori indicator of the behavior of Y; and (3) one or more indirect indicators of Y based on observation of other system parameters. The application of the algorithm to the SEASAT-A attitude determination under conditions of sensor field-of-view limitations and sun occultations is presented.

Treder, A. J.↗
NASA NTRS1978

Acoustic scattering from locally homogeneous turbulence

An equation is derived for acoustic scattering from the velocity and temperature fields of a drifting blob of air turbulence. The equation is solved for scattering from the mean and turbulent portions of both fields, and the time autocorrelation and power spectral density of the received signal are calculated. The mean velocity and temperature fields, as well as the corresponding turbulence intensity distributions, are allowed to be spatially nonuniform. The turbulent fields are required to be only locally homogeneous and locally stationary. It is shown that the common practice of representing antenna patterns by either truncating the flow or tapering the strength of the flow is, at least in the case of the transmitting pattern, not a bad approximation. Spectral broadening of the receiver signal due to convection of the small scattering eddies by macroeddies and by the mean flow in modeled, and the broadening by macroeddy convection is seen to render negligible the broadening which arises from the drift of the target flow through the antenna beam. The analysis reveals that only for target flows having a high degree of spatial uniformity and/or symmetry does the received positive-frequency power spectral density turn out to be symmetrical about a center frequency.

Ramsey, V. W.↗
NASA NTRS1978

Noise spectra for monolithic detector arrays

Following a brief review of nonstationary processes, the paper deals with mixed stationary processes as they apply to the determination of the power density spectrum for monolithic detector arrays. A rather direct method is developed for realizing the noise power spectrum by transforming the corresponding autocorrelation function. Although the measurements presented pertain to a 96-element photodiode linear array, the method outlined should be applicable to any monolithic device. Theoretical and experimental results are provided.

Finkel, M. W.↗
NASA NTRS1978

Experimental clean combustor program: Phase 3: Turbulence measurement addendum

Airflow turbulence parameters were measured in the high pressure, high temperature flow stream leaving the compressor of an operating gas turbine engine. A water cooled hot film turbulence probe was used to determine the turbulence intensity and length scale in the compressor exit flow stream of the CF6-50 engine. Data were obtained only at idle operating conditions. At engine power levels above idle conditions, durability problems and erratic data readings were encountered with the turbulence measurement probes. Turbulence measurements were made at three radial immersions at a point 15.5 cm downstream of the compressor outlet guide vanes in the compressor exit diffuser. The passage height at this point is 5.54 cm. Data reduction was accomplished with a time-data fast Fourier transform (FFT) system. This system finds the power spectral density distribution (PSD) of a large number of data samples using a direct Fourier transform algorithm and finds the autocorrelation parameter for these data by doing an FFT analysis of the PSD curves for a series of time delay intervals.

Taylor, J. R.↗
NASA NTRS1979

A model for the noise radiated by turbulent boundary layers and its interaction with laminar layers in supersonic flow

Motivated by the noise-boundary layer transition problem in supersonic flow, the interaction between incoming aerodynamic noise and the laminar boundary layer was studied. The noise field was modeled as a Mach wave radiation field consisting of discrete waves emanating from coherent turbulent entities moving downstream within the supersonic turbulent boundary layer. The individual disturbances are likened to miniature sonic booms and the laminar boundary layer is strafed by the waves as the sources move downstream. The mean, autocorrelation, and power spectral density of the field are expressed in terms of the wave shapes and their average arrival rates. The emphasis in the interaction analysis is on the behavior of the shocklets in the noise field. The shocklets are shown to be focused by the laminar boundary layer in its outer region. Borrowing wave-propagation terminology, this region is termed the caustic region. Using scaling laws from sonic boom work, focus factors at the caustic are estimated to vary from 2 to 6 for incoming shocklet strengths of 1 to .01 percent of the free stream pressure level. The situation regarding experimental evidence of the caustic region is reviewed. It is not clear whether the focusing effectively promotes early transition. Additional observations concerning the interaction problem are presented.

Schopper, M. R.↗
NASA NTRS1978

Coded aperture imaging - Predicted performance of uniformly redundant arrays

It is noted that uniformly redundant arrays (URAs) have autocorrelation functions with perfectly flat sidelobes. A generalized signal-to-noise equation has been developed to predict URA performance. The signal-to-noise value is formulated as a function of aperture transmission or density, the ratio of the intensity of a resolution element to the integrated source intensity, and the ratio of detector background noise to the integrated intensity. It is shown that the only two-dimensional URAs known have a transmission of one half. This is not a great limitation because a nonoptimum transmission of one half never reduces the signal-to-noise ratio more than 30%. The reconstructed URA image contains practically uniform noise, regardless of the object structure. URA's improvement over the single-pinhole camera is much larger for high-intensity points than for low-intensity points.

Fenimore, E. E.↗
NASA NTRS1978

A method for determining the weak statistical stationarity of a random process

A method for determining the weak statistical stationarity of a random process is presented. The core of this testing procedure consists of generating an equivalent ensemble which approximates a true ensemble. Formation of an equivalent ensemble is accomplished through segmenting a sufficiently long time history of a random process into equal, finite, and statistically independent sample records. The weak statistical stationarity is ascertained based on the time invariance of the equivalent-ensemble averages. Comparison of these averages with their corresponding time averages over a single sample record leads to a heuristic estimate of the ergodicity of a random process. Specific variance tests are introduced for evaluating the statistical independence of the sample records, the time invariance of the equivalent-ensemble autocorrelations, and the ergodicity. Examination and substantiation of these procedures were conducted utilizing turbulent velocity signals.

Sadeh, W. Z.↗
NASA NTRS1979

Ozone in the upper troposphere from Gasp measurements

Several aspects of tropospheric ozone variations are examined by using ozone measurements made from commercial airliners (Gasp data). Through visual inspection of the autocorrelation function it is shown that the east-west variations of ozone have a predominant wavelength near 2400 km, while temperature and wind have predominant wavelengths near 3300 km. Distance-lagged correlation functions of ozone with temperature and wind show a definite periodicity with wavelengths near 2400 km. Attention is given to the tropical tropospheric ozone values above 100 parts per billion by volume, which appear to be associated with meridional transport from middle latitudes, and in some cases, relatively large tropical ozone values are coincident with clouds.

Nastrom, G. D.↗
NASA NTRS1979

A model for the noise radiated by turbulent boundary layers and its interaction with laminar layers in supersonic flow

Motivated by the noise-boundary layer transition problem in supersonic flow, the interaction between incoming aerodynamic noise and the laminar boundary layer has been studied. The noise field has been modeled as a Mach wave radiation field consisting of discrete waves emanating from coherent turbulent entities moving downstream within the supersonic turbulent boundary layer. The individual disturbances are likened to miniature sonic booms and the laminar boundary layer is strafed by the waves as the sources move downstream. The mean, autocorrelation, and power spectral density of the field are expressed in terms of the wave shapes and their average arrival rates. The emphasis in the interaction analysis is on the behavior of the shocklets in the noise field. The shocklets are shown to be focused by the laminar boundary layer in its outer region. Borrowing wave-propagation terminology, this region is termed the caustic region. Using scaling laws from sonic boom work, focus factors at the caustic are estimated to vary from 2 to 6 for incoming shocklet strengths of 1 to .01 percent of the free stream pressure level.

Schopper, M. R.↗
NASA NTRS1979

Laser velocimetry measurements in a gas turbine research combustor

The effects of turbulence on the production of pollutant species in a gas-turbine research combustor are studied using laser diffraction velocimetry (LDV) techniques. Measurements that were made in the primary combustion zone include mean velocity, rms velocity fluctuations, velocity probability distributions, and autocorrelation functions. A unique combustor design provides relatively uniform flow conditions and independent control of drop size, equivalence ratio, inlet temperature, and combustor pressure. Parameters which characterize the nature of the spray combustion (i.e., whether single droplet or group combustion occurs), were determined from the LDV data. Turbulent diffusivity (eddy viscosity) reaches a value of 2930 sq cm/sec, corresponding to a convective integral length scale of 1.8 cm. The group combustion number, based on turbulent diffusivity, is measured to be 6.2

Driscoll, J. F.↗
NASA NTRS1979

An upper limit on ultraviolet shot noise from Cygnus X-1

Rapid photometry of Cygnus X-1 through an ultraviolet filter centered on 0.35 micron has been obtained at 100-ms sampling intervals. The autocorrelation function of these data has been examined for shot noise analogous to the behavior of the X-ray light curve. The ultraviolet data are entirely consistent with white noise. Considering randomly occurring ultraviolet shots with the same duration (0.5 s) and average rate (1 per sec) as the X-ray shots, a 3-sigma upper limit on the ratio of optical to X-ray energies per shot is estimated to be 0.13, before the ultraviolet light is attenuated by interstellar dust. This limit is then generalized for shots of arbitrary duration and rate.

Duthie, J. G.↗
NASA NTRS1980

RF spectrometers for heterodyne receivers

Several types of spectrometers developed for radio astronomy receivers which utilize RF filters, multiple oscillators and mixers, digital autocorrelators and acoustic/optic devices are considered. The RF spectrometer developed at GSFC to provide wide bandwidths (greater than 1 GHz) as well as high resolution (5MHz) is described. The 128 channel filter bank is divided into high and low resolution sections. The high resolution section is tunable by providing a second mixer ahead of the filter bank. This is necessary because infrared receivers which use gas lasers as local oscillators are only tunable to specific laser frequencies. To compensate for astronomical Doppler shifts and molecule frequency differences a second local oscillator and mixer is needed. A diagram of the RF section of the filter bank is shown. The RF spectrometer is shown to be the best means of achieving ultra-wide bandwidths for infrared heterodyne receivers. For high resolution with a large number of channels, the acousto/optical spectrometer is the principle instrument, particularly for balloon or space flight applications.

Buhl, D.↗