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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 415 records · Page 23
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

CCD charge collection efficiency and the photon transfer technique

The charge-coupled device (CCD) has shown unprecendented performance as a photon detector in the areas of spectral response, charge transfer, and readout noise. Recent experience indicates, however, that the full potential for the CCD's charge collection efficiency (CCE) lies well beyond that which is realized in currently available devices. A definition of CCE performance is presented and a standard test tool (the photon transfer technique) for measuring and optimizing this important CCD parameter is introduced. CCE characteristics for different types of CCDs are compared; the primary limitations in achieving high CCE performance are discussed, and the prospects for future improvement are outlined.

Janesick, J.↗
NASA NTRS1987

Numerical solution of turbulent flow past a backward facing step using a nonlinear K-epsilon model

The problem of turbulent flow past a backward facing step is important in many technological applications and has been used as a standard test case to evaluate the performance of turbulence models in the prediction of separated flows. It is well known that the commonly used kappa-epsilon (and K-l) models of turbulence yield inaccurate predictions for the reattachment points in this problem. By an analysis of the mean vorticity transport equation, it will be argued that the intrinsically inaccurate prediction of normal Reynolds stress differences by the Kappa-epsilon and K-l models is a major contributor to this problem. Computations using a new nonlinear kappa-epsilon model (which alleviates this deficiency) are made with the TEACH program. Comparisons are made between the improved results predicted by this nonlinear kappa-epsilon model and those obtained from the linear kappa-epsilon model as well as from second-order closure models.

Speziale, C. G.↗
NASA NTRS1988

Delamination durability of composite materials for rotorcraft

Delamination is the most commonly observed failure mode in composite rotorcraft dynamic components. Although delamination may not cause immediate failure of the composite part, it often precipitates component repair or replacement, which inhibits fleet readiness, and results in increased life cycle costs. A fracture mechanics approach for analyzing, characterizing, and designing against delamination will be outlined. Examples of delamination problems will be illustrated where the strain energy release rate associated with delamination growth was found to be a useful generic parameter, independent of thickness, layup, and delamination source, for characterizing delamination failure. Several analysis techniques for calculating strain energy release rates for delamination from a variety of sources will be outlined. Current efforts to develop ASTM standard test methods for measuring interlaminar fracture toughness and developing delamination failure criteria will be reviewed. A technique for quantifying delamination durability due to cyclic loading will be presented. The use of this technique for predicting fatigue life of composite laminates and developing a fatigue design philosophy for composite structural components will be reviewed.

Obrien, T. Kevin↗
NASA NTRS1988

A crystallographic model for nickel base single crystal alloys

The purpose of this research is to develop a tool for the mechanical analysis of nickel-base single-crystal superalloys, specifically Rene N4, used in gas turbine engine components. This objective is achieved by developing a rate-dependent anisotropic constitutive model and implementing it in a nonlinear three-dimensional finite-element code. The constitutive model is developed from metallurgical concepts utilizing a crystallographic approach. An extension of Schmid's law is combined with the Bodner-Partom equations to model the inelastic tension/compression asymmetry and orientation-dependence in octahedral slip. Schmid's law is used to approximate the inelastic response of the material in cube slip. The constitutive equations model the tensile behavior, creep response and strain-rate sensitivity of the single-crystal superalloys. Methods for deriving the material constants from standard tests are also discussed. The model is implemented in a finite-element code, and the computed and experimental results are compared for several orientations and loading conditions.

Dame, L. T.↗
NASA NTRS1988

Numerical solution of turbulent flow past a backward facing step using a nonlinear K-epsilon model

The problem of turbulent flow past a backward facing step is important in many technological applications and has been used as a standard test case to evaluate the performance of turbulence models in the prediction of separated flows. It is well known that the commonly used kappa-epsilon (and K-l) models of turbulence yield inaccurate predictions for the reattachment points in this problem. By an analysis of the mean vorticity transport equation, it will be argued that the intrinsically inaccurate prediction of normal Reynolds stress differences by the kappa-epsilon and K-l models is a major contributor to this problem. Computations using a new nonlinear Kappa-epsilon model (which alleviates this deficiency) are made with the TEACH program. Comparisons are made between the improved results predicted by this nonlinear kappa-epsilon model and those obtained from the linear kappa-epsilon model as well as from second-order closure models.

Speziale, C. G.↗
NASA NTRS1990

Photovoltaic array space power plus diagnostics experiment

The objective of the Photovoltaic Array Space Power Plus Diagnostics (PASP Plus) experiment is to measure the effects of the interaction of the low- to mid-altitude space environment on the performance of a diverse set of small solar-cell arrays (planar and concentrator, representative of present and future military technologies) under differing conditions of velocity-vector orientation and simulated (by biasing) high-voltage operation. Solar arrays to be tested include Si and GaAs planar arrays and several types of GaAs concentrator arrays. Diagnostics (a Langmuir probe and a pressure gauge) and a transient pulse monitor (to measure radiated and conducted EMI during arcing) will be used to determine the impact of the environment on array operation to help verify various interactions models. Results from a successful PASP Plus flight will furnish answers to important interactions questions and provide inputs for design and test standards for photovoltaic space-power subsystems.

Guidice, Donald A.↗
NASA NTRS1991

An approach for configuring space photovoltaic tandem arrays based on cell layer performance

Meeting solar array performance goals of 300 W/Kg requires use of solar cells with orbital efficiencies greater than 20 percent. Only multijunction cells and cell layers operating in tandem produce this required efficiency. An approach for defining solar array design concepts that use tandem cell layers involve the following: transforming cell layer performance at standard test conditions to on-orbit performance; optimizing circuit configuration with tandem cell layers; evaluating circuit sensitivity to cell current mismatch; developing array electrical design around selected circuit; and predicting array orbital performance including seasonal variations.

Flora, C. S.↗
NASA NTRS1991

Design of lightning protection for a full-authority digital engine control

The steps and procedures are described which are necessary to achieve a successful lightning-protection design for a state-of-the-art Full-Authority Digital Engine Control (FADEC) system. The engine and control systems used as examples are fictional, but the design and verification methods are real. Topics discussed include: applicable airworthiness regulation, selection of equipment transient design and control levels for the engine/airframe and intra-engine segments of the system, the use of cable shields, terminal-protection devices and filter circuits in hardware protection design, and software approaches to minimize upset potential. Shield terminations, grounding, and bonding are also discussed, as are the important elements of certification and test plans, and the role of tests and analyses. Also included are examples of multiple-stroke and multiple-burst testing. A review of design pitfalls and challenges, and status of applicable test standards such as RTCA DO-160, Section 22, are presented.

Dargi, M.↗
NASA NTRS1991

A statistical model of carbon/carbon composite failure

A failure model which considers the stochastic nature of the damage accumulation process is essential to assess reliability and to accurately scale the results from standard test specimens to composite structures. A superior filamentary composite for high temperature applications is composed of carbon fibers in a carbon matrix. Carbon-carbon composites are the strongest known material at very high temperatures. Since there appears to be a significant randomness in C-C material strength which cannot be controlled or detected with current technology, a better model of the material failure based upon statistical principles should be used. Simple applications of the model based upon the limited data provide encouraging results that indicate that better design of test specimens would provide a substantially higher prediction for the design strength of C-C composites. An A-basis strength for the C-C tensile rings from a first stage D-5 billets was estimated. A statistical failure model was developed for these rings which indicates that this strength may be very conservative for larger C-C parts. The analysis may be improved by use of a heterogeneous/noncontinuum finite element approach on the minimechanical level.

Slattery, Kerry T.↗
NASA NTRS1992

Analysis of an RNG based turbulence model for separated flows

A two-equation turbulence model of the K-epsilon type was recently derived by using Renormalization Group (RNG) methods. It was later reported that this RNG based model yields substantially better predictions than the standard K-epsilon model for turbulent flow over a backward facing step - a standard test case used to benchmark the performance of turbulence models in separated flows. The improvements obtained from the RNG K-epsilon model were attributed to the better treatment of near wall turbulence effects. In contrast to these earlier claims, it is shown in this paper that the original version of the RNG K-epsilon model substantially underpredicts the reattachment point in the backstep problem. This is a deficiency that is traced to the modeling of the production of dissipation term. However, with the most recent improvements in the RNG K-epsilon model, excellent results for the backstep problem are now obtained.

Speziale, C. G.↗
NASA NTRS1992

PASP Plus: An experiment to measure space-environment effects on photovoltaic power subsystems

The Photovoltaic Array Space Power Plus Diagnostic experiment (PASP Plus) was accepted as part of the APEX Mission payload aboard a Pegastar satellite to be orbited by a Pegasus launch vehicle in late 1992. The mission's elliptical orbit will allow us to investigate both space plasma and space radiation effects. PASP Plus will have eleven types of solar arrays and a full complement of environmental and interactions diagnostic sensors. Measurements of space-plasma interactions on the various solar arrays will be made at large negative voltages (to investigate arcing parameters) and at large positive voltages (to investigate leakage currents) by biasing the arrays to various levels up to -500 and +500 volts. The long-term deterioration in solar array performance caused by exposure to space radiation will also be investigated; radiation dosage will be measured by an electron/proton dosimeter included in the environmental sensor complement. Experimental results from PASP Plus will help establish cause-and-effect relationships and lead to improved design guidelines and test standards for new-technology solar arrays.

Guidice, Donald A.↗
NASA NTRS1992

Investigation of advancing front method for generating unstructured grid

The advancing front technique is used to generate an unstructured grid about simple aerodynamic geometries. Unstructured grids are generated using VGRID2D and VGRID3D software. Specific problems considered are a NACA 0012 airfoil, a bi-plane consisting of two NACA 0012 airfoil, a four element airfoil in its landing configuration, and an ONERA M6 wing. Inviscid time dependent solutions are computed on these geometries using USM3D and the results are compared with standard test results obtained by other investigators. A grid convergence study is conducted for the NACA 0012 airfoil and compared with a structured grid. A structured grid is generated using GRIDGEN software and inviscid solutions computed using CFL3D flow solver. The results obtained by unstructured grid for NACA 0012 airfoil showed an asymmetric distribution of flow quantities, and a fine distribution of grid was required to remove this asymmetry. On the other hand, the structured grid predicted a very symmetric distribution, but when the total number of points were compared to obtain the same results it was seen that structured grid required more grid points.

Thomas, A. M.↗
NASA NTRS1993

An assessment of space shuttle flight software development processes

In early 1991, the National Aeronautics and Space Administration's (NASA's) Office of Space Flight commissioned the Aeronautics and Space Engineering Board (ASEB) of the National Research Council (NRC) to investigate the adequacy of the current process by which NASA develops and verifies changes and updates to the Space Shuttle flight software. The Committee for Review of Oversight Mechanisms for Space Shuttle Flight Software Processes was convened in Jan. 1992 to accomplish the following tasks: (1) review the entire flight software development process from the initial requirements definition phase to final implementation, including object code build and final machine loading; (2) review and critique NASA's independent verification and validation process and mechanisms, including NASA's established software development and testing standards; (3) determine the acceptability and adequacy of the complete flight software development process, including the embedded validation and verification processes through comparison with (1) generally accepted industry practices, and (2) generally accepted Department of Defense and/or other government practices (comparing NASA's program with organizations and projects having similar volumes of software development, software maturity, complexity, criticality, lines of code, and national standards); (4) consider whether independent verification and validation should continue. An overview of the study, independent verification and validation of critical software, and the Space Shuttle flight software development process are addressed. Findings and recommendations are presented.

Source record↗
NASA NTRS1921

Performance of a 300-horsepower Hispano-Suiza airplane engine

The National Bureau of Standards tested a 300-horsepower Hispano-Suiza engine to determine the characteristic performance of the engine at various altitudes. The engine was operated at the ground, at 25,000 feet, and at intermediate altitudes, both at full loads similar to those that would be imposed upon the engine at various speeds by a propeller whose normal full-load speed was 1,800 r.p.m. Friction horsepower also was determined in order that the mechanical efficiency of the engine might be calculated. From the test data there were computed the brake horsepower, brake mean effective pressure, specific fuel consumption, mixture ratio, jacket loss, exhaust loss, and thermal, mechanical, and volumetric efficiencies. A record of jacket water temperatures, oil temperatures, manifold pressures, etc., shows the conditions under which the test was made.

Sparrow, S W↗
NASA NTRS1924

The Resistance of Spheres in Wind Tunnels and In Air

To supplement the standardization tests now in progress at several laboratories, a broad investigation of the resistance of spheres in wind tunnels and free air has been carried out by the National Advisory Committee for Aeronautics. The subject has been classed in aerodynamic research, and in consequence there is available a great mass of data from previous investigations. This material was given careful consideration in laying out the research, and explanation of practically all the disagreement between former experiments has resulted. A satisfactory confirmation of Reynolds law has been accomplished, the effect of means of support determined, the range of experiment greatly extended by work in the new variable density wind tunnel, and the effects of turbulence investigated by work in the tunnels and by towing and dropping tests in free air. It is concluded that the erratic nature of most of the previous work is due to support interference and differing turbulence conditions. While the question of support has been investigated thoroughly, a systematic and comprehensive study of the effects of scale and quality of turbulence will be necessary to complete the problem, as this phase was given only general treatment.

Bacon, D L↗
NASA NTRS1994

Measuring Differential Delays With Sine-Squared Pulses

Technique for measuring differential delays among red, green, and blue components of video signal transmitted on different parallel channels exploits sine-squared pulses that are parts of standard test signals transmitted during vertical blanking interval of frame period. Technique does not entail expense of test-signal generator. Also applicable to nonvideo signals including sine-squared pulses.

Hurst, Robert N.↗
NASA NTRS1993

Design and application of electromechanical actuators for deep space missions

The annual report Design and Application of Electromechanical Actuators for Deep Space Missions is presented. The reporting period is 16 Aug. 1992 to 15 Aug. 1993. However, the primary focus will be work performed since submission of our semi-annual progress report in Feb. 1993. Substantial progress was made. We currently feel confident in providing guidelines for motor and control strategy selection in electromechanical actuators to be used in thrust vector control (TVC) applications. A small portion was presented in the semi-annual report. At this point, we have implemented highly detailed simulations of various motor/drive systems. The primary motor candidates were the brushless dc machine, permanent magnet synchronous machine, and the induction machine. The primary control implementations were pulse width modulation and hysteresis current control. Each of the two control strategies were applied to each of the three motor choices. With either pulse width modulation or hysteresis current control, the induction machine was always vector controlled. A standard test position command sequence for system performance evaluation is defined. Currently, we are gathering all of the necessary data for formal presentation of the results. Briefly stated for TVC application, we feel that the brushless dc machine operating under PWM current control is the best option. Substantial details on the topic, with supporting simulation results, will be provided later, in the form of a technical paper prepared for submission and also in the next progress report with more detail than allowed for paper publication.

Haskew, Tim A.↗