Langley Research Center

About: Langley Research Center is a facility organization based out in Hampton, Virginia, United States. It is known for research contribution in the topics: Mach number & Wind tunnel. The organization has 15945 authors who have published 37602 publications receiving 821623 citations. The organization is also known as: NASA Langley & NASA Langley Research Center.

Judd-Ofelt theory: principles and practices

Abstract: previous ideas and forming a complete picture. So it was in August 1962 when there appeared in the literature, simultaneously and independently, two identical formulations of a theory. One by Brian R. Judd at the University of California at Berkeley and the other by a Ph.D. student, George S. Ofelt, at the Johns Hopkins University in Baltimore. Judd and Ofelt had never met personally, and were not aware of each other’s interest in the intensities of rare earth ions in solids. While there are some differences in the two formulations, the approach and the assumptions used to arrive at the final result are remarkably similar. The titles of the two articles reflect a thought along similar lines. Judd referred to Optical Absorption, while Ofelt referred to Crystal Spectra, and each to Intensities of Rare-Earth Ions. The formulations as originally published by Judd and Ofelt came to be known as the Judd-Ofelt theory of the intensities of rare earth ions. Regarding these publications, the late Professor Brian G. Wybourne has said, “I suggest that the coincidence of discovery was indicative that the time was right for the solution of the problem” [1]. In the light of the advancement in the understanding of complex atomic spectra in the quarter century preceding the 1962 publications of Judd and Ofelt, this suggestion of Wybourne is based on very sound reasoning. The rare earths comprise Y, Sc, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu. The last 15 make up the lanthanide series. Most of these elements were discovered over a period of time stretching from the late 18th century to the early 20th century. Promethium (Pm) was the last to be discovered in 1947 at Oak Ridge National Laboratory. So, with the exception of Promethium, all the rare earths were discovered in the span of a little more than a century. Part of the reason why they are called rare earths is two-fold. First, they are very difficult to chemically extract from the earth. Second, they do not exist in nature in high abundance. In the universe, the rare earths are approximately 10 6 times less abundant than the

A detailed evaluation of the heating efficiency in the middle atmosphere

Abstract: The role of airglow losses in reducing the efficiency of solar heating in the Hartley, Huggins, and Chappuis bands of ozone in the Herzberg, Ly alpha, Schumann-Runge continuum, and in Schumann-Runge bands of molecular oxygen is investigated together with the role of heating due to seven chemical reactions in the middle atmosphere. The results of calculations of bulk efficiencies demonstrate that airglow and chemiluminescent emission significantly reduce the amount of energy available for heat throughout the mesosphere and lower thermosphere.

Aligned single‐wall carbon nanotube polymer composites using an electric field

Abstract: While high shear alignment has been shown to improve the mechanical properties of single-wall carbon nanotube (SWNT)-polymer composites, this method does not allow for control over the electrical and dielectric properties of the composite and of- ten results in degradation of these properties. Here, we report a novel method to actively align SWNTs in a polymer matrix, which permits control over the degree of alignment of the SWNTs without the side effects of shear alignment. In this process, SWNTs were aligned via AC field-induced dipolar interactions among the nanotubes in a liquid matrix followed by immobilization by photopolymerization under continued application of the electric field. Alignment of SWNTs was controlled as a function of magnitude, frequency, and application time of the applied electric field. The degree of SWNT alignment was assessed using optical microscopy and polarized Raman spectroscopy, and the morphol- ogy of the aligned nanocomposites was investigated by high-resolution scanning electron microscopy. The structure of the field induced aligned SWNTs was intrinsically different from that of shear aligned SWNTs. In the present work, SWNTs are not only aligned along the field, but also migrate laterally to form thick, aligned SWNT percolative col- umns between the electrodes. The actively aligned SWNTs amplify the electrical and dielectric properties of the composite. All of these properties of the aligned nanocomposites exhibited anisotropic characteristics, which were controllable by tuning the applied field parameters. V C 2006 Wiley Periodicals, Inc.* J Polym Sci Part B: Polym Phys 44: 1751-1762, 2006

Bulk Scattering Properties for the Remote Sensing of Ice Clouds. Part I: Microphysical Data and Models.

Abstract: This study reports on the use of in situ data obtained in midlatitude and tropical ice clouds from airborne sampling probes and balloon-borne replicators as the basis for the development of bulk scattering models for use in satellite remote sensing applications. Airborne sampling instrumentation includes the twodimensional cloud (2D-C), two-dimensional precipitation (2D-P), high-volume precipitation spectrometer (HVPS), cloud particle imager (CPI), and NCAR video ice particle sampler (VIPS) probes. Herein the development of a comprehensive set of microphysical models based on in situ measurements of particle size distributions (PSDs) is discussed. Two parameters are developed and examined: ice water content (IWC) and median mass diameter Dm. Comparisons are provided between the IWC and Dm values derived from in situ measurements obtained during a series of field campaigns held in the midlatitude and tropical regions and those calculated from a set of modeled ice particles used for light-scattering calculations. The ice particle types considered in this study include droxtals, hexagonal plates, solid columns, hollow columns, aggregates, and 3D bullet rosettes. It is shown that no single habit accurately replicates the derived IWC and Dm values, but a mixture of habits can significantly improve the comparison of these bulk microphysical properties. In addition, the relationship between Dm and the effective particle size Deff, defined as 1.5 times the ratio of ice particle volume to projected area for a given PSD, is investigated. Based on these results, a subset of microphysical models is chosen as the basis for the development of ice cloud bulk scattering models in Part II of this study.

A Finite-Element Analysis of Fatigue Crack Closure

Abstract: Experiments have shown that fatigue cracks close at positive loads during constant-amplitude load cycling. The crack-closure phenomenon is caused by residual plastic deformations remaining in the wake of an advancing crack tip. The present paper is concerned with the application of a two-dimensional, nonlinear, finite-element analysis for predicting crack-closure and crack-opening stresses during cyclic loading. A two-dimensional finite-element computer program, which accounts for both elastic-plastic material behavior and changing boundary conditions associated with crack extension and intermittent contact of the crack surfaces under cyclic loading, has been developed. An efficient technique to account for changing boundary conditions was also incorporated into the nonlinear analysis program. This program was subsequently used to study crack extension and crack closure under constant-amplitude and two-level block loading. The calculated crack-closure and crack-opening stresses were qualitatively consistent with experimental observations.