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At least 703 records · Page 39

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NASA NTRS1987

Kinetic aspects of magnetotail dynamics - Observations

The observed kinetic (as opposed to fluid) aspects of particle behavior in the geomagnetic tail are discussed. Such behavior has been observed from thermal plasma energies to energies a few MeV or more. The importance of recognizing and analyzing the kinetic behavior of tail particles is that it provides a powerful diagnostic tool for inferring the state of the local and remote environment and the mechanisms responsible for observed tail dynamics, and a way of determining when fluid analyses (relying upon moments such as bulk velocity and temperature) may be misleading. Thus kinetic analyses provide a valuable complement to fluid-dynamics-based studies of the collective behavior of geomagnetic tail particles. Observed kinetic effects can be analyzed to examine spatial gradients; convection electric fields, distances to source regions, source-region characteristics, magnetic-field topology, heating/acceleration mechanism, boundary characteristics, and remote mechanisms affecting local particle distributions.

Mitchell, D. G.↗

NASA NTRS1989

Test aspects of the JPL Viterbi decoder

The generation of test vectors and design-for-test aspects of the Jet Propulsion Laboratory (JPL) Very Large Scale Integration (VLSI) Viterbi decoder chip is discussed. Each processor integrated circuit (IC) contains over 20,000 gates. To achieve a high degree of testability, a scan architecture is employed. The logic has been partitioned so that very few test vectors are required to test the entire chip. In addition, since several blocks of logic are replicated numerous times on this chip, test vectors need only be generated for each block, rather than for the entire circuit. These unique blocks of logic have been identified and test sets generated for them. The approach employed for testing was to use pseudo-exhaustive test vectors whenever feasible. That is, each cone of logid is tested exhaustively. Using this approach, no detailed logic design or fault model is required. All faults which modify the function of a block of combinational logic are detected, such as all irredundant single and multiple stuck-at faults.

Breuer, M. A.↗

NASA NTRS1989

Some low-speed flutter characteristics of simple low-aspect-ratio delta wing models

Some results from a combined experimental and analytical study of the low-speed flutter characteristics of low-aspect-ratio delta wings are presented. Data are presented which show the effects of sweep angle on the flutter characteristics of some simple plate models of constant planform area. The range of sweep angles studied was from 30 to 72 degrees. In addition, flutter results are presented for two 30 deg-sweep clipped-delta wing models. Further, results are presented that show the effects of root clamping (percentage length of the root chord that is cantilevered) for a 45 deg-sweep delta wing. The experimental data are compared with analytical results obtained by using kernel function and doublet lattice subsonic unsteady lifting surface theories.

Doggett, Robert V., Jr.↗

NASA NTRS1989

Computational aspects of multibody dynamics

Computational aspects are addressed which impact the requirements for developing a next generation software system for flexible multibody dynamics simulation which include: criteria for selecting candidate formulation, pairing of formulations with appropriate solution procedures, need for concurrent algorithms to utilize computer hardware advances, and provisions for allowing open-ended yet modular analysis modules.

Park, K. C.↗

NASA NTRS1989

Optimum design of swept-forward high-aspect-ration graphite-epoxy wings

An analytical investigation of a swept-forward high-aspect-ratio graphite-epoxy transport wing is described. The objectives of this investigation are to illustrate an effective usage of the unique properties of composite materials by exploiting material tailoring and to demonstrate an integrated multidisciplinary approach for conducting this investigation.

Shuart, Mark J.↗

NASA NTRS1989

Evaluation of a strain-gage load calibration on a low-aspect-ratio wing structure at elevated temperature

The environmental aspect of elevated temperature and its relationship to the science of strain gage calibrations of aircraft structures are addressed. A section of a wing designed for a high-speed aircraft structure was used to study this problem. This structure was instrumented with strain gages calibrated at both elevated and room temperatures. Load equations derived from a high-temperature load calibration were compared with equations derived from an identical load calibration at room temperature. The implications of the high temperature load calibration were studied from the viewpoint of applicability and necessity. Load equations derived from the room temperature load calibration resulted in generally lower equation standard errors than equations derived from the elevated temperature load calibration. A distributed load was applied to the structure at elevated temperature and strain gage outputs were measured. This applied load was then calculated using equations derived from both the room temperature and elevated temperature calibration data. It was found that no significant differences between the two equation systems existed in terms of computing this applied distributed load, as long as the thermal shifts resulting from thermal stresses could be identified. This identification requires a heating of the structure. Therefore, it is concluded that for this structure, a high temperature load calibration is not required. However, a heating of the structure is required to determine thermal shifts.

Reardon, Lawrence F.↗

NASA NTRS1985

Boundary-layer measurements on a transonic low-aspect ratio wing

Tabulations and plots are presented of boundary-layer velocity and flow-direction surveys from wind-tunnel tests of a large-scale (0.90 m semi-span) model of the NASA/Lockheed Wing C. This wing is a generic, transonic, supercritical, highly three-dimensional, low-aspect-ratio configuration designed with the use of a three-dimensional, transonic full-potential-flow wing code (FLO22). Tests were conducted at the design angle of attack of 5 deg over a Mach number range from 0.25 to 0.96 and a Reynolds number range of 3.4x10 to the 6th power. Wing pressures were measured at five span stations, and boundary-layer surveys were measured at the midspan station. The data are presented without analysis.

Keener, Earl R.↗

NASA NTRS1988

Systems aspects of a space nuclear reactor power system

Various system aspects of a 300-kW nuclear reactor power system for spacecraft have been investigated. Special attention is given to the cases of a reusable OTV and a space-based radar. It is demonstrated that the stowed length of the power system is important to mission design, and that orbital storage for months to years may be needed for missions involving orbital assembly.

Jaffe, L.↗

NASA NTRS1989

Some low-speed flutter characteristics of simple low-aspect-ratio delta wing models

Some results from a combined experimental and analytical study of the low-speed flutter characteristics of low-aspect-ratio delta wings are presented. Data are presented which show the effects of sweep angle on the flutter characteristics of some simple plate models of constant planform area. The range of sweep angles studied was from 30 to 72 degrees. In addition, flutter results are presented for two 30 deg-sweep clipped-delta wing models. Further results are presented that show the effects of root clamping (percentage length of the root chord that is cantilevered) for a 45 deg-sweep delta wing. The experimental data are compared with analytical results obtained by using kernel function and doublet lattice subsonic unsteady lifting surface theories.

Doggett, Robert V., Jr.↗

NASA NTRS1989

Three dimensional flow field measurements of a 4:1 aspect ratio subsonic jet

Flow field measurements for a subsonic rectangular cold air jet with an aspect ratio of 4:1 (12.7 x 50.8 mm) at a Mach number of 0.09 and Re of 100,000 have been carried out using a three-dimensional laser Doppler anemometer system. Mean velocity measurements show that the jet width spreads more rapidly along the minor axis than along the major axis. The outward velocities, however, are not significantly different for the two axes, indicating the presence of enhanced mixing along the minor axis. The jet slowly changes from a rectangular jet to a circular jet as the flow progresses downstream.

Morrison, G. L.↗

NASA NTRS1988

Thermosolutal convection in high-aspect-ratio enclosures

Convection in high-aspect-ratio rectangular enclosures with combined horizontal temperature and concentration gradients is studied experimentally. An electrochemical system is employed to impose the concentration gradients. The solutal buoyancy force either opposes or augments the thermal buoyancy force. Due to a large difference between the thermal and solutal diffusion rates the flow possesses double-diffusive characteristics. Various complex flow patterns are observed with different experimental conditions.

Wang, L. W.↗

NASA NTRS1990

Fundamental aspects of and failure modes in high-temperature composites

Fundamental aspects of and attendant failure mechanisms for high temperature composites are summarized. These include: (1) in-situ matrix behavior; (2) load transfer; (3) limits on matrix ductility to survive a given number of cyclic loadings; (4) fundamental parameters which govern thermal stresses; (5) vibration stresses; and (6) impact resistance. The resulting guidelines are presented in terms of simple equations which are suitable for the preliminary assessment of the merits of a particular high temperature composite in a specific application.

Chamis, Christos C.↗

NASA NTRS1990

Some Aspects of Essentially Nonoscillatory (ENO) Formulations for the Euler Equations, Part 3

An essentially nonoscillatory (ENO) formulation is described for hyperbolic systems of conservation laws. ENO approaches are based on smart interpolation to avoid spurious numerical oscillations. ENO schemes are a superset of Total Variation Diminishing (TVD) schemes. In the recent past, TVD formulations were used to construct shock capturing finite difference methods. At extremum points of the solution, TVD schemes automatically reduce to being first-order accurate discretizations locally, while away from extrema they can be constructed to be of higher order accuracy. The new framework helps construct essentially non-oscillatory finite difference methods without recourse to local reductions of accuracy to first order. Thus arbitrarily high orders of accuracy can be obtained. The basic general ideas of the new approach can be specialized in several ways and one specific implementation is described based on: (1) the integral form of the conservation laws; (2) reconstruction based on the primitive functions; (3) extension to multiple dimensions in a tensor product fashion; and (4) Runge-Kutta time integration. The resulting method is fourth-order accurate in time and space and is applicable to uniform Cartesian grids. The construction of such schemes for scalar equations and systems in one and two space dimensions is described along with several examples which illustrate interesting aspects of the new approach.

Chakravarthy, Sukumar R.↗

NASA NTRS1989

Some aspects of transient cooling of a radiating rectangular medium

The emission from a gray radiating medium is analyzed for transient cooling in surroundings at a low temperature. The medium is rectangular with no variations in the direction normal to the cross section. The integral equation for the transient temperature distribution is solved numerically using a two-dimensional Gaussian integration subroutine. The emissive ability for a rectangle at uniform temperature is compared with that for transient cooling where the temperature distribution of the region has reached a fully developed shape, as shown by a separation of variables solution. The two solutions provide the upper and lower bounds for the emittance of a rectangle during transient cooling. The emittances for various aspect ratios are presented as a function of the mean length of the rectangle and are compared with results for a plane layer and a cylinder.

Siegel, Robert↗

NASA NTRS1990

Effects of spoiler surfaces on the aeroelastic behavior of a low-aspect-ratio rectangular wing

The relationship between spoiler surfaces and flutter for a low-aspect ratio, rectangular wing is examined. The model design is a rigid wing mounted to the wing tunnel wall using a flexible, rectangular beam. The spoiler surface is composed of thin, rectangular aluminum plates that are vertically mounted to the wing surface. The effects of changes in spoiler geometry and location on wing flutter are studied using wind tunnel testing. It is determined that increases in spoiler height and weight cause an increase in the flutter dynamic pressure; however, variations in the location of the spoiler surfaces have little effect on flutter. Diagrams of the wing model are provided.

Cole, Stanley R.↗

NASA NTRS1990

Aspects of the Kelvin wave response to episodic wind forcing

Episodes of westerly wind are an important aspect of surface stress variability in the western Pacific. During ENSO periods, the presence of such wind episodes comprises much of the LF relaxation of the trades over the central and western Pacific. This paper describes the oceanic Kelvin pulse response to a single idealized episode of westerly wind stress, using results from linear theory as well as from a 27-level general circulation model. When stratification typical of the western and eastern Pacific is used, the conservation of energy flux predicts a reduction of surface currents associated with the first baroclinic mode and an enhancement of surface currents associated with the second baroclinic mode. The idealized wind anomaly is also used to drive an ocean general circulation model. When the wind anomaly is weak, the model Kelvin response agrees with predictions of linear theory. For more realistic strong forcing there are three important deviations from linear theory: the amplitude of low baroclinic modes increases; the amplitude of higher baroclinic modes decreases; and the phase speed increases.

Giese, Benjamin S.↗

NASA NTRS1990

Manual control aspects of Space Station docking maneuvers

Due to an increase in spacecraft traffic forecasted for the Space Station era, researchers are investigating manual control and other aspects of docking operations with hopes of increasing safety, productivity, and likelihood of success while decreasing cost. Experiments have been performed which revealed the effect of approach velocity, in-flight anomalies, and control mode. Displays have been designed to enable flight planners to more easily overcome the difficulties presented by orbital mechanics. Improved understanding of human factors in the docking mission and other orbital maneuvers will play a significant role in design tradeoffs concerning thruster size, docking fixture style and mass, and on-board trajectory planning displays. Incorporating both empirical and analytic results into current and future planning of missions occurring not only in earth orbit, but also for missions in lunar and Mars orbit, will expand the performance envelopes of the astronauts who participate in these missions.

Brody, Adam R.↗

NASA NTRS1990

Fundamental aspects and failure modes in high-temperature composites

Fundamental aspects and attendant failure mechanisms for high-temperature composites are summarized. These include in situ matrix behavior, load transfer, limits on matrix ductility to survive a given number of cyclic loadings, fundamental parameters which govern thermal stresses, vibration stresses and impact resistance, as well as their attendant failure mechanisms and failure sequences. The resulting guidelines are presented in terms of simple equations which are suitable for the preliminary assessment of the merits of a particular high-temperature composite in a specific application.

Chamis, C. C.↗