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At least 703 records · Page 39
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NASA NTRS1994

On the hydrodynamic interaction of shock waves with interstellar clouds. 1: Nonradiative shocks in small clouds

The interstellar medium (ISM) is inhomogeneous, with clouds of various temperatures and densities embedded in a tenuous intercloud medium. Shocks propagating through the ISM can ablate or destroy the clouds, at the same time significantly altering the properties of the intercloud medium. This paper presents a comprehensive numerical study of the simplest case of the interaction between a shock wave and a spherical cloud, in which the shock far from the cloud is steady and planar, and in which radiative losses, thermal conduction, magnetic fields, and gravitational forces are all neglected. As a result, the problem is completely specified by two numbers: the Mach number of the shock, M, and the ratio of the density of the cloud to that of the intercloud medium, Chi. For strong shocks we show that the dependence on M scales out, so the primary independent parameter is Chi. Variations from this simple case are also considered: the potential effect of radiative losses is assessed by calculations in which the ratio of specific heats in the cloud is 1.1 instead of 5/3; the effect of the initial shape of the cloud is studied by using a cylindrical cloud instead of a spherical one; and the role of the initial shock is determined by considering the case of a cloud embedded in a wind. Local adaptive mesh refinement techniques with a second-order, two-fluid, two-dimensional Godunov hydrodynamic scheme are used to address these problems, allowing heretofore unobtainable numerical resolution. Convergence studies to be described in a subsequent paper demonstrate that about 100 zones per cloud radius are needed for accurate results; previous calculations have generally used about a third of this number. The results of the calculations are analyzed in terms of global quantities which provide an overall description of te shocked cloud: the size and shape of the cloud, the mean density, the mean pressure, the mean velocity, the velocity dispersion, and the total circulation.

Klein, Richard I.↗
NASA NTRS1994

Circumprotostellar environments. 2: Envelopes, activity, and evolution

We have obtained 800 and 1100 micron photometric observations of a complete, flux-limited, IRAS-selected sample of cold sources in Taurus, whose infrared and molecular properties indicate them to be low-mass protostars which are younger than T Tauri stars. The goal of this study is to understand the role of the circumstellar envelope and disk during low-mass star formation and to search for signs of evolution. We find a good correlation between the submillimeter flux density (as well as of envelope mass) with both the mechanical luminosity of the molecular outflow and with the far-infrared luminosity, presumably produced by accretion onto the protostar. This suggests that the mass of the disk/envelope may be linked to the rate of accretion onto the protostar and to the generation of outflows. However, in contrast to the results of Cabrit & Andre (1991) we see no evidence for a lower limit to the disk/envelope mass which can produce outflow. We also find a significant difference between the spectral energy distributions at submillimeter wavelengths of embedded protostars compared to optically visible T Tauri stars. The more evolved (T Tuari) objects tend to have shallower spectral indices than embedded sources, suggesting (1) an evolution of dust grains, and/or (2) a more compact envelope and optically thick disk in the older objects.

Moriarty-Schieven, G. H.↗
NASA NTRS1994

A detailed examination of a X-line region in the distant tail: ISEE-3 observations of jet flow and B(sub z) reversals and a pair of slow shocks

We report an observation of Petschek-type magnetic reconnection at a distant neutral line (X = -230 R(sub e)) with a full set of signatures of the magnetic merging process. These features include a reversal of plasma flows from earthward to tailward, a pair of slow shocks and the magnetic field X-type line. These two slow shocks are shown to satisfy the shock criteria used by Feldman et al. (1987). The spacecraft first crosses a slow shock to enter the earthward flowing plasmasheet with velocity of about 440 km/s. The embedded magnetic field has a positive B(sub z) component. The spacecraft next enters a region of tailward plasma flow with speed approximately 670 km/s and an embedded negative B(sub z), indicating entry into the plasmasheet tailward of the X-line. These observed velocities are comparable to calculated velocities based on Rankine-Hugoniot conservation relationships. The spacecraft subsequently returns into the south tail lobe by crossing another slow shock. Coplanarity analyses shows that the two slow shocks have orientations consistent with that predicted by the Petschek reconnection model. We note that this event occurs during northward interplanetary magnetic fields. Thus, a magnetic stress built-up in the distant tail may be responsible for this reconnection process.

Ho, C. M.↗
NASA NTRS1995

Evidence for chemical processing of precometary icy grains in circumstellar environments of pre-main-sequence stars

We report the detection of a broad absorption feature near 2166/cm in the spectrum of the Taurus cloud cource Elias 18. This pre-main-sequence source is the second in Taurus, the third in our survey, and the fifth known in the sky to show the broad 2166/cm absorption feature. Of equal importance, this feature is not seen toward several other embedded sources in our survey, nor is it seen toward the source Elias 16, located behind the Taurus cloud. Laboratory experiments with interstellar ice analogs show that such a feature is associated with a complex C triple bonded to N containing compound (called X(C triple bonded to N)) that results from high-energy processing (ultraviolet irradiation or ion bombardment) of simple ice components into more complex, organic components. We find a nonlinear anticorrelation between the abundance of X(C triple bonded to N) and frozen CO components in nonpolar lattices. We find no correlation between the abundance of X(C triple bonded to N) and frozen CO in polar lattices. Because the abundances of frozen CO and H2O are strongly correlated with each other and with visual extinction toward sources embedded in and located behind the Taurus molecular cloud, these ice components usually are associated with intracloud material. Our results indicate that X(C triple bonded to N) molecules result from chemical processing of dust grains dominated by nonpolar icy mantles in the local environments of pre-main-sequence stars. Such processing of icy grains in the early solar system may be an important source of organic compounds observed in minor solar system bodies. The delivery of these organic compounds to the surface of the primitive Earth through comet impacts may have provided the raw materials for prebiotic chemistry.

Tegler, Stephen C.↗
NASA NTRS1995

Person-Locator System Based On Wristband Radio Transponders

Computerized system based on wristband radio frequency (RF), passive transponders is being developed for use in real-time tracking of individuals in custodial institutions like prisons and mental hospitals. Includes monitoring system that contains central computer connected to low-power, high-frequency central transceiver. Transceiver connected to miniature transceiver nodes mounted unobtrusively at known locations throughout the institution. Wristband transponders embedded in common hospital wristbands. Wristbands tamperproof: each contains embedded wire loop which, when broken or torn off and discarded, causes wristband to disappear from system, thus causing alarm. Individuals could be located in a timely fashion at relatively low cost.

Mintz, Frederick W.↗
NASA NTRS1990

Building distributed rule-based systems using the AI Bus

The AI Bus software architecture was designed to support the construction of large-scale, production-quality applications in areas of high technology flux, running heterogeneous distributed environments, utilizing a mix of knowledge-based and conventional components. These goals led to its current development as a layered, object-oriented library for cooperative systems. This paper describes the concepts and design of the AI Bus and its implementation status as a library of reusable and customizable objects, structured by layers from operating system interfaces up to high-level knowledge-based agents. Each agent is a semi-autonomous process with specialized expertise, and consists of a number of knowledge sources (a knowledge base and inference engine). Inter-agent communication mechanisms are based on blackboards and Actors-style acquaintances. As a conservative first implementation, we used C++ on top of Unix, and wrapped an embedded Clips with methods for the knowledge source class. This involved designing standard protocols for communication and functions which use these protocols in rules. Embedding several CLIPS objects within a single process was an unexpected problem because of global variables, whose solution involved constructing and recompiling a C++ version of CLIPS. We are currently working on a more radical approach to incorporating CLIPS, by separating out its pattern matcher, rule and fact representations and other components as true object oriented modules.

Schultz, Roger D.↗
NASA NTRS1995

The envelopes around young stellar objects: What can far-infared observations tell us?

High resolution, far-infrared spatial observations offer a way to probe the density structure of the envelopes around young stellar objects, and thus provide a test of star formation theories. As part of a long term program, we are using the Kuiper Airborne Observatory with the Yerkes Far-Infrared Camera system to study the far-infrared emission (60, 100, 160, and 200 microns) from a number of low mass embedded objects in Taurus. By comparing the observations with the results of radiative transfer models, we are able to estimate the envelope density distribution around these systems. We present our initial results for 4 embedded objects (L1551-IRS 5, L1489-IR, L1551-NE, and L1527-IR).

Butner, H. M.↗
NASA NTRS1995

The 16-45 micron observations of the Galactic Center

Existing observations of the Galactic Center at infrared and radio wavelengths challenge our understanding of the detailed morphology and energy balance of the inner few parsec, including the Galactic Center and the infrared torus. The distribution and nature of the sources heating this region are still not well understood; existing determinations of dust temperature and ionization do not provide us with consistent pictures of the relative important of the central source and the embedded stars in this dusty region. The composite IR emission of the Galactic Center can be crudely divided into three categories: (1) hot dust heated directly by an incident UV field along the inner region of the Galactic Center torus; (2) warm dust heated by te non-ionizing radiation of the embedded stars and re-radiated NIR dust emission; (3) cooler absorbing dust located along the galactic line of sight. The apparent inconsistencies between the observations and theoretical expectations may stem from the interplay of various physical process and source-cloud geometries. Observations with increased spatial and spectral resolution are clearly needed to provide the information necessary to address the various problems. Therefore, we made 15-45 micron spectrophotometric observations of the inner 80 min (3 pc) regions surrounding the Galactic Center with the 20 min aperture of Goddard Cryogenic Grating Spectrometer No. 2 in May 1994 from the KAO. We measured nine points, including the 50 and 90 micron peaks of Davidson et al. and points between them and SgrA,. The wavelength coverage of our instrument ensures sensitivity to the hot dust component, silicate emission and/or absorption features, and cooler dust at longer wavelengths. Our observations will be used to set limits on the luminosity of any central sources, or give an independent estimate of central luminosity, and to set limits on the range of acceptable dust parameters for this region.

Chan, Kin-Wing↗
NASA NTRS1995

An overview of engineering concepts and current design algorithms for probabilistic structural analysis

The article begins by examining the fundamentals of traditional deterministic design philosophy. The initial section outlines the concepts of failure criteria and limit state functions two traditional notions that are embedded in deterministic design philosophy. This is followed by a discussion regarding safety factors (a possible limit state function) and the common utilization of statistical concepts in deterministic engineering design approaches. Next the fundamental aspects of a probabilistic failure analysis are explored and it is shown that deterministic design concepts mentioned in the initial portion of the article are embedded in probabilistic design methods. For components fabricated from ceramic materials (and other similarly brittle materials) the probabilistic design approach yields the widely used Weibull analysis after suitable assumptions are incorporated. The authors point out that Weibull analysis provides the rare instance where closed form solutions are available for a probabilistic failure analysis. Since numerical methods are usually required to evaluate component reliabilities, a section on Monte Carlo methods is included to introduce the concept. The article concludes with a presentation of the technical aspects that support the numerical method known as fast probability integration (FPI). This includes a discussion of the Hasofer-Lind and Rackwitz-Fiessler approximations.

Duffy, S. F.↗
NASA NTRS1996

Data and Time Transfer Using SONET Radio

The need for precise knowledge of time and frequency has become ubiquitous throughout our society. The areas of astronomy, navigation, and high speed wide-area networks are among a few of the many consumers of this type of information. The Global Positioning System (GPS) has the potential to be the most comprehensive source of precise timing information developed to date; however, the introduction of selective availability has made it difficult for many users to recover this information from the GPS system with the precision required for today's systems. The system described in this paper is a 'Synchronous Optical NetWORK (SONET) Radio Data and Time Transfer System'. The objective of this system is to provide precise time and frequency information to a variety of end-users using a two-way data and time-transfer system. Although time and frequency transfers have been done for many years, this system is unique in that time and frequency information are embedded into existing communications traffic. This eliminates the need to make the transfer of time and frequency informatio a dedicated function of the communications system. For this system SONET has been selected as the transport format from which precise time is derived. SONET has been selected because of its high data rates and its increasing acceptance throughout the industry. This paper details a proof-of-concept initiative to perform embedded time and frequency transfers using SONET Radio.

Graceffo, Gary M.↗
NASA NTRS1996

Control of Thermal Deflection, Panel Flutter and Acoustic Fatigue at Elevated Temperatures Using Shape Memory Alloys

The High Speed Civil Transport (HSCT) will have to be designed to withstand high aerodynamic load at supersonic speeds (panel flutter) and high acoustic load (acoustic or sonic fatigue) due to fluctuating boundary layer or jet engine acoustic pressure. The thermal deflection of the skin panels will also alter the vehicle's configuration, thus it may affect the aerodynamic characteristics of the vehicle and lead to poor performance. Shape memory alloys (SMA) have an unique ability to recover large strains completely when the alloy is heated above the characteristic transformation (austenite finish T(sub f)) temperature. The recovery stress and elastic modulus are both temperature dependent, and the recovery stress also depends on the initial strain. An innovative concept is to utilize the recovery stress by embedding the initially strained SMA wire in a graphite/epoxy composite laminated panel. The SMA wires are thus restrained and large inplane forces are induced in the panel at elevated temeperatures. By embedding SMA in composite panel, the panel becomes much stiffer at elevated temperatures. That is because the large tensile inplane forces induced in the panel from the SMA recovery stress. A stiffer panel would certainly yield smaller dynamic responses.

Mei, Chuh↗
NASA NTRS1996

Fiber Optic Sensors for Smart Materials and Structures

In this paper we describe recently developed fiber sensors which are capable of monitoring the health of smart-structures. The unobstrusive geometry of these sensors make them an excellent choice for embedding the sensor in composite materials to measure internal states of strain in structures and materials. Some of these sensors have gage lengths that can be tailored from tens of microns to many meters. We will describe various demodulation schemes (Pseudo-Heterodyne, Synthetic-Heterodyne, Homodyne, Differential-Cross Multiplier, and Single Channel Phase-Tracker) to obtain high bandwidth measurements, enabling measurement of static to high frequency impact generated strains with a dynamic response exceeding tens of thousands of microstrains. In addition, we will show that we can tailor the fiber sensor to either measure only strain and reject temperature response or measure only the temperature, or measure both temperature and strain simultaneously. We will also demonstrate the ability to measure multiple strain components inside a host simultaneously using a single fiber sensor embedded in the host using a certain sensor type and transverse strain immunity using another sensor type. Additionally we will show the ability to measure temperature up to 100 C using fiber optic sensors.

Singh, H.↗
NASA NTRS1994

Geometry and Unoccupied Electronic States of Ba and BaO on W(001)

A study aimed at understanding the geometrical and electronic properties of barium and oxygen coadsorbed on the tungsten(001) surface has been carried out by means of work-function measurements (Delta-phi), Auger-electron spectroscopy, low-energy electron diffraction, inverse photoelectron spectroscopy, and relativistic-electronic-structure calculations. A report of the experimental measurements and a comparison with theoretical results from embedded-cluster-model calculations are presented. Our experimental studies show that the work function of the W(001) surface (phi = 4.63 eV) is lowered to approximately 2.3-2.4 eV by coadsorption of 1 ML of Ba and O regardless of the order of deposition of these two species. The technique of IPS in the isochromat mode was used to determine the unoccupied electronic-energy band structure for ordered c (2 X 2) Ba and O layers on W(001). Several spectral features are observed above the Fermi level (E(F)), which we assign to transitions into Ba and W d-states. The measured two-dimensional electronic band structure is independent of the order of Ba and O deposition. Using embedded-cluster-model calculations, we investigated two possible adsorption configurations of an ordered c(2 X 2) adlayer of Ba and O on W(001): 'tilted,' where Ba and O are placed on alternate fourfold-hollow sites, and 'upright,' where the adsorbed atoms lay above the same site with Ba outer-most. The calculated densities of states for the tilted geometry show distinct peaks above E(F) originating from Ba and W d-orbitals and are in good agreement with the experimental results.

Lamouri, A.↗
NASA NTRS1997

Micromechanical Modeling of Woven Metal Matrix Composites

This report presents the results of an extensive micromechanical modeling effort for woven metal matrix composites. The model is employed to predict the mechanical response of 8-harness (8H) satin weave carbon/copper (C/Cu) composites. Experimental mechanical results for this novel high thermal conductivity material were recently reported by Bednarcyk et al. along with preliminary model results. The micromechanics model developed herein is based on an embedded approach. A micromechanics model for the local (micro-scale) behavior of the woven composite, the original method of cells (Aboudi), is embedded in a global (macro-scale) micromechanics model (the three-dimensional generalized method of cells (GMC-3D) (Aboudi). This approach allows representation of true repeating unit cells for woven metal matrix composites via GMC-3D, and representation of local effects, such as matrix plasticity, yarn porosity, and imperfect fiber-matrix bonding. In addition, the equations of GMC-3D were reformulated to significantly reduce the number of unknown quantities that characterize the deformation fields at the microlevel in order to make possible the analysis of actual microstructures of woven composites. The resulting micromechanical model (WCGMC) provides an intermediate level of geometric representation, versatility, and computational efficiency with respect to previous analytical and numerical models for woven composites, but surpasses all previous modeling work by allowing the mechanical response of a woven metal matrix composite, with an elastoplastic matrix, to be examined for the first time. WCGMC is employed to examine the effects of composite microstructure, porosity, residual stresses, and imperfect fiber-matrix bonding on the predicted mechanical response of 8H satin C/Cu. The previously reported experimental results are summarized, and the model predictions are compared to monotonic and cyclic tensile and shear test data. By considering appropriate levels of porosity, residual stresses, and imperfect fiber-matrix debonding, reasonably good qualitative and quantitative correlation is achieved between model and experiment.

Bednarcyk, Brett A.↗
NASA NTRS1997

X-Ray Gas Temperatures in the Arc Clusters MS0440+204 and MS0302+1658

The cluster of galaxies MS0440+02, originally discovered through its X-ray emission, was part of an optical observational program to search for arcs and arclets in a complete sample of X-ray luminous, medium-distant clusters of galaxies. Mauna Kea CCD images of MS0440+02 showed a remarkable optical morphology. The core of the cluster contains 6 bright galaxies and numerous fainter ones embedded in a low surface brightness halo. Besides, MS0440+02 is the most spectacular example that we have found of an arc system in a compact condensed cluster, with arcs symmetrically distributed to draw almost perfect circles around the cluster center. Giant arcs are magnified images of distant galaxies, gravitationally distorted by massive foreground clusters. It is of great importance to compare the results of the lensing studies with those derived from X-ray observations, as the two are independent methods of studying the mass distribution. Thus MS0440+02 was the ideal target to obtain temperature measurement with ASCA and good spatial resolution X-ray observations with ROSAT. The X-ray data have been used in conjunction with Hubble Space Telescope observations to put more stringent constrains on the mass estimates. Most of the different wavelength datasets have been reduced and analyzed. Mass determinations have been separately obtained from galaxy virial motions and X-ray profile fitting using the cluster gas temperature as measured by the ASCA satellite. Assuming that the hot gas is in hydrostatic equilibrium and in a spherical potential, we find from the X-ray data a mass distribution profile that is well described by a Beta model. From the multiple images formed by gravitational lensing (HST data) using the modelling of the gravitational lensed arcs, we have derived Beta model. To reconcile the mass estimates we have explored the possibility of having a supercluster surrounding the MOS0440 cluster, that is a model with two isothermal spheres, one embedded inside the other. These results have been published or are in press.

Gioia, Isabella M.↗
NASA NTRS1998

Analysis and Testing of Plates with Piezoelectric Sensors and Actuators

Piezoelectric material inherently possesses coupling between electrostatics and structural dynamics. Utilizing linear piezoelectric theory results in an intrinsically coupled pair of piezoelectric constitutive equations. One equation describes the direct piezoelectric effect where strains produce an electric field and the other describes the converse effect where an applied electrical field produces strain. The purpose of this study is to compare finite element analysis and experiments of a thin plate with bonded piezoelectric material. Since an isotropic plate in combination with a thin piezoelectric layer constitutes a special case of a laminated composite, the classical laminated plate theory is used in the formulation to accommodated generic laminated composite panels with multiple bonded and embedded piezoelectric layers. Additionally, the von Karman large deflection plate theory is incorporated. The formulation results in laminate constitutive equations that are amiable to the inclusion of the piezoelectric constitutive equations yielding in a fully electro-mechanically coupled composite laminate. Using the finite element formulation, the governing differential equations of motion of a composite laminate with embedded piezoelectric layers are derived. The finite element model not only considers structural degrees of freedom (d.o.f.) but an additional electrical d.o.f. for each piezoelectric layer. Comparison between experiment and numerical prediction is performed by first treating the piezoelectric as a sensor and then again treating it as an actuator. To assess the piezoelectric layer as a sensor, various uniformly distributed pressure loads were simulated in the analysis and the corresponding generated voltages were calculated using both linear and nonlinear finite element analyses. Experiments were carried out by applying the same uniformly distributed loads and measuring the resulting generated voltages and corresponding maximum plate deflections. It is found that a highly nonlinear relationship exists between maximum deflection and voltage versus pressure loading. In order to assess comparisons of predicted and measured piezoelectric actuation, sinusoidal excitation voltages are simulated/applied and maximum deflections are calculated/measured. The maximum deflection as a function of time was determined using the linear finite elements analysis. Good correlation between prediction and measurement was achieved in all cases.

Bevan, Jeffrey S.↗
NASA NTRS1999

Effects of Precipitation on Ocean Mixed-Layer Temperature and Salinity as Simulated in a 2-D Coupled Ocean-Cloud Resolving Atmosphere Model

A two-dimensional coupled ocean-cloud resolving atmosphere model is used to investigate possible roles of convective scale ocean disturbances induced by atmospheric precipitation on ocean mixed-layer heat and salt budgets. The model couples a cloud resolving model with an embedded mixed layer-ocean circulation model. Five experiment are performed under imposed large-scale atmospheric forcing in terms of vertical velocity derived from the TOGA COARE observations during a selected seven-day period. The dominant variability of mixed-layer temperature and salinity are simulated by the coupled model with imposed large-scale forcing. The mixed-layer temperatures in the coupled experiments with 1-D and 2-D ocean models show similar variations when salinity effects are not included. When salinity effects are included, however, differences in the domain-mean mixed-layer salinity and temperature between coupled experiments with 1-D and 2-D ocean models could be as large as 0.3 PSU and 0.4 C respectively. Without fresh water effects, the nocturnal heat loss over ocean surface causes deep mixed layers and weak cooling rates so that the nocturnal mixed-layer temperatures tend to be horizontally-uniform. The fresh water flux, however, causes shallow mixed layers over convective areas while the nocturnal heat loss causes deep mixed layer over convection-free areas so that the mixed-layer temperatures have large horizontal fluctuations. Furthermore, fresh water flux exhibits larger spatial fluctuations than surface heat flux because heavy rainfall occurs over convective areas embedded in broad non-convective or clear areas, whereas diurnal signals over whole model areas yield high spatial correlation of surface heat flux. As a result, mixed-layer salinities contribute more to the density differences than do mixed-layer temperatures.

Li, Xiaofan↗
NASA NTRS1999

Future X Pathfinder: Quick, Low Cost Flight Testing for Tomorrow's Launch Vehicles

The DC-X and DC-XA Single Stage Technology flight program demonstrated the value of low cost rapid prototyping and flight testing of launch vehicle technology testbeds. NASA is continuing this important legacy through a program referred to as Future-X Pathfinder. This program is designed to field flight vehicle projects that cost around $100M each, with a new vehicle flying about every two years. Each vehicle project will develop and extensively flight test a launch vehicle technology testbed that will advance the state of the art in technologies directly relevant to future space transportation systems. There are currently two experimental, or "X" vehicle projects in the Pathfinder program, with additional projects expected to follow in the near future. The first Pathfinder project is X-34. X-34 is a suborbital rocket plane capable of flights to Mach 8 and 75 kilometers altitude. There are a number of reusable launch vehicle technologies embedded in the X-34 vehicle design, such as composite structures and propellant tanks, and advanced reusable thermal protection systems. In addition, X-34 is designed to carry experiments applicable to both the launch vehicle and hypersonic aeronautics community. X-34 is scheduled to fly later this year. The second Pathfinder project is the X-37. X-37 is an orbital space plane that is carried into orbit either by the Space Shuttle or by an expendable launch vehicle. X-37 provides NASA access to the orbital and orbital reentry flight regimes with an experimental testbed vehicle. The vehicle will expose embedded and carry-on advanced space transportation technologies to the extreme environments of orbit and reentry. Early atmospheric approach and landing tests of an unpowered version of the X-37 will begin next year, with orbital flights beginning in late 2001. Future-X Pathfinder is charting a course for the future with its growing fleet of low-cost X- vehicles. X-34 and X-37 are leading the assault on high launch costs and enabling the flight testing of technologies that will lead to affordable access to space.

London, John, III↗