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At least 217 records · Page 12

OTVE turbopump condition monitoring, task E.5

Recent work has been carried out on development of isotope wear analysis and optical and eddy current technologies to provide bearing wear measurements and real time monitoring of shaft speed, shaft axial displacement and shaft orbit of the Orbit Transfer Vehicle hydrostatic bearing tester. Results show shaft axial displacement can be optically measured (at the same time as shaft orbital motion and speed) to within 0.3 mils by two fiberoptic deflectometers. Evaluation of eddy current probes showed that, in addition to measuring shaft orbital motion, they can be used to measure shaft speed without having to machine grooves on the shaft surface as is the usual practice for turbomachinery. The interim results of this condition monitoring effort are presented.

Coleman, Paul T.↗

Development of a Practical Methodology for Elastic-Plastic and Fully Plastic Fatigue Crack Growth

A practical engineering methodology has been developed to analyze and predict fatigue crack growth rates under elastic-plastic and fully plastic conditions. The methodology employs the closure-corrected effective range of the J-integral, delta J(sub eff) as the governing parameter. The methodology contains original and literature J and delta J solutions for specific geometries, along with general methods for estimating J for other geometries and other loading conditions, including combined mechanical loading and combined primary and secondary loading. The methodology also contains specific practical algorithms that translate a J solution into a prediction of fatigue crack growth rate or life, including methods for determining crack opening levels, crack instability conditions, and material properties. A critical core subset of the J solutions and the practical algorithms has been implemented into independent elastic-plastic NASGRO modules. All components of the entire methodology, including the NASGRO modules, have been verified through analysis and experiment, and limits of applicability have been identified.

McClung, R. C.↗

Development of a Practical Methodology for Elastic-Plastic and Fully Plastic Fatigue Crack Growth

A practical engineering methodology has been developed to analyze and predict fatigue crack growth rates under elastic-plastic and fully plastic conditions. The methodology employs the closure-corrected effective range of the J-integral, (Delta)J(sub eff), as the governing parameter. The methodology contains original and literature J and (Delta)J solutions for specific geometries, along with general methods for estimating J for other geometries and other loading conditions, including combined mechanical loading and combined primary and secondary loading. The methodology also contains specific practical algorithms that translate a J solution into a prediction of fatigue crack growth rate or life, including methods for determining crack opening levels, crack instability conditions, and material properties. A critical core subset of the J solutions and the practical algorithms has been implemented into independent elastic-plastic NASGRO modules. All components of the entire methodology, including the NASGRO modules, have been verified through analysis and experiment, and limits of applicability have been identified.

McClung, R. C.↗

Evaluation of Learning Associated with Multiple Exposures to Computerized Dynamic Posturography

Computerized dynamic posturography has been used to quantitatively assess the time course of functional sensorimotor recovery after exposure to spaceflight or to groundbased analogs such as head-down bed rest. An assessment of balance recovery may be confounded as subjects develop new strategies through repeated exposures to test paradigms. The purpose of this control study was to characterize the learning effects of sensory organization and motor control tests across multiple sessions. METHODS: Twenty-eight healthy subjects were tested over four sessions. To examine the effects of between-session interval, subjects were assigned to one of four groups in which the interval between the 1 st and 2nd sessions was 7 (+/- 1) days, 14 (+/-1) days, 28 (+/-2) days, or 56 (+/-3) days. The interval between remaining sessions was 28 (+/-4) days. Peak-to-peak anterior-posterior sway was measured during standard Sensory Organization Tests (SOTs) using either fixed or unstable sway-referenced support with eyes open, eyes closed, or sway-referenced vision. Sway was also measured during modified SOTs using eyes-closed conditions with either static or dynamic head tilts. Postural recovery to unexpected support surface perturbations (translations or rotations) was measured during Motor Control Tests. The test order was block randomized across subjects. RESULTS: The learning effects varied with test condition. There were no measurable differences with a stable support surface. The more challenging conditions (unstable support surface with and without head tilts) led to greater differences and took more trials to stabilize. The effect of time interval between the first two sessions was negligible across conditions. Evidence suggested that learning carried across similar conditions (such as unstable support SOTs). DISCUSSION: Familiarization session and/or trials are recommended to minimize learning effects when characterizing functional recovery after exposure to altered sensory environments. The number of practice trials required depends on task difficulty and similarity across conditions. Learning statement: This presentation will review the learning effects of computerized d

Dean, S. Lance↗

Upward And Downward Flame Spreading And Extinction In Partial Gravity Environments

The premise of this research effort has been to begin exploring the gap in the literature between studies of material flammability and flame spread phenomena in normal-gravity and those conducted in the microgravity environment, with or without forced flows. From a fundamental point of view, flame spreading in upward (concurrent) buoyant flow is considerably different from concurrent forced flow. The flow accelerates throughout the length of the buoyant flame bringing the streamlines and the flame closer to the fuel surface and strengthening the interaction between the flame and fuel. Forced flows are diverted around the flame and away from the fuel surface, except where the flow might be constrained by a finite duct. The differences may be most clearly felt as the atmospheric conditions, viz. pressure or oxygen content, approach the flammability limit. From a more practical point of view, flame spreading and material flammability behavior have not been studied under the partial gravity conditions that are the natural state in space exploration destinations such as the Moon and Mars. This effort constitutes the beginning of the research needed to engineer fire safety provisions for such future missions. In this program we have performed partial-gravity experiments (from 0.1 to 1 g/g(sub Earth)) considering both upward and downward flame spread over thin solid fuels aboard the NASA KC-135 aircraft. In those tests, the atmospheric pressure and the fuel sample width were varied. Steady flame spread rates and approximate extinction boundaries were determined. Flame images were recorded using video cameras and two-dimensional fuel surface temperature distributions were determined using an IR camera. These results are available, and complement our earlier work in downward spread in partial gravity varying oxygen content. In conjunction with the experiment, three-dimensional models of flame spreading in buoyant flow have been developed. Some of the computed results on upward spreading have been presented. A derivative three-dimensional model of downward spreading has been developed. It is currently being used to evaluate the standard limiting oxygen index (LOI) measuring device and its potential performance in different gravity levels.

Sacksteder, Kurt R.↗

HRP's Healthcare Spin-Offs Through Computational Modeling and Simulation Practice Methodologies

Spaceflight missions expose astronauts to novel operational and environmental conditions that pose health risks that are currently not well understood, and perhaps unanticipated. Furthermore, given the limited number of humans that have flown in long duration missions and beyond low Earth-orbit, the amount of research and clinical data necessary to predict and mitigate these health and performance risks are limited. Consequently, NASA's Human Research Program (HRP) conducts research and develops advanced methods and tools to predict, assess, and mitigate potential hazards to the health of astronauts. In this light, NASA has explored the possibility of leveraging computational modeling since the 1970s as a means to elucidate the physiologic risks of spaceflight and develop countermeasures. Since that time, substantial progress has been realized in this arena through a number of HRP funded activates such as the Digital Astronaut Project (DAP) and the Integrated Medical Model (IMM). Much of this success can be attributed to HRP's endeavor to establish rigorous verification, validation, and credibility (VV&C) processes that ensure computational models and simulations (M&S) are sufficiently credible to address issues within their intended scope. This presentation summarizes HRP's activities in credibility of modeling and simulation, in particular through its outreach to the community of modeling and simulation practitioners. METHODS: The HRP requires all M&S that can have moderate to high impact on crew health or mission success must be vetted in accordance to NASA Standard for Models and Simulations, NASA-STD-7009 (7009) [5]. As this standard mostly focuses on engineering systems, the IMM and DAP have invested substantial efforts to adapt the processes established in this standard for their application to biological M&S, which is more prevalent in human health and performance (HHP) and space biomedical research and operations [6,7]. These methods have also generated substantial interest by the broader medical community though institutions like the National Institutes of Health (NIH) and the Food and Drug Administration (FDA) to develop similar standards and guidelines applicable to the larger medical operations and research community. DISCUSSION: Similar to NASA, many leading government agencies, health institutions and medical product developers around the world are recognizing the potential of computational M&S to support clinical research and decision making. In this light, substantial investments are being made in computational medicine and notable discoveries are being realized [8]. However, there is a lack of broadly applicable practice guidance for the development and implementation of M&S in clinical care and research in a manner that instills confidence among medical practitioners and biological researchers [9,10]. In this presentation, we will give an overview on how HRP is working with the NIH's Interagency Modeling and Analysis Group (IMAG), the FDA and the American Society of Mechanical Engineers (ASME) to leverage NASA's biomedical VV&C processes to establish a new regulatory standard for Verification and Validation in Computational Modeling of Medical Devices, and Guidelines for Credible Practice of Computational Modeling and Simulation in Healthcare.

Mulugeta, Lealem↗

On A Stabilization of the Ingard-Myers Impedance Boundary Condition

It has been well-known that the Ingard-Myers impedance condition, while simple to apply, is subject to the hydrodynamic Kelvin-Helmholtz instability due to its use of a vortex sheet in modeling the flow at the liner boundary. Recently, in the development of a time domain boundary element method for acoustic scattering by treated surfaces, it was found that by neglecting a certain second-order spatial derivative term in the Ingard-Myers formulation, the hydrodynamic instability can be avoided. The present paper aims to provide further analysis of this modified condition, hereby referred to as Truncated Ingard-Myers Impedance Boundary Condition (TIMIBC). It will be shown, based on the dispersion relations of linear waves, that the instability intrinsic to the Ingard-Myers condition is eliminated in the proposed new formulation. Quantitative assessments on the accuracy of TIMIBC for scattering of acoustic waves by lined surfaces will be carried out, and its effectiveness will be demonstrated by numerical examples. Specifically, the accuracy is assessed by comparing solutions obtained by the Ingard-Myers condition with that by the proposed TIMIBC where theoretical reflection coefficients at a lined surface are derived for cases of plane and spherical incident waves. It is found that the TIMIBC provides a good approximation to the original Ingard-Myers condition for flows of low to mid subsonic Mach numbers. Time domain implementations of TIMIBC are also discussed and illustrated with a numerical example using a finite difference scheme. As many studies have shown the Ingard-Myers condition to be the correct limit of boundary layer thickness going to zero, the proposed TIMIBC can offer a practical solution for overcoming the intrinsic instability associated with the Ingard-Myers condition.

Time Domain BEM↗

On A Stabilization of the Ingard-Myers Impedance Boundary Condition

It has been well-known that the Ingard-Myers impedance condition, while simple to apply, is subject to the hydrodynamic Kelvin-Helmholtz instability due to its use of a vortex sheet in modeling the flow at the liner boundary. Recently, in the development of a time domain boundary element method for acoustic scattering by treated surfaces, it was found that by neglecting a certain second-order spatial derivative term in the Ingard-Myers formulation, the hydrodynamic instability can be avoided. The present paper aims to provide further analysis of this modified condition, hereby referred to as Truncated Ingard-Myers Impedance Boundary Condition (TIMIBC). It will be shown, based on the dispersion relations of linear waves, that the instability intrinsic to the Ingard-Myers condition is eliminated in the proposed new formulation. Quantitative assessments on the accuracy of TIMIBC for scattering of acoustic waves by lined surfaces will be carried out, and its effectiveness will be demonstrated by numerical examples. Specifically, the accuracy is assessed by comparing solutions obtained by the Ingard-Myers condition with that by the proposed TIMIBC where theoretical reflection coefficients at a lined surface are derived for cases of plane and spherical incident waves. It is found that the TIMIBC provides a good approximation to the original Ingard-Myers condition for flows of low to mid subsonic Mach numbers. Time domain implementations of TIMIBC are also discussed and illustrated with a numerical example using a finite difference scheme. As many studies have shown the Ingard-Myers condition to be the correct limit of boundary layer thickness going to zero, the proposed TIMIBC can offer a practical solution for overcoming the intrinsic instability associated with the Ingard-Myers condition.

Time Domain BEM↗

Suppression of Soot Formation and Shapes of Laminar Jet Diffusion Flames

Laminar nonpremixed (diffusion) flames are of interest because they provide model flame systems that are far more tractable for analysis and experiments than practical turbulent flames. In addition, many properties of laminar diffusion flames are directly relevant to turbulent diffusion flames using laminar flamelet concepts. Finally, laminar diffusion flame shapes have been of interest since the classical study of Burke and Schumann because they involve a simple nonintrusive measurement that is convenient for evaluating flame shape predictions. Motivated by these observations, the shapes of round hydrocarbon-fueled laminar jet diffusion flames were considered, emphasizing conditions where effects of buoyancy are small because most practical flames are not buoyant. Earlier studies of shapes of hydrocarbon-fueled nonbuoyant laminar jet diffusion flames considered combustion in still air and have shown that flames at the laminar smoke point are roughly twice as long as corresponding soot-free (blue) flames and have developed simple ways to estimate their shapes. Corresponding studies of hydrocarbon-fueled weakly-buoyant laminar jet diffusion flames in coflowing air have also been reported. These studies were limited to soot-containing flames at laminar smoke point conditions and also developed simple ways to estimate their shapes but the behavior of corresponding soot-free flames has not been addressed. This is unfortunate because ways of selecting flame flow properties to reduce soot concentrations are of great interest; in addition, soot-free flames are fundamentally important because they are much more computationally tractable than corresponding soot-containing flames. Thus, the objectives of the present investigation were to observe the shapes of weakly-buoyant laminar jet diffusion flames at both soot-free and smoke point conditions and to use the results to evaluate simplified flame shape models. The present discussion is brief.

Xu, F.↗

Stability analysis of numerical boundary conditions and implicit difference approximations for hyperbolic equations

Implicit, noniterative, finite difference schemes were recently developed by several authors for multidimensional systems of nonlinear hyperbolic partial differential equations. When applied to linear model equations with periodic boundary conditions those schemes are unconditionally stable (A-stable). As applied in practice the algorithms often face a severe time step restriction. A major source of the difficulty is the treatment of the numerical boundary conditions. One conjecture was that unconditional stability requires implicit numerical boundary conditions. An apparent counter example was the space time extrapolation considered by Gustafsson, Kreiss, and Sunstrom. Spatial (implicit) and space time (explicit) extrapolation using normal mode analysis for a finite and infinite number of spatial mesh intervals are examined. The results indicate that for unconditional stability with a finite number of spatial mesh intervals, the numerical boundary conditions must be implicit.

Beam, R. M.↗

Dry film lubricants

The Space Age has necessitated accelerated development of many technologies which have direct application to commercial, earthbound hardware. One striking example is dry film lubrication. The stimulus for the development of dry film lubricants was provided by the unsatisfactory condition created when commercial liquid lubricants were exposed to the multiple environs of space. In the search for a satisfactory solution to lubrication in space, dry films were investigated. The dry film lubrication concept has proven most desirable for space, and a natural fallout is the potential shown by such films for consumer item application. On the acceptance that the coefficient of friction of dry films is not equivalent to that of a liquid lubricant under ideal conditions, this paper discusses how the ideal condition for liquid lubrication is rarely achieved in practical applications. Comparisons are made between the performance of dry film and liquid lubricants as a function of temperature, environmental pressure, and life. Furthermore, consideration is given to the comparative costs of liquid versus dry film lubricants to show the practicality and desirability of dry film lubrication to consumer items. Although no data on the performance of consumer items lubricated with dry films are presented, the comparisons clearly establish the potential of dry film lubricants in a competitive consumer market.

SOLID LUBRICANT↗

A General Theory of Unsteady Compressible Potential Aerodynamics

The general theory of potential aerodynamic flow around a lifting body having arbitrary shape and motion is presented. By using the Green function method, an integral representation for the potential is obtained for both supersonic and subsonic flow. Under small perturbation assumption, the potential at any point, P, in the field depends only upon the values of the potential and its normal derivative on the surface, sigma, of the body. Hence, if the point P approaches the surface of the body, the representation reduces to an integro-differential equation relating the potential and its normal derivative (which is known from the boundary conditions) on the surface sigma. For the important practical case of small harmonic oscillation around a rest position, the equation reduces to a two-dimensional Fredholm integral equation of second-type. It is shown that this equation reduces properly to the lifting surface theories as well as other classical mathematical formulas. The question of uniqueness is examined and it is shown that, for thin wings, the operator becomes singular as the thickness approaches zero. This fact may yield numerical problems for very thin wings.

Morino, L.↗

A fast-reacting and versatile optokinetic stimulus pattern by computer graphics with application examples

A computer graphics method for the generation of horizontally moving vertical stripes which serve as stimuli for optokinetic nystagmus is presented. The pattern is generated by the successive addition of a constant in a digital register, leading to its periodic overflow. Digital to analog conversion of the register contents results in a staircase waveform, which is converted into a pattern of vertical lines on a CRT. The addition of any number to the reference buffer shifts all lines uniformly, so that the group velocity of the pattern is easily controlled. The computer controlled display allows an accurate realization of a commanded velocity profile with practically no time delay and facilitates study under conditions of variable feedback. The display software can be easily modified to limit stimulation to a selected area of the moving retina, and the method has been used to simulate central scotomata. It is pointed out that the technique, while more accurate, versatile and responsive than a rotating drum device, may be less useful in experiments requiring a wide field display.

Yasui, S.↗

Spinning for safety's sake

NASA Langley is currently engaged in a comprehensive research program to improve the stall/spin characteristics of general aviation airplanes. Part of this program has included systematic flight tests of a fully instrumented airplane to determine the effects of various tail, wing, and fuselage modifications, and variation of inertia, center-of-gravity, and control positions on the spin and recovery characteristics. Airplane equipment, onboard instrumentation ground equipment, test area, and test procedures have been combined to provide a safe, practical means for conducting research in preplanned hazardous conditions. Configuration changes produced spins varying from easily recoverable to unrecoverable, and wind section design significantly influenced airplane stall, spin, and recovery characteristics.

Stough, H. P., III↗

Prediction of sound radiated from different practical jet engine inlets

Existing computer codes for calculating the far field radiation patterns surrounding various practical jet engine inlet configurations under different excitation conditions were upgraded. The computer codes were refined and expanded so that they are now more efficient computationally by a factor of about three and they are now capable of producing accurate results up to nondimensional wave numbers of twenty. Computer programs were also developed to help generate accurate geometrical representations of the inlets to be investigated. This data is required as input for the computer programs which calculate the sound fields. This new geometry generating computer program considerably reduces the time required to generate the input data which was one of the most time consuming steps in the process. The results of sample runs using the NASA-Lewis QCSEE inlet are presented and comparison of run times and accuracy are made between the old and upgraded computer codes. The overall accuracy of the computations is determined by comparison of the results of the computations with simple source solutions.

Zinn, B. T.↗

USDA/federal user of LANDSAT remote sensing

Developed and potential uses of remote sensing in crop condition and acreage assessment, renewable resources inventories, conservation practices, and water and forest management applications are described. Operational approaches, the adaptation of procedures to needs, and the agency's concern about data continuity and cost are discussed as well as support for future technology development for enhanced sensing capability. The use of improved camera systems for soil mapping and conservation monitoring from space shuttle, and of aerospace radar to improve soil moisture monitoring are mentioned.

Allen, R.↗

Some design guidelines for discrete-time adaptive controllers

There have been many algorithms proposed for adaptive control which will provide globally asymptotically stable controllers if some stringent conditions on the plant are met. The conditions on the plant cannot be met in practice as all plants will contain high frequency unmolded dynamics therefore, blind implementation of the published algorithms can lead to disastrous results. This paper uses a linearization analysis of a non-linear adaptive controller to demonstrate analytically design guidelines which aleviate some of the problems associated with adaptive control in the presence of unmodeled dynamics.

Rohrs, C. E.↗

The prediction and confirmation of critical epitaxial parameters

The coherency-incoherency transition in epitaxial crystals is said to take place at a critical misfit f(c) or, for a system in which a monolayer is subcritical, at a critical thickness h(c). In this paper, the physical principles and models used to predict critical parameters are analyzed and put into perspective. The dependence of the relevant principles on the equilibrium-nonequilibrium conditions under which the quantities are measured in practice is stressed. The advantages and disadvantages of the models used (essentially the Frenkel-Kontorowa and Volterra models) are highlighted.

Van Der Merwe, Jan↗