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At least 559 records · Page 31

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NASA NTRS1989

Hypersonic blunt body computations including real gas effects

Various second-order explicit and implicit TVD shock-capturing methods, a generalization of Roe's approximate Riemann solver, and a generalized flux-vector splitting scheme are used to study two-dimensional hypersonic real-gas flows. Special attention is given to the identification of some of the elements and parameters which can affect the convergence rate for high Mach numbers or real gases, but have negligible effect for low Mach numbers, for cases involving steady-state inviscid blunt flows. Blunt body calculations at Mach numbers of greater than 15 are performed to treat real-gas effects, and impinging shock results are obtained to test the treatment of slip surfaces and complex structures. Even with the addition of improvements, the convergence rate of algorithms in the hypersonic flow regime is found to be generally slower for a real gas than for a perfect gas.

Montagne, J.-L.↗

NASA NTRS1990

Welding And Cutting A Nickel Alloy By Laser

Technique effective and energy-efficient. Report describes evaluation of laser welding and cutting of Inconel(R) 718. Notes that electron-beam welding processes developed for In-718, but difficult to use on large or complex structures. Cutting of In-718 by laser fast and produces only narrow kerf. Cut edge requires dressing, to endure fatigue.

Banas, C. M.↗

NASA NTRS1989

Efficient numerical method for computation of thermohydrodynamics of laminar lubricating films

The purpose of this paper is to describe an accurate, yet economical, method for computing temperature effects in laminar lubricating films in two dimensions. The procedure presented here is a sequel to one presented in Leeds in 1986 that was carried out for the one-dimensional case. Because of the marked dependence of lubricant viscosity on temperature, the effect of viscosity variation both across and along a lubricating film can dwarf other deviations from ideal constant-property lubrication. In practice, a thermohydrodynamics program will involve simultaneous solution of the film lubrication problem, together with heat conduction in a solid, complex structure. The extent of computation required makes economy in numerical processing of utmost importance. In pursuit of such economy, we here use techniques similar to those for Gaussian quadrature. We show that, for many purposes, the use of just two properly positioned temperatures (Lobatto points) characterizes well the transverse temperature distribution.

Elrod, Harold G.↗

NASA NTRS1989

ASCIE: An integrated experiment to study CSI in large segmented optical systems

A description of the advanced structures/control integrated experiment (ASCIE) experimental setup, a generic test bed for several essential technologies was presented. In particular its multi-input, multi-output, non-collocated control system and its complex structural dynamics, characteristic of large segmented systems make it an ideal test bed for Control-Structure Interaction (CSI) experiments. The high accuracy of its measurement system will make it possible to investigate the dynamics of microvibrations and its implication for the CSI phenomenon.

Aubrun, Jean-Noel↗

NASA NTRS1990

PCI: A PATRAN-NASTRAN model translator

The amount of programming required to develop a PATRAN-NASTRAN translator was surprisingly small. The approach taken produced a highly flexible translator comparable with the PATNAS translator and superior to the PATCOS translator. The coding required varied from around ten lines for a shell element to around thirty for a bar element, and the time required to add a feature to the program is typically less than an hour. The use of a lookup table for element names makes the translator also applicable to other versions of NASTRAN. The saving in time as a result of using PDA's Gateway utilities was considerable. During the writing of the program it became apparent that, with a somewhat more complex structure, it would be possible to extend the element data file to contain all data required to define the translation from PATRAN to NASTRAN by mapping of data between formats. Similar data files on property, material and grid formats would produce a completely universal translator from PATRAN to any FEA program, or indeed any CAE system.

Sheerer, T. J.↗

NASA NTRS1990

Obtaining an equivalent beam

In modeling a complex structure the researcher was faced with a component that would have logical appeal if it were modeled as a beam. The structure was a mast of a robot controlled gantry crane. The structure up to this point already had a large number of degrees of freedom, so the idea of conserving grid points by modeling the mast as a beam was attractive. The researcher decided to make a separate problem of of the mast and model it in three dimensions with plates, then extract the equivalent beam properties by setting up the loading to simulate beam-like deformation and constraints. The results could then be used to represent the mast as a beam in the full model. A comparison was made of properties derived from models of different constraints versus manual calculations. The researcher shows that the three-dimensional model is ineffective in trying to conform to the requirements of an equivalent beam representation. If a full 3-D plate model were used in the complete representation of the crane structure, good results would be obtained. Since the attempt is to economize on the size of the model, a better way to achieve the same results is to use substructuring and condense the mast to equivalent end boundary and intermediate mass points.

Butler, Thomas G.↗

NASA NTRS1989

Analysis of Thematic Mapper data for studying the suspended matter distribution in the coastal area of the German Bight (North Sea)

Thematic Mapper data were analyzed with respect to its capability for mapping the complex structure and dynamics of suspended matter distribution in the coastal area of the German Bight (North Sea). Three independent pieces of information were found by factor analysis of all seven TM channels: suspended matter concentration, atmospheric scattering, and sea surface temperature. For the required atmospheric correction, the signal-to-noise ratios of Channels 5 and 7 have to be improved by averaging over 25 x 25 pixels, which also makes it possible to monitor the aerosol optical depth and aerosol type over cloud-free water surfaces. Near-surface suspended matter concentrations may be detected with an accuracy of factor less than 2 by using an algorithm derived from radiative transfer model calculation. The patchiness of suspended matter and its relation to underwater topography was analyzed with autocorrelation and cross-correlation.

Doerffer, R.↗

NASA NTRS1989

Failure processes within ceramic coatings at high temperatures

Plasma sprayed coatings have a complex structure which is produced by the overlaying of many molten or semi-molten particles in the diameter range of 20 to 120 microns. The failure behavior of coatings has been characterized by using acoustic emission (AE) methodology. Coatings of NiCrAlY bond coat with a zirconia-12 wt pct yttria overlay were applied to disk-shaped specimens of U-700 alloy. A waveguide of 1-mm diameter platinum was TlG welded to the specimen and allowed it to be suspended in a tubular furnace. The specimen was thermally cycled to 1150 C and the AE monitored. Analyses have been carried out with the aim of establishing parameters which are related to the crack size population. These studies have been used to postulate types of cracking mechanisms which may occur in plasma sprayed coatings during thermal cycling. It is shown that microcracking gave rise to a large amount of AE. However, this coating still survived more thermal cycles than a coating which exhibited macrocracking events.

Berndt, Christopher↗

NASA NTRS1989

Magnetic fields at Neptune

The Voyager 2 magnetic field experiment discovered a complex and powerful magnetic field in Neptune, as well as an associated magnetosphere and magnetic tail. As the spacecraft exited the magnetosphere, the magnetic tail appeared to be monopolar. The auroral zones are probably located far from the rotation poles, and may possess complex geometry. The Neptune rings and all its known moons are imbedded deep within the magnetosphere (except for Nereid, which is outside when it lies sunward of the planet); the radiation belts have a complex structure due to the absorption of energetic particles by the moons and rings of Neptune, as well as losses associated with the significant changes in the diurnally varying magnetosphere configuration.

Ness, Norman F.↗

NASA NTRS1990

Characteristics of wave-particle interactions during sudden commencements. II - Spacecraft observations

Wave data from the DE 1 spacecraft for 50 sudden commencements were analyzed for amplitude and spectral modifications and correlated with data from ground-based observatories. Changes in wave activity were identified in 14 of the events studied. The changes, which were commonly observed at frequencies below the local electron gyrofrequency, exhibited a complex structure both in frequency and in time and varied considerably among events. The typical event lasted 1-10 min with a maximum narrowband amplitude increase or decrease of 10-30 dB. The onset time of the wave growth was correlated with the arrival of the magnetic disturbance at the spacecraft. Changes were most commonly observed at L shells in the range of L values between 3 and 6. Growth was observed on the nightside as well as the dayside, and no clear local time dependence was found. In several cases, growth at frequencies above the local gyrofrequency was noted and identified as auroral kilometric radiation.

Gail, W. B.↗

NASA NTRS1990

Microwave and X-ray observations of a major confined solar flare

Observations of an X4 flare of May 19, 1984 using the VLA and the SMM X-ray Polychromator are discussed. The 6 cm radio source remained stationary throughout the decay phase. Combined with other evidence, this indicates that the flare was magnetically confined and did not disrupt the complex structure of its region. The observed structures may be associated with a neutral sheet connecting the two bipoles.

Schmahl, E. J.↗

NASA NTRS1990

On lightly damped linear systems

A simple engineering approximation using undamped modes is shown to be valid for determining the effect of small viscous or equivalent viscous-type damping forces on the forced vibration response of a lightly damped linear systems. It is shown that intermodal coupling of the undamped modes may be neglected with negligible error, except when the system has degenerate or quasi-degenerate modes. This can occur in complex structural systems such as aircraft or spacecraft. In such cases, one of these two modes has virtually no damping, whereas the other effectively has the damping of both.

Young, M. I.↗

NASA NTRS1990

Spatially resolved optical and near infrared spectroscopy of I Zw 18

For a long time, I Zw 18 has been recognized as the lowest abundance extra-galactic HII region (with the possible recent exception of SBS 0335-052; Izotov 1989). As such, it is important for many studies, including the determination of the primordial helium abundance. Recent imaging studies of I Zw 18 have revealed a more complex structure to I Zw 18 than the simple two-component model previously assumed. This has given rise to concern about the reliability of chemical abundance measurements derived for I Zw 18. Researchers have obtained long-slit spectra covering the wavelength range lambda 3650 to lambda 10,000, which allow us to measure physical parameters and chemical abundances as a function of position. With these new data we can investigate the SE component, which has not been studied previously, and we can address some of the concerns about abundance uncertainties.

Skillman, Evan D.↗

NASA NTRS1990

Multiband radar characterization of forest biomes

The utility of airborne and orbital SAR in classification, assessment, and monitoring of forest biomes is investigated through analysis of orbital synthetic aperature radar (SAR) and multifrequency and multipolarized airborne SAR imagery relying on image tone and texture. Preliminary airborne SAR experiments and truck-mounted scatterometer observations demonstrated that the three dimensional structural complexity of a forest, and the various scales of temporal dynamics in the microwave dielectric properties of both trees and the underlying substrate would severely limit empirical or semi-empirical approaches. As a consequence, it became necessary to develop a more profound understanding of the electromagnetic properties of a forest scene and their temporal dynamics through controlled experimentation coupled with theoretical development and verification. The concatenation of various models into a physically-based composite model treating the entire forest scene became the major objective of the study as this is the key to development of a series of robust retrieval algorithms for forest biophysical properties. In order to verify the performance of the component elements of the composite model, a series of controlled laboratory and field experiments were undertaken to: (1) develop techniques to measure the microwave dielectric properties of vegetation; (2) relate the microwave dielectric properties of vegetation to more readily measured characteristics such as density and moisture content; (3) calculate the radar cross-section of leaves, and cylinders; (4) improve backscatter models for rough surfaces; and (5) relate attenuation and phase delays during propagation through canopies to canopy properties. These modeling efforts, as validated by the measurements, were incorporated within a larger model known as the Michigan Microwave Canopy Scattering (MIMICS) Model.

Dobson, M. Craig↗

NASA NTRS1990

AEOSS runtime manual for system analysis on Advanced Earth-Orbital Spacecraft Systems

Advanced earth orbital spacecraft system (AEOSS) enables users to project the required power, weight, and cost for a generic earth-orbital spacecraft system. These variables are calculated on the component and subsystem levels, and then the system level. The included six subsystems are electric power, thermal control, structure, auxiliary propulsion, attitude control, and communication, command, and data handling. The costs are computed using statistically determined models that were derived from the flown spacecraft in the past and were categorized into classes according to their functions and structural complexity. Selected design and performance analyses for essential components and subsystems are also provided. AEOSS has the feature permitting a user to enter known values of these parameters, totally and partially, at all levels. All information is of vital importance to project managers of subsystems or a spacecraft system. AEOSS is a specially tailored software coded from the relational database program of the Acius' 4th Dimension with a Macintosh version. Because of the licensing agreements, two versions of the AEOSS documents were prepared. This version, AEOSS Runtime Manual, is permitted to be distributed with a finite number of the restrictive 4D Runtime version. It can perform all contained applications without any programming alterations.

Lee, Hwa-Ping↗

NASA NTRS1991

Mechanical response tissue analyzer for estimating bone strength

One of the major concerns for extended space flight is weakness of the long bones of the legs, composed primarily of cortical bone, that functions to provide mechanical support. The strength of cortical bone is due to its complex structure, described simplistically as cylinders of parallel osteons composed of layers of mineralized collagen. The reduced mechanical stresses during space flight or immobilization of bone on Earth reduces the mineral content, and changes the components of its matrix and structure so that its strength is reduced. Currently, the established clinical measures of bone strength are indirect. The measures are based on determinations of mineral density by means of radiography, photon absorptiometry, and quantitative computer tomography. While the mineral content of bone is essential to its strength, there is growing awareness of the limitations of the measurement as the sole predictor of fracture risk in metabolic bone diseases, especially limitations of the measurement as the sole predictor of fracture risk in metabolic bone diseases, especially osteoporosis. Other experimental methods in clinical trials that more directly evaluate the physical properties of bone, and do not require exposure to radiation, include ultrasound, acoustic emission, and low-frequency mechanical vibration. The last method can be considered a direct measure of the functional capacity of a long bone since it quantifies the mechanical response to a stimulus delivered directly to the bone. A low frequency vibration induces a response (impedance) curve with a minimum at the resonant frequency, that a few investigators use for the evaluation of the bone. An alternative approach, the method under consideration, is to use the response curve as the basis for determination of the bone bending stiffness EI (E is the intrinsic material property and I is the cross-sectional moment of inertia) and mass, fundamental mechanical properties of bone.

Arnaud, Sara B.↗

NASA NTRS1989

Independent modal space control with positive position feedback

An independent modal space control (IMSC) algorithm is presented, whose modal control forces are generated from a positive position feedback (PPF) strategy. The proposed algorithm combines the attributes of both the IMSC and the PPF, and maintains the simplicity of the IMSC as it designs the controller of a complex structure at the uncoupled modal level. The effectiveness of the algorithm in damping out the vibration of flexible structures is validated experimentally. A simple cantilevered beam is employed as an example of a flexible structure whose multimodes of vibration are controlled by a single actuator. Performance of the active control system is determined in the frequency and the time domains. The experimental results indicate the potential of the proposed methodology as a viable method for controlling the vibration of large flexible structures.

Baz, A.↗

NASA NTRS1991

Circular polarization in 1612 MHz OH maser emission from OH/IR stars

In order to study the incidence of circular polarization in the 1612 MHz OH emission from envelopes of OH/IR stars, the Arecibo radio telescope was used to obtain high-resolution, high signal-to-noise 1612 MHz spectra in both circular polarizations for several OH/IR stars. The method of Troland and Heiles (1982) was used to estimate the intensities of magnetic fields in the envelopes. The fields appear to have complex structures, and intensities on the order of 1-100 microG. The spectra are weakly polarized, probably as a result of weakly polarized emission from individual emitting elements. Integration over the Stokes parameter V yields nonzero results for three of the stars, suggesting that another polarizing mechanism (in addition to simple Zeeman splittings) is at work.

Zell, Philip↗