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At least 397 records · Page 22
All sourcesNASA NTRSDOE OSTI
NASA NTRS1983

Combustion response to compositional fluctuations

A mechanism involving compositional fluctuations at certain frequencies is examined by which the heterogeneity of a composite solid propellant may contribute directly to the combustion response function of combustion instability theory. A combustion model appropriate to ammonium perchlorate (AP) is used to derive the combustion response to compositional fluctuations, and the properties of the combustion response are discussed in terms of the theoretical results obtained. The concentration exponent, i.e., the dependence of the burn rate on AP concentration, is found to be a signficant combustion parameter and to have a tremendous range of variability. It is suggested that the combustion response to compositional fluctuations may be a dominating factor in driving combustion instability.

Cohen, N. S.↗
NASA NTRS1983

The production of turbulent stress in a shear flow by irrotational fluctuations

Attention is given to the way in which external turbulence affects an initially turbulence-free region in which there is a mean velocity gradient. External turbulence induces irrotational fluctuations in the sheared region which interact with the shear to produce rotational velocity fluctuations and mean Reynolds stresses. Since the actual front between the initial external turbulence and the shear flow is a randomly contorted surface, the turbulence near the front is intermittent, and is presently included in the form of a simple statistical model. In wind tunnel tests, turbulent shear stress was found to grow from zero to significant values in the interaction region. Observed stress magnitude and extent agrees with predictions, and it is concluded that turbulent stresses can be produced by irrotational fluctuations in a region of mean shear.

Gartshore, I. S.↗
NASA NTRS1985

Muon fluctuation studies of EAS 10(17) eV

Fluctuation studies need to compare a parameter which is sensitive to longitudinal fluctuations against a parameter which is insensitive. Cascade calculations indicate that the shower size parameter at Haverah Park, rho (500), and the muon density are insensitive while parameters that significantly reflect the longitudinal development of a particular extensive air shower (EAS) include the muon/water Cerenkov response ratio and the muon arrival time dispersion. This paper presents conclusions based on muon fluctuation studies of EAS measured between 1976 and 1981 at Haverah Park.

Blake, P. R.↗
NASA NTRS1985

Study of the energy spectrum of primary cosmic rays: EAS size fluctuations at a fixed primary energy

During the initial period of the Samarkand EAS array operations the showers were selected on the basis of charged-particle flux density, and during the subsequent periods the showers were selected on the basis of Cerenkov light flux density. This procedure made it possible to measure the shower energy, to estimate the EAS size fluctuations at a fixed primary energy, and to experimentally obtain the scaling factor K(Ne, Eo) from the EAS size spectrum to the primary energy spectrum. Six scintillators of area S = 2 sq m each were added to the array. The fluctuations of EAS sizes in the showers of fixed primary energies and the scaling factors K(Ne, Eo) were inferred from the data obtained. The showers with zenith angles 30 deg were selected. The EAS axis positions were inferred from the amplitude data of the scintillators. The primary energy Eo was determined by the method of least squares for the known EAS axis position using the data of the Cerenkov detector located at 80 to 150 m EAS axis. It is shown that the Cerenkov light flux fluctuations at 100 m from EAS axis, q sub 100, do not exceed 10% at a fixed EAS energy, so the parameter q sub 100 may be used to estimate the EAS-generating primary particle-energy.

Allev, N.↗
NASA NTRS1983

The origin of density fluctuations in the 'new inflationary universe'

Cosmological mysteries which are not explained by the Big Bang hypothesis but may be approached by a revamped inflationary universe model are discussed. Attention is focused on the isotropy, the large-scale homogeneity, small-scale inhomogeneity, the oldness/flatness of the universe, and the baryon asymmetry. The universe is assumed to start in the lowest energy state, be initially dominated by false vacuum energy, enter a de Sitter phase, and then cross a barrier which is followed by the formation of fluctuation regions that lead to structure. The scalar fields (perturbation regions) experience quantum fluctuations which produce spontaneous symmetry breaking on a large scale. The scalar field value would need to be much greater than the expansion rate during the de Sitter epoch. A supersymmetric (flat) potential which satisfies the requirement, yields fluctuations of the right magnitude, and allows inflation to occur is described.

Turner, M. S.↗
NASA NTRS1985

Massive superclusters as a probe of the nature and amplitude of primordial density fluctuations

It is pointed out that correlation studies of galaxy positions have been widely used in the search for information about the large-scale matter distribution. The study of rare condensations on large scales provides an approach to extend the existing knowledge of large-scale structure into the weakly clustered regime. Shane (1975) provides a description of several apparent massive condensations within the Shane-Wirtanen catalog, taking into account the Serpens-Virgo cloud and the Corona cloud. In the present study, a description is given of a model for estimating the frequency of condensations which evolve from initially Gaussian fluctuations. This model is applied to the Corona cloud to estimate its 'rareness' and thereby estimate the rms density contrast on this mass scale. An attempt is made to find a conflict between the density fluctuations derived from the Corona cloud and independent constraints. A comparison is conducted of the estimate and the density fluctuations predicted to arise in a universe dominated by cold dark matter.

Kaiser, N.↗
NASA NTRS1986

The effect of turbulence on lidar backscattered signal fluctuations

CO2 lidar measurements being subjected to uncertainties caused by strong fluctuations were explained to appear primarily due to speckle effects whereas turbulence was to be negligible. Spectral cross correlation measurements performed in the near IR and in the visible show that the main contribution to the fluctuation for the above lidar wavelengths originates from the atmospheric turbulence. The standard deviation of the fluctuation signal was measured for the two wavelengths simultaneously. A typical curve representing mid-summer day changes in the value of sigma as a function of time is shown. The results, indicating the strong dependence on the turbulence, is analyzed and discussed.

Fastig, S.↗
NASA NTRS1986

The generation of rapid solar flare hard X-ray and microwave fluctuations in current sheets

The generation of rapid fluctuations, or spikes, in hard X-ray and microwave bursts via the disruption of electron heating and acceleration in current sheets is studied. It is found that 20 msec hard X-ray fluctuations can be thermally generated in a current sheet if the resistivity in the sheet is highly anomalous, the plasma density in the emitting region is relatively high, and the volume of the emitting region is greater than that of the current sheet. A specific mechanism for producing the fluctuations, involving heating in the presence of ion acoustic turbulence and a constant driving electric field, and interruption of the heating by a strong two-stream instability, is discussed. Variations upon this mechanism are also discussed. This mechanism also modulates electron acceleration, as required for the microwave spike emission. If the hard X-ray emission at energies less than approx. 1000 keV is nonthermal bremsstrahlung, the coherent modulation of electron acceleration in a large number of current sheets is required.

Holman, Gordon D.↗
NASA NTRS1986

Systematic errors in determining the propagation direction of interplanetary Alfvenic fluctuations

Analyses of Alfvenic fluctuations in the solar wind contain a systematic effect in which outward propagation is favored. The effect is generally small at and beyond 1 AU in a Mach 10 flow but can become important where the Mach number is lower, e.g., inside 1 AU. If the propagation speeds of the inward and outward propagating Alfvenic fluctuations are properly included, this systematic effect can be removed. A technique for doing this is illustrated using both artificial and spacecraft data. The results suggest that time intervals originally thought to be outward propagating sometimes contain wave number bands in the cross helicity spectrum of inward propagating Alfvenic fluctuations.

Goldstein, M. L.↗
NASA NTRS1987

Quasi-Lagrangian measurements of density surface fluctuations and power spectra in the stratosphere

Pressure and temperature data from eight superpressure balloon flights at 26 km in the southern hemisphere stratosphere are analyzed. The balloons, which float on a constant density surface, travel steadily westward during summer and eastward during winter, as expected from local climatology. Two types of fluctuations are observed: neutral buoyancy oscillations (NBO) of around 4 min, and 0.1- to 1-hour oscillations that are characterized as small-amplitude density surface fluctuations. Lapse rates and densities are calculated and found to agree well with the expected values. Examples of wave damping and simultaneous fluctuation at two nearby balloons are presented. Spectral analysis is performed clearly showing the NBO and that the majority of the power is in the mesoscale range. Spectral slopes of power versus frequency are measured to be on the average -2.18 + or - 0.24 for pressure and -1.72 + or - 0.24 for temperature. These slopes are compared to the predictions of turbulence theories and the theory of a universal gravity wave spectrum.

Quinn, Elizabeth P.↗
NASA NTRS1987

Large-scale fluctuations between 13 AU and 25 AU and their effects on cosmic rays

The reasons for the temporal and latitudinal large-scale fluctuations in the magnetic-field strength observed between 13 and 25 AU by Voyagers 1 and 2 in 1984-1985, and their effect on the cosmic ray intensity profile are investigated. The study of low-frequency and intermediate-frequency fluctuations determined from magnetic-field-intensity profile suggests the presence of discontinuities near the equator during both years. The intensity of cosmic ray nuclei of greater than 75 MeV during 1984 was found to fluctuate about a constant value, because the decreases of intensity caused by the passage of large merged interaction regions balanced the increases of intensity associated with rarefied regions. The intensity of cosmic ray nuclei above 75 MeV increased during 1985 because there were few large interactions then. The cosmic ray intensity increased more rapidly at low than at high latitudes. The model of Burlaga et al. (1985) was found to provide good first-order fits to the cosmic ray intensity profiles.

Burlaga, L. F.↗
NASA NTRS1987

Low-frequency magnetic field fluctuations in comet P/Halley's magnetosheath - Giotto observations

The interaction region between comets and the solar wind is characterized by large amplitude, low frequency magnetic field fluctuations, both within the upstream region as well as in the magnetosheath. Magnetosheath observations of the magnetic field experiment onboard Giotto indicate values of delta B/B equal to about O(1). Power spectral peaks appear at frequencies of 10mHz with the spectrum exhibiting a power law dependence with an exponent of the order 2. Radial variation of the fluctuation level does not clearly increase with decreasing distance from the cometary nucleus as observed by the magnetometer onboard Vega-1 and as expected from quasi-linear theory. The entrance into the cometary bow shock is furthermore characterized by an order of magnitude increase of the fluctuation level, both on the in- and outbound pass of Giotto.

Glassmeier, K. H.↗
NASA NTRS1988

An observational study of MHD wave-induced density fluctuations upstream of the earth's bow shock

An investigation of plasma density fluctations upstream of the earth's bow shock and their association with MHD waves is reported. The normalized density fluctuation was 14 percent on a day when the plasma beta was less than unity and the waves were circularly polarized and of relatively low amplitude, and 17 percent on a day when the plasma beta was in excess of unity and the waves were elliptically polarized and of large fractional amplitude. On both days there was a feature of the density power spectrum at the MHD carrier wave frequency attributable to oblique propagation of the waves with propagation angles of a few degrees with respect to the mean field. Ponderomotive effects due to spatial gradient in the MHD wave energy density are proposed as responsible for a correlation betweeen density and transverse wave intensity on both days. Generation of density fluctuations by linear polarized MHD waves is not an important contributor to the observed density fluctuations.

Spangler, Steven↗
NASA NTRS1988

Measurements of the free-stream fluctuations above a turbulent boundary layer

The paper describes an investigation of the velocity fluctuations in the free stream above an incompressible turbulent boundary layer developing at constant pressure. Measurements were performed in a wind tunnel with an rms level of axial velocity fluctuations of about 0.2 percent. The possibility of assessing the contributions to the rms level of the velocity fluctuations without using the high-pass filtering technique is demonstrated.

Wood, D. H.↗
NASA NTRS1989

Large-scale fluctuations in the solar wind at 1 AU - 1978-1982

The large-scale fluctuations in the strength of the magnetic field B and the bulk speed V at 1 AU were investigated on the basis of hourly averages of IMP 8 and ISEE 3 data compiled by Couzens and King (1986). The power spectra of the intermediate frequency fluctuations in B at 1 AU had the form of a power law with exponent 1.92 + or - 0.06 during the 1978-1982 interval. The contributions of jumps to the spectra of the large-scale fluctuations in B and V were studied for the interval from days 226 to 365, 1978, which is representative of other such intervals. Of the jumps in B, 22 percent have the signature of a forward or reverse shock.

Burlaga, L. F.↗
NASA NTRS1989

Lower thermospheric density fluctuations during the time period of Typhoon Dinah

High frequency Doppler sounder arrays were used to study the thermospheric density fluctuations caused by Typhoon Dinah in August, 1987. The results show that the maximum density fluctuations caused by the typhoon at altitudes of 150 to 350 km were close to + or - 30 percent deviation from the quiet background. The time-dependent density fluctuations were in phase with the horizontal phase velocity of major gravity waves excited by the storm. It is suggested that the study demonstrates the use of HF Doppler sounder measurement of gravity waves, horizontal wind velocity in the direction of gravity wave propagation, and density perturbations at thermospheric heights.

Hung, R. J.↗
NASA NTRS1989

Propagation of magnetosheath hydromagnetic fluctuations into the magnetosphere

A study of the transmission of magnetosheath hydromagnetic fluctuations with f of about 23 mHz and f of about 13 mHz through the magnetopause is reported. The study involves measurements of the fluctuations inside the magnetosphere on the ISEE-2 spacecraft near the equator and on the ground near the boundary flux tube connecting with the spacecraft. The magnetic energy of the fluctuations inside the magnetopause is about 1 percent of those in the magnetosheath. This ratio is consistent with that expected for the transmission of fast-mode waves through a magnetopause with a tangential discontinuity.

Lanzerotti, L. J.↗
NASA NTRS1990

Magnetic properties of high-T(sub c) superconductors: Rigid levitation, flux pinning, thermal depinning, and fluctuation

The levitation of high-T(sub c) superconductors is quite conspicuous: Above magnets of low symmetry a disk of these ceramics floats motionless, without vibration or rotation; it has a continuous range of stable positions and orientations as if it were stuck in sand. Some specimens may even be suspended above or below the same magnet. This fascinating stability, inherent to no other type of levitation, is caused by the pinning of magnetic flux lines by inhomogeneities inside these extreme type-2 superconductors. The talk deals with pinning of magnetic flux in these materials, with flux flow, flux creep, thermally activated depinning, and the thermal fluctuation of the vortex positions in the flux line lattice (often called flux lattice melting). Also discussed are the fluctuations of the (nearly periodic) magnetic field inside these superconductors which are caused by random pinning sites and by the finite temperature. These fluctuations broaden the van-Hove singularities observed in the density of the magnetic field by nuclear magnetic resonance and by muon spin rotation.

Brandt, E. H.↗