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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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NASA NTRS1982

Approximating a finite element model of a hybrid deployable truss by a discrete mass model

The NASTRAN finite element model of the MSFC/hybrid deployable truss is approximated by a model consisting of four bodies serially connected by a spring hinge suspension. The truss is decomposed into four serially connected modules determining the total mass and rotational inertias for a rigid body with its mass center on the axis of the truss for each module. A five-body cantilever truss model is developed along with an undamped form of this model. Spring coefficient values are generated to match eigenvalues of the latter model with those of the NASTRAN model. Damping coefficients are then determined that provide n percent of critical damping for the model.

Chichester, F. D.↗
NASA NTRS1984

Helicopter Pilot Performance for Discrete-maneuver Flight Tasks

This paper describes a current study of several basic helicopter flight maneuvers. The data base consists of in-flight measurements from instrumented helicopters using experienced pilots. The analysis technique is simple enough to apply without automatic data processing, and the results can be used to build quantitative matah models of the flight task and some aspects of the pilot control strategy. In addition to describing the performance measurement technqiue, some results are presented which define the aggressiveness and amplitude of maneuvering for several lateral maneuvers including turns and sidesteps.

Heffley, R. K.↗
NASA NTRS1984

Discrete models for the numerical analysis of time-dependent multidimensional gas dynamics

A possible technique is explored for extending to multidimensional flows some of the upwind-differencing methods that are highly successful in the one-dimensional case. Emphasis is on the two-dimensional case, and the flow domain is assumed to be divided into polygonal computational elements. Inside each element, the flow is represented by a local superposition of elementary solutions consisting of plane waves not necessarily aligned with the element boundaries.

Roe, P. L.↗
NASA NTRS1985

Capabilities of the GRO/BATSE for monitoring of discrete sources

Although the Burst and Transient Source Experiment (BATSE) to be flown on the Gamma Ray Observatory has as its primary objective the detection of gamma ray bursts, its uncollimated design will enable it to serve a unique function as an all-sky monitor for bright hard X-ray and low-energy gamma ray sources. Pulsating sources may be detected by conventional techniques such as summed-epoch and Fourier analyses. The BATSE will, in addition, be able to use Earth occultation in an unprecedented way to monitor sufficiently bright sources as often as several times per day over approx. 85% of the sky. Estimates of the expected BATSE sensitivity using both of these techniques are presented.

Wilson, R. B.↗
NASA NTRS1985

Two-dimensional quasineutral description of particles and fields above discrete auroral arcs

Stationary hot and cool particle distributions in the auroral magnetosphere are modelled using adiabatic assumptions of particle motion in the presence of broad-scale electrostatic potential structure. The study has identified geometrical restrictions on the type of broadscale potential structure which can be supported by a multispecies plasma having specified sources and energies. Without energization of cool thermal ionospheric electrons, a substantial parallel potential drop cannot be supported down to altitudes of 2000 km or less. Observed upward-directed field-aligned currents must be closed by return currents along field lines which support little net potential drop. In such regions the plasma density appears significantly enhanced. Model details agree well with recent broad-scale implications of satellite observations.

Newman, A. L.↗
NASA NTRS1985

Assessment of discretization schemes to reduce numerical diffusion in the calculation of complex flows

Comprehensive criteria for evaluating finite-difference schemes to reduce numerical diffusion in the solution of the Reynolds averaged Navier-Stokes equations have been established in this study. Several candidate schemes have been examined, based on these criteria. Taylor series analyses, model problem studies and examination of the difference equations have been used to assess the schemes. Two schemes, which showed the most promise, and satisfied a majority of the criteria, are recommended for incorporation into two dimensional and three dimensional computer codes for further evaluation.

Syed, S. A.↗
NASA NTRS1984

Stability and accuracy analysis of some fully-discrete algorithms for the one-dimensional second-order wave equation

The present investigation is concerned with some basic results for a predictor-multicorrector algorithm applied to the one-dimensional wave equation, giving particular attention to so-called 2-pass explicit schemes in which both lumped and coupled mass matrices are employed. In an assessment of the accuracy and stability properties of the algorithms, use is made of the one-dimensional, second-order wave equation. The maximum stable time step of the lumped right-hand-side mass, 2-pass explicit algorithm is twice that of the 1-pass explicit algorithm. Improved accuracy is obtained by employing higher-order, or consistent, right-hand-side, mass.

Hughes, T. J. R.↗
NASA NTRS1985

Some design guidelines for discrete-time adaptive controllers

There have been many algorithms proposed for adaptive control which will provide globally asymptotically stable controllers if some stringent conditions on the plant are met. The conditions on the plant cannot be met in practice as all plants will contain high frequency unmolded dynamics therefore, blind implementation of the published algorithms can lead to disastrous results. This paper uses a linearization analysis of a non-linear adaptive controller to demonstrate analytically design guidelines which aleviate some of the problems associated with adaptive control in the presence of unmodeled dynamics.

Rohrs, C. E.↗
NASA NTRS1986

Two-dimensional quasi-neutral description of particles and fields above discrete auroral arcs

Models are presented for particle distributions, electric fields and currents in an adiabatic treatment of auroral electrostatic potential distributions in order to describe the quiet-time evening auroral arcs featuring both upward and return currents. The models are consistent with current continuity and charge balance requirements for particle populations controlled by adiabatic invariants and quasi-neutrality in the magnetosphere. The effective energy of the cool electron population is demonstrated to have a significant effect on the latitudinal breadth of the auroral electrostatic potential structure and the extent of the penetration of the accelerating potential into the ionosphere. Another finding is that the energy of any parallel potential drop in the lowest few thousand kilometers of the field line is of the same order of magnitude as the thermal energy of the cool electrons. Additional predictions include density cavities along field lines that support large potential drops, and density enhancements along field lines at the edge of an inverted V with a small potential drop.

Newman, A. L.↗
NASA NTRS1986

Discrete models for the numerical analysis of time-dependent multidimensional gas dynamics

A possible technique is explored for extending to multidimensional flows some of the upwind-differencing methods that are highly successful in the one-dimensional case. Emphasis is on the two-dimensional case, and the flow domain is assumed to be divided into polygonal computational elements. Inside each element, the flow is represented by a local superposition of elementary solutions consisting of plane waves not necessarily aligned with the element boundaries.

Roe, P. L.↗
NASA NTRS1987

Discrete element weld model, phase 2

A numerical method was developed for analyzing the tungsten inert gas (TIG) welding process. The phenomena being modeled include melting under the arc and the flow in the melt under the action of buoyancy, surface tension, and electromagnetic forces. The latter entails the calculation of the electric potential and the computation of electric current and magnetic field therefrom. Melting may occur at a single temperature or over a temperature range, and the electrical and thermal conductivities can be a function of temperature. Results of sample calculations are presented and discussed at length. A major research contribution has been the development of numerical methodology for the calculation of phase change problems in a fixed grid framework. The model has been implemented on CHAM's general purpose computer code PHOENICS. The inputs to the computer model include: geometric parameters, material properties, and weld process parameters.

Prakash, C.↗
NASA NTRS1986

The discrete one-sided Lipschitz condition for convex scalar conservation laws

Physical solutions to convex scalar conservation laws satisfy a one-sided Lipschitz condition (OSLC) that enforces both the entropy condition and their variation boundedness. Consistency with this condition is therefore desirable for a numerical scheme and was proved for both the Godunov and the Lax-Friedrichs scheme--also, in a weakened version, for the Roe scheme, all of them being only first order accurate. A new, fully second order scheme is introduced here, which is consistent with the OSLC. The modified equation is considered and shows interesting features. Another second order scheme is then considered and numerical results are discussed.

Brenier, Yann↗
NASA NTRS1987

Discrete X-ray sources and the X-ray background

Since the discovery of a highly uniform X-ray background (XRB) in the 2 to 10 keV range, its nature is not yet fully explained. It appears clear from the results of Einstein medium and deep surveys that at least 50% of the XRB is due to individual extragalactic sources when their contribution is integrated to Z = 3. This includes contribution from Quasi Stellar Objects (QSOs), Active Galactic Nuclei (AGNs), galaxies, and clusters of galaxies. The average spectrum of each of the individual contributing sources is softer than that of the observed XRB. Therefore, the remaining contribution must have a rather hard spectrum of alpha nearly equal to 0.0 to 0.2. It is unlikely that this spectrum can be produced by diffuse processes. Therefore, the remainder of the XRB must be due to individual sources with the appropriate spectrum. This requires either that the spectrum of the already identified sources changes at early epochs or a new class of objects. Advanced X-ray Astrophysics Facility (AXAF) observations will extend survey sensitivity to limiting fluxes of order of 3 x 10 to the minus 16 erg/sq cm/s, some 50 times fainter than any previous survey. There will be sufficient sensitivity and angular resolution to permit identification and study of these objects.

Giacconi, Riccardo↗
NASA NTRS1988

Discrete fluorescent saturation regimes in multilevel systems

Using models of multilevel atoms, the fluorescent process was examined for the ratio of the photooxidation rate, Pij, to the collisional oxidation rate, Cij, in the pumped resonance transition i-j. It is shown that, in the full range of the parameter Pij/Cij, there exist three distinct regimes (I, II, and III) which may be usefully exploited. These regimes are defined, respectively, by the following conditions: Pij/Cij smaller than about 1; Pij/Cij much greater than 1 and Pij much lower than Cki; and Pij/Cij much greater than 1 and Pij much higher than Cki, where Cki is the collisional rate populating the source level i. The only regime which is characterized by the sensitivity of fluorescent-fluorescent line intensity ratios to Pij is regime I. If regime III is reached, even fluorescent-nonfluorescent line ratios become independent of Pij. The analysis is applied to the resonant photoexcitation of a carbonlike ion.

Kastner, S. O.↗
NASA NTRS1988

Discrete analysis of spatial-sensitivity models

Procedures for reducing the computational burden of current models of spatial vision are described, the simplifications being consistent with the prediction of the complete model. A method for using pattern-sensitivity measurements to estimate the initial linear transformation is also proposed which is based on the assumption that detection performance is monotonic with the vector length of the sensor responses. It is shown how contrast-threshold data can be used to estimate the linear transformation needed to characterize threshold performance.

Nielsen, Kenneth R. K.↗
NASA NTRS1988

Numerically stable algorithm for discrete-ordinate-method radiative transfer in multiple scattering and emitting layered media

The transfer of monochromatic radiation in a scattering, absorbing, and emitting plane-parallel medium with a specified bidirectional reflectivity at the lower boundary is considered. The equations and boundary conditions are summarized. The numerical implementation of the theory is discussed with attention given to the reliable and efficient computation of eigenvalues and eigenvectors. Ways of avoiding fatal overflows and ill-conditioning in the matrix inversion needed to determine the integration constants are also presented.

Stamnes, Knut↗