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At least 1,531 records · Page 85

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NASA NTRS1994

Closed-loop carrier phase synchronization techniques motivated by likelihood functions

This article reexamines the notion of closed-loop carrier phase synchronization motivated by the theory of maximum a posteriori phase estimation with emphasis on the development of new structures based on both maximum-likelihood and average-likelihood functions. The criterion of performance used for comparison of all the closed-loop structures discussed is the mean-squared phase error for a fixed-loop bandwidth.

Tsou, H.↗

NASA NTRS2019

Multifunctional Polymers and Composites for Aerospace Applications

System needs and challenges for commercial aviation and space exploration are constantly changing. Multifunctional materials are ideal for aerospace applications because their integration into structural components has potential to not only improve efficiency, but it can also increase payload and mission capabilities without the burden of significant additions to mass and volume. Multi-constituent materials will play an important role in enabling structures with the potential to demonstrate true multi-functionality; however, modifications to many mature polymer processing methods may be necessary. This presentation will cover multifunctional materials development efforts underway at the NASA Glenn Research Center related to formulating and characterizing novel electrical insulation for high voltage aircraft. Incorporating functionality into the reinforcement material is another materials development technique that will be discussed, which includes braiding with hybrid fibers to enhance toughness in composite structures, stitching with carbon nanotube yarns to make durable electrically conductive textiles, and treating fabrics with shear thickening fluid suspensions to produce impact-resistant inflatable habitats.

multi-functional composites↗

NASA NTRS1975

Stability of localized plasma model in two and three dimensions

Stability against catastrophic radial collapse is demonstrated for initially well-behaved, localized solutions in a class of two- and three-dimensional wave equations characterized by saturating nonlinearities, which model various electrostatic and electromagnetic 'caviton' structures in an asymptotically uniform plasma. This result contrasts sharply with mathematical predictions of equations with low-order nonlinearities. Discrete classes of possible quasistationary modes exhibit features comparable to atomic wave functions and differ qualitatively from one-dimensional structures.

Wilcox, J. Z.↗

NASA NTRS1995

Sensitivity Analysis of Wing Aeroelastic Responses

Design for prevention of aeroelastic instability (that is, the critical speeds leading to aeroelastic instability lie outside the operating range) is an integral part of the wing design process. Availability of the sensitivity derivatives of the various critical speeds with respect to shape parameters of the wing could be very useful to a designer in the initial design phase, when several design changes are made and the shape of the final configuration is not yet frozen. These derivatives are also indispensable for a gradient-based optimization with aeroelastic constraints. In this study, flutter characteristic of a typical section in subsonic compressible flow is examined using a state-space unsteady aerodynamic representation. The sensitivity of the flutter speed of the typical section with respect to its mass and stiffness parameters, namely, mass ratio, static unbalance, radius of gyration, bending frequency, and torsional frequency is calculated analytically. A strip theory formulation is newly developed to represent the unsteady aerodynamic forces on a wing. This is coupled with an equivalent plate structural model and solved as an eigenvalue problem to determine the critical speed of the wing. Flutter analysis of the wing is also carried out using a lifting-surface subsonic kernel function aerodynamic theory (FAST) and an equivalent plate structural model. Finite element modeling of the wing is done using NASTRAN so that wing structures made of spars and ribs and top and bottom wing skins could be analyzed. The free vibration modes of the wing obtained from NASTRAN are input into FAST to compute the flutter speed. An equivalent plate model which incorporates first-order shear deformation theory is then examined so it can be used to model thick wings, where shear deformations are important. The sensitivity of natural frequencies to changes in shape parameters is obtained using ADIFOR. A simple optimization effort is made towards obtaining a minimum weight design of the wing, subject to flutter constraints, lift requirement constraints for level flight and side constraints on the planform parameters of the wing using the IMSL subroutine NCONG, which uses successive quadratic programming.

Issac, Jason Cherian↗

NASA NTRS2012

Validation Tests of Fiber Optic Strain-Based Operational Shape and Load Measurements

Aircraft design has been progressing toward reduced structural weight to improve fuel efficiency, increase performance, and reduce cost. Lightweight aircraft structures are more flexible than conventional designs and require new design considerations. Intelligent sensing allows for enhanced control and monitoring of aircraft, which enables increased structurally efficiency. The NASA Dryden Flight Research Center (DFRC) has developed an instrumentation system and analysis techniques that combine to make distributed structural measurements practical for lightweight vehicles. Dryden's Fiber Optic Strain Sensing (FOSS) technology enables a multitude of lightweight, distributed surface strain measurements. The analysis techniques, referred to as the Displacement Transfer Functions (DTF) and Load Transfer Functions (LTF), use surface strain values to calculate structural deflections and operational loads. The combined system is useful for real-time monitoring of aeroelastic structures, along with many other applications. This paper describes how the capabilities of the measurement system were demonstrated using subscale test articles that represent simple aircraft structures. Empirical FOSS strain data were used within the DTF to calculate the displacement of the article and within the LTF to calculate bending moments due to loads acting on the article. The results of the tests, accuracy of the measurements, and a sensitivity analysis are presented.

FOSS↗

NASA NTRS2024

FRAAME Version 1.0 User Manual

The code for Forced Response Aeromechanics Analysis in a MATLAB-based (The MathWorks, Inc.) Environment (FRAAME Version 1.0) was developed for internal use in aeromechanics efforts undertaken at the NASA Glenn Research Center for computing turbomachine component forced response and Goodman diagrams via modal summation method. The main working script (FRAAMEv1.m) allows users to input case-specific manual inputs while the triple-nested loop invokes functions to compute forced response per blade, per nodal diameter, and per mode. Secondly, forced response values are applied to modal stresses to compute complex Von Mises stress values and generate Goodman diagrams per blade, per nodal diameter, and per mode using a linear modal summation method. Currently, this code functions in the Windows (Microsoft Corporation) operating system using MATLAB Version R2023a, but it can be adapted for use in the Linux (Linus Torvalds) operating system by changing the appropriate file path structure in the main script, as well as functions that call external results files.

Aeromechanics↗

NASA NTRS1979

Pyrolysis of polymeric materials. I - Effect of chemical structure, temperature, heating rate, and air flow on char yield and toxicity

Various polymeric materials, including synthetic polymers and cellulosic materials, were evaluated at different temperatures, heating rates and air flow rates for thermophysical and toxicological responses. It is shown that char yields appeared to be a function of air access as much as of the chemical structure of the material. It is stated that the sensitivity of the apparent thermal stability of some materials to air access is so marked that thermogravimetric studies in oxygen-free atmospheres may be a consistently misleading approach to comparing synthetic polymers intended to increase fire safety. Toxicity also appeared to be a function of temperature and air access as much as of the chemical structure of the material. Toxicity of the gases evolved seemed to increase with increasing char yield for some polymers.

Hilado, C. J.↗

NASA NTRS2004

Macromolecular Crystallization in Microgravity

The key concepts that attracted crystal growers, macromolecular or solid state, to microgravity research is that density difference fluid flows and sedimentation of the growing crystals are greatly reduced. Thus, defects and flaws in the crystals can be reduced, even eliminated, and crystal volume can be increased. Macromolecular crystallography differs from the field of crystalline semiconductors. For the latter, crystals are harnessed for their electrical behaviors. A crystal of a biological macromolecule is used instead for diffraction experiments (X-ray or neutron) to determine the three-dimensional structure of the macromolecule. The better the internal order of the crystal of a biological macromolecule then the more molecular structure detail that can be extracted. This structural information that enables an understanding of how the molecule functions. This knowledge is changing the biological and chemical sciences with major potential in understanding disease pathologies. Macromolecular structural crystallography in general is a remarkable field where physics, biology, chemistry, and mathematics meet to enable insight to the basic fundamentals of life. In this review, we examine the use of microgravity as an environment to grow macromolecular crystals. We describe the crystallization procedures used on the ground, how the resulting crystals are studied and the knowledge obtained from those crystals. We address the features desired in an ordered crystal and the techniques used to evaluate those features in detail. We then introduce the microgravity environment, the techniques to access that environment, and the theory and evidence behind the use of microgravity for crystallization experiments. We describe how ground-based laboratory techniques have been adapted to microgravity flights and look at some of the methods used to analyze the resulting data. Several case studies illustrate the physical crystal quality improvements and the macromolecular structural advances. Finally, limitations and alternatives to microgravity and future directions for this research are covered.

Snell, Edward H.↗

NASA NTRS1984

An aeroelastician's perspective of wind tunnel and flight experiences with active control of structural response and stability

Active controls technology is assessed based on a review of most of the wind-tunnel and flight tests and actual applications of active control concepts since the late sixties. The distinction is made between so-called ""rigid-body'' active control functions and those that involve significant modification of structural elastic response or stability. Both areas are reviewed although the focus is on the latter area. The basic goals and major results of the various studies or applications are summarized, and the anticipated use of active controls on current and near-future research and demonstration aircraft is discussed. Some of the ""holes'' remaining in the feasbility/benefits demonstration of active controls technology are examined.

Hanson, P. W.↗

NASA NTRS2023

Influence of Cation Species on Thermal Expansion of Y2Si2O7–Gd2Si2O7 Solid Solutions

Mixtures of Y 2 Si 2 O 7 and Gd 2 Si 2 O 7 were synthesized by solid-state reaction at 1600°C and characterized via in situ x-ray diffraction (XRD) to determine their coefficients of thermal expansion (CTE). All solid solutions within the system exhibited the orthorhombic δ-RE 2 Si 2 O 7 (Pna2 1 ) structure. Thermal expansion measurements of Y 2 Si 2 O 7 and Gd 2 Si 2 O 7 correlated well with reported values in literature, and all synthesized solid solutions exhibited CTEs between Y 2 Si 2 O 7 and Gd 2 Si 2 O 7 . Generally, there was a slight decrease in CTE exhibited by the materials with increasing Gd 2 Si 2 O 7 content, with Gd 2 Si 2 O 7 having the lowest CTEs and Y 2 Si 2 O 7 the highest CTEs. The decrease in CTE was attributed to stronger bonds of Gd-O over Y-O, as determined by calculated crystal orbital Hamilton populations using density functional theory. However, such differences were very small and crystal structure was the dominating factor in CTE trends.

rare earth silicates↗

NASA NTRS1999

Rendering Three-Dimensional Solar Coronal Structures of Active Region 8227

Coronal X-ray and EUV synthesized images are constructed of Active Region 8227 (May-June 1996) and are compared with Yohkoh/SXT, SOHO/EIT, and TRACE observations. Using the rendering technique of Gary (1997) and Alexander, Gary, and Thompson (1998), specific geometric and physical models are used to integrated the plasma emission along the line of sight to obtain a rendered image. The specific instrumental profiles are convolved in the integration process as well as specific heating functions. We analyze coronal X-ray and EUV structures by constructing synthesized image and comparison with observations provide test of specific physical models. We investigate how different pressure distributions within the active region loop system affect the emission characteristics and compare the various results with coronal observations. We investigate how the different heating functions in the active region are reflected in the effect of overall structure of the region. Specific heating rates are tested.

Gary, G. A.↗

NASA NTRS1977

Room-temperature-operation visible-emission semiconductor diode lasers

There were two main approaches taken to develop shorter wavelength lasers. (1) Based on (AlGa)As and liquid-phase epitaxy, significant new results were obtained: Properties of these laser diodes (power output, spectra, and beam patterns), materials considerations, laser theory, and growth problems are discussed. The design of (AlGa)As layers is discussed from the vertical point of view, and various design curves are given. Horizontal structural requirements are also discussed. Experimental results from measurements done as a function of hydrostatic pressure are correlated with other results. (2) The first heterojunction laser structures using GaAs sub l-x P sub x and In sub y Ga sub l-y P at compositions, where the lattice constants are matched, were grown using vapor-phase growth technology and are described in detail, including experimental device results. Threshold current densities from 3,000 to 5,000 A per sq cm. and emission wavelengths from 6,520 A to 6,640 A were obtained at 77 K. The limiting factor in these devices is nonradiative recombination at the heterojunctions. Life tests on facet-coated (AlGa)As CW diodes are reported.

Ladany, I.↗

NASA NTRS1954

Calibration of strain-gage installations in aircraft structures for the measurement of flight loads

A general method has been developed for calibrating strain-gage installations in aircraft structures, which permits the measurement in flight of the shear or lift, the bending moment, and the torque or pitching moment on the principal lifting or control surfaces. Although the stress in structural members may not be a simple function of the three loads of interest, a straightforward procedure is given for numerically combining the outputs of several bridges in such a way that the loads may be obtained. Extensions of the basic procedure by means of electrical combination of the strain-gage bridges are described which permit compromises between strain-gage installation time, availability of recording instruments, and data reduction time. The basic principles of strain-gage calibration procedures are illustrated by reference to the data for two aircraft structures of typical construction, one a straight and the other a swept horizontal stabilizer.

Skopinski, T H↗

NASA NTRS2006

Stable Carbon Isotopic Signatures of Abiotic Organics from Hydrothermal Synthesis Experiments

Stable carbon isotopes can be powerful biogeochemical markers in the study of life's origins. Biogenic carbon fixation produces organics that are depleted in C-13 by about -20 to -30%0. Less attention has been paid to the isotopic signatures of abiotic processes. The possibility of abiotic processes producing organics with morphologies and isotopic signatures in the biogenic range has been at the center of recent debate over the Earth's earliest microfossils. The abiotic synthesis of organic compounds in hydrothermal environments is one possible source of endogenous organic matter to the prebiotic earth. Simulated hydrothermal settings have been shown to synthesize, among other things, single chain amphiphiles and simple lipids from a mix of CO, CO2, and H2. A key characteristic of these amphiphilic molecules is the ability to self-assemble in aqueous phases into more organized structures called vesicles, which form a selectively permeable boundary and serve the function of containing and concentrating other organic molecules. The ability to form cell like structures also makes these compounds more likely to be mistaken for biogenic. Hydrothermal simulation experiments were conducted from oxalic or formic acid in water at 175 C for 72 hr. The molecular and isotopic composition of the products of these reactions were determined and compared to biogenic fractionations . Preliminary results indicate isotopic fractionation during abiotic hydrocarbon synthesis in hydrothermal environments is on par with biological carbon fixation.

Stern, Jennifer C.↗

NASA NTRS2005

Identification of Cyclically Symmetric Resonance in Experimental Data for Engine Failure Analysis

Many structural components are rotationally symmetric. For a perfectly manufactured structure, the modal wave shapes are exactly sinusoidal. These structures can be excited by similarly-shaped forcing functions such as those found frequently in or near turbomachinery. Due to orthogonality, resonance occurs only when the integer wave number of the structure equals the integer wave number of the excitation as well as when the excitation frequency is close to the natural frequency of the mode. This resonance can lead to high-cycle fatigue (HCF) cracking of the structure. To help understand this phenomenon, it is desirable to determine the modal composition of the response and excitation. Once the modal composition is known, either the excitation or the structure can be modified to reduce the response and therefore prevent HCF cracking.

Medley, Steve↗

NASA NTRS1998

Modeling and Simulation of Semiconductor Quantum Well Structures and Lasers

In this talk I will cover two aspects of modeling and simulation efforts at NASA Ames Research Center. In the quantum well structure simulation, we typically start from the quantum mechanical calculation of the quantum well structures for the confined/and unconfined eigen states and functions. A bandstructure calculation of the k*p type is then performed for the confined valence states. This information is then used to computer the optical gain and refractive index of the quantum well structures by solving the linearized multiband semiconductor Bloch equations with the many-body interactions included. In our laser simulation, we typically solve the envelope equations for the laser field in space-time domain, coupled with a reduced set of material equations using the microscopic calculation of the first step. Finally I will show some examples of both aspects of simulation and modeling.

Ning, Cun-Zheng↗

NASA NTRS2011

Vertical Distribution of Aersols and Water Vapor Using CRISM Limb Observations

Near-infrared spectra taken in a limb-viewing geometry by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) on-board the Mars Reconnaissance Orbiter (MRO) provide a useful tool for probing atmospheric structure. Specifically, the observed radiance as a function of wavelength and height above the limb allows the vertical distribution of both dust and ice aerosols to be retrieved. These data serve as an important supplement to the aerosol profiling provided by the MRO/MCS instrument allowing independent validation and giving additional information on particle physical and scattering properties through multi-wavelength studies. A total of at least ten CRISM limb observations have been taken so far covering a full Martian year. Each set of limb observations nominally contains about four dozen scans across the limb giving pole-to-pole coverage for two orbits at roughly 100 and 290 W longitude over the Tharsis and Syrtis/Hellas regions, respectively. At each longitude, limb scans are spaced roughly 10 degrees apart in latitude, with a vertical spatial resolution on the limb of roughly 800 m. Radiative transfer modeling is used to model the observations. We compute synthetic CRISM limb spectra using a discrete-ordinates radiative transfer code that accounts for multiple scattering from aerosols and accounts for spherical geometry of the limb observations by integrating the source functions along curved paths in that coordinate system. Retrieved are 14-point vertical profiles for dust and water ice aerosols with resolution of 0.4 scale heights between one and six scale heights above the surface. After the aerosol retrieval is completed, the abundances of C02 (or surface pressure) and H20 gas are retrieved by matching the depth of absorption bands at 2000 nm for carbon dioxide and at 2600 run for water vapor. In addition to the column abundance of water vapor, limited information on its vertical structure can also be retrieved depending on the signal available from aerosol scattering.

Smith, Michael D.↗

NASA NTRS1987

Adaptation, plant evolution, and the fossil record

The importance of adaptation in determining patterns of evolution has become an important focus of debate in evolutionary biology. As it pertains to paleobotany, the issue is whether or not adaptive evolution mediated by natural selection is sufficient to explain the stratigraphic distributions of taxa and character states observed in the plant fossil record. One means of addressing this question is the functional evaluation of stratigraphic series of plant organs set in the context of paleoenvironmental change and temporal patterns of floral composition within environments. For certain organ systems, quantitative estimates of biophysical performance can be made on the basis of structures preserved in the fossil record. Performance estimates for plants separated in time or space can be compared directly. Implicit in different hypotheses of the forces that shape the evolutionary record (e.g. adaptation, mass extinction, rapid environmental change, chance) are predictions about stratigraphic and paleoenvironmental trends in the efficacy of functional performance. Existing data suggest that following the evolution of a significant structural innovation, adaptation for improved functional performance can be a major determinant of evolutionary changes in plants; however, there are structural and development limits to functional improvement, and once these are reached, the structure in question may no longer figure strongly in selection until and unless a new innovation evolves. The Silurian-Devonian paleobotanical record is consistent with the hypothesis that the succession of lowland floodplain dominants preserved in the fossil record of this interval was determined principally by the repeated evolution of new taxa that rose to ecological importance because of competitive advantages conferred by improved biophysical performance. This does not seem to be equally true for Carboniferous-Jurassic dominants of swamp and lowland floodplain environments. In these cases, environmental disruption appears to have been a major factor in shaping the fossil record. This does not mean that continuing adaptation was not important during this interval, but it may indicate that adaptive evolution was strongest in environments other than those best represented in the paleobotanical record.

NASA Discipline Exobiology↗