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Nomenclatura Unesco > (55) Historia

Mostrando recursos 108,521 - 108,540 de 154,011

108521. Shock diffraction computations over complex structures - Kutler, P.; Mark, A.
This work contains the results of a study aimed at the development of two- and three-dimensional numerical procedures for computing the flowfield generated by the interaction of a blast wave and a rigid body. A number of numerical procedures were applied to two-dimensional problems including both implicit and explicit algorithms. Each was tried on the blast wave-cylinder interaction problem. MacCormack's (1969) method with added fourth-order dissipation yielded the best results and was then applied to the blast wave-truck interaction problems in two dimensions. MacCormack's method was also used in three dimensions to determine the flowfield that results when a blast...

108522. Nonlinear constitutive theory for turbine engine structural analysis - Thompson, R. L.
A number of viscoplastic constitutive theories and a conventional constitutive theory are evaluated and compared in their ability to predict nonlinear stress-strain behavior in gas turbine engine components at elevated temperatures. Specific application of these theories is directed towards the structural analysis of combustor liners undergoing transient, cyclic, thermomechanical load histories. The combustor liner material considered in this study is Hastelloy X. The material constants for each of the theories (as a function of temperature) are obtained from existing, published experimental data. The viscoplastic theories and a conventional theory are incorporated into a general purpose, nonlinear, finite element computer program....

108523. Numerical study of ribbon-induced transition in Blasius flow - Spalart, Philippe R.; Yang, Kyung-Soo
The early three-dimensional stages of transition in the Blasius boundary layer are studied by numerical solution of the Navier-Stokes equations. A finite-amplitude two-dimensional wave and low-amplitude three-dimensional random disturbances are introduced. Rapid amplification of the three-dimensional components is observed and leads to transition. For intermediate amplitudes of the two-dimensional wave the breakdown is of subharmonic type, and the dominant spanwise wavenumber increases with the amplitude. For high amplitudes the energy of the fundamental mode is comparable to the energy of the subharmonic mode, but never dominates it; the breakdown is of mixed type. Visualizations, energy histories, and spectra are presented....

108524. Secondary ionization in a flat universe - Atrio-Barandela, F.; Doroshkevich, A. G.
We analyze the effect of a secondary ionization on the evolution of temperature fluctuations in cosmic background radiation. The main results presented in this paper are appropriate analytic expressions of the transfer function relating temperature fluctuations to matter density perturbations at recombination for all possible recombination histories. Furthermore, we particularize our calculation to the standard cold dark matter model, where we study the erasure of primordial temperature fluctuations and calculate the magnitude and angular scale of the damping induced by a late recombination.

108525. Validation of structural analysis methods using the in-house liner cyclic rigs - Thompson, R. L.
Test conditions and variables to be considered in each of the test rigs and test configurations, and also used in the validation of the structural predictive theories and tools, include: thermal and mechanical load histories (simulating an engine mission cycle; different boundary conditions; specimens and components of different dimensions and geometries; different materials; various cooling schemes and cooling hole configurations; several advanced burner liner structural design concepts; and the simulation of hot streaks. Based on these test conditions and test variables, the test matrices for each rig and configurations can be established to verify the predictive tools over as wide...

108526. Chaotic motion of Europa and Ganymede and the Ganymede-Callisto dichotomy - Tittemore, William C.
Europa and Ganymede may have undergone an episode of chaotic motion before the establishment of the current Laplace resonance involving the three inner GAlilean satellites. During this episode, the orbital eccentricities of both satellites may have increased dramatically. As a result, the mechanical stresses due to tidal deformation of the satellites' icy lithospheres may have been large enough to result in extensive fracturing, and tidal heating may have melted water ice in the mantles of both satellites, triggering the geological activity that has modified their surfaces since the heavy cratering period. The tidal effects on Ganymede during this episode provide...

108527. Chaotic motion of Europa and Ganymede and the Ganymede-Callisto dichotomy - Tittemore, William C.
Europa and Ganymede may have undergone an episode of chaotic motion before the establishment of the current Laplace resonance involving the three inner GAlilean satellites. During this episode, the orbital eccentricities of both satellites may have increased dramatically. As a result, the mechanical stresses due to tidal deformation of the satellites' icy lithospheres may have been large enough to result in extensive fracturing, and tidal heating may have melted water ice in the mantles of both satellites, triggering the geological activity that has modified their surfaces since the heavy cratering period. The tidal effects on Ganymede during this episode provide...

108528. He D3 as a diagnostic for the hard and soft X-rays from solar flares - Feldman, U.; Liggett, M.; Zirin, H.
The time comparison of He D3 and X-ray emission has been studied in a number of medium-sized flares. In most cases there is a good agreement between the time histories of the He D3 emission and the high-temperature (about 2 keV) thermal source. The most intense He D3 emission comes from two small regions on either side of the neutral line, which are identified with the footpoints of magnetic structures in which a hot (2.3 x 10 to the 7th K) thermal plasma is formed. The impulsive X-ray event is marked by the transient brightening of a number of (usually...

108529. Science/Engineering: Open Doors - Arnold, James O.; White, Susan
Trends in American society are changing the role of women in science and engineering, but all the elements in our society change at different rates. Women, like men, must choose during their teenage years to continue their training in math or science, or they close the door that can lead them to futures in the interesting and satisfying fields of science and engineering. The key is to keep girls involved in the hard sciences through the adolescent crisis. Many mentoring and outreach programs exist to help young women cross this threshold. These programs include hands-on science experiences, mentoring or putting...

108530. The Average Intensity and Spectral Evolution of Batse Cosmic Gamma-Ray Bursts - Pozanenko, Alexei S.; Meegan, Charles A.; Sagdeev, Roald Z.; Briggs, Michael S.; Paciesas, William S.; Mitrofanov, Igor G.; Fishman, Gerald J.; Chernenko, Anton M.
Statistical studies of BATSE gamma-ray bursts are presented: we average the time profiles, aligning the events at their highest peaks. Using the time histories in different energy channels, we summarize the general features of the average intensity and spectral evolution of gamma-ray bursts (GRBS) and compare the average evolution of bright and dim events. While no differences are found between the average intensity histories, the average hardness ratio histories demonstrate a hardness/brightness correlation. The observed lack of differences between the intensity histories of bright and dim bursts is incompatible with cosmological time dilation effects predicted by simple cosmological models. The...

108531. Component-specific modeling - Mcknight, R. L.
A series of interdisciplinary modeling and analysis techniques that were specialized to address three specific hot section components are presented. These techniques will incorporate data as well as theoretical methods from many diverse areas including cycle and performance analysis, heat transfer analysis, linear and nonlinear stress analysis, and mission analysis. Building on the proven techniques already available in these fields, the new methods developed will be integrated into computer codes to provide an accurate, efficient and unified approach to analyzing combustor burner liners, hollow air-cooled turbine blades and air-cooled turbine vanes. For these components, the methods developed will predict temperature,...

108532. The X-ray emission of M81 and its nucleus - Fabbiano, G.
X-ray observations of M81 made with the Einstein satellite are analyzed, and the results are reported. Eight bright sources other than the nucleus were detected, with luminosities in excess of about 2 x 10 to the 38th ergs/s. These are probably young, massive, accreting binary systems. A comparison of M81 with the morphologically similar galaxy M31 shows that the former is overluminous in both the radio continuum and the X-ray emission and to a lesser extent the far-infared emission, relative to the optical luminosity. This points to a difference in the star-formation histories of the two galaxies. The individual X-ray...

108533. Was core formation violent enough to homogenize the early mantle? - Cooperman, S. A.; Kaula, W. M.
The dynamics of iron, its thermal state and its phase in the accreting Earth probably played a major role in the Earth's early thermal evolution. Plausible impact thermal histories make it possible that pure iron was molten in the accreting Earth after it was about 10% grown. Hence, iron eutectic alloys (FeS, FeO) certainly were. Additionally, the initial temperature of the core is an important constraint on the secular cooling of the early Earth and on the strength of the early geodynamo. Whether iron is solid or molten would influence geochemical equilibria in the upper and lower mantle; the mode...

108534. FASTRAN II - FATIGUE CRACK GROWTH STRUCTURAL ANALYSIS (UNIX VERSION) - Newman, J. C.
Predictions of fatigue crack growth behavior can be made with the Fatigue Crack Growth Structural Analysis (FASTRAN II) computer program. As cyclic loads are applied to a selected crack configuration with an initial crack size, FASTRAN II predicts crack growth as a function of cyclic load history until either a desired crack size is reached or failure occurs. FASTRAN II is based on plasticity-induced crack-closure behavior of cracks in metallic materials and accounts for load-interaction effects, such as retardation and acceleration, under variable-amplitude loading. The closure model is based on the Dugdale model with modifications to allow plastically deformed material...

108535. STS External Tank Reentries Through STS46 - Wheaton, K. D.
This memo documents the analytic procedures used to reconstruct the External Tank (ET) reentry altitude history. Several NORAD sensors provide metrics, and with NASA-provided ET separation state vectors, the ET reentries can be reconstructed. The specific purpose of the reconstruction is to estimate two significant altitude regions: the hydrogen tank rupture altitude and the altitude at which the ET fin0y breaks up. The metric data is examined for the characteristic rupture and breakup signatures associated with the ET reentry. A trajectory reconstruction of the ET from separation through reentry is accomplished by solving for minor modifications to the NASA-supplied day-of-...

108536. Functional test case generation for real-time systems - Mandrioli, Dino; Morasca, Sandro; Morzenti, Angelo
The problem of automatic derivation of functional test cases for real time systems is addressed. Techniques for deriving test cases from formal specification of such systems coded in TRIO, a language which extends classical temporal logic to deal explicitly with time measures, are presented. Essentially, the proposed technique consists of generating models (histories) of the axioms specifying the system by means of a suitably specialized interpreter of the language TRIO. Such models are used as test cases by separating input and output events. The problem of testing nondeterministic systems is solved by means of an a posteriori verification performed by...

108537. Large Torque Variations in Two Soft Gamma Repeaters - Swank, Jean; vanderKlis, Michiel; Finger, Mark H.; Woods, Peter M.; Kouveliotou, Chryssa; Gogus, Ersin; Hurley, Kevin; Six, N. Frank; Markwardt, Craig B.
We have monitored the pulse frequencies of the two soft gamma repeaters SGR 1806-20 and SGR 1900+14 through the beginning of year 2001 using primarily Rossi X-ray Timing Explorer Proportional Counter Array observations. In both sources, we observe large changes in the spin-down torque up to a factor of approximately 4, which persist for several months. Using long baseline phase-connected timing solutions as well as the overall frequency histories, we construct torque noise power spectra for each SGR. The power spectrum of each source is very red (power-law slope approximately -3.5). These power spectra are consistent in normalization with some...

108538. Combined micromechanical and fabrication process optimization for metal-matrix composites - Saravanos, D. A.; Morel, M.; Chamis, C. C.
A method is presented to minimize the residual matrix stresses in metal matrix composites. Fabrication parameters such as temperature and consolidation pressure are optimized concurrently with the characteristics (i.e., modulus, coefficient of thermal expansion, strength, and interphase thickness) of a fiber-matrix interphase. By including the interphase properties in the fabrication process, lower residual stresses are achievable. Results for an ultra-high modulus graphite (P100)/copper composite show a reduction of 21 percent for the maximum matrix microstress when optimizing the fabrication process alone. Concurrent optimization of the fabrication process and interphase properties show a 41 percent decrease in the maximum microstress. Therefore,...

108539. The North Alabama Lightning Mapping Array: Recent Results and Future Prospects - Blakeslee, R.; Goodman, S. J.; Bailey, J.; Koshak, W.; Hall, J.; Bateman, M.; Boccippio, D.; Christian, H.; McCaul, E.; Buechler, D.
The North Alabama Lightning Mapping Array became operational in November 2001 as a principal component of a severe weather test bed to infuse new science and technologies into the short-term forecasting of severe and hazardous weather and the warning decision-making process. The LMA project is a collaboration among NASA scientists, National Weather Service (NWS) weather forecast offices (WFOs), emergency managers, and other partners. The time rate-of-change of storm characteristics and life-cycle trending are accomplished in real-time through the second generation Lightning Imaging Sensor Data Applications Display (LISDAD II) system, initially developed in T997 through a collaboration among NASA/MSFC, MIT/Lincoln Lab...

108540. Vibrational relaxation of the bending mode of shock-heated CO2 by laser-absorption measurements. - Eckstrom, D. J.; Bershader, D.
Study of the vibrational relaxation characteristics of shock-heated CO2 using a tuned CO2 laser absorption technique. Absorption-coefficient histories were obtained for a single rotational state in each of the -10 0- and -02 0- levels over the temperature range from 500 to 2000 K, and for 21 rotational states of the -10 0- level at 1000 K. These histories have been combined with translational-rotational temperature histories based on interferometer measurements to calculate vibrational relaxation times for the bending mode. The results verify the mutual equilibrium of the bending and symmetric-stretch modes due to Fermi resonance. The bending mode relaxation times...

 

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