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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 1,153 records · Page 64
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

Strong fluctuation theory for scattering, attenuation, and transmission of microwaves through snowfall

The strong fluctuation theory is applied to the study of the atmospheric snowfall which is modeled as a layer of random discrete-scatterers medium. As functions of size distribution, fractional volume, and radius of scatterers, the relationship is illustrated between the reflectivity factor and precipitation rate, the attenuation of the centimeter and millimeter waves, and the line-of-sight transmission of coherent and incoherent wave components. The theoretical results are shown to match favorably with experimental data.

Jin, Y.-Q.↗
NASA NTRS1984

Measurement of temperature and density fluctuations in turbulence using an ultraviolet laser

Noninvasive measurement of density and temperature fluctuations in turbulent air flow was examined. The approach used fluorescence of oxygen molecules which are selectively excited by a tunable vacuum ultraviolet laser beam. The strength of the fluorescence signal and its dependence on laser wavelength vary with the density and temperature of the air in the laser beam. Because fluorescence can be detected at 90 degrees from the beam propagation direction, spatial resolution in three dimensions, rather than path-integrated measurements can be achieved. With spatial resolutions of the order of a millimeter and at supersonic air velocities it is necessary to perform each measurement in a time of the order of a microsecond; this is possible by by using laser pulses of ten nanosecond duration. In this method atmospheric O2 is excited by the emission of a tunable ArF excimer laser, and the fluorescence, which spans the 210 to 420 range, is detected by an ultraviolet phototube.

Massey, G. A.↗
NASA NTRS1986

Isocurvature cold dark matter fluctuations

According to Preskill et al. (1983), the axion field represents a particularly attractive candidate for the dark matter in the universe. In many respects it behaves like other forms of cold dark matter, such as massive gravitinos, photinos, and monopoles. It is, however, a pseudo-Goldstone boson of very low mass, and it is only because of rapid coherent oscillations of the field that it can dominate the mass density of the universe. In the present paper it is assumed that the isocurvature mode is dominant. The linear evolution calculations conducted do not depend upon specific details of particle physics. For this reason, the conducted discussion is applicable to any cold dark matter model with isocurvature perturbations. The results of the study lead to the conclusion that scale-invariant isocurvature perturbations do not seem an attractive possibility for the origin of large-scale structure. The findings strengthen the review that primordial adiabatic perturbations were the dominant fluctuations in the early stages of the Big Bang.

Efstathiou, G.↗
NASA NTRS1986

Turbulent boundary-layer wall pressure fluctuations downstream of a tandem LEBU

An initial measurement of turbulent wall pressure fluctuations downstream of tandem large eddy breakup devices (LEBUs) indicates a significant reduction by comparison with the reference case of a flat plate; the average magnitude of the reduction is 12.5 percent. Peak reduction is at 7-8 kHz, and is of the order of the C(f) reduction due to the tandem LEBUs. These data indicate a secondary benefit derivable from LEBUs, in addition to their skin friction reduction effect.

Beeler, G. B.↗
NASA NTRS1986

Grain temperature fluctuations - A key to infrared spectra

Interstellar grains of radius less than 003 microns are known to have a fluctuating temperature because their internal energy is comparable to the energy of UV and visible photons. The exact resultant grain temperature distribution is given by studying the multiphoton absorption processes. This distribution satisfies an homogeneous integral equation which can be solved by an iteration technique. These small grains generally emit at shorter wavelengths than big cold grians and therefore could help in the interpretation of the infrared spectra (1-1000 microns) of various astrophysical objects.

Desert, F. X.↗
NASA NTRS1986

Generation of microwave background fluctuations from nonlinear perturbations at the era of galaxy formation

The production of temperature distortions of the microwave background by primordial galaxy formation is investigated theoretically. The temperature anisotropies induced by the presence of hot ionized gas during the formation of structure are roughly approximated and found to be greater than the initial anisotropies predicted by hierarchical models, and close to the present upper limit (delta (T/T = about 0.00001 at angular scale 1 arcmin) for standard cold-dark-matter models with Omega(0) = Omega(b) = H/(100 km/s) = 1. It is suggested that sensitive small-scale microwave-fluctuation measurements can impose significant constraints on models of primordial galaxy formation.

Ostriker, J. P.↗
NASA NTRS1986

The use of the experimentally deduced Brunt-Vaisala frequency and turbulent velocity fluctuations to estimate the eddy diffusion coefficient

The determination of the turbulent energy dissipation rate or the eddy diffusion coefficient from radar observations can be done through the turbulence refractive index structure constant, deduced from calibrated echo power measurements, or through the turbulent velocity fluctuations, deduced from the echo spectrum width. Besides the radar parameters, power and spectrum width, the first approach needs knowledge of profiles of temperature and electron density in the mesosphere and the fraction of the radar volume filled with turbulence. The latter approach needs knowledge of the temperature profile, namely, the Brunt-Vaisala frequency. The use of this latter approach is demonstrated.

Rottger, J.↗
NASA NTRS1987

Some features of surface pressure fluctuations in turbulent boundary layers with zero and favorable pressure gradients

Measurements of surface pressure fluctuation spectra, coherence and convective wave speeds from zero and favorable pressure gradient turbulent boundary layers are reported for momentum Reynolds numbers from 3000 to 18,800. The acceleration parameter K is near 2 x 10 to the -7 power for the favorable pressure gradient flow. The outer variables, U sub e, tau sub w and delta sub 1 non-dimensionalize and collapse the spectra for the low to middle range of frequencies for most test cases. The grouping using the inner variable, U sub tau and gamma, collapse the spectra for the middle to high range of frequencies for all test cases. The value of p'/tau sub w was near 3.8 and 2.8 for the smallest values of d+ in the zero and favorable pressure gradient flows, respectively. The coherence exhibits a decay that is not exponential in some cases, but the Corcos similarity parameters omega Delta x/U sub c and omega Delta z/U sub c collapse the data for all test cases. The ratio of U sub c/U sub e increases with omega delta sub 1/U sub e up to omega delta sub 1/U sub e on the order of unity, where U sub c/U sub e becomes nearly constant. This was observed in the present results for both streamwise pressure gradient flows. The experimental results presented show good agreement with previous research.

Mcgrath, B. E.↗
NASA NTRS1986

Hard X-ray imaging observation of fluctuating bursts

Measurements were done to obtain the one-dimensional sizes of rapidly fluctuating bursts with fast spikes whose rise times are typically about one second, and in some extreme cases less than 0.1 second. The results of two bursts with fast spikes are presented. One has a soft spectrum, and the other has a very hard spectrum. The measured one-dimensional size of both events indicates relatively a small size and simple structure. It can be said, however, that the source size is not so small as expected from its rapid time variations. Therefore, a thermal explanation of these bursts seems to be excluded.

Ohki, K.↗
NASA NTRS1986

Position measurements of rapidly fluctuating microwave bursts

It was investigated whether microwave source positions change while the total fluxes of hard X-rays and microwaves show remarkable rapid fluctuations of the order of seconds. The position measurements were made in one dimension (east-west direction) with the 17 GHz interferometer at Nobeyama. Position changes greater than a few arc seconds can be detected. The result shows that significant position changes are found for five of seven bursts but that no position changes greater than 3 arc seconds are found for the remaining two bursts.

Kai, K.↗
NASA NTRS1986

Density fluctuations in the interstellar medium: Evidence for anisotropic magnetogasdynamic turbulence. II - Stationary structures

A model of anisotropic, plasma-fluid variations was used to investigate the unknown origin of the power spectra of interstellar electron fluctuations inferred by Armstrong, Cordes, and Rickett (1981). The modeled electron variations are interpreted as density components of an anisotropic stationary mode of nonlinear magnetogasdynamics-tangential pressure balances. It is suggested that the wavenumber spectra of electron variations are identical to the spectra of the convecting velocity fields over a wide range of wavenumbers.

Higdon, J. C.↗
NASA NTRS1986

Magnetic pumping by magnetosonic waves in the presence of noncompressive electromagnetic fluctuations

Numerical simulations of the damping of magnetosonic waves via magnetic pumping in the presence of electromagnetic fluctuations that can pitch-angle scatter the plasma particles are presented. From the first simulation it is found that the magnetosonic-wave energy is transferred to high-energy particles. In the second type of simulation, magnetosonic waves produce a hot surface layer on the plasma that is ablated by the wave energy. Solution of a Fokker-Planck equation for the magnetic-pumping process is found to adequately represent magnetic pumping by small-amplitude magnetosonic waves.

Borovsky, Joseph E.↗
NASA NTRS1987

Density fluctuation spectra in magnetohydrodynamic turbulence

It is shown that within the framework of nearly incompressible magnetohydrodynamics, but not within that of neutral-fluid hydrodynamics, a k exp -5/3 inertial-range wave number density fluctuation spectrum is to be expected at the same times that k exp -5/3 kinetic energy and magnetic energy cascade spectra are present. A previous discrepancy between theory and observation in the local interstellar medium and solar wind is thereby resolved.

Montgomery, D.↗
NASA NTRS1987

An analysis of solar wind fluctuations between 1 and 10 AU

A cross-correlation analysis between different parameters of the solar wind plasma based on Voyager 1 and 2 data from 1 to 9.5 AU evidences a persistent presence of time periods characterized by high anitcorrelation between the magnetic field magnitude and the proton density. These events are present at least 25 percent of the total time, and their frequency of occurrence progressively increases with increasing heliocentric distance. These results have been satisfactorily interpreted as being due to the presence of nonpropagating structures with internal pressure balance and characteristic time scale of less than about 10 hours. Alfvenic fluctuations are confirmed to dominate the microscale at 1-2 AU, but they seem to become less important at larger heliocentric distances.

Vellante, M.↗
NASA NTRS1987

On helicity fluctuations and the energy cascade in turbulence

Recent conjectures concerning the correlation between regions of high local helicity and low dissipation are examined from a rigorous theoretical standpoint based on the Navier-Stokes equations. It is proven that only the solenoidal part of the Lamb vector omega x u (which is directly tied to the nonlocal convection and stretching of vortex lines) contributes to the energy cascade in turbulence. Consequently, it is shown that regions of low dissipation can be associated with either low or high helicity, a result which disproves earlier speculations concerning this direct connection between helicity and the energy cascade. Some brief examples are given along with a discussion of the consistency of these results with the most recent computations of helicity fluctuations in incompressible turbulent flows.

Speziale, Charles G.↗
NASA NTRS1987

Fluctuating pressure loads under high speed boundary layers

Aeroacoustic fatigue is anticipated to control the design of significant portions of the structures of high-speed vehicles. This is due to contemplated long-duration flights at high dynamic pressures and Mach numbers with related high skin temperatures. Fluctuating pressure loads are comparatively small beneath attached turbulent boundary layers, but become important in regions of flow separation such as compression and expansion corners on elevons and rudders. The most intense loads are due to shock/boundary-layer interaction. These flows may occur in the engine-exhaust wall jet and in flows over control surfaces. A brief review is given of available research in these areas with a description of work under way at Langley Research Center.

Zorumski, William E.↗
NASA NTRS1987

Outer magnetospheric fluctuations and pulsar timing noise

The Cheng, Ho, and Ruderman (1986) outer-magnetosphere gap model was used to investigate the stability of Crab-type outer magnetosphere gaps for pulsars having the parameter (Omega-square B) similar to that of the Crab pulsar. The Lamb, Pines, and Shaham (1978) fluctuating magnetosphere noise model was applied to the Crab pulsar to examine the type of the equation of state that best describes the structure of the neutron star. The noise model was also applied to other pulsars, and the theoretical results were compared with observational data. The results of the comparison are consistent with the stiff equation of state, as suggested by the vortex creep model of the neutron star interior. The timing-noise observations also contribute to the evidence for the existence of superfluid in the core of the neutron star.

Cheng, K. S.↗
NASA NTRS1986

On the fine-structure constant in a plasma model of the fluctuating vacuum substratum

The existence of an intimate connection between the quivering motion of electrons and positrons (Zitterbewegung), predicted by the Dirac equation, and the zero-point fluctuations of the vacuum is suggested. The nature of the proposed connection is discussed quantitatively, and an approximate self-consistency relation is derived, supplying a purely mathematical expression that relates the dimensionless coupling strengths (fine-structure constants) alpha sub e and alpha sub g of electromagnetism and gravity. These considerations provide a tentative explanation for the heretofore puzzling number 1/alpha sub e of about 137.036 and suggest that attempts to unify gravity with the electroweak and strong interactions will ultimately prove successful.

Cragin, B. L.↗