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At least 271 records · Page 15

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NASA NTRS1985

Viscous dissipation effects on thermophoretically augmented aerosol particle transport across laminar boundary layers

The effect of viscous dissipation on mass transport across nonisothermal low-mass-loading laminar boundary layers of dusty gas is investigated theoretically by means of numerical simulations. The derivation of the model is outlined, and numerical results are presented in extensive graphs and characterized in detail. The dissipation effects are found to be significant, increasing total particle-deposition rates; the intensity of the effects depends on the ratio of wall temperature to mainstream static temperature.

Gokoglu, S. A.↗

NASA NTRS1990

Bipolar supernova remnants and the obliquity dependence of shock acceleration

The diffusive shock acceleration mechanism proposed to explain the bipolarity observed in the synchrotron radio emission of young adiabatically expanding shell SNRs is investigated by means of numerical simulations. The theoretical basis of the SNR models and the numerical computation methods are explained, and the results are presented in graphs and synthetic radio maps and discussed in detail. It is found that the efficiency of the acceleration process depends on the obliquity angle theta(Bn) between the shock normal and the uniform magnetic field: models with theta(Bn) of about 90 deg can reproduce the observed azimuthal intensity ratios in most cases, but models with theta(Bn) near 0 deg cannot.

Fulbright, Michael S.↗

NASA NTRS1991

Ion dynamics in the Venus ionosphere

Measurement data on the ion velocity in the Venus ionosphere (mainly from the Pioneer Venus Orbiter Retarding Potential Analyzer) are summarized, and theoretical models developed to explain them are reviewed. Data and theoretical predictions are compared in extensive graphs and diagrams and discussed in detail. It is shown that the predominant flow is away from the subsolar point, at up to 3 km/sec in the terminator region. A model of axisymmetric flow based on momentum, energy, and mass conservation laws is found to reproduce the observed ion velocities at solar zenith angles less than about 140 deg, but not the high velocities and chaotic behavior seen near the antisolar point. Also discussed are significant differences between the flow above and below about 400 km and the effects of changes in the dynamic pressure of the solar wind.

Miller, K. L.↗

NASA NTRS2000

Analysis of Complexity Evolution Management and Human Performance Issues in Commercial Aircraft Automation Systems

Autoflight systems in the current generation of aircraft have been implicated in several recent incidents and accidents. A contributory aspect to these incidents may be the manner in which aircraft transition between differing behaviours or 'modes.' The current state of aircraft automation was investigated and the incremental development of the autoflight system was tracked through a set of aircraft to gain insight into how these systems developed. This process appears to have resulted in a system without a consistent global representation. In order to evaluate and examine autoflight systems, a 'Hybrid Automation Representation' (HAR) was developed. This representation was used to examine several specific problems known to exist in aircraft systems. Cyclomatic complexity is an analysis tool from computer science which counts the number of linearly independent paths through a program graph. This approach was extended to examine autoflight mode transitions modelled with the HAR. A survey was conducted of pilots to identify those autoflight mode transitions which airline pilots find difficult. The transitions identified in this survey were analyzed using cyclomatic complexity to gain insight into the apparent complexity of the autoflight system from the perspective of the pilot. Mode transitions which had been identified as complex by pilots were found to have a high cyclomatic complexity. Further examination was made into a set of specific problems identified in aircraft: the lack of a consistent representation of automation, concern regarding appropriate feedback from the automation, and the implications of physical limitations on the autoflight systems. Mode transitions involved in changing to and leveling at a new altitude were identified across multiple aircraft by numerous pilots. Where possible, evaluation and verification of the behaviour of these autoflight mode transitions was investigated via aircraft-specific high fidelity simulators. Three solution approaches to concerns regarding autoflight systems, and mode transitions in particular, are presented in this thesis. The first is to use training to modify pilot behaviours, or procedures to work around known problems. The second approach is to mitigate problems by enhancing feedback. The third approach is to modify the process by which automation is designed. The Operator Directed Process forces the consideration and creation of an automation model early in the design process for use as the basis of the software specification and training.

Vakil, Sanjay S.↗

NASA NTRS1989

Photochemical stability of CO2 in the Martian atmosphere - Reevaluation of the eddy diffusion coefficient and the role of water vapor

Theoretical models of CO2 photodissociation in the atmosphere of Mars are developed and analyzed, with a focus on the conditions prevailing during the Mariner 6/7 experiments. The aeronomy of Martian CO2 is reviewed; the atomic O distribution in the upper atmosphere is characterized; the construction of the basic model is described; and simulation results are presented in extensive tables and graphs and discussed in detail. The vertical eddy-diffusion coefficient corresponding to the estimated upper-atmosphere O density (1-2 percent at 135 km) is found to be about 5 x 10 to the 7th sq cm/sec, and HO(x) is found to play a significant role in catalyzing the recombination of CO and O to form CO2. The reasons why the model overpredicts the production of HO(x) and hence of CO2 are explored.

Shimazaki, Tatsuo↗

NASA NTRS1988

Methane rain on Titan

The atmosphere of Titan is characterized by means of model computations based on Voyager IRIS IR spectra and published data from laboratory determinations of absorption coefficients and cloud refractive indices. The results are presented in tables and graphs, and it is pointed out that the presence of Ar is not required in the model. Particular attention is given to the role of CH4, which is found to form patchy clouds (with particle radii of 50 microns or greater and visible/IR optical depths of 2-5) at altitudes up to about 30 km. The mechanisms by which such rain-sized particles could form are discussed, and it is suggested that the observed 500-600/cm spectrum is affected much less by the CH4 clouds than by H2 or variations in the temperature of the high-altitude haze.

Toon, Owen B.↗

NASA NTRS1983

The construction of life prediction models for the design of Stirling engine heater components

The service life of Stirling-engine heater structures of Fe-based high-temperature alloys is predicted using a numerical model based on a linear-damage approach and published test data (engine test data for a Co-based alloy and tensile-test results for both the Co-based and the Fe-based alloys). The operating principle of the automotive Stirling engine is reviewed; the economic and technical factors affecting the choice of heater material are surveyed; the test results are summarized in tables and graphs; the engine environment and automotive duty cycle are characterized; and the modeling procedure is explained. It is found that the statistical scatter of the fatigue properties of the heater components needs to be reduced (by decreasing the porosity of the cast material or employing wrought material in fatigue-prone locations) before the accuracy of life predictions can be improved.

Petrovich, A.↗

NASA NTRS1991

Software reliability report

There are many software reliability models which try to predict future performance of software based on data generated by the debugging process. Unfortunately, the models appear to be unable to account for the random nature of the data. If the same code is debugged multiple times and one of the models is used to make predictions, intolerable variance is observed in the resulting reliability predictions. It is believed that data replication can remove this variance in lab type situations and that it is less than scientific to talk about validating a software reliability model without considering replication. It is also believed that data replication may prove to be cost effective in the real world, thus the research centered on verification of the need for replication and on methodologies for generating replicated data in a cost effective manner. The context of the debugging graph was pursued by simulation and experimentation. Simulation was done for the Basic model and the Log-Poisson model. Reasonable values of the parameters were assigned and used to generate simulated data which is then processed by the models in order to determine limitations on their accuracy. These experiments exploit the existing software and program specimens which are in AIR-LAB to measure the performance of reliability models.

Wilson, Larry↗

NASA NTRS1988

A multicomponent model of the infrared emission from Comet Halley

The 3-200-micron emission of Comet Halley is characterized theoretically by means of numerical simulations. Models with different populations of refractory dust grains are developed, applying compositional and spectral constraints based on ground-based and spacecraft observations. The results are presented in extensive graphs and discussed in detail. Best agreement between predicted and observed Halley emission is obtained for models with 8-40 times more crystalline silicate grains (by mass) than amorphous carbon grains. For the silicate grains, the predominance of large grains and thin carbonaceous coatings are found to account for the lack of a 10-micron silicate feature and the dark appearance of the comet, respectively.

Swamy, Krishna K. S.↗

NASA NTRS1987

Superluminal motion in the double-lobed quasar 3C 245

VLBI observations of 3C 245 obtained at 10.7 GHz with the Mark III array at five epochs in 1981-1986 are reported. The data-reduction and model-fitting procedures are explained, and the results are presented in tables, graphs, and maps along with previously published data on 11 other superluminal sources. The central component of the quasar is found to have an apparent transverse velocity of (3.1 + or - 1.4) c/h, assuming H0 = 100h km/s Mpc and q0 = 0.5. From the apparent source orientation (based on R, variability, apparent curvature, and projected linear size) it is inferred that the apparent transverse velocity is anticorrelated with the angle to the line of sight, as predicted by relativistic-beaming models of superluminal motion.

Hough, D. H.↗

NASA NTRS1988

Graph-based real-time fault diagnostics

A real-time fault detection and diagnosis capability is absolutely crucial in the design of large-scale space systems. Some of the existing AI-based fault diagnostic techniques like expert systems and qualitative modelling are frequently ill-suited for this purpose. Expert systems are often inadequately structured, difficult to validate and suffer from knowledge acquisition bottlenecks. Qualitative modelling techniques sometimes generate a large number of failure source alternatives, thus hampering speedy diagnosis. In this paper we present a graph-based technique which is well suited for real-time fault diagnosis, structured knowledge representation and acquisition and testing and validation. A Hierarchical Fault Model of the system to be diagnosed is developed. At each level of hierarchy, there exist fault propagation digraphs denoting causal relations between failure modes of subsystems. The edges of such a digraph are weighted with fault propagation time intervals. Efficient and restartable graph algorithms are used for on-line speedy identification of failure source components.

Padalkar, S.↗

NASA NTRS1974

Study of cryogenic propellant systems for loading the space shuttle

Computer programs were written to model the liquid oxygen loading system for the space shuttle. The programs allow selection of input data through graphic displays which schematically depict the part of the system being modeled. The computed output is also displayed in the form of graphs and printed messages. Any one of six computation options may be selected. The first four of these pertain to thermal stresses, pressure surges, cooldown times, flow rates and pressures during cooldown. Options five and six deal with possible water hammer effects due to closing of valves, steady flow and transient response to changes in operating conditions after cooldown. Procedures are given for operation of the graphic display unit and minicomputer.

Voth, R. O.↗

NASA NTRS1989

An integrated aerodynamic/propulsion study for generic aero-space planes based on waverider concepts

The design of a unified aero-space plane based on waverider technology is analyzed. The overall aerodynamic design and performance of an aero-space plane are discussed in terms of the forebody, scramjet, and afterbody. Other subjects considered in the study are combustion/nozzle optimization, the idealized tip-to-tail waverider model, and the two-dimensional minimum length nozzle. Charts and graphs are provided to show the results of the preliminary investigations.

Rasmussen, M. L.↗

NASA NTRS1989

A simple element for aeroelastic analysis of undamaged and damaged wings

The effects of material damage on the aeroelastic response of an anisotropic composite wing are investigated analytically. The wing is modeled as a Timoshenko beam of arbitrary cross section, applying a 24-DOF one-dimensional beam element and Cartesian coordinates. The formulations for the element stiffness matrix, the strain-displacement relations, the box-beam constitutive relations, the contributions of stringers and web, and the warping correction factor are outlined; the application of strip theory in deriving the aerodynamic model is briefly explained; and numerical results are presented in extensive graphs. It is shown that asymmetry and damage can induce bending-stretching coupling phenomena and thereby affect the stability of the aircraft either positively or negatively. The need to take such effects into account in the design of advanced composite wings is stressed.

Kapania, Rakesh K.↗

NASA NTRS1990

Radiative forcing of climate by changes in the vertical distribution of ozone

Results from a one-dimensional radiative-convective equilibrium model (similar to that of Lacis et al., 1981) of surface-temperature changes induced by changes in the vertical distribution of O3 in the atmosphere are parameterized and applied to measurement data for the northern midlatitudes during the 1970s. The construction of the model is outlined, and the results are presented in tables and graphs and discussed in detail. It is found that decreases in O3 in the lower stratosphere in the 1970s caused atmospheric cooling greater than that due to CO2 (thus possibly obscuring the CO2 greenhouse effect) and surface cooling which outweighed the warming effect of increased O3 in the troposphere. The net surface cooling is shown to be equal to about half of the estimated CO2 warming effect for the period.

Lacis, Andrew A.↗

NASA NTRS1992

Simulator for concurrent processing data flow architectures

A software simulator capability of simulating execution of an algorithm graph on a given system under the Algorithm to Architecture Mapping Model (ATAMM) rules is presented. ATAMM is capable of modeling the execution of large-grained algorithms on distributed data flow architectures. Investigating the behavior and determining the performance of an ATAMM based system requires the aid of software tools. The ATAMM Simulator presented is capable of determining the performance of a system without having to build a hardware prototype. Case studies are performed on four algorithms to demonstrate the capabilities of the ATAMM Simulator. Simulated results are shown to be comparable to the experimental results of the Advanced Development Model System.

Malekpour, Mahyar R.↗

NASA NTRS1993

A survey of program slicing for software engineering

This research concerns program slicing which is used as a tool for program maintainence of software systems. Program slicing decreases the level of effort required to understand and maintain complex software systems. It was first designed as a debugging aid, but it has since been generalized into various tools and extended to include program comprehension, module cohesion estimation, requirements verification, dead code elimination, and maintainence of several software systems, including reverse engineering, parallelization, portability, and reuse component generation. This paper seeks to address and define terminology, theoretical concepts, program representation, different program graphs, developments in static slicing, dynamic slicing, and semantics and mathematical models. Applications for conventional slicing are presented, along with a prognosis of future work in this field.

Beck, Jon↗

NASA NTRS1989

Power flow for spacecraft power systems

A method for constructing the generalized system-level admittance matrix for use with a Newton-Raphson power flow is presented. The network modeling technique presented does not use the standard pi-equivalent models, which assume a lossless return path, for the transmission line and transformer. If the return path cannot be assumed lossless, then the standard algorithms for constructing the system admittance matrix cannot be used. The method presented here uses concepts from linear graph theory to combine network modules to form the system-level admittance matrix. The modeling technique is presented, and the resulting matrix is used with a standard Newton-Raphson power flow to calculate all system voltages and current (power) flows.

Halpin, S. M.↗