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At least 289 records · Page 16

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NASA NTRS1974

Experimental evaluation of fluctuating density and radiated noise from a high-temperature jet

An experimental investigation has been conducted to characterize the fluctuating density within a high temperature (1100 K) subsonic jet and to characterize the noise radiated to the surroundings. Cross correlations obtained by introducing time delay to the signals detected from spatially separated crossed laser beams set up as a schlieren system were used to determine radial and axial distributions of the convection velocity of the moving noise sources (eddies). In addition, the autocorrelation of the fluctuating density was evaluated in the moving frame of reference of the eddies. Also, the autocorrelation of the radiated noise in the moving reference frame was evaluated from cross correlations by introducing time delay to the signals detected by spatially separated pairs of microphones. The radiated noise results are compared with Lighthill's theory and with the data of Lush. Radial distributions of the mean velocity were obtained from measurements of the stagnation temperature, and stagnation and static pressures with the use of probes.

Massier, P. F.↗

NASA NTRS1973

Faculae and the '25-day' solar fluctuation

The evidence presented suggests that the 25-d period of the fluctuation is caused, inadvertently, by the irregular nature of the observing pattern. The effects of irregular sampling as against regular sampling of the facular signal are compared in order to show that the characteristics of Dicke's (1972) fluctuations can be caused by the observing pattern provided that faculae are an important source of the excess solar oblateness.

Chapman, G. A.↗

NASA NTRS1974

A comparison of the geomagnetic ap index with fluctuations in B and V - Mariner 5

Mariner 5 data for the interplanetary magnetic field and the solar wind velocity are compared with terrestrial ap values for the period from June 15, to August 22, 1967. It is concluded that fluctuations in the interplanetary magnetic field are more closely coupled to geomagnetic activity as measured with the ap index than are fluctuations in the solar wind velocity. During periods of high-level activity, the data support the Dessler and Walters (1964) model where the magnetic field is the primary causal agent of geomagnetic activity.

Jones, D. E.↗

NASA NTRS1974

A relation between cosmic-ray fluctuations, gradient, and diffusion coefficient

The motion of charged particles in a stochastic magnetic field is considered via a generalized quasi-linear expansion of Liouville's equation. The result is an equation relating cosmic-ray scintillations to particle gradients and to magnetic-field fluctuations (or diffusion coefficient). The resulting theory may be regarded as an example of a fluctuation-dissipation phenomenon, in which the diffusion coefficient plays the role of the dissipative parameter. The resonant interaction between particles and the random interplanetary magnetic field is considered explicitly, and it is shown that observed scintillations of high-energy (about 1 GeV) cosmic rays may be reasonably explained by the model.

Owens, A. J.↗

NASA NTRS1974

Vibration response of spacecraft shrouds to in-flight fluctuating pressures

Results are presented for the vibration response of a spacecraft shroud to a range of in-flight fluctuation pressures. An Atlas-Agena 15 degree cone-cylinder shroud was analyzed during the present study, and three critical flight Mach numbers were considered. At transonic Mach numbers, the aerodynamic flow over the shroud is complex, involving zones of regular attached flow, separated flow, shock wave oscillation, and modified attached flows induced by local thickening of the boundary layer. The overall shroud vibration levels for a particular Mach number were determined by initially calculating the mean square acceleration levels induced by the fluctuating pressures distributed over an individual zone, and then summing mean square acceleration levels in one-third octave bands over all zones.

Cockburn, J. A.↗

NASA NTRS1974

A comparison of the turbulent fluctuations in clear air convection measured simultaneously by aircraft and Doppler radar

Good agreement was found between the velocity spectra obtained from Doppler radar measurements of the variations in the mean wind field and the spectra of velocity fluctuations simultaneously measured by an aircraft-borne hot-wire anemometer. This experiment is the first in which such a comparison was attempted where the radar return was from fine-scale refractivity fluctuations due to turbulent mixing and not from precipitation particles of artificial scatterers.

Robison, F. L.↗

NASA NTRS1974

Simulation of random wind fluctuations

A technique was developed for the simulation of random wind fluctuations for use in computer studies of the space shuttle ascent control. The simulated wind fluctuations were generated using the techniques of control theory that have statistical characteristics similar to the characteristics obtained from wind data at Kennedy Space Center.

Perlmutter, M.↗

NASA NTRS1975

Preliminary measurements of plasma fluctuations in an 8-cm mercury ion thruster

The investigation considered included an observation of the effects on the plasma fluctuations of a variation of several neutralizer parameters. The apparatus used in the investigation and the experimental techniques are discussed, taking into account the thruster, the thruster power supply, questions of thruster operation, transducers, and an analysis of time-varying data. The results presented are related to rms magnitudes, spectral data, and fluctuation correlations.

Serafini, J. S.↗

NASA NTRS1975

Fluctuating pressures on aircraft wing and flap surfaces associated with powered-lift systems

The present work presents results from two research studies that provide information on the fluctuating pressures generated by the use of powered-lift systems in STOL aircraft. Data are given for several chordwise and spanwise locations on large-scale models of an externally blown flap (EBF) configuration and an upper surface blown flap (USB) configuration in which actual jet engines were used. Pressure levels were high enough to indicate that special design effort will be required to avoid acoustic fatigue failures of wing and flap sturctures. The observation that pressure levels did not decrease very much with increased distance from the engine exhaust center line suggests that a STOL aircraft fuselage, which is in relatively close proximity to the engines for aerodynamic reasons, will be subjected to unusually high external overall fluctuating pressure levels (OAFPLs) that may cause difficulty in control of the cabin noise level.

Mixson, J. S.↗

NASA NTRS1975

Density fluctuations and radiated noise for a high-temperature supersonic jet

Experimental data on density fluctuations were obtained by the laser Schlieren method in a supersonic jet which at the nozzle exit had a Mach number of 1.43 and a stagnation temperature of about 1090 K. The jet emerged into the ambient atmosphere in an anechoic chamber, correctly expanded from a nozzle which had an exit diameter of 10.8 cm. Using the information on the density fluctuations and the mean shear obtained by probes, the autocorrelation of the radiated noise was calculated by a theory that is suitable for Mach wave emission. This theory is a modification of that developed by Ffowcs Williams and Maidanik (1965). The calculated noise field agrees well with that obtained by using microphones outside the jet.

Parthasarathy, S. P.↗

NASA NTRS1975

Pulsar fluctuation spectra and the generalized drifting-subpulse phenomenon. II

Pulsar observations at meter wavelengths have been analyzed to investigate pulse-to-pulse variations and to identify regions of the pulse profile which display distinct statistical properties. Fluctuation spectra of eight recently discovered pulsars are presented along with a description of the drifting-subpulse phenomenon in several objects with quasiperiodic responses in their fluctuation spectra. A quantitative analysis of the drifting-subpulse phenomenon in PSR 0031-07 and PSR 0809+74 is given which emphasizes the broad-band nature of the phenomenon. A summary of the memory exhibited by pulsars with time scales of about 50 periods is given.

Backer, D. C.↗

NASA NTRS1975

Mean and fluctuating flow measurements of a fully-developed, non-adiabatic, hypersonic boundary layer

Extensive boundary-layer measurements have been made on a cone-ogive-cylinder model at a free-stream Mach number of 7.0 and momentum-thickness Reynolds number of 8500. Mean flow transformations and calculated turbulence correlations are presented which are in good agreement with previous incompressible results. New quantitative turbulence measurements including measurements of the first higher moment and probability density of fluctuations in mass flow and total temperature in hypersonic flow are also presented. The higher moment and probability density data show that the characters of the fluctuation modes of the mass flow and total temperature are significantly different in the wall region and in the outer part of the boundary layer. These differences together with data on the turbulence scale and lifetime obtained from autocorrelation and space-time correlation measurements are discussed.

Owen, F. K.↗

NASA NTRS1975

Fluctuating surface pressure measurements on USB wing using two types of transducers

Measurements of the fluctuating pressures on the wing surface of an upper-surface-blown powered-lift model and a JT15 engine were obtained using two types of pressure transducers. The pressures were measured using overall-fluctuating pressures and power spectral density analyses for various thrust settings and two jet impingement angles. Comparison of the data from the two transducers indicate that similar results are obtained in the lower frequency ranges for both transducers. The data also indicate that for this configuration, the highest pressure levels occur at frequencies below 2000 Hz.

Reed, J. B.↗

NASA NTRS1976

Noise reduction from the redesign of a fan stage to minimize stator lift fluctuations

An existing fan stage, redesigned to reduce stator lift fluctuations, was acoustically tested for reduced noise generation. The lift fluctuations on the stator were reduced by increasing the stator chord, adjusting incidence angles, and by adjusting the rotor velocity diagrams. The experiments showed significantly reduced broadband noise levels in the middle to high frequencies. Blade passage tone power was not reduced, but decreases in the harmonics were observed. Aerodynamic improvements in both performance and efficiency were obtained.

Dittmar, J. H.↗

NASA NTRS1975

Strong scintillations in astrophysics. III - The fluctuations in intensity

Fluctuations in intensity due to wave propagation in a wide range of turbulent media, are discussed on the basis of a previously developed theory of strong scintillations. An asymptotic analytic solution to the fourth-moment equation is presented, and its range of validity is discussed. Various parameters characterizing the intensity fluctuations, such as scintillation index and correlation scale, are expressed in terms of turbulence parameters for both Gaussian and power-law irregularity spectra as well as for thick and thin turbulent media.

Lee, L. C.↗

NASA NTRS1975

Two-component model for cosmic-ray fluctuations at neutron monitor energies

A formal model for the fluctuations of the cosmic-ray intensity near earth is presented. For frequencies exceeding 5 x 10 to the -6 Hz, the fluctuations are due to field-aligned-anisotropy induced interplanetary 'scintillations' of cosmic rays, while for lower frequencies the effects of non-field-aligned anisotropies dominate. Cross-correlations between neutron monitor counting rates and the interplanetary magnetic field components are presented which support the model.

Jokipii, J. R.↗

NASA NTRS1976

Noise reduction from the redesign of a fan stage to minimize stator lift fluctuations

Dittmar, J. H.↗

NASA NTRS1976

Effects of stream-associated fluctuations upon the radial variation of average solar-wind parameters

The effects of nonlinear fluctuations due to solar wind streams upon radial gradients of average solar wind parameters are computed, using a numerical MHD model for both spherically symmetric time dependent and corotating equatorial flow approximations. Significant effects of correlations are found between fluctuations upon the gradients of azimuthal magnetic fields, radial velocity, density and azimuthal velocity. Between 400 to 900 solar radii stream interactions have transferred the major portion of the angular momentum flux to the magnetic field; at even greater distances the plasma again carries the bulk of the angular momentum flux. The average azimuthal component of the magnetic field may decrease as much as 10% faster than the Archimedean spiral out to 6 AU due to stream interactions, but this result is dependent upon inner boundary conditions.

Goldstein, B. E.↗