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At least 1,567 records · Page 87
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NASA NTRS1996

A Combined FEM/MoM/GTD Technique To Analyze Elliptically Polarized Cavity-Backed Antennas With Finite Ground Plane

Radiation pattern prediction analysis of elliptically polarized cavity-backed aperture antennas in a finite ground plane is performed using a combined Finite Element Method/Method of Moments/Geometrical Theory of Diffraction (FEM/MoM/GTD) technique. The magnetic current on the cavity-backed aperture in an infinite ground plane is calculated using the combined FEM/MoM analysis. GTD, including the slope diffraction contribution, is used to calculate the diffracted fields caused by both soft and hard polarizations at the edges of the finite ground plane. Explicit expressions for regular diffraction coefficients and slope diffraction coefficients are presented. The slope of the incident magnetic field at the diffraction points is derived and analytical expressions are presented. Numerical results for the radiation patterns of a cavity-backed circular spiral microstrip patch antenna excited by a coaxial probe in a finite rectangular ground plane are computed and compared with experimental results.

Reddy, C. J.↗
NASA NTRS1993

The Environmental Impact of Intra-Cluster Medium on the Interstellar Medium in Early Type Galaxies

Draft versions of three articles submitted for publication are presented. The first two articles address high resolution X-ray images of early type galaxies observed with the ROSAT HRI and PSPC. Data for NGC 1553 and NGC 5846 indicate that the emission is highly irregular, with interesting features at different scales. The gas temperatures also vary both with the galactocentric radius and in correspondence to regions of higher emission and denser material. Strikingly similar features are observed in the X-ray and H-alpha morphologies of NGC 1553 and NGC 5846, while smooth, regular isophotes are observed in NGC 4649 at both wavelengths. The third article addresses ROSAT PSPC observations of 5 X-ray bright early type galaxies.

Trinchieri, Ginevra↗
NASA NTRS1995

Spacecraft Availability Enhancement by In-flight Testing of Spare Parts

This paper describes an approach to improve availability by testing redundant parts at pre-determined intervals. The purpose of the testing is to detect nonfunctional back-up equipment and develop work-around measures or replacement spacecraft before a failure of the primary equipment reduces availability. The work reported here is an outgrowth of the NASA Space Network's program for the maintenance and replenishment of the Tracking and Data Relay Satellite System. The approach is based on a standby factor and a cyclic stress factor. The standby factor accounts for the effects of adverse storage conditions encountered as part of the in-flight environment. The stress factor accounts for the effects of physical or thermal cycling due to the application of force or power that characterizes the operation or use of a component. By the quantitative consideration of standby and cycling risks, a regular testing interval can be calculated - an interval for testing that furnishes information for availability planning but does not subject the spacecraft to undue risk. This paper includes quantitative solutions for appropriate testing intervals for equipment configurations and failure rates that are representative of a Tracking and Relay Data Satellite. The effect of a single test on the availability of certain equipment is also illustrated.

Cutler, Robert M.↗
NASA NTRS1995

Sound Generation by Flow over a Two-Dimensional Cavity

Sound generated by flow over a cavity at a Mach number of 0.1 and a Reynolds number based on cavity length of 5000 is calculated. The computation utilizes a two part technique where the time-dependent incompressible flow is first obtained and then a second calculation is performed for the compressible aspects of the flow. This second calculation utilizes a grid and numerical scheme designed for resolution of acoustic waves. The cavity flowfield is observed to oscillate quite regularly at the Strouhal number of 0.58 which produces an acoustic source of the same frequency. Time histories, spectra, and directivity of the sound radiation are computed.

Hardin, Jay C.↗
NASA NTRS1997

Direct Numerical Simulation of Incompressible Pipe Flow Using a B-Spline Spectral Method

A numerical method based on b-spline polynomials was developed to study incompressible flows in cylindrical geometries. A b-spline method has the advantages of possessing spectral accuracy and the flexibility of standard finite element methods. Using this method it was possible to ensure regularity of the solution near the origin, i.e. smoothness and boundedness. Because b-splines have compact support, it is also possible to remove b-splines near the center to alleviate the constraint placed on the time step by an overly fine grid. Using the natural periodicity in the azimuthal direction and approximating the streamwise direction as periodic, so-called time evolving flow, greatly reduced the cost and complexity of the computations. A direct numerical simulation of pipe flow was carried out using the method described above at a Reynolds number of 5600 based on diameter and bulk velocity. General knowledge of pipe flow and the availability of experimental measurements make pipe flow the ideal test case with which to validate the numerical method. Results indicated that high flatness levels of the radial component of velocity in the near wall region are physical; regions of high radial velocity were detected and appear to be related to high speed streaks in the boundary layer. Budgets of Reynolds stress transport equations showed close similarity with those of channel flow. However contrary to channel flow, the log layer of pipe flow is not homogeneous for the present Reynolds number. A topological method based on a classification of the invariants of the velocity gradient tensor was used. Plotting iso-surfaces of the discriminant of the invariants proved to be a good method for identifying vortical eddies in the flow field.

Loulou, Patrick↗
NASA NTRS1996

High-Performance Parallel Analysis of Coupled Problems for Aircraft Propulsion

This research program dealt with the application of high-performance computing methods to the numerical simulation of complete jet engines. The program was initiated in January 1993 by applying two-dimensional parallel aeroelastic codes to the interior gas flow problem of a bypass jet engine. The fluid mesh generation, domain decomposition and solution capabilities were successfully tested. Attention was then focused on methodology for the partitioned analysis of the interaction of the gas flow with a flexible structure and with the fluid mesh motion driven by these structural displacements. The latter is treated by a ALE technique that models the fluid mesh motion as that of a fictitious mechanical network laid along the edges of near-field fluid elements. New partitioned analysis procedures to treat this coupled three-component problem were developed during 1994 and 1995. These procedures involved delayed corrections and subcycling, and have been successfully tested on several massively parallel computers, including the iPSC-860, Paragon XP/S and the IBM SP2. For the global steady-state axisymmetric analysis of a complete engine we have decided to use the NASA-sponsored ENG10 program, which uses a regular FV-multiblock-grid discretization in conjunction with circumferential averaging to include effects of blade forces, loss, combustor heat addition, blockage, bleeds and convective mixing. A load-balancing preprocessor tor parallel versions of ENG10 was developed. During 1995 and 1996 we developed the capability tor the first full 3D aeroelastic simulation of a multirow engine stage. This capability was tested on the IBM SP2 parallel supercomputer at NASA Ames. Benchmark results were presented at the 1196 Computational Aeroscience meeting.

Felippa, C. A.↗
NASA NTRS1996

Wind Variability in Intermediate Luminosity B Supergiants

This study used the unique spectroscopic diagnostics of intermediate luminosity B supergiants to determine the ubiquity and nature of wind variability. Specifically, (1) A detailed analysis of HD 64760 demonstrated massive ejections into its wind, provided the first clear demonstration of a 'photospheric connection' and ionization shifts in a stellar wind; (2) The international 'IUE MEGA campaign' obtained unprecedented temporal coverage of wind variability in rapidly rotating stars and demonstrated regularly repeating wind features originating in the photosphere; (3) A detailed analysis of wind variability in the rapidly rotating B1 Ib, gamma Ara demonstrated a two component wind with distinctly different mean states at different epochs; (4) A follow-on campaign to the MEGA project to study slowly rotating stars was organized and deemed a key project by ESA/NASA, and will obtain 30 days of IUE observations in May-June 1996; and (5) A global survey of archival IUE time series identified recurring spectroscopic signatures, identified with different physical phenomena. Items 4 and 5 above are still in progress and will be completed this summer in collaboration with Raman Prinja at University College, London.

Massa, Derck↗
NASA NTRS1995

The IUE Mega Campaign. Modulated Structure in the Wind of HD 64760 (B0.5 Ib)

We highlight systematic variability in the stellar wind of the early B type supergiant, HD 64760, whose UV line profiles were monitored for almost 16 days in 1995 January as part of the IUE 'MEGA Campaign.' The extensive coverage reveals a pattern of rapidly evolving discrete optical depth changes which typically migrate from approx. - 200 km/s to approx. -1500 km/s in less than 12 hr. These features coexist with more slowly evolving structures lasting several days. Time-series analysis of the Si(IV), Si(III), and N(V) profile variations presents a clear 1.2 day periodicity, which is a quarter of the estimated maximum rotation period of HD 64760. The line profile changes are consistent with an interpretation in terms of a set of corotating wind features which occult the stellar disk at least 3 times during the observing run. These data are combined with UV observations collected in 1993 March to argue in favor of rotationally modulated wind variations in HD 64760. The basic result of very regular, large-scale optical depth variations points to a 'clock' whose origin is on the stellar surface, rather than a mechanism that is entirely intrinsic to the stellar wind.

Prinja, Raman K.↗
NASA NTRS1993

Three-Dimensional Reconstruction of Asteroids from Radar Observations

A program of research has recently been initiated under the NASA Innovative Research Program to develop and test techniques for reconstructing the three-dimensional shapes of asteroids from ground-based radar observations. Presently, no methods exist to do this and the full scientific potential of current radar data sets lies untapped. The situation will become even more pronounced in approximately two years when the upgraded Arecibo radar system becomes capable of regularly producing asteroid data sets with very fine spatial resolution and high signal-to-noise ratios. If successful, the proposed research will significantly increase our knowledge of these objects. The potential and limitations of the techniques developed will be rigorously tested in a series of computer simulations and optical scattering experiments with scale models.

Hudson, Scott↗
NASA NTRS1991

Relationship Between Intraseasonal Oscillation and Subtropical Wind Maxima Over the South Pacific Ocean

The significance of tropical heat sources on higher latitude jet streams has been examined by numerous investigators. Hurrell and Vincent (1990) provide a summary of many of these investigations in their observational case study of the relationship between tropical heating and subtropical wind maxima in the Southern Hemisphere during SOP-1, FGGE. They showed that the divergent outflow from tropical heating associated with the South Pacific Convergence Zone (SPCZ), acted on by the coriolis force, was an important factor in maintaining the subtropical jet on the poleward side of the SPCZ during the period, 6-20 January 1979. They found a similar, but weaker relationship, over the southern Indian Ocean from 3-17 February 1979, a period when the SPCZ heating was greatly reduced and the jet was essentially non-existent. Since their findings were based on a case study and involved the use of the highly-specialized FGGE data set, the natural questions which arose were: (1) Is this relationship a regular feature of the circulation over the South Pacific? and, (2) If so, can it be detected with a routine data set? Another question posed by Hurrell and Vincent in their papers was:(3) How important was the intraseasonal oscillation in causing the enhanced heating and divergent outflow in the Pacific Ocean in January and southern Indian Ocean in February? The purpose of the present paper is to address the answer to these three questions. To accomplish this, some circulation features for an entire warm season in the Southern Hemisphere were examined. The year selected was 1984-85, and the warm season consisted of the 6-month period, 1 November 1984 - 30 April 1985. This period was chosen because there were numerous cases of the westerly wind maxima over the South Pacific and the intraseasonal oscillation was well documented.

Vincent, Dayton G.↗
NASA NTRS1997

Analysis of Solar Spectral Irradiance Measurements from the SBUV/2-Series and the SSBUV Instruments

The NOAA-9 SBEV/2 instrument has made the first regular measurements ot solar UV activity over a complete solar cycle, beginning in March 1985 and continuing as of this writing. The NOAA-9 solar irradiance data set includes the minimum between Cycles 21-22 and the current minimum at the end of Cycle 22. Although overall solar activity is low during these periods, 27-day rotational modulation is frequently present. The episode of 13-day periodicity observed during September 1994 - March 1995 shows that phenomena previously associated with high levels of solar activity can occur at any point in the solar cycle. The 205 nm irradiance and Mg II index measured by NOAA-9 showed very similar behavior during the Cycle 21-22 minimum in 1985-1986, when 27-day periodicity dominated short-term solar variations, but behaved differently in 1994-1995 during the episode of 13-day periodicity. We plan further investigations into the physical causes of this result, since it affects the extent to which the Mg II index is an accurate proxy for 205 nm irradiance variations during such episodes. The NOAA-9 Mg II data are available.

Cebula, Richard P.↗
NASA NTRS1997

Error Analysis System for Spacecraft Navigation Using the Global Positioning System (GPS)

The Flight Dynamics Division (FDD) at the National Aeronautics and Space Administration (NASA) Goddard Space Flight Center (GSFC) is currently developing improved space-navigation filtering algorithms to use the Global Positioning System (GPS) for autonomous real-time onboard orbit determination. In connection with a GPS technology demonstration on the Small Satellite Technology Initiative (SSTI)/Lewis spacecraft, FDD analysts and programmers have teamed with the GSFC Guidance, Navigation, and Control Branch to develop the GPS Enhanced Orbit Determination Experiment (GEODE) system. The GEODE system consists of a Kalman filter operating as a navigation tool for estimating the position, velocity, and additional states required to accurately navigate the orbiting Lewis spacecraft by using astrodynamic modeling and GPS measurements from the receiver. A parallel effort at the FDD is the development of a GPS Error Analysis System (GEAS) that will be used to analyze and improve navigation filtering algorithms during development phases and during in-flight calibration. For GEAS, the Kalman filter theory is extended to estimate the errors in position, velocity, and other error states of interest. The estimation of errors in physical variables at regular intervals will allow the time, cause, and effect of navigation system weaknesses to be identified. In addition, by modeling a sufficient set of navigation system errors, a system failure that causes an observed error anomaly can be traced and accounted for. The GEAS software is formulated using Object Oriented Design (OOD) techniques implemented in the C++ programming language on a Sun SPARC workstation. The Phase 1 of this effort is the development of a basic system to be used to evaluate navigation algorithms implemented in the GEODE system. This paper presents the GEAS mathematical methodology, systems and operations concepts, and software design and implementation. Results from the use of the basic system to evaluate navigation algorithms implemented on GEODE are also discussed. In addition, recommendations for generalization of GEAS functions and for new techniques to optimize the accuracy and control of the GPS autonomous onboard navigation are presented.

Truong, S. H.↗
NASA NTRS1996

Proceedings of 15th International Conference on Thermoelectrics

The proceedings include all the technical papers presented at the 15th International Conference on Thermoelectrics, held on 26-29 March 1996 at Pasadena, California, USA. There were 10 invited presentations and over 110 regular presentations in this conference. The presentations were made in 20 technical sessions and are organized accordingly in this proceedings.

Fleurial, Jean-Pierre↗
NASA NTRS1996

Novel Afferent Terminal Structure in the Crista Ampullaris of the Goldfish, Carassius auratus

Using transmission electron microscopy, we have identified a new type of afferent terminal structure in the crista ampullaris of the goldfish Carassius auratus. In addition to the bouton-type afferent terminals previously described in the ear of this species, the crista also contained enlarged afferent terminals that enveloped a portion of the basolateral hair cell membrane. The hair cell membrane was evaginated and protruded into the afferent terminal in a glove-and-finger configuration. The membranes of the two cells were regularly aligned in the protruded region of the contact and had a distinct symmetrical electron density. The electron-dense profiles of these contacts were easily identified and were present in every crista sampled. In some cases, efferent terminals synapsed onto the afferents at a point where the hair cell protruded into the terminal. The ultrastructural similarities of the goldfish crista afferents to calyx afferents found in amniotes (birds, reptiles, and mammals) are discussed. The results of the study support the hypothesis that structural variation in the vertebrate inner ear may have evolved much earlier in evolution than previously supposed.

Lanford, Pamela J.↗
NASA NTRS1997

Reactions of Residents to Long-Term Sonic Boom Noise Environments

A combined social survey and noise measurement program has been completed in 14 communities in two regions of the western United States that have been regularly exposed to sonic booms for many years. A total of 1,573 interviews were completed. Three aspects of the sonic booms are most disturbing: being startled, noticing rattles or vibrations, and being concerned about the possibility of damage from the booms. Sonic boom annoyance is greater than that in a conventional aircraft environment with the same continuous equivalent noise exposure. The reactions in the two study regions differ in severity.

Fields, James M.↗
NASA NTRS1997

Combustion Noise at Elevated Pressures in a Liquid-Fueled Premixed Combustor

Noise generated in gas turbine combustors can exist in several forms-broadband noise, sharp resonant peaks, and regular or intermittent nonlinear pulsing. In the present study, dynamic pressure measurements were made in several JP-5-fueled combustor configurations, at various mean pressures and temperatures. The fluctuating pressure was measured at mean pressures from 6 to 14 atm and inlet temperatures from 550 K to 850 K. The goal of the present work was to study the effect of changes in mean flow conditions on combustor noise: both broadband noise and sharp tones were considered. In general, the shape of the broadband noise spectrum was consistent from one configuration to another. The shape of the spectrum was influenced by the acoustic filtering of the combustion zone. This filtering ensured the basic consistency of the spectra. In general, the trends in broadband noise observed at low mean pressures were also seen at high mean pressures; that is, the total sound level decreased with both increasing equivalence ratio and increasing inlet temperature. The combustor configurations without a central pilot experienced higher broadband noise levels and were more susceptible to narrow peak resonances than configurations with a central pilot. The sharp peaks were more sensitive to the mean flow than was the broadband noise, and the effects were not always the same. In some situations, increasing the equivalence ratio made the sharp peaks grow, while at other conditions, increasing the equivalence ratio made the sharp peaks shrink. Thus, it was difficult to predict when resonances would occur; however, they were reproducible. Acoustic coupling between the upstream and downstream regions of the combustor may play a role in the sharp-peaked oscillations. Noise was also observed near lean blow out. As with other types of noise, lean blow out noise was affected by the combustion chamber acoustics, which apparently maintains the fluctuations at a uniform frequency. However, the actual conditions when this type of noise was experienced appeared to simply follow the lean blow out limit as it varied with mean temperature and pressure.

Darling, Douglas↗
NASA NTRS1993

Topographic Characterization

The National Aeronautics and Space Administration (NASA), other government agencies, and private industry have requirements to map and analyze 3-dimensional surfaces of varying regularity and material composition. This requires a high fidelity, 3-dimensional description of the work space. In cases where complete and current information about the space is not available, topographic characterization allows on-line initialization and/or modification of the work space database. The mapping environment is often challenging with regard to lighting, radiation, temperature, atmosphere, and causticity. This paper describes a system that provides topographic characterization based on fusing intensity and depth information, and describes the application of this technique for inspection of Shuttle thermal tiles.

Goode, Plesent W.↗
NASA NTRS1997

Risk Management of NASA Projects

Various NASA Langley Research Center and other center projects were attempted for analysis to obtain historical data comparing pre-phase A study and the final outcome for each project. This attempt, however, was abandoned once it became clear that very little documentation was available. Next, extensive literature search was conducted on the role of risk and reliability concepts in project management. Probabilistic risk assessment (PRA) techniques are being used with increasing regularity both in and outside of NASA. The value and the usage of PRA techniques were reviewed for large projects. It was found that both civilian and military branches of the space industry have traditionally refrained from using PRA, which was developed and expanded by nuclear industry. Although much has changed with the end of the cold war and the Challenger disaster, it was found that ingrained anti-PRA culture is hard to stop. Examples of skepticism against the use of risk management and assessment techniques were found both in the literature and in conversations with some technical staff. Program and project managers need to be convinced that the applicability and use of risk management and risk assessment techniques is much broader than just in the traditional safety-related areas of application. The time has come to begin to uniformly apply these techniques. The whole idea of risk-based system can maximize the 'return on investment' that the public demands. Also, it would be very useful if all project documents of NASA Langley Research Center, pre-phase A through final report, are carefully stored in a central repository preferably in electronic format.

Sarper, Hueseyin↗