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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 1,063 records · Page 59
All sourcesNASA NTRSDOE OSTI
NASA NTRS1987

A canopy reflectance model based on an analytical solution to the multiple scattering equation

An approximation to the radiative transfer equation for solar radiation in relatively full, homogeneous plant canopies is presented and solved analytically for solar zenith angles less than 60 deg. The model predicts reflectance at any depth in the canopy and in any direction and may be inverted with bidirectional reflectance measurements. The model was fit to data at two sun angles and two wavebands (visible and NIR) to within the assumed errors on the reflectance data. The calculated albedos are insensitive to achievable measurement errors. Some of the parameter values themselves found by the inversion agree reasonably well with independent measurements, but the uncertainties introduced by the data noise are rather large. However, the agreement is good enough to demonstrate that the model is physically realistic.

Camillo, Peter↗
NASA NTRS1988

Existence, uniqueness and regularity of a time-periodic probability density distribution arising in a sedimentation-diffusion problem

The sedimentation and diffusion of a nonneutrally buoyant Brownian particle in vertical fluid-filled cylinder of finite length which is instantaneously inverted at regular intervals are investigated analytically. A one-dimensional convective-diffusive equation is derived to describe the temporal and spatial evolution of the probability density; a periodicity condition is formulated; the applicability of Fredholm theory is established; and the parameter-space regions are determined within which the existence and uniqueness of solutions are guaranteed. Numerical results for sample problems are presented graphically and briefly characterized.

Nitsche, Ludwig C.↗
NASA NTRS1988

The dual nature of the supernova remnant G351.2 + 0.1

The Galactic SNR G351.2 + 0.1 has been imaged at frequencies of 1.4, 5, and 15 GHz. The images reveal a source composed of two distinct components: a broken elongated shell measuring 4 x 6 arcmin and a central core about 15 arcsec in diameter. The two components are spectrally distinct. The shell has a nonthermal spectrum (alpha = -0.4) typical for Galactic SNRs, while the central core has an inverted spectrum (alpha = +0.27). The central core may be an analog of the Crab Nebula (a synchrotron nebula powered by a pulsar), but if so it is not typical of such objects.

Becker, R. H.↗
NASA NTRS1987

CMOS detector cells for holographic optical interconnects in microcircuits

This paper reports on the characterization of CMOS detectors for holographic optical interconnects in micro circuits. A VLSI temporal response system has been built, which has high-magnification viewing capabilities to facilitate the identification of the sample area under investigation. An isolated photodiode and load circuit has been characterized to determine responsivity, response time, and light spot positioning effects. The threshold of optical gate cells incorporating the above detectors and a transistor inverter stage to couple to other circuitry has been determined. The rise time and fall time of the optical gate cells have also been determined experimentally. The results were compared with the results of SPICE simulation, and show satisfactory agreement. The time delay of optical gate cell output was thus determined to be 70 ns at 10 microW light input. Threshold power was 0.5 microW light input.

Wu, W. H.↗
NASA NTRS1988

On the spectrum of the secondary auroral electrons

Results are presented from a statistical study of the spectral characteristics of the (power-law-portion) secondary electrons associated with inverted-V electron precipitation events (IVEs). A total of 106 IVEs observed at low altitudes (below 1000 km) by the DE 2 spacecraft are included in data base. Spectral parameters for the electrons between 20 eV and 100 eV are plotted against the different parameters pertaining to the locations of the IVEs and the primary beams to determine their empirical dependences. It is found that the differential spectrum of the secondary electrons can be modeled by a power law, J proportional to E exp -gamma, with an averaged gamma of 1.85. This spectral parameter is relatively insensitive to the variation of locations of the IVEs, but it depends on the primary beam parameters. These results are compared with predictions, and theories on the generation of the low-energy auroral power-law secondary electrons are assessed. The analysis shows that collisional processses are the dominant source of the secondary electrons.

Fung, S. F.↗
NASA NTRS1988

Stochastic estimation of organized turbulent structure - Homogeneous shear flow

A generalization of the conditional-eddy concept is proposed in which the conditional event specifies the local kinematic state in terms of the velocity and the deformation. Results are presented for stochastically estimated conditional eddies given the local kinematics. The equation governing the probability density function of a kinematic state has been derived for constant-property incompressible flow, providing a link between coherent flow structures corresponding to the conditional eddies and the modelling of turbulent transport. The primary contributions to the second-quadrant and fourth-quadrant Reynolds-stress events in homogeneous shear flow are shown to come from flow induced through the 'legs' and close to the 'heads' of upright and inverted 'hairpins', respectively.

Adrian, Ronald J.↗
NASA NTRS1988

Fast Euler solver for transonic airfoils. I - Theory. II - Applications

Equations written in terms of generalized Riemann variables are presently integrated by inverting six bidiagonal matrices and two tridiagonal matrices, using an implicit Euler solver that is based on the lambda-formulation. The solution is found on a C-grid whose boundaries are very close to the airfoil. The fast solver is then applied to the computation of several flowfields on a NACA 0012 airfoil at various Mach number and alpha values, yielding results that are primarily concerned with transonic flows. The effects of grid fineness and boundary distances are analyzed; the code is found to be robust and accurate, as well as fast.

Dadone, Andrea↗
NASA NTRS1988

Determination of the area and mass distribution of orbital debris fragments

An important factor in modeling the orbital debris environment is the loss rate of debris due to atmospheric drag and lunisolar perturbations. An accurate knowledge of the area-to-mass ratio of debris fragments is required to calculate the effects of atmospheric drag. It is shown here that the orbital elements as a function of time can be used to invert any propagation algorithm to yield the area-to-mass ratio of an orbiting object. From these calculations and the observed radar cross-section of the object, the mass can be calculated to an accuracy of about 30 percent. It is shown that the mass is related to the effective cross-section area by a power-law relation, but for a given area the mass distribution is very broad. An expression is given for the cumulative mass distribution.

Badhwar, G. D.↗
NASA NTRS1989

Current Regulator For Sodium-Vapor Lamps

Regulating circuit maintains nearly-constant alternating current in sodium-vapor lamp. Regulator part of dc-to-ac inverter circuit used to supply power to street lamp from battery charged by solar-cell array.

Mclyman, W. T.↗
NASA NTRS1989

Timing Sampler For Delay Measurements

Integrated circuit called timing sampler used to measure delays in complementary metal oxide/silicon logic gates. Circuit chip contains chains of inverters and metal traces having minimum parasitic delays (delay chains), circuits to compare pulse timings, and output multiplexer. Timing sampler enables delay measurements for both positive and negative logic-level transitions by use of variety of circuit geometries representing tentative practical designs.

Blaes, Brent R.↗
NASA NTRS1989

Improved Coplanar Waveguides

Three new types of coplanar waveguide transmission lines proposed: suspended, striplinelike suspended, and inverted. Numerical analyses of propagation of electromagnetic fields of new and of more-conventional plain and conductor-backed coplanar waveguides showed new waveguides offered several advantages over conventional ones. Configurations promise lower attenuation and reduce effects of manufacturing errors.

Simons, Rainee N.↗
NASA NTRS1989

High-Performance Power-Semiconductor Packages

A 600-V, 50-A transistor and 1,200-V, 50-A diode in rugged, compact, lightweight packages intended for use in inverter-type power supplies having switching frequencies up to 20 kHz. Packages provide low-inductance connections, low loss, electrical isolation, and long-life hermetic seal. Low inductance achieved by making all electrical connections to each package on same plane. Also reduces high-frequency losses by reducing coupling into inherent shorted turns in packaging material around conductor axes. Stranded internal power conductors aid conduction at high frequencies, where skin effect predominates. Design of packages solves historical problem of separation of electrical interface from thermal interface of high-power semiconductor device.

Renz, David↗
NASA NTRS1988

A model for radiative transfer in heterogeneous three-dimensional canopies

A model, dubbed TRIM, for the interaction of electromagnetic radiation with inhomogeneous vegetation canopies is presented. It is based on the four flux theory for homogeneous canopies. The canopy is assumed to consist of ellipsoidal subcanopies located on the ground at periodic intervals. The model is inverted with field measured data for reflectance from corn canopies in the near infrared region. It is shown that TRIM correctly identifies the architecture of the canopy (homogeneous or row canopy, percentage of ground cover) and gives a good estimate of leaf area index.

Goel, N. S.↗
NASA NTRS1987

Sensor-based diagnosis using knowledge of structure and function

A system for fault detection and isolation called LES, developed at the Kennedy Space Center for the Space Shuttle's Launch Processing System, is a well-developed diagnostic system that is simultaneously model-based and sensor-based. This experiment has led to a surprising result: the failure of a sensor can not only be handled in precisely the same way as the failure of any other object, but may present an especially easy case. Classical rule-based diagnostic systems need to find out whether or not their sensors are telling them the truth before they can safely draw inferences from them. By contrast, while LES does treat sensors as a special case, it does so only because there may exist a short cut that allows them to be handled more simply than other objects. LES uses both structural and functional knowledge, and has found cases in which the structural knowledge can be economically replaced by the judicious use of functional relationships; LES' functional relationships are stored in exactly one place, so they must be inverted to determine hypothetical values for possibly faulty objects. The inversion process has been extended to include conditional functions not normally considered to have inverses.

Scarl, Ethan A.↗
NASA NTRS1988

Passive microwave remote sensing of asteroids using the VLA

Precise flux density measurements made with the Very Large Array (VLA) of the National Radio Astronomy Observatory will be used to define the microwave continuum spectra of asteroids. These spectra will be inverted in order to estimate the near-surface bulk properties (radii, roughness, composition) independent of previous optical or infrared spectroscopy. The results for 2 Pallas, 4 Vesta and 10 Hygeia will be submitted for publication. The review chapter will be published and the results on Asteroid photometeric diameters will be published.

Webster, William J., Jr.↗
NASA NTRS1988

The effect of simulated weightlessness on performance and mood

The performance results of the bedrest study at Ames were not what were expected. The Air Combat Maneuvering performance test was tested to assure its reliability. However, the results from this study show a continued increase in performance. One would assume that scores would become constant if not decrease by the first days of bedrest because an inverted position would affect performance. It is also interesting to observe that while the subject's moods deteriorated, their performance improved. Although the performance results were surprising, the mood results were as expected.

Rosenberg, Bonnie↗
NASA NTRS1989

Intelligent control of robotic arm/hand systems for the NASA EVA retriever using neural networks

Adaptive/general learning algorithms using varying neural network models are considered for the intelligent control of robotic arm plus dextrous hand/manipulator systems. Results are summarized and discussed for the use of the Barto/Sutton/Anderson neuronlike, unsupervised learning controller as applied to the stabilization of an inverted pendulum on a cart system. Recommendations are made for the application of the controller and a kinematic analysis for trajectory planning to simple object retrieval (chase/approach and capture/grasp) scenarios in two dimensions.

Mclauchlan, Robert A.↗
NASA NTRS1989

Estimation of velocity perturbations in satellite fragmentation events

The magnitude, variance and directionality of the velocity perturbations of the fragments of a satellite can shed valuable information regarding the nature and intensity of the fragmentation. Up until now, the only method used to calculate the three orthogonal components of the velocity change consisted of inverting the process of evaluating the changes in the orbital elements of the fragments due to velocity perturbing forces. But the traditional method failed in five different cases: (1) when the parent satellite's orbit was circular; (2) and (3) when the true anomaly of the parent was either 0 deg or 180 deg; and (4) and (5) when the argument of latitude of the parent was 90 deg or 270 deg. Described here is a new method of calculating the velocity perturbations which is free from the shortcomings of the traditional method and could be used in all occasions, provided the fragmentation data and the orbital elements data are consistent with one another.

Tan, Arjun↗