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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 1,171 records · Page 65
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NASA NTRS1987

Minimal cosmic background fluctuations implied by streaming motions

The minimal cosmic background radiation (CBR) anisotropy implied by the presence of peculiar motions of a given amplitude on some specified scale is calculated using a new, power spectrum-independent approach. If the tentative evidence for deviations from the Hubble flow of magnitude delta V/V roughly 0.1 at V roughly 5000 km/s is confirmed, microwave background fluctuations with a coherence scale of about 2 deg and dispersion delta T/T greater than 10 to the -5th are predicted. It is found that the existing upper limits on delta T/T are not inconsistent with v(r) = 500 km/s at r = 50/h Mpc. A reduction of the observational limits on the CBR anisotropy below the authors' minimal predictions for delta T/T would challenge the current interpretation of measurements of deviations from the Hubble flow. Gravitational instability without reheating as a mechanism for generation of the large-scale structure of the universe would be in severe difficulty.

Juszkiewicz, Roman↗
NASA NTRS1987

Statistical treatment of fluctuations in the gravitational focusing of light due to stellar masses within a gravitational lens

When light from small, distant sources in the universe is gravitationally focused by an intervening galaxy, the gravitational lens can be influenced by the granularity of the matter distribution which is caused by the stellar (or other compact) masses in the galaxy. A largely analytic, statistical calculation for a gravitational lens due to a collection of compact masses - valid for sources of finite size and for large (as well as small) 'optical depths' for the lens - is developed to treat fluctuations in the light caused by such 'microfocusing' effects. Previous treatments have been either numerical simulations of the Monte Carlo type or limited to single-star (i.e., low-optical-depth) effects.

Deguchi, Shuji↗
NASA NTRS1988

Simultaneous density and electric field fluctuation spectra associated with velocity shears in the auroral oval

A detailed study is presented of simultaneous density and electric field fluctuation spectra over a large-scale length range seen in association with large structured convective plasma flows, field-aligned currents, and particle precipitation at high latitudes. The data were obtained for two Dynamics Explorer 2 orbits at two different altitudes within the F region and the topside ionosphere. The observations are compared with results of nonlinear simulations of shear flow-driven instabilities and predictions based on two-dimensional turbulence arguments, with particular reference to the Kelvin-Helmholtz process.

Basu, Sunanda↗
NASA NTRS1988

Effects of eddy viscosity and thermal conduction and Coriolis force in the dynamics of gravity wave driven fluctuations in the OH nightglow

The chemical-dynamical model of Walterscheid et al. (1987), which describes wave-driven fluctuations in OH nightglow, was modified to include the effects of both eddy thermal conduction and viscosity, as well as the Coriolis force (with the shallow atmosphere approximation). Using the new model, calculations were performed for the same nominal case as used by Walterscheid et al. but with only wave periods considered. For this case, the Coriolis force was found to be unimportant at any wave period. For wave periods greater than 2 or 3 hours, the inclusion of thermal conduction alone greatly modified the results (in terms of a complex ratio 'eta' which expresses the relationship between the intensity oscillation about the time-averaged intensity and the temperature oscillation about the time-averaged temperature); this effect was reduced with the further inclusion of the eddy viscosity.

Hickey, M. P.↗
NASA NTRS1988

Magnetic ULF fluctuations in the compressional zone of AMPTE's artificial comets

The two Ba releases in the solar wind, termed artificial comets (the first in interplanetary space, the second in the terrestrial magnetosheath), showed a number of common magnetic features. Both times a magnetic cavity was detected and a field pile-up observed with magnitudes several times the ambient field strength upstream of the cavity. The magnetic fluctuations in the compressed region are generally enhanced by a factor comparable to the amount of field pile-up. There is a peak in the spectra of both events around 0.1 Hz in the component which is perpendicular to the solar-wind velocity vector and almost field-aligned. This magnetosonic wave has been interpreted as the fundamental mode of resonance oscillations of the compression zone. For frequencies beyond 2 Hz the spectra decrease more steeply in the case of the second 'comet' than in the first. This is due to the interaction of the supersonic solar wind with the first comet. The spectra obtained during the first comet at the solar-wind/ion-cloud interface are identical to those observed during bow-shock crossings.

Kloecker, N.↗
NASA NTRS1988

Surface pressure fluctuations due to impinging vortical flows upon an airfoil

A vortical flow impinging upon an airfoil is studied for the case of a strong vortical flow passing close by the airfoil leading and trailing edge. The vortical flow, having a nonuniform vorticity distribution in the core, is distorted and splits as it nears the leading edge of the airfoil. Significant pressure fluctuation occurs near the leading edge, which becomes a source of noise and vibration. A vortex method and a panel method are used to calculate the highly nonlinear, unsteady and rotational flow during the interaction. The flow is assumed to be two dimensional, incompressible and inviscid. The nonuniform vorticity in the vortex core is represented by multiple, discrete vortex elements whose strengths are variable depending on the initial velocity profile. Detailed surface pressure and vortex velocity vectors are calculated. The surface pressure is decomposed into quasi-steady and unsteady pressure, sometimes called 'impulsive pressure'. The first time derivative of the pressure related the strength of the noise, and its r.m.s. values are also calculated.

Lee, D. J.↗
NASA NTRS1989

Improving Estimates Of Phase Parameters When Amplitude Fluctuates

Adaptive inverse filter applied to incoming signal and noise. Time-varying inverse-filtering technique developed to improve digital estimate of phase of received carrier signal. Intended for use where received signal fluctuates in amplitude as well as in phase and signal tracked by digital phase-locked loop that keeps its phase error much smaller than 1 radian. Useful in navigation systems, reception of time- and frequency-standard signals, and possibly spread-spectrum communication systems.

Vilnrotter, V. A.↗
NASA NTRS1988

Springtime surface ozone fluctuations at high Arctic latitudes and their possible relationship to atmospheric bromine

At high Arctic stations such as Barrow, Alaska, springtime near-surface ozone amounts fluctuate between the highest and lowest values seen during the course of the year. Episodes when the surface ozone concentration is essentially zero last up to several days during this time of year. In the Arctic Gas and Aerosol Sampling Program (AGASP-I and AGASP-II) in 1983 and 1986, it was found that ozone concentrations often showed a very steep gradient in altitude with very low values near the surface. The cold temperatures, and snow-covered ground make it unlikely that the surface itself would rapidly destroy significant amounts of ozone. The AGASP aircraft measurements that found low ozone concentrations in the lowest layers of the troposphere also found that filterable excess bromine (the amount of bromine in excess of the sea salt component) in samples collected wholly or partially beneath the temperature inversion had higher bromine concentrations than other tropospheric samples. Of the four lowest ozone minimum concentrations, three of them were associated with the highest bromine enrichments. Surface measurements of excess filterable bromine at Barrow show a strong seasonal dependence with values rising dramatically early in March, then declining in May. The concentration of organic bromine gases such as bromoform rise sharply during the winter and then begin to decline after March with winter and early spring values at least three times greater than the summer minimum.

Oltmans, Samuel J.↗
NASA NTRS1988

A high speed data acquisition and analysis system for transonic velocity, density, and total temperature fluctuations

The high speed Dynamic Data Acquisition System (DDAS) is described which provides the capability for the simultaneous measurement of velocity, density, and total temperature fluctuations. The system of hardware and software is described in context of the wind tunnel environment. The DDAS replaces both a recording mechanism and a separate data processing system. The data acquisition and data reduction process has been combined within DDAS. DDAS receives input from hot wires and anemometers, amplifies and filters the signals with computer controlled modules, and converts the analog signals to digital with real-time simultaneous digitization followed by digital recording on disk or tape. Automatic acquisition (either from a computer link to an existing wind tunnel acquisition system, or from data acquisition facilities within DDAS) collects necessary calibration and environment data. The generation of hot wire sensitivities is done in DDAS, as is the application of sensitivities to the hot wire data to generate turbulence quantities. The presentation of the raw and processed data, in terms of root mean square values of velocity, density and temperature, and the processing of the spectral data is accomplished on demand in near-real-time- with DDAS. A comprehensive description of the interface to the DDAS and of the internal mechanisms will be prosented. A summary of operations relevant to the use of the DDAS will be provided.

Clukey, Steven J.↗
NASA NTRS1989

The effect of spatial and temporal wet-troposphere fluctuations on connected element interferometry

Numerical integrations of the structure function of tropospheric inhomogeneities have been performed to assess the impact of water vapor fluctuations on connected element interferometry (CEI). The expectation value of the RMS troposphere error for a differential spacecraft-quasar observation is derived by integrating the spatial refractivity structure function along raypaths to both the spacecraft and quasar from two spatially separated sites. Correlations between the tropospheric conditions at the two sites, which can become significant for short baseline observations, are fully accounted for in this calculation. Temporal effects are treated by assuming a frozen-flow model in which a fixed spatial distribution blows over both sites. Two nominal observation scenarios are considered, along with variations to study the dependence of the resultant differential troposphere errors on baseline length, observation time, source separation angle, and elevation. Consecutive differential observations are found to be almost completely uncorrelated, implying that averaging many repeated differential observations can quickly reduce the troposphere error.

Edwards, C. D.↗
NASA NTRS1989

Mean, tidal, and fluctuating winds in the middle atmosphere and lower thermosphere observed during MAP/WINE in Northern Scandinavia

During the MAP/WINE campaign in winter 1983 to 1984 several instrumental techniques, such as meteorological rockets, sounding rockets, MST radar and incoherent scatter radar, were applied to measure wind velocities in the middle atmosphere. Profiles of mean, tidal and fluctuating wind velocities were obtained up to 90 to 100 km altitude. These are compared with profiles from models, measurements at other locations and at other times as well as satellite derived data. The results are discussed in terms of ageostropic winds, planetary waves, tidal modes and the possibility of a saturated gravity wave spectrum in the mesosphere.

Roettger, J.↗
NASA NTRS1989

Spectral analysis of temperature and Brunt-Vaisala frequency fluctuations observed by radiosondes

Recent studies have revealed that vertical wave number spectra of wind velocity and temperture fluctuations in the troposphere and the lower stratosphere are fairly well explained by a saturated gravity wave spectrum. But N(2) (N:Brunt-Vaisala (BV) frequency) spectra seem to be better for testing the scaling of the vertical wave number spectra in layers with different stratifications, beause its energy density is proportional only to the background value of N(2), while that for temperature depends on both the BV frequency and the potential temperature. From temperature profiles observed in June to August 1987 over the MU Observatory, Japan, by using a radiosonde with 30 m height resolution, N(2) spectra are determined in the 2 to 8.5 km (troposphere) and 18.5 to 25 km (lower stratosphere) ranges. Although individual spectra show fairly large day-by-day variability, the slope of the median of 34 spectra agrees reasonably with the theoretical value of -1 in the wave number range of 6 x 10(-4) similar to 3 x 10(-3) (c/m). The ratio of the spectral energy between these two height regions is about equal to the ratio of N(2), consistent with the prediction of saturated gravity wave theory.

Tsuda, T.↗
NASA NTRS1988

Fluctuation statistics of outgoing longwave radiation in a general circulation model and in satellite data

Fluctuation statistics of Northern-Hemisphere-winter outgoing longwave radiation (OLR) obtained from data collected by Nimbus 7 and NOAA AVHRR are compared with those computed by a general circulation model (GCM). It was found that the GCM OLR has more variance than does the satellite, but that the GCM clear-sky OLR has less variance than the satellite OLR. The variance of the GCM outgoing full-sky OLR was found to be forced by clouds. While the standard deviation of the GCM full OLR is larger than that of the satellite OLR, the geographical patterns of both are similar. The autocorrelations of the modeled and the observed OLR also display similar geographical distributions, and they are in turn correlated with the dynamics.

Charlock, Thomas P.↗
NASA NTRS1989

The autocorrelation properties of fluctuations in the cosmic X-ray background

The HEAO 1 A-2 all-sky survey data have been analyzed to investigate the autocorrelation function of the surface brightness fluctuations of the extragalactic 2-10 keV X-ray background on angular scales ranging from 3 to 27 deg. The derived upper limits are compatible with optical estimates of the spatial correlation function of Abell's clusters and set interesting constraints on possible AGN clustering.

Persic, M.↗
NASA NTRS1989

Fluctuating pressures on wing surfaces in the slipstream of a single-rotor propfan

Measurements of the fluctuating pressure levels (FPLs) induced on a Propfan Test Assessment wing by the SR-7L propfan slipstream within the airplane flight envelope were obtained as a function of propfan operating conditions. It is shown that FPLS were high over most of the flight envelope, and that the spectra were dominated by the propfan first-order blade passage frequency tone. The highest FPLs were found at the lowest aircraft test altitudes and Mach numbers and for propfan conditions of lowest rotational tip speed and highest power.

Swift, G.↗
NASA NTRS1988

A high speed data acquisition system for the analysis of velocity, density, and total temperature fluctuations at transonic speeds

The use of a high-speed Dynamic Data Acquisition System (DDAS) to measure simultaneously velocity, density, and total temperature fluctuations is described. The DDAS is used to automate the acquisition of hot-wire calibration data. The data acquisition, data handling, and data reporting techiques used by DDAS are described. Sample data are used to compare results obtained with the DDAS with those obtained from the FM tape and post-test digitization method.

Clukey, Steven J.↗
NASA NTRS1989

Non-WKB evolution of solar wind fluctuations - A turbulence modeling approach

Recent observations indicate systematic changes in the interplanetary inertial range velocity-magnetic field correlation with increasing heliocentric distance. Here, a multiple length scale model for the evolution of the small scale fluctuations due to local nonlinear couplings as well as couplings to large scale gradients is outlined, which is similar to that used in turbulence modeling of inhomogeneous shear flows. A simple special case solution is given, indicating that couplings to large scale flow gradients can cause radial evolution of the type seen in observations. The strongest couplings of this type are linear and do not appear in the usual WKB orderings. The relationship of the present approach to WKB theory is not fully understood at present.

Ye, Zou↗