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At least 325 records · Page 18

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NASA NTRS1973

Remote sensing of stream flow rates - Correlation of meander and discharge spectra

Results of a study of river meander patterns and discharges, in which attempt was made to correlate the discharge spectrum of a river with the river meander power spectrum determined from aerial and satellite imagery. Some significant characteristics of both the discharge and the meander spectra have been discovered. Discharge frequency spectra based on long-term records of daily streamflow are found to have an inverse power-law dependence on discharge. This is shown to reflect the short-term decay of individual floods which are found to have an inverse power-law dependence on time. Meander power spectra for a number of river reaches, digitized from aerial photography, also show significant structure, the power spectral density having an inverse power-law dependence on wave number over one or more portions of the spectrum with breaks in the spectra at characteristic wave numbers. A number of examples of typical discharge and meander spectra are shown.

Lingenfelter, R. E.↗

NASA NTRS1974

Nonlinear panel response by a Monte Carlo approach

The vibration of clamped and simply supported elastic panels due to subsonic and supersonic turbulent boundary-layer flows is investigated by a Monte Carlo technique. The resulting generalized random forces are simulated numerically from boundary-layer turbulence spectra, and the response analysis is performed in the time domain. The mutual interaction between panel motion and external and/or internal airflow is included. Response studies are performed with respect to rms response, probability structure, peak distribution, threshold crossing and spectral density. The effect on the response statistics of in-plane loading, static pressure differential, and cavity pressure is investigated.

Vaicaitis, R.↗

NASA NTRS1974

Correlation analysis of X-ray emission from Cygnus X-1

Normalized autocorrelation and cross-correlation coefficients have been calculated for the X-ray emission of Cygnus X-1 in two energy bins, 2.1-5.1 keV and 5.1-12 keV. The analysis shows a strong correlation between the pulsations in the two energy bins, with the low-energy pulsations lagging behind the high-energy ones, or alternatively a shorter duration of the pulse trains in the high-energy bin. The power spectral density derived from the autocorrelation functions indicates a larger contribution of the high-frequency components for the higher-energy X-rays.

Brinkman, A. C.↗

NASA NTRS1974

Nonstationary atmospheric boundary layer turbulence simulation

Report on a new and general technique for simulating atmospheric turbulence-like random processes which are statistically homogeneous along the horizontal and nonhomogeneous along the vertical. This technique is general in the sense that it can be used for a broad class of similar problems. Like the other presently available schemes, the techniques presented are based on the Dryden hypothesis and Taylor's frozen eddy hypothesis; however, they go a step further by utilizing certain self-similarity properties of the Dryden spectral density function which permits the development of height invariant filters. These filters are in turn used to generate vertically homogeneous (statistically) random processes from which turbulence at any specified level in the boundary layer can be simulated, thus facilitating the simulation of a nonstationary turbulence process along the flight path of an aircraft during take-off or landing.

Fichtl, G. H.↗

NASA NTRS1974

Signal-to-noise ratios in coherent soft limiters

Expressions for the output signal-to-noise power ratio of a bandpass soft limiter followed by a coherent detection device are presented and discussed. It is found that a significant improvement in the output signal-to-noise ratio (SNR) at low input SNR's can be achieved by such soft limiters as compared to hard limiters. This indicates that the soft limiter may be of some use in the area of threshold extension. Approximation methods for determining output signal-to-noise spectral densities are also presented.

Lesh, J. R.↗

NASA NTRS1974

Wall pressure spectra calculations for equilibrium boundary layers

Calculation of the flow direction wave-number spectrum of pressure fluctuations on the wall under a turbulent boundary layer. Particular attention is paid to finding the spectral density of the wall pressure fluctuations as a function of the streamwise wave number. For this purpose a five-dimensional integration is employed in which the equilibrium boundary layers are assumed to have velocity profiles given by the law of the wall plus Cole's wake function.

Panton, R. L.↗

NASA NTRS1974

Sound radiation from a subsonic rotor subjected to turbulence

The broadband sound radiated by a subsonic rotor subjected to turbulence in the approach stream has been analyzed. The power spectral density of the sound intensity has been found to depend on a characteristic time scale-namely, the integral scale of the turbulence divided by the axial flow velocity-as well as several length-scale ratios. These consist of the ratio of the integral scale to the acoustic wavelength, rotor radius, and blade chord. Due to the simplified model chosen, only a limited number of cascade parameters appear. Limited comparisons with experimental data indicate good agreement with predicted values.

Sevik, M.↗

NASA NTRS1974

Effects of vibration and shock on the performance of gas-bearing space-power Brayton cycle turbomachinery. Part 4: Suppression of rotor-bearing system vibrations through flexible bearing support damping

A bearing damper, operating on the support flexure of a pivoted pad in a tilting-pad type gas-lubricated journal bearing, has been designed, built, and tested under externally-applied random vibrations. The NASA Brayton Rotating Unit (BRU), a 36,000 rpm, 10-Kwe turbogenerator had previously been subjected in the MTI Vibration Test Laboratory to external random vibrations, and vibration response data had been recorded and analyzed for amplitude distribution and frequency content at a number of locations in the machine. Based on data from that evaluation, a piston-type damper was designed and developed for each of the two flexibly-supported journal bearing pads (one in each of the two three-pad bearings). A modified BRU, with dampers installed, has been re-tested under random vibration conditions. Root-mean-square vibration amplitudes were determined from the test data, and displacement power spectral density analyses have been performed. Results of these data reduction efforts have been compared with vibration tolerance limits. Results of the tests indicate significant reductions in vibration levels in the bearing gas-lubricant films, particularly in the rigidly-mounted pads. The utility of the gas-lubricated damper for limiting rotor-bearing system vibrations in high-speed turbomachinery has thus been demonstrated.

Tessarzik, J. M.↗

NASA NTRS1973

The muscle spindle as a feedback element in muscle control

The muscle spindle, the feedback element in the myotatic (stretch) reflex, is a major contributor to muscular control. Therefore, an accurate description of behavior of the muscle spindle during active contraction of the muscle, as well as during passive stretch, is essential to the understanding of muscle control. Animal experiments were performed in order to obtain the data necessary to model the muscle spindle. Spectral density functions were used to identify a linear approximation of the two types of nerve endings from the spindle. A model reference adaptive control system was used on a hybrid computer to optimize the anatomically defined lumped parameter estimate of the spindle. The derived nonlinear model accurately predicts the behavior of the muscle spindle both during active discharge and during its silent period. This model is used to determine the mechanism employed to control muscle movement.

Andrews, L. T.↗

NASA NTRS1974

Estimations of the smoothing operator response characteristics

The mean response characteristic of the graphical smoothing method is discussed. The method is illustrated by analysis of latitude observations at Washington from 1915.9 to 1941.0. Spectral density, frequency distribution, and distribution functions are also discussed.

Yatskiv, Y. S.↗

NASA NTRS1975

Refinement and validation of two digital Microwave Landing System (MLS) theoretical models

Two digital microwave landing system theoretical models are considered which are generic models for the Doppler and scanning-beam frequency reference versions of the MLS. These models represent errors resulting from both system noise and discrete multipath. The data used for the validation effort were obtained from the Texas Instrument conventional scanning beam and the Hazeltine Doppler feasibility hardware versions of the MLS. Topics discussed include tape read software, time history plots, computation of power spectral density, smoothed power spectra, best-fit models, different equations for digital simulation, and discrete multipath errors.

Duff, W. G.↗

NASA NTRS1975

The effects of localized damping on structural response

The effect of localized structural damping on the excitability of higher order normal modes of the large space telescope was investigated. A preprocessor computer program was developed to incorporate Voigt structural joint damping models in a NASTRAN finite-element dynamic model. A postprocessor computer program was developed to select critical modes for low-frequency attitude control problems and for higher frequency fine-stabilization problems. The mode selection is accomplished by ranking the flexible modes based on coefficients for rate gyro, position gyro, and optical sensors, and on image-plane motions due to sinusoidal or random power spectral density force and torque inputs.

Merchant, D. H.↗

NASA NTRS1975

Large space telescope control moment gyro test program

The underlying theory and design of test instrumentation and computer techniques used to determine the power spectral density (PSD) of the third generation gyro are described in detail. Acceptance test results on the completed instrument are presented as well as composite PSD plots. The measured RMS noise over the frequency band 0.0025 - 10 Hz was 0.012 arc seconds. Results obtained at Martin Company indicate an RMS noise of 0.009 arc seconds (0.05 - 10 Hz). The results obtained showed the TGG performance was close to the proposed LST requirements but that additional development work would be needed to reduce the TGG noise to within more desirable limits.

Salamin, E.↗

NASA NTRS1975

Experimental evaluation of trailing edge and incidence fluctuation noise theories

Tests were conducted to evaluate conflicting theories for trailing edge noise and for incidence fluctuation noise. A flat-plate airfoil with flush-mounted surface pressure transducers was tested in an anechoic wind tunnel at velocities from 31.5 to 177 m/sec and nominal 4 and 6% grid-generated turbulence levels. In one series of runs, the airfoil was faired into the tunnel nozzle and extended beyond the nozzle lip for studies of trailing edge noise without a leading edge and with flow on only one side. Such noise was found to vary with velocity to the fifth power and turbulence level squared as predicted by Ffowcs Williams and Hall (1970) and by Chase (1972). Power spectral density at high frequencies decayed approximately inversely with frequency to the 10/3 power as predicted by Chase. The data were poorly predicted by Hayden's correlation (1969, 1972).

Fink, M. R.↗

NASA NTRS1975

Electrostatic and electromagnetic turbulence associated with the earth's bow shock

Simultaneous measurements were made of the electric and magnetic field spectral densities in the earth's bow shock by a plasma wave experiment on the Imp 6 spacecraft. The frequency range of the plasma wave detector was 20 Hz to 200 kHz. Electric fields were measured with high-sensitivity 100-m long dipole antennas and magnetic fields were measured with single-turn loop antennas. Two components are distinguished in the electric field spectrum in the bow shock: one component has a broad peak centered in the region 200-800 Hz, while the other component increases monotonically with decreasing frequency. The magnetic field spectrum has only one component that increases monotonically with decreasing frequency and has an upper cutoff frequency near the local electron gyrofrequency. This magnetic field turbulence is judged to be caused by whistler mode waves. The monotonic component of the electric field spectrum is thought to be the electric field spectrum of these whistler mode waves.

Rodriguez, P.↗

NASA NTRS1975

Acoustic characteristics of a large upper-surface blown configuration with turbofan engines

This paper presents acoustic results from static and simulated low forward velocity tests of a large-scale model of an aircraft configuration with two turbofan engines in the full-scale tunnel at the Langley Research Center. The turbofan engines were integrated into the model and equipped with rectangular nozzles to provide the upper-surface blowing. Results indicate that the upper-surface-blowing noise problem can be characterized, primarily, by the unsymmetrical radiation pattern due mainly to shielding of the high-frequency engine noise and the production of low-frequency noise by jet-surface interaction. The directivity of the low-frequency noise was found to depend on the trailing-edge flap angle when the thrust levels were low. Normalized sound pressure level spectral density data showed good agreement at low Strouhal number with other small- and large-scale model data from tests using simulated wing-flap systems. Forward speed effects were undetectable at the low tunnel speeds used during the tests.

Preisser, J. S.↗

NASA NTRS1975

Effects of transducer flushness on fluctuating surface pressure measurements

The installation flushness of pressure transducers has been found to strongly influence the quality of fluctuating surface pressure data in supersonic-attached turbulent boundary-layer flow. A systematic investigation of the effects of flushness on surface pressure fluctuations beneath a 4-in.-thick boundary layer was conducted in a supersonic wind tunnel at Mach numbers of 1.68, 2.0, and 2.5. Flushness effects on transducer signal amplitude, power spectral density, coherence, and narrowband convection velocity was evaluated. Results show that a very slightly protruding transducer produces adverse effects on the desired pressure fluctuation measurements, whereas a submerged transducer produces small effects. Indications are that a more viable method for enhancing the uniformity and spatial correlation of fluctuating surface pressure data can be achieved by deliberately submerging transducers beneath a surface orifice about 0.010 in. deep.

Hanly, R. D.↗

NASA NTRS1974

The MAT project - Atmospheric turbulence measurements with emphasis on long wavelengths

The significant aspects of the NASA MAT (measurement of atmospheric turbulence) program are described. A principal objective of the project is to determine if the Von Karman equation is appropriate for clear-air turbulence in various meteorological conditions, and if so, to define the appropriate L-values in the equation. The current work is primarily a status report describing the measurement system and analysis procedure, some meteorological aspects, and several preliminary results. Measurements of gust velocity will be made over a wide altitude range and in different meteorological conditions. Power spectral density values will be obtained for the three components of gust velocity to wavelengths of 30,000 ft and greater. Principal meteorological parameters of interest include the jet stream, mountain waves, and flight at low altitude (or in the earth's boundary layer).

Murrow, H. N.↗