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At least 343 records · Page 19
All sourcesNASA NTRSDOE OSTI
NASA NTRS2003

Efficient Processing of Data for Locating Lightning Strikes

Two algorithms have been devised to increase the efficiency of processing of data in lightning detection and ranging (LDAR) systems so as to enable the accurate location of lightning strikes in real time. In LDAR, the location of a lightning strike is calculated by solving equations for the differences among the times of arrival (DTOAs) of the lightning signals at multiple antennas as functions of the locations of the antennas and the speed of light. The most difficult part of the problem is computing the DTOAs from digitized versions of the signals received by the various antennas. One way (a time-domain approach) to determine the DTOAs is to compute cross-correlations among variously differentially delayed replicas of the digitized signals and to select, as the DTOAs, those differential delays that yield the maximum correlations. Another way (a frequency-domain approach) to determine the DTOAs involves the computation of cross-correlations among Fourier transforms of variously differentially phased replicas of the digitized signals, along with utilization of the relationship among phase difference, time delay, and frequency.

Medelius, Pedro J.↗
NASA NTRS2003

The Role of Convection and Growth Competition on Phase Selection in Microgravity

For a wide range of compositions, Fe-Cr-Ni alloys can solidify by a two-step process: a metastable solid forms first, subsequently transforming into the stable solid phase. This two-step solidification produces a profoundly different microstructure, and therefore different properties, in the final solid. The delay time between the two steps of the solidification process varies from microseconds to seconds. This delay time is a strong function of composition and a weak function of the undercooling of the melt below the metastable liquidus. From the results obtained during the MSL-1 mission and in ground based electrostatic levitation testing at NASA MSFC, we know that convection also has a significant effect on the delay time. By examining the effect of convection on delay time over a wide range of convection, for several different alloys, we can gain a better understanding and better control of the microstructural evolution and final structure of stainless steel alloys. MSFC provides ESL for ground-based testing in a low-convection state, and the numerical modeling that enables estimation of the convection levels under the different process conditions.

Source record↗
NASA NTRS1985

Modulational development of nonlinear gravity-wave groups

Observations of the development of nonlinear surface gravity-wave groups are presented, and the amplitude and phase modulations are calculated using Hilbert-transform techniques. With increasing propagation distance and wave steepness, the phase modulation develops local phase reversals whose locations correspond to amplitude minima or nodes. The concomitant frequency modulation develops jumps or discontinuities. The observations are compared with recent similar results for wavetrains. The observations are modelled numerically using the cubic nonlinear Schroedinger equation. The motivation is twofold: to examine quantitatively the evolution of phase as well as amplitude modulation, and to test the inviscid predictions for the asymptotic behavior of groups versus long-time observations. Although dissipation rules out the recurrence, there is a long-time coherence of the groups. The phase modulation is found to distinguish between dispersive and soliton behavior.

Chereskin, T. K.↗
NASA NTRS1984

Crystallization of fluorozirconate glasses

The crystallization of a number of glasses of the fluorozirconate family has been studied (using powder X-ray diffraction and DSC) as a function of time and temperature of heating. The main crystalline phases were beta BaZrF6 and beta BaZr2F10. Stable and metastble transformations to the low-temperature alpha phases were also investigated. The size of crystallites in fully devitrified glasses was calculated (from line broadening of the X-ray diffraction peaks) to be about 60 nm.

Bansal, Narottam P.↗
NASA NTRS1998

Optical encryption interface

An analog optical encryption system based on phase scrambling of two-dimensional optical images and holographic transformation for achieving large encryption keys and high encryption speed. An enciphering interface uses a spatial light modulator for converting a digital data stream into a two dimensional optical image. The optical image is further transformed into a hologram with a random phase distribution. The hologram is converted into digital form for transmission over a shared information channel. A respective deciphering interface at a receiver reverses the encrypting process by using a phase conjugate reconstruction of the phase scrambled hologram.

Jackson, Deborah J.↗
NASA NTRS1974

Fourier transform techniques for the inference of cloud motion

The development and evaluation are reported of phase shift techniques based on the Fourier transform for the estimation of cloud motion from geosynchronous meteorological satellite photographs. An alternative approach to cloud motion estimation, involving thresholding, was proposed and studied.

Lo, R. C.↗
NASA NTRS2005

Characterization of Ternary NiTiPt High-Temperature Shape Memory Alloys

Pt additions substituted for Ni in NiTi alloys are known to increase the transformation temperature of the alloy but only at fairly high Pt levels. However, until now only ternary compositions with a very specific stoichiometry, Ni50-xPtxTi50, have been investigated and then only to very limited extent. In order to learn about this potential high-temperature shape memory alloy system, a series of over twenty alloys along and on either side of a line of constant stoichiometry between NiTi and TiPt were arc melted, homogenized, and characterized in terms of their microstructure, transformation temperatures, and hardness. The resulting microstructures were examined by scanning electron microscopy and the phase compositions quantified by energy dispersive spectroscopy."Stoichiometric" compositions along a line of constant stoichiometry between NiTi to TiPt were essentially single phase but by any deviations from a stoichiometry of (Ni,Pt)50Ti50 resulted in the presence of at least two different intermetallic phases, depending on the overall composition of the alloy. Essentially all alloys, whether single or two-phase, still under went a martensitic transformation. It was found that the transformation temperatures were depressed with initial Pt additions but at levels greater than 10 at.% the transformation temperature increased linearly with Pt content. Also, the transformation temperatures were relatively insensitive to alloy stoichiometry within the range of alloys examined. Finally, the dependence of hardness on Pt content for a series of Ni50-xPtxTi50 alloys showed solution softening at low Pt levels, while hardening was observed in ternary alloys containing more than about 10 at.% Pt. On either side of these "stoichiometric" compositions, hardness was also found to increase significantly.

Rios, Orlando↗
NASA NTRS1985

High temperature oxidation of beta-NiAl

The oxidation of single crystal beta-NiAl has been studied primarily using electron microscopy. Oriented metastable Al2O3 phases form during transient oxidation at 800 C. Specific orientation relationships exist on all metal orientations studied and are a result of the small mismatch along aligned close-packed directions in the cation sublattices of the metal and oxide. Transformation of the metastable Al2O3 phases at 1100 C results in an oxide morphology described as the 'lacey' structure of alpha-Al2O3 scales. This structure results from impingement of oriented patches of alpha-Al2O3 as the transformation initiates and moves radially parallel to the surface. Scale growth occurs by diffusion along high angle grain boundaries. A drastic reduction in oxidation rate accompanies the change in oxide morphology.

Koychak, J. K.↗
NASA NTRS1977

Range safety signal propagation through the SRM exhaust plume of the space shuttle

Theoretical predictions of plume interference for the space shuttle range safety system by solid rocket booster exhaust plumes are reported. The signal propagation was calculated using a split operator technique based upon the Fresnel-Kirchoff integral, using fast Fourier transforms to evaluate the convolution and treating the plume as a series of absorbing and phase-changing screens. Talanov's lens transformation was applied to reduce aliasing problems caused by ray divergence.

Boynton, F. P.↗
NASA NTRS1988

High resolution infrared absorption spectra of various trace gases present in the upper atmosphere of the Earth

The objective of NASA Grant NsG 7473 was to obtain and analyze high resolution infrared absorption spectra of various trace gases present in the Earth's upper atmosphere. The goal of the spectral analysis was to obtain values of absorption line strengths, widths and frequencies of sufficient accuracy for use in upper atmosphere trace gas monitoring. During the early phase of the grant, high resolution spectra were obtained from two instruments. One was the 0.02/cm resolution vacuum grating spectrometer at the Florida State University and the other was the 0.01/cm resolution Fourier transform spectrometer at the McMath solar telescope at the Kitt Peak Observatory. Using these instruments, a considerable amount of spectra of methane and hydrogen peroxide were obtained and analyzed. During the latter years of the project, data taking was halted while efforts were devoted to building a new 0.0025/cm resolution vacuum Fourier transform spectrometer. Progress during this phase of the grant then became greatly slowed due to a lack of suitable graduate students in the program. However, the instrument was completed and brought to the point of producing interferograms.

Hunt, Robert H.↗
NASA NTRSDate not supplied

TPSAS-NF1676L-34417-DND

A boron nitride nanotube (BNNT), an electrically-insulating counterpart of a carbon nanotube (CNT), is a great nanofiller candidate for ferroelectric polymeric nanocomposites due to BNNT?s exceptional properties in piezoelectricity, thermal stability, mechanical strength, and radiation shielding capability. Fast and high-yield polar crystallization of polyvinylidene fluoride (PVDF) was accomplished by incorporating BNNTs as a nucleating agent. As-fabricated BNNT-PVDF nanocomposites were characterized by Fourier-transform infrared spectroscopy (FTIR), wide-angle X-ray diffraction (XRD), and differential scanning calorimetry (DSC) to identify and quantify chain conformations. Besides, piezoelectric constants, d33 of BNNT-PVDF nanocomposites were measured and compared with that of a mechanically drawn beta phase polar PVDF. The polar transformation due to BNNT incorporation has a great advantage over conventional beta or gamma transformation methods that require (1) mechanical drawing often resulting in defects or (2) high-temperature annealing for an extended time. In addition to the fast polar crystallization, BNNTs had a role of reinforcement of the polymer matrix. The improved Young?s modulus and electromechanical coupling coefficient of BNNT-PVDF nanocomposites indicated potential applications in energy harvesting under harsh environments such as large deformation, wide temperature cycles, and high radiation.

Dongwon Lee↗
NASA NTRS2003

Aging Wire Insulation Assessment by Phase Spectrum Examination of Ultrasonic Guided Waves

Wire integrity has become an area of concern to the aerospace community including DoD, NASA, FAA, and Industry. Over time and changing environmental conditions, wire insulation can become brittle and crack. The cracks expose the wire conductor and can be a source of equipment failure, short circuits, smoke, and fire. The technique of using the ultrasonic phase spectrum to extract material properties of the insulation is being examined. Ultrasonic guided waves will propagate in both the wire conductor and insulation. Assuming the condition of the conductor remains constant then the stiffness of the insulator can be determined by measuring the ultrasonic guided wave velocity. In the phase spectrum method the guided wave velocity is obtained by transforming the time base waveform to the frequency domain and taking the phase difference between two waveforms. The result can then be correlated with a database, derived by numerical model calculations, to extract material properties of the wire insulator. Initial laboratory tests were performed on a simple model consisting of a solid cylinder and then a solid cylinder with a polymer coating. For each sample the flexural mode waveform was identified. That waveform was then transformed to the frequency domain and a phase spectrum was calculated from a pair of waveforms. Experimental results on the simple model compared well to numerical calculations. Further tests were conducted on aircraft or mil-spec wire samples, to see if changes in wire insulation stiffness can be extracted using the phase spectrum technique.

Anastasi, Robert F.↗
NASA NTRS1979

The upper mantle transition region - Eclogite

The upper mantle transition region is usually considered to be peridotite which undergoes a series of phase changes involving spinel and post-spinel assemblages. There are difficulties associated with attempts to explain the 220, 400 and 670 km discontinuities in terms of phase changes in a peridotitic mantle. Moreover, in a differentiated earth there should be large quantities of eclogite in the upper mantle. Eclogite is denser than Al2O3-poor mantle to depths of 670 km, but it stays in the garnet stability field to pressures in excess of those required to transform depleted mantle to denser phases such as ilmenite and perovskite. Eclogite, therefore, remains above 670 km. The seismic properties of the transition region are more consistent with eclogite than peridotite. Most of the mantle's inventory of incompatible trace elements may be in this layer, which is a potential source region for some basalt magmas. The radioactivity in this layer is the main source of mantle heat flow, 0.7 microcalorie/sq cm-sec, and drives upper mantle convection.

Anderson, D. L.↗
NASA NTRS1977

High-resolution electron microscope

Employing scanning transmission electron microscope as interferometer, relative phases of diffraction maximums can be determined by analysis of dark field images. Synthetic aperture technique and Fourier-transform computer processing of amplitude and phase information provide high resolution images at approximately one angstrom.

Nathan, R.↗
NASA NTRS1994

Regional-scale flows in mountainous terrain. Part 1: A numerical and observational comparison

This study uses observed data and a numerical simulation to examine the generation of thermally driven flows across the Colorado mountain barrier on meso-beta to meso-alpha scales. The observations were collected from remote surface observing systems at exposed mountaintop locations throughout the state of Colorado, over the summers of 1984-88, as part of the Rocky Mountain Peaks Experiment (ROMPEX). The data show the development of a recurrent circulation system across the Colorado mountain barrier, operating on a diurnal timescale. From the observations, the basic structure of the flow system appears as a daytime inflow toward the highest terrain, and a nocturnal outflow away from it. However, when examined in detail, the flow system exhibits more unusual behavior, especially west of the barrier crest. Here, winds in the early evening are occasionally observed to onset abruptly from an easterly direction, generally counter to the upper-level winds. Observations from ROMPEX for 26 August 1985 are used to provide comparison data for a numerical simulation with the Regional Atmospheric Modeling System (RAMS). This three-dimensional case study experiment is initialized with data from the National Meteorological Center and incorporates two-way interactive grid nesting. From the observed data and case study simulation, four distinct phases of the regional-scale circulation system have been identified. In the development phase, a deep mountain-plains solenoid is generated through terrain heating along the Front Range. This circulation system transforms in the late afternoon transition phase into a westward-propagating density current (WPDC). The third phase, called the 'density-current propagation phase,' occurs as the WPDC moves westward across the mountains, leaving in its wake strong southeasterly flow at the mountaintop level. This current appears to be the cause of the peculiar easterly component winds found in the ROMPEX mountaintop observations along the western slope. In the final late-night adjustment phase, the WPDC dissipates near the western edge of the Colorado mountains and steady southerly flow evolves over the high mountain terrain. This southerly flow is the steady response to the differential heating that develops between the low-lying plains and the intermountain region.

Bossert, James E.↗
NASA NTRS1989

Optical joint transform correlation on the DMD

Initial experimental investigation of the deformable mirror device (DMD) in a joint optical transform correlation is reported. The inverted cloverleaf version of the DMD, in which form the DMD is phase-mostly but of limited phase range, is used. Binarized joint Fourier transforms were calculated for similar and dissimilar objects and written onto the DMD. Inverse Fourier transform was done in a diffraction order for which the DMD shows phase-mostly modulation. Matched test objects produced sharp correlation, distinct objects did not. Further studies are warranted and they are outlined.

Knopp, Jerome↗
NASA NTRS1989

Formation of Fourier phase shifts in the solar Ni I 6768 A line

A formalism is developed to understand better how Doppler shifts of spectrum lines as inferred from phase shifts in the Fourier transforms of line profiles are related to the underlying velocity structures which they are intended to measure. With a standard model atmosphere and a simplified, quasi-LTE treatment of line formation, the formalism is applied to the Ni I 6768 A line, which has been selected for use with a network of imaging interferometers under development by the Global Oscillations Network Group for research in helioseismology. Fourier phase shifts are found to be a remarkably linear measure of velocity even in the presence of gradients and unresolved lateral variations in the assumed velocity field. An assumed outward increase in amplitude of a model oscillatory velocity is noticeably reflected in the center-to-limb behavior of the simulated velocity measure, and a sample model of solar granulation is found to have a strong influence on the formation of the Fourier phase.

Jones, Harrison P.↗
NASA NTRS1992

Shock-induced transformations in the system NaAlSiO4-SiO2 - A new interpretation

New internally consistent interpretations of the phases represented by the high pressure phase shock wave data for an albite-rich rock, jadeite, and nepheline in the system NaAlSiO4-SiO2, are obtained using the results of static high pressure investigations, and the recent discovery of the hollandite phase in a shocked meteorite. We conclude that nepheline transforms directly to the calcium ferrite structure, whereas albite transforms possibly to the hollandite structure. Shock Hugoniots for the other plagioclase and alkali feldspars also indicate that these transform to hollandite structures. The pressure-volume data at high pressure could alternatively represent the compression of an amorphous phase. Moreover, the shock Hugoniot data are expected to reflect the properties of the melt above shock stresses of 60-80 GPa. The third order Birch-Murnaghan equation of state parameters are given for the calcium ferrite type NaAlSiO4 and for albite-rich, orthoclase-rich, and anorthite-rich hollandites.

Sekine, Toshimori↗