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At least 1,783 records · Page 99
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

Test particle calculations of pick-up ions in the vicinity of Comet Giacobini-Zinner

Ionization of cometary neutral molecules produces ions which are picked-up by the solar wind. The cometary ion pick-up process for Comet Giacobini-Zinner is studied using a test particle method in which trajectories are numerically calculated for several thousand ions whose initial locations were chosen randomly with probability proportional to the neutral density. The cometary ion distribution function was calculated at several locations upstream of the bow shock. Several types of magnetic fluctuations were included. Circularly and linearly polarized monochromatic waves result in less pitch-angle scattering of pick-up ions than turbulent fluctuations which have about the same amplitude. Some anisotropy remains in the distribution function just upstream of the shock, even for the turbulent fluctuations.

Cravens, T. E.↗
NASA NTRS1990

Propulsion system filter sizing considerations for the Galileo spacecraft

Filtration capacity of the Galileo propulsion system has been investigated. Ground testing of flight spare components has been conducted to determine the representative number and size distribution of particulate matter collected in flow tests. For these tests and for filter qualification tests, a number of effects were examined. The influence of gravitational particle settling on ground test results vs the flight environment was considered. Centrifugal effects inherent in the test apparatus and/or the propulsion system could have an influence. Variability among particle sizing techniques was explored. The influence of size distribution, particle shape, and density in the choice of a particulate test material was studied. Characteristics for AC Coarse Dust (an industry standard filter test media), microscopic polymer beads, and aluminum oxide particulate test matter are listed. Filtration tests were conducted using AC Coarse Dust and mixtures containing components of AC Coarse Dust. These results are expected to be useful data for filter sizing of future spacecraft.

Jan, Darrell L.↗
NASA NTRS1991

FD-TD numerical simulation of an entire lightning strike on the C160 aircraft

Experimental transient electromagnetic field measurements were performed on a Transall C160 aircraft during in-flight lightning strikes. The data allow a test of the predictive capabilities of a three dimensional time domain finite difference code (ALICE) developed at ONERA in order to investigate lightning-aircraft interactions. Using a transfer function technique in the 3D code, it is shown that a bi-leader attached to an aircraft can be simulated by a linear model, and so the electromagnetic fields can be calculated anywhere on the vehicle. Comparison of experimental and numerical results were made for several lightning strikes. Skin current density and electromagnetic field distributions are discussed in detail.

Alliot, J. C.↗
NASA NTRS1990

Electric current in a unipolar sunspot with an untwisted field

The return flux (RF) sunspot model is applied to a round, unipolar sunspot observed by H. Kawakami (1983). Solving the magnetohydrostatic problem using the gas pressure deficit between the umbral and quiet-sun atmospheres as a source function, a distribution of electric current density in an untwisted, unipolar sunspot as a function of height and radial distance from the sunspot center is observed. Maximum electric current density is about 32 mA/sq m at the bottom of the sunspot.

Osherovich, V. A.↗
NASA NTRS1992

A range-resolved bistatic lidar using a high-sensitive CCD-camera

Until now monostatic type lidar systems have been mainly utilized in the field of lidar measurements of the atmosphere. We propose here a range-resolved bistatic lidar system using a high-sensitive cooled charge coupled device (CCD) camera. This system has the ability to measure the three dimensional distributions of aerosol, atmospheric density, and cloud by processing the image data of the laser beam trajectory obtained by a CCD camera. Also, this lidar system has a feature that allows dual utilization of continuous wave (CW) lasers and pulse lasers. The scheme of measurement with this bistatic lidar is shown. A laser beam is emitted vertically and the image of its trajectory is taken with a remote high-sensitive CCD detector using an interference filter and a camera lens. The specifications of the bistatic lidar system used in the experiments are shown. The preliminary experimental results of our range-resolved bistatic lidar system suggest potential applications in the field of lidar measurements of the atmosphere.

Yamaguchi, K.↗
NASA NTRS1992

Topology of large-scale structure in seeded hot dark matter models

The topology of the isodensity surfaces in seeded hot dark matter models, in which static seed masses provide the density perturbations in a universe dominated by massive neutrinos is examined. When smoothed with a Gaussian window, the linear initial conditions in these models show no trace of non-Gaussian behavior for r0 equal to or greater than 5 Mpc (h = 1/2), except for very low seed densities, which show a shift toward isolated peaks. An approximate analytic expression is given for the genus curve expected in linear density fields from randomly distributed seed masses. The evolved models have a Gaussian topology for r0 = 10 Mpc, but show a shift toward a cellular topology with r0 = 5 Mpc; Gaussian models with an identical power spectrum show the same behavior.

Beaky, Matthew M.↗
NASA NTRS1992

Linear velocity fields in non-Gaussian models for large-scale structure

Linear velocity fields in two types of physically motivated non-Gaussian models are examined for large-scale structure: seed models, in which the density field is a convolution of a density profile with a distribution of points, and local non-Gaussian fields, derived from a local nonlinear transformation on a Gaussian field. The distribution of a single component of the velocity is derived for seed models with randomly distributed seeds, and these results are applied to the seeded hot dark matter model and the global texture model with cold dark matter. An expression for the distribution of a single component of the velocity in arbitrary local non-Gaussian models is given, and these results are applied to such fields with chi-squared and lognormal distributions. It is shown that all seed models with randomly distributed seeds and all local non-Guassian models have single-component velocity distributions with positive kurtosis.

Scherrer, Robert J.↗
NASA NTRS1992

A source model for the L134N molecular cloud

The dark molecular cloud L134N is observed at millimeter wavelengths in the CS (J = 3-2), OCS (J = 7-6), and HDO (1 sub 11-1 sub 10) transitions. The CS (J = 3-2) transition was observed at four positions within the cloud, while the other two transitions were observed at one position each. Fractional abundances in the LTE approximation are calculated for each emission line detected. L134N appears to have a high-density core characterized by NH3, C3H2, and H(C-13)O(+) emission maps. A lower density envelope characterized by C(0-18), CS (J = 2-1), and SO emission surrounds the core. There appears to be a gas-phase oxygen abundance gradient in L134N with atomic oxygen depleted in the high-density core. Observed molecular distributions within L134N can be explained by a model in which chemical and physical processes in icy-dust-grain mantles influence the gas-phase molecular abundances.

Swade, Daryl A.↗
NASA NTRS1992

Probabilistic material degradation model for aerospace materials subjected to high temperature, mechanical and thermal fatigue, and creep

A probabilistic general material strength degradation model has been developed for structural components of aerospace propulsion systems subjected to diverse random effects. The model has been implemented in two FORTRAN programs, PROMISS (Probabilistic Material Strength Simulator) and PROMISC (Probabilistic Material Strength Calibrator). PROMISS calculates the random lifetime strength of an aerospace propulsion component due to as many as eighteen diverse random effects. Results are presented in the form of probability density functions and cumulative distribution functions of lifetime strength. PROMISC calibrates the model by calculating the values of empirical material constants.

Boyce, L.↗
NASA NTRS1993

Spectroscopic constraints on the properties of dust in active galactic nuclei

The optical properties of graphite, silicate, and SiC grains over the wavelength range 1000 microns - 1 A are calculated for grains in the 0.005-10-micron size range. Both graphite + silicate (MRN) and graphite + SiC grain mixtures are considered, with various grain size distributions. A detailed radiative transfer calculation is performed to obtain constraints on the emission properties of dust in AGN in either 'optically thin' or optically thick configurations. Warm graphite + silicate or graphite + SiC dust (T is greater than 200 K) with the MRN size distribution and a column density less than about 10 exp 23/sq cm produces a strong silicate or SiC emission feature. Such dust cannot be responsible for the observed IR emission from AGN. MRN dust with a high optical depth at 10 microns produces an emission feature with an amplitude of about 57 percent, in excess of the typical observational limit for most objects. Reddening of the broad emission lines and continuum is unlikely to be common.

Laor, Ari↗
NASA NTRS1993

Probabilistic simulation of stress concentration in composite laminates

A computational methodology is described to probabilistically simulate the stress concentration factors in composite laminates. This new approach consists of coupling probabilistic composite mechanics with probabilistic finite element structural analysis. The probabilistic composite mechanics is used to probabilistically describe all the uncertainties inherent in composite material properties while probabilistic finite element is used to probabilistically describe the uncertainties associated with methods to experimentally evaluate stress concentration factors such as loads, geometry, and supports. The effectiveness of the methodology is demonstrated by using it to simulate the stress concentration factors in composite laminates made from three different composite systems. Simulated results match experimental data for probability density and for cumulative distribution functions. The sensitivity factors indicate that the stress concentration factors are influenced by local stiffness variables, by load eccentricities and by initial stress fields.

Chamis, C. C.↗
NASA NTRS1993

Optimization of energy transfer in microwave electrothermal thrusters

Results are presented from preliminary tests conducted to evaluate the performance of a prototype microwave electrothermal thruster. The primary component of the device is a microwave resonant cavity. The device produces stable axial plasmas within a pressurized section of the cavity with the plasma positioned in the inlet region of the nozzle. Plasma stability is enhanced by axial power coupling, an optimal distribution of electric power density within the cavity, and a propellant gas flow which has a large vortical velocity component. The thruster has been operated with a number of propellant gases: helium, nitrogen, ammonia, and hydrogen. Plasmas can be formed in a reliable manner at cavity pressures of 1 kPa and incident power levels ranging from 50 W to 350 W, depending on the gas used, and can be operated at pressures up to 300 kPa at power levels up to 2200 W. Ideal performance results of vacuum Isp and thermal efficiency vs. specific power are presented for each gas. Representative results of this preliminary work are: He - Isp = 625 s, eta-thermal = 90 percent; N2 - Isp = 270 s, eta-thermal = 41 percent; NH3 - Isp = 475 s, eta-thermal= 55 percent; H2 - Isp = 1040 s, eta-thermal = 53 percent.

Sullivan, D. J.↗
NASA NTRS1993

A critical evaluation of theories for predicting microcracking in composite laminates

We present experimental results on 21 different layups of Hercules AS4 carbon fiber/3501-6 epoxy laminates. All laminates had 90 deg plies; some had them in the middle, while some had them on a free surface. During tensile loading, the first form of damage in all laminates was microcracking of the 90 deg plies. For each laminate, we recorded both the crack density and the complete distribution of crack spacings as a function of the applied load. By rearranging various microcracking theories, we developed a master-curve approach that permitted plotting the results from all laminates on a single plot. By comparing master-curve plots for different theories, it was possible to critically evaluate the quality of those theories. We found that a critical-energy-release-rate criterion calculated using a 2D variational stress analysis gave the best results. All microcracking theories based on a strength-failure criteria gave poor results. All microcracking theories using 1D stress analyses, regardless of the failure criterion, also gave poor results.

Nairn, John A.↗
NASA NTRS1994

Probabilistic Simulation of Stress Concentration in Composite Laminates

A computational methodology is described to probabilistically simulate the stress concentration factors (SCF's) in composite laminates. This new approach consists of coupling probabilistic composite mechanics with probabilistic finite element structural analysis. The composite mechanics is used to probabilistically describe all the uncertainties inherent in composite material properties, whereas the finite element is used to probabilistically describe the uncertainties associated with methods to experimentally evaluate SCF's, such as loads, geometry, and supports. The effectiveness of the methodology is demonstrated by using is to simulate the SCF's in three different composite laminates. Simulated results match experimental data for probability density and for cumulative distribution functions. The sensitivity factors indicate that the SCF's are influenced by local stiffness variables, by load eccentricities, and by initial stress fields.

Chamis, C. C.↗
NASA NTRS1994

Survey of electron and ion bulk flows in the distant magnetotail with the Geotail spacecraft

A histogramic survey is reported for the electron and ion bulk flows in Earth's magnetotail at geocentric radial distances in the range of about 10 to 210 R(sub E) from the Comprehensive Plasma Instrumentation on board the Geotail spacecraft. Specifically the X-components of the bulk flows are examined for high ion temperatures, greater than 5 x 10(exp 6) K, and for the lower ion temperatures. This separation of plasmas provides a reasonably effective criterion for identifying the hot plasmas associated with the plasma sheet and its boundary layers and the colder plasmas found in the magnetotail lobes and magnetopause boundary layer. The survey shows that the distributions of earthward bulk flows of ions and electrons are similar for each of the two temperature regimes, respectively. However, average speeds for the ions and electrons as a function of radial distance are dissimilar in that the electron bulk speeds are greater than those for the ions. All average electron and ion bulk flows are directed tailward at geocentric radial distances greater than 30 R(sub E). The most striking dissimilarity in the occurrence frequencies of the bulk flows is the large number with high tailward speeds for the electrons, greater than 500 km/s, relative to those for the ions. It is found that the disparity in bulk flows is due to the presence of two ion velocity distributions, one with high temperatures and a bulk speed similar to that of the electrons and the second, colder distribution with higher number density and considerably lesser bulk flow speed.

Frank, L. A.↗
NASA NTRS1995

Subcluster mergers and galaxy infall in A2151

We have obtained a 12.5 ks image of the Hercules Cluster, A2151, with the ROSAT PSPC. Comparison of the optical and X-ray emission coincides with the highest-density peak in the distribution, and is bimodal. The northern subclummp, distinct in position and velocity, has no detectable X-ray gas. The eastern subclump, apparent in the optical contour map, is indistinguishable from the clump in velocity space, but is clearly visible in the X-ray image. X-ray spectra derived from the central peak of emission yield a best-fit temperature of 1.6 keV. The emission coincident with the eastern clump of galaxies is cooler, 0.8 keV, and is outside the 90% confidence intervals of the central peak temperature. We suggest that the eastern and central subclusters have recently undergone a merger event. The lack of X-ray emission to the north suggests that those galaxies do not form a physically distinct structure (i.e., they are not located within a distinct gravitational potential), but rather that they are falling into the cluster core along the filament defined by the Hercules Supercluster.

Bird, Christina M.↗
NASA NTRS1996

Planetary Atmosphere Dynamics and Radiative Transfer

This research program has dealt with two projects in the field of planetary atmosphere dynamics and radiative energy transfer, one theoretical and one experimental. The first project, in radiative energy transfer, incorporated the capability to isolate and quantify the contribution of individual atmospheric components to the Venus radiative balance and thermal structure to greatly improve the current understanding of the radiative processes occurring within the Venus atmosphere. This is possible by varying the mixing ratios of each gas species, and the location, number density and aerosol size distributions of the clouds. This project was a continuation of the work initiated under a 1992 University Consortium Agreement. Under the just completed grant, work has continued on the use of a convolution-based algorithm that provided the capability to calculate the k coefficients of a gas mixture at different temperatures, pressures and spectral intervals from the separate k-distributions of the individual gas species. The second primary goal of this research dealt with the Doppler wind retrieval for the Successful Galileo Jupiter probe mission in December, 1995. In anticipation of the arrival of Galileo at Jupiter, software development continued to read the radioscience and probe/orbiter trajectory data provided by the Galileo project and required for Jupiter zonal wind measurements. Sample experiment radioscience data records and probe/orbiter trajectory data files provided by the Galileo Radioscience and Navigation teams at the Jet Propulsion Laboratory, respectively, were used for the first phase of the software development. The software to read the necessary data records was completed in 1995. The procedure by which the wind retrieval takes place begins with initial consistency checks of the raw data, preliminary data reductions, wind recoveries, iterative reconstruction of the probe descent profile, and refined wind recoveries. At each stage of the wind recovery consistency is checked and maintained between the orbiter navigational data, the radioscience data, and the probe descent profile derived by the Atmospheric Instrument Team. Preliminary results show that the zonal winds at Jupiter increase with depth to approximately 150 m/s.

Atkinson, David H.↗
NASA NTRS1995

Large-scale structure of solar wind as observed on the Prognoz 7 satellite

Properties of different solar wind streams depend on the large scale structure of coronal magnetic field and dynamical phenomena in the solar atmosphere. We present average values and distributions of MHD parameters (density, velocity, temperature, fluxes of mass, momentum and energy, ratio of thermal and magnetic pressures, as well as helium abundance) as observed on board the Prognoz 7 satellite in the different types of the solar wind streams connected with solar corona structure and phenomena: (1) heliospheric current sheet, (2) streams from coronal holes, (3) streams from coronal streamers, (4) plasma disturbed by interplanetary shocks, and (5) coronal mass ejections. As for quasistationary streams of solar wind, maximum mass flux is recorded in the streams emanating from the coronal streamers while maximum thermal and kinetic energy fluxes are observed in the streams from the coronal holes. The momentum fluxes are equal in both types of streams. Maximum ratio of thermal and magnetic pressures is observed in heliospheric current sheet. The maximum helium abundance is observed in coronal mass ejection, it is higher in streams from coronal holes than in streams from streamers, and its dependences on density and mass flux are different in different types of the streams. Dynamics of alpha-particle velocity and temperature relative to protons in different streams is discussed.

Yermolaev, Yu. I.↗