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At least 739 records · Page 41

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NASA NTRS1992

Integrated multidisciplinary analysis of segmented reflector telescopes

The present multidisciplinary telescope-analysis approach, which encompasses thermal, structural, control and optical considerations, is illustrated for the case of an IR telescope in LEO; attention is given to end-to-end evaluations of the effects of mechanical disturbances and thermal gradients in measures of optical performance. Both geometric ray-tracing and surface-to-surface diffraction approximations are used in the telescope's optical model. Also noted is the role played by NASA-JPL's Integrated Modeling of Advanced Optical Systems computation tool, in view of numerical samples.

Briggs, Hugh C.↗

NASA NTRS1993

Navier-Stokes simulation of external/internal transonic flow on the forebody/inlet of the AV-8B Harrier II

In this work, the computation of combined external/internal transonic flow on the complex forebody/inlet configuration of the AV-8B Harrier II is performed. The actual aircraft has been measured and its surface and surrounding domain, in which the fuselage and inlet have a common wall, have been described using structured grids. The 'thin-layer' Navier-Stokes equations were used to model the flow along with the Chimera embedded multi-block technique. A fully conservative, alternating direction implicit (ADI), approximately factored, partially fluxsplit algorithm was employed to perform the computation. Comparisons to some experimental wind tunnel data yielded good agreement for flow at zero incidence and angle of attack. The aim of this paper is to provide a methodology or computational tool for the numerical solution of complex external/internal flows.

Mysko, Stephen J.↗

NASA NTRS1993

Structured system engineering methodologies used to develop a nuclear thermal propulsion engine

To facilitate the development of a space nuclear thermal propulsion engine for manned flights to Mars, requirements must be established early in the technology development cycle. The long lead times for the acquisition of the engine system and nuclear test facilities demands that the engine system size, performance and safety goals be defined at the earliest possible time. These systems are highly complex and require a large multidisciplinary systems engineering team to develop and track requirements, and to ensure that the as-built system reflects the intent of the mission. A methodology has been devised which uses sophisticated computer tools to effectively develop and interpret functional requirements, and furnish these to the specification level for implementation.

Corban, R.↗

NASA NTRS1993

Comment on tangential wave motion in unstable combustors

An attempt is made to gain insight into the injection boundary instability mechanism as captured by a simple computational tool developed for liquid rocket engines. Considerable evidence is generated supporting a hypothesis that destructive tangential-mode instabilities in liquid-fuel combustors are driven by combustion processes occurring at the propellant-wetted injector face. The driving action is found to be directly analogous to a burning solid propellant surface mechanism, responsible for solid rocket motor instabilities. Since the proposed instability mechanism has not been fundamentally confirmed, experimental research is strongly recommended.

Litchford, Ron J.↗

NASA NTRS1990

Integration of viscous effects into inviscid computational methods

A variety of practical fluid dynamic problems related to the low-speed, high Reynolds number flow over aircraft and ground vehicles fall in a category where some simplified mathematical models become applicable. This provides the fluid dynamicists with a more economical computational tool, compared to the alternative solution of the Navier Stokes equations. The objective was to provide a brief survey of some of the viscous boundary layer solution methods and to propose a method for coupling between the inviscid outer flow and the viscous boundary layer solutions. Results of this survey and details of the viscous/inviscid flow coupling efforts are presented.

Katz, Joseph↗

NASA NTRS1993

A comparison between coherent and noncoherent mobile systems in large Doppler shift, delay spread, and C/I environment

The performance and implementation complexity of coherent and of noncoherent QPSK and GMSK modulation/demodulation techniques in a complex mobile satellite systems environment, including large Doppler shift, delay spread, and low C/I, are compared. We demonstrate that for large f(sub d)T(sub b) products, where f(sub d) is the Doppler shift and T(sub b) is the bit duration, noncoherent (discriminator detector or differential demodulation) systems have a lower BER floor than their coherent counterparts. For significant delay spreads, e.g., tau(sub rms) greater than 0.4 T(sub b), and low C/I, coherent systems outperform noncoherent systems. However, the synchronization time of coherent systems is longer than that of noncoherent systems. Spectral efficiency, overall capacity, and related hardware complexity issues of these systems are also analyzed. We demonstrate that coherent systems have a simpler overall architecture (IF filter implementation-cost versus carrier recovery) and are more robust in an RF frequency drift environment. Additionally, the prediction tools, computer simulations, and analysis of coherent systems is simpler. The threshold or capture effect in low C/I interference environment is critical for noncoherent discriminator based systems. We conclude with a comparison of hardware architectures of coherent and of noncoherent systems, including recent trends in commercial VLSI technology and direct baseband to RF transmit, RF to baseband (0-IF) receiver implementation strategies.

Feher, Kamilo↗

NASA NTRS1993

Turbulence modeling for non-equilibrium flows

Two projects are reported. The first is the development and testing of an eddy viscosity transport model. This project also is a starting point for our work on developing computational tools for solving turbulence models in complex geometries. The second project is a stochastic analysis of the realizability of Reynolds stress transport models.

Durbin, Paul A.↗

NASA NTRS1993

Computing in space: Issues and answers

Future plans for space exploration call for scientific instruments whose data is increasingly voluminous, far exceeding constraints set by telemetry rates, and therefore, an increased role for spaceborne computing. Three issues that greatly influence the realization of spaceborne computing systems are addressed. The first involves the identification of those research problems which are unique to space computing. Resources must be expended on these problems if efforts are to add value to, and have impact on, the technology of computing in space. The second addresses the transfer of new technology from the research laboratory to use in space. What can be done to expedite this process and shrink the technology gap that separates Earth and space based systems must be assessed. The third issue involves the user's point of view of what is required of new space computing systems. That is, what are the driving forces that most influence customers' decisions to use or not use new technology? Issues of design environment for spaceborne computers, tools for modeling and evaluation of flight systems, and validation of space flight hardware and software are also discussed.

Davidson, John M.↗

NASA NTRS1994

Theoretical investigation of gas-surface interactions

The goal of this project was to develop computational tools for the calculation of the electronic structure of molecules containing heavy atoms, and to use these tools in the study of catalytic processes, with the overall objective of gaining an understanding of the catalytic process which could be used to design more efficient catalysts. The main catalytic system of interest was the combustion of hydrogen on platinum surfaces. Under this project, a flexible Dirac-Hartree-Fock (DHF) program has been developed, a code to calculate DHF correlation energies at the second-order Moeller-Plesset perturbation (PP2) level is almost complete, and code to include correlation at the MCSCF and MCSCF/MP2 level is planned. The tools so far developed have been validated and used to calibrate some more approximate methods, and applied to investigate the importance of relativistic effects in the bonding of hydrogen to platinum.

Dyall, Kenneth G.↗

NASA NTRS1994

New approaches for real time decision support systems

NCCOSC RDT&E Division (NRaD) is conducting research into ways of improving decision support systems (DSS) that are used in tactical Navy decision making situations. The research has focused on the incorporation of findings about naturalistic decision-making processes into the design of the DSS. As part of that research, two computer tools were developed that model the two primary naturalistic decision-making strategies used by Navy experts in tactical settings. Current work is exploring how best to incorporate the information produced by those tools into an existing simulation of current Navy decision support systems. This work has implications for any applications involving the need to make decisions under time constraints, based on incomplete or ambiguous data.

Hair, D. Charles↗

NASA NTRS1994

Development and application of computational aerothermodynamics flowfield computer codes

Research was performed in the area of computational modeling and application of hypersonic, high-enthalpy, thermo-chemical nonequilibrium flow (Aerothermodynamics) problems. A number of computational fluid dynamic (CFD) codes were developed and applied to simulate high altitude rocket-plume, the Aeroassist Flight Experiment (AFE), hypersonic base flow for planetary probes, the single expansion ramp model (SERN) connected with the National Aerospace Plane, hypersonic drag devices, hypersonic ramp flows, ballistic range models, shock tunnel facility nozzles, transient and steady flows in the shock tunnel facility, arc-jet flows, thermochemical nonequilibrium flows around simple and complex bodies, axisymmetric ionized flows of interest to re-entry, unsteady shock induced combustion phenomena, high enthalpy pulsed facility simulations, and unsteady shock boundary layer interactions in shock tunnels. Computational modeling involved developing appropriate numerical schemes for the flows on interest and developing, applying, and validating appropriate thermochemical processes. As part of improving the accuracy of the numerical predictions, adaptive grid algorithms were explored, and a user-friendly, self-adaptive code (SAGE) was developed. Aerothermodynamic flows of interest included energy transfer due to strong radiation, and a significant level of effort was spent in developing computational codes for calculating radiation and radiation modeling. In addition, computational tools were developed and applied to predict the radiative heat flux and spectra that reach the model surface.

Venkatapathy, Ethiraj↗

NASA NTRS1994

Propulsion/airframe interference for ducted propfan engines with ground effect

The advanced propfan propulsion systems design of the next-generation subsonic transport aircraft has been of interest to many airline companies in the past several years. This is due to the studies which indicate that an efficient ducted propfan engine technology offers a significant reduction in aircraft fuel consumption. However, because of the geometric complexity of the configuration, one challenge is the integration of the ducted propfan engine with the airframe so that aerodynamic interference effects frequently encountered near the nacelle can be minimized, or perhaps, optimized. To understand this interaction phenomenon better, it is desirable to have a reliable and efficient computational tool that can predict propeller effects on the flowfield around complex configurations.

Arabshahi, Abdollah↗

NASA NTRS1994

Modeling of stress/strain behavior of fiber-reinforced ceramic matrix composites including stress redistribution

A computational simulation procedure is presented for nonlinear analyses which incorporates microstress redistribution due to progressive fracture in ceramic matrix composites. This procedure facilitates an accurate simulation of the stress-strain behavior of ceramic matrix composites up to failure. The nonlinearity in the material behavior is accounted for at the constituent (fiber/matrix/interphase) level. This computational procedure is a part of recent upgrades to CEMCAN (Ceramic Matrix Composite Analyzer) computer code. The fiber substructuring technique in CEMCAN is used to monitor the damage initiation and progression as the load increases. The room-temperature tensile stress-strain curves for SiC fiber reinforced reaction-bonded silicon nitride (RBSN) matrix unidirectional and angle-ply laminates are simulated and compared with experimentally observed stress-strain behavior. Comparison between the predicted stress/strain behavior and experimental stress/strain curves is good. Collectively the results demonstrate that CEMCAN computer code provides the user with an effective computational tool to simulate the behavior of ceramic matrix composites.

Mital, Subodh K.↗

NASA NTRS1994

Formalizing procedures for operations automation, operator training and spacecraft autonomy

The generation and validation of operations procedures is a key task of mission preparation that is quite complex and costly. This has motivated the development of software applications providing support for procedures preparation. Several applications have been developed at MATRA MARCONI SPACE (MMS) over the last five years. They are presented in the first section of this paper. The main idea is that if procedures are represented in a formal language, they can be managed more easily with a computer tool and some automatic verifications can be performed. One difficulty is to define a formal language that is easy to use for operators and operations engineers. From the experience of the various procedures management tools developed in the last five years (including the POM, EOA, and CSS projects), MMS has derived OPSMAKER, a generic tool for procedure elaboration and validation. It has been applied to quite different types of missions, ranging from crew procedures (PREVISE system), ground control centers management procedures (PROCSU system), and - most relevant to the present paper - satellite operation procedures (PROCSAT developed for CNES, to support the preparation and verification of SPOT 4 operation procedures, and OPSAT for MMS telecom satellites operation procedures).

Lecouat, Francois↗

NASA NTRS1993

Condition monitoring requirements for the development of a space nuclear propulsion module

To facilitate the development of a space nuclear propulsion module for manned flights to Mars, requirements must be established early in the technology cycle. The long lead times for the acquisition of the engine system and nuclear test facilities demands that the engine system, size, performance, safety goals and condition monitoring philosophy be defined at the earliest possible time. These systems are highly complex and require a large multi-disciplinary systems engineering team to develop and track the requirements and to ensure that the as-built system reflects the intent of the mission. An effective methodology has been devised coupled with sophisticated computer tools to effectivly develop and interpret the functional requirements. These requirements can then be decomposed down to the specification level for implementation. This paper discusses the application of the methodology and the analyses to develop condition monitoring requirements under a contract with the National Aeronautics and Space Administration (NASA) Lewis Research Center (LeRC) Nuclear Propulsion Office (NPO).

Wagner, Robert C.↗

NASA NTRS1995

Flow in serpentine coolant passages with trip strips

Under the subject contract, an effort is being conducted at Scientific Research Associates, Inc. (SRA) to obtain flow field measurements in the coolant passage of a rotating turbine blade with ribbed walls, both in the stationary and rotating frames. The data obtained will be used for validation of computational tools and assessment of turbine blade cooling strategies. The configuration of the turbine blade passage model is given, and the measuring plane locations are given. The model has a four-pass passage with three 180 turns. This geometry was chosen to allow analyses of the velocity measurements corresponding to the heat transfer results obtained by Wagner. Two passes of the passage have a rectangular cross-section of 1.0 in x 0.5 in. Another two passes have a square cross-section of 0.5 in x 0.5 in. Trips with a streamwise pitch to trip height (P/e) = 5 and trip height to coolant passage width (e/Z) = 0.1, were machined along the leading and trailing walls. These dimensions are typical of those used in turbine blade coolant passages. The trips on these walls are staggered by the half-pitch. The trips are skewed at +/- 45 deg, and this allows the effect of trip orientation to be examined. Experiments will be conducted with flow entering the model through the 1.0 in x 0.5 in rectangular passage (Configuration C) and the 0.5 in x 0. 5 in square passage (Configuration D) to examine the effect of passage aspect ratio. Velocity measurements were obtained with a Reynolds number (Re) of 25,000, based on the hydraulic diameter of and bulk mean velocity in the half inch square passage. The coordinate system used in presenting the results for configurations C and D, respectively, is shown. The first, second and third passes of the passage will be referred to as the first, second and third passages, respectively, in later discussion. Streamwise distance (x) from the entrance is normalized by the hydraulic diameter (D). Vertical (y) and tangential (z) distances are normalized by the half passage height (H) and width (Z), respectively. The x coordinate and U component are positive in the streamwise direction. The y coordinate and V component are positive against gravity. The z coordinate and W component are positive in the direction of rotation. The velocities are normalized by the bulk mean velocity (Ub) of 3.44 m/s based on the half-inch square passage. The contours of the 1.0 in x 0.5 in and 0.5 in x 0.5 in passages were evaluated from 11 x 30 and 9 x 30 measurement grids, respectively.

Tse, D. G.-N.↗

NASA NTRS1995

Spray combustion stability

The central purpose of this project is the improvement of liquid-fueled rocket motor design technology in order to assist the establishment of economical commercial access to space through the development of engines with enhanced performance and reliability. Specific research effort in the project is focused on spray physics and associated combustion instability phenomena. Results garnered from this work will contribute to the development of new computational tools for design of stable liquid propellant rocket engines. The specific objectives of the research effort include identifying and evaluating physical submodels which pertain to spray combustion stability with the idea of enhancing or refining existing submodels with a more comprehensive approach. In particular, any refinements to the spray combustion physical submodels which are achieved during the project will be channeled back to Rocketdyne for incorporation in their ARICC liquid rocket combustor code as second generation improvements. Also, as the ARICC code forms the basis or future CFD development, some effort is devoted to an evaluation of the code's capability for modeling oscillating pressure waves within the combustor.

Liang, Pak-Yan↗

NASA NTRS1995

Tensoral Present and Future

The coding of high-performance fluids simulations requires significant knowledge of both numerical and computational details. The magnitude and complexity of low-level details is often enough to discourage many users of turbulence data wishing to study more important, higher-level fluid dynamical questions. These same complexities are often a practical barrier to simulation experts who develop, verify and maintain the codes which generate this data. Future fluids codes, with high resolution and complex geometries, are likely to involve far more coding complexity. The research--the design and implementation of the Tensoral computer language--aims to greatly ease the coding of today's simulation and post-processing codes and at the same time provide a general computational tool for future simulations.

Dresselhaus, Eliot↗