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At least 649 records · Page 36
All sourcesNASA NTRSDOE OSTI
NASA NTRS2003

Mechanisms for the Crystallization of ZBLAN

The objective of this ground based study is to test the hypothesis that shear thinning (the non-Newtonian response of viscosity to shear rate) is a viable mechanism to explain the observation of enhanced glass formation in numerous low-g experiments. In 1-g, fluid motion results from buoyancy forces and surface tension driven convection. This fluid flow will introduce shear in undercooled liquids in 1-g. In low-g it is known that fluid flows are greatly reduced so that the shear rate in fluids can be extremely low. It is believed that some fluids may have weak structure in the absence of flow. Very small shear rates could cause this structure to collapse in response to shear resulting in a lowering of the viscosity of the fluid. The hypothesis of this research is that: Shear thinning in undercooled liquids decreases the viscosity, increasing the rate of nucleation and crystallization of glass forming melts. Shear in the melt can be reduced in low-g, thus enhancing undercooling and glass formation. The viscosity of a model glass (lithium di-silicate, L2S) often used for crystallization studies has been measured at very low shear rates using a dynamic mechanical thermal analyzer. Our results are consistent with increasing viscosity with a lowering of shear rates. The viscosity of L2S may vary as much as an order of magnitude depending on the shear rate in the temperature region of maximum nucleation and crystal growth. Classical equations for nucleation and crystal growth rates, are inversely related to the viscosity and viscosity to the third power respectively. An order of magnitude variation in viscosity (with shear) at a given temperature would have dramatic effects on glass crystallization Crystallization studies with the heavy metal fluoride glass ZBLAN (ZrF2-BaF2-LaF3-AlF3-NaF) to examine the effect of shear on crystallization are being initiated. Samples are to be melted and quenched under quiescent conditions at different shear rates to determine the effect on crystallization. The results from this study are expected to advance the current scientific understanding of glass formation in low-g and glass crystallization under glass molding conditions and will improve the scientific understanding of technological processes such as fiber pulling, bulk amorphous alloys, and glass fabrication processes.

Ethridge, Edwin C.↗
NASA NTRS1955

Altitude-Test-Chamber Investigation of the Endurance and Performance Characteristics of the J65-W-7 Engine at a Mach Number of 2.0

An investigation of the endurance characteristics, at high Mach number, of the J65-W-7 engine was made in an altitude chamber at the Lewis laboratory. The investigation was made to determine whether this engine can be operated at flight conditions of Mach 2 at 35,000-feet altitude (inlet temperature, 250 F) as a limited-service-life engine Failure of the seventh-stage aluminum compressor blades occurred in both engines tested and was attributed to insufficient strength of the blade fastenings at the elevated temperatures. For the conditions of these tests, the results showed that it is reasonable to expect 10 to 15 minutes of satisfactory engine operation before failure. The high temperatures and pressures imposed upon the compressor housing caused no permanent deformation. In general, the performance of the engines tested was only slightly affected by the high ram conditions of this investigation. There was no discernible depreciation of performance with time prior to failure.

Biermann, A.E.↗
NASA NTRS1994

Middle- and low-latitude emissions from energetic neutral atom precipitation seen from ATLAS 1 under quiet magnetic conditions

During the ATLAS 1 mission spectral observations were made at middle and low latitudes of features expected from the precipitation of energetic neutral atoms. The Imaging Spectrometric Observatory was used at night in the UV and visible with maximum gain. The tangent ray heights of the look directions ranged from near 100 km to near 200 km, and the geomagnetic conditions were quiet during the observations, which were made March 28 to April 3, 1992. The N2(+) 1N 391.4-nm and O I 130.4 and 135.6-nm emissions were observed at all latitudes, with lower emission rates at lower magnetic dip latitudes, except that enhancements in the O I lines were seen within 30 deg of the dip equator to radiative recombination of ionospheric plasma. The latitude profile observed for the N2(+) 1N emission did not show an equatorial or midlatitude peak. This implies that the source of energetic neutrals is more consistent with prompt charge exchange loss of freshly injected trapped ions with relatively low mirror heights (i.e., ions on higher L shells with equatorial pitch angle distributions nearly isotropic to the loss cone) than loss of highly eroded populations of particles with high mirror heights (i.e., ions on lower L shells with pancake equatorial pitch angle distributions). The N2(+) 1N emission rates have been compared with models of atmospheric emission due to fluxes of O/O(+) and H/H(+) in the thermosphere, as produced by energetic neutral oxygen or hydrogen atom precipitation. Energy deposition rates are inferred.

Tinsley, B. A.↗
NASA NTRS1983

Fluid signatures of rotational discontinuities at Earth's magnetopause

Fluid signatures in the MHD approximation at rotational discontinuities (RD) of finite width called rotational shear layers (RSL) are examined for general flow and magnetic geometries. Analytical and geometrical arguments illustrate that the fluid speed can either go up or down across an RSL for a fixed normal mass flux. The speed profile may or may not be monotonic depending on the boundary conditions. The flow velocity may or may not be field aligned or ""jetting'' as a result of traversing the RSL. In general, significant ""convection'' is expected in the layer. The observable signatures of (MHD) RSL's depend on 7 (boundary condition) parameters are (1) the mass density, (2 to 5) the incident normal and transverse components of the magnetic field and fluid velocity, (6) the angle epsilon between the incident tangential flow velocity and tangential magnetic field, and (7) the size of the magnetic angular rotation implemented by the layer delta phi.

Scudder, J. D.↗
NASA NTRS1984

Fluid signatures of rotational discontinuities at the earth's magnetopause

Fluid signatures in the MHD approximation at rotational discontinuities (RD) of finite width called rotational shear layers (RSL) are examined for general flow and magnetic geometries. Analytical and geometrical arguments illustrate that the fluid speed can either go up or down across an RSL for a fixed normal mass flux. The speed profile may or may not be monotonic depending on the boundary conditions. The flow velocity may or may not be field aligned or 'jetting' as a result of traversing the RSL. In general, significant 'convection' is expected in the layer. The observable signatures of (MHD) RSL's depend on 7 (boundary condition) parameters are (1) the mass density, (2 to 5) the incident normal and transverse components of the magnetic field and fluid velocity, (6) the angle epsilon between the incident tangential flow velocity and tangential magnetic field, and (7) the size of the magnetic angular rotation implemented by the layer delta phi.

Scudder, J. D.↗
NASA NTRS1987

Flight experience of solar mesosphere explorer's power system over high temperatures ranges

The performance of the power system on the Solar Mesosphere Explorer (SME) satellite for the life of the mission and the techniques used to ensure power system health are summarized. Early in the mission high cell imbalances in one of the batteries resulted in a loading scheme which attempted to minimize the cell imbalances without causing an undervoltage condition. A short term model of the power system allowed planners to predict depth of discharge using the latest available data. Due to expected orbital shifts the solar arrays experience extended periods of no eclipse. This has required special conditioning schemes to keep the batteries healthy when the eclipses return. Analysis of the SME data indicates long term health of the SME power system as long as the conditioning scheme is continued.

Faber, Jack↗
NASA NTRS1994

N2 Lyman-Birge-Hopfield dayglow from ATLAS 1

Spectral measurements of the far ultraviolet dayglow were made from the ATLAS 1 shuttle mission using an intensified charge coupled device (CCD) imaging spectrometer array. The instrument imaged relatively large (177 A) segments of the spectrum simultaneously and had the capability to image altitude in the second dimension of the two-dimensional focal plane detector when the entrance slit of the instrument was oriented perpendicular to the limb of the Earth. The far ultraviolet (FUV) channel of the instrument used a CsTe photocathode rather than the more conventionally used and more solar blind CsI photocathode material. As a result, the spectra were also measured with good sensitivity at FUV wavelengths longer than 1500 A. In this paper, data are selected of specific interest to a study of the N2 Lyman-Birge-Hopfield system using two shuttle attitudes: In the first the field of view was held at a fixed tangent ray height of approximately 140 km while scanning in wavelength, providing repeated FUV spectra over a range of latitudes and allowing comparison with our thermospheric airglow model. In the second observing sequence the line of sight of the instrument was scanned down through the atmosphere but covered about 75% of the full wavelength range. This sequence allows comparison of height dependence,including O2 absorption effects, with the thermospheric model. In all cases, relatively good agreement is obtained with the model (run strictly in a predictive mode) in comparisions in which the line of sight must be intergrated along a path that spans a considerable range of conditions (height, local time, latitude, solar zenith angle). In the case of the data sets examined in this study the vibrational population distributions show mixed results. In one case the distribution agrees well with a theoretical model based on direct electron impact excitation without a significant cascade contribution. A second case indicated a lower population in nu' = 3 and a higher population in nu' = 6 than would be expected from direct impact excitation alone. This supports previous indications that conditions resulting in the cascade mechanism may not always be operative.

Torr, Marsha R.↗
NASA NTRS2006

Towards Robust Designs Via Multiple-Objective Optimization Methods

Fabricating and operating complex systems involves dealing with uncertainty in the relevant variables. In the case of aircraft, flow conditions are subject to change during operation. Efficiency and engine noise may be different from the expected values because of manufacturing tolerances and normal wear and tear. Engine components may have a shorter life than expected because of manufacturing tolerances. In spite of the important effect of operating- and manufacturing-uncertainty on the performance and expected life of the component or system, traditional aerodynamic shape optimization has focused on obtaining the best design given a set of deterministic flow conditions. Clearly it is important to both maintain near-optimal performance levels at off-design operating conditions, and, ensure that performance does not degrade appreciably when the component shape differs from the optimal shape due to manufacturing tolerances and normal wear and tear. These requirements naturally lead to the idea of robust optimal design wherein the concept of robustness to various perturbations is built into the design optimization procedure. The basic ideas involved in robust optimal design will be included in this lecture. The imposition of the additional requirement of robustness results in a multiple-objective optimization problem requiring appropriate solution procedures. Typically the costs associated with multiple-objective optimization are substantial. Therefore efficient multiple-objective optimization procedures are crucial to the rapid deployment of the principles of robust design in industry. Hence the companion set of lecture notes (Single- and Multiple-Objective Optimization with Differential Evolution and Neural Networks ) deals with methodology for solving multiple-objective Optimization problems efficiently, reliably and with little user intervention. Applications of the methodologies presented in the companion lecture to robust design will be included here. The evolutionary method (DE) is first used to solve a relatively difficult problem in extended surface heat transfer wherein optimal fin geometries are obtained for different safe operating base temperatures. The objective of maximizing the safe operating base temperature range is in direct conflict with the objective of maximizing fin heat transfer. This problem is a good example of achieving robustness in the context of changing operating conditions. The evolutionary method is then used to design a turbine airfoil; the two objectives being reduced sensitivity of the pressure distribution to small changes in the airfoil shape and the maximization of the trailing edge wedge angle with the consequent increase in airfoil thickness and strength. This is a relevant example of achieving robustness to manufacturing tolerances and wear and tear in the presence of other objectives.

Man Mohan, Rai↗
NASA NTRS1995

Global aspects of the motion of the heliospheric termination shock: A gasdynamic model

The heliospheric termination shock is expected to move in response to variation in upstream solar wind conditions. Using numerical techniques, we extend an earlier strictly one-dimensional analytic gas dynamic model of shock motion to two dimensions, to investigate the qualitative features of global behavior of shock motion, and the consequences of latitudinal variation in dynamic pressure. The boundary conditions of the calculation are given by the solar wind parameters as a function of latitude and time on an inner spherical boundary, and a constant pressure (roughly simulating the effect of the local interstellar medium) on an outer boundary. Density variations, specified at the inner boundary as a function of time, are convected into the termination shock. Immediately after the interaction, the shock moves with speeds given by the earlier analytic model. However, as the termination shock propagates outward (or inward), it begins to slow down. After about 2 to 10 years, depending on details of boundary conditions, the signal from the shock interaction has reached the outer boundary and propagates inward to the position of the termination shock, strongly affecting the behavior of the shock. Assuming no further disturbances in the solar wind, the termination shock will reach its new equilibrium after some tens of years. In reality, large-scale variations in solar wind dynamic pressure occur on time scales short in comparison with the eleven year solar cycle, so that one expects that the termination shock is never in an equilibrium position, but rather oscillates inward and outward; this oscillation will vary with heliographic latitude. The effects of a variety of types of solar wind disturbances are investigated and summarized.

Ratkiewicz, R.↗
NASA NTRS2023

Multiphysics Modeling of Electrode Dissolution under Hydrodynamic Conditions in Low Conductivity Water

Efficient plug-flow electrochemical reactors are characterized by their ability to effectively convert species with significant ionic strength and/or with the assistance of a well-behaved electrolyte. Because of the classic electroconductivity requirement, electrochemical reactors are not particularly attractive options for applications with low-conductivity conditions, especially for low-pressure feeds. However, these reactors can still find application in dedicated processes such as ultra-clean water conditioning and biological-coupled separation systems. Consequently, a multi-physics model was developed using COMSOL Multiphysics to characterize different reactor-design conditions based on the Tertiary Current Distribution which accounts for the effects of expected variations in electrolyte composition and ionic strength on the electrochemical process, as well as solution resistance and electrode kinetics. The modelling framework presented here employs single-phase laminar fluid flow, the Nernst-Planck equation, the water-based electroneutrality condition, and concentration-dependent overpotentials. This investigation particularly explores the electrolysis process of “anodic dissolution,” in which elemental species dissociate from the solid matrix of the electrode and enter the liquid phase of the electrolyte as soluble ions. The reactor configuration of interest has a rectangular parallel-electrode design and is operated galvanostatically. A series of parametric studies are carried out to inspect the response of the system when hydrodynamic, kinetic, and geometric conditions are changed. The parametric sweep resulted in variable effluent concentrations and system voltage, which reveal underlaying optimization patterns for electrode dissolution in low conductivity water.

Multiphysics↗
NASA NTRS1989

The use of vestibular models for design and evaluation of flight simulator motion

Quantitative models for the dynamics of the human vestibular system are applied to the design and evaluation of flight simulator platform motion. An optimal simulator motion control algorithm is generated to minimize the vector difference between perceived spatial orientation estimated in flight and in simulation. The motion controller has been implemented on the Vertical Motion Simulator at NASA Ames Research Center and evaluated experimentally through measurement of pilot performance and subjective rating during VTOL aircraft simulation. In general, pilot performance in a longitudinal tracking task (formation flight) did not appear to be sensitive to variations in platform motion condition as long as motion was present. However, pilot assessment of motion fidelity by means of a rating scale designed for this purpose, were sensitive to motion controller design. Platform motion generated with the optimal motion controller was found to be generally equivalent to that generated by conventional linear crossfeed washout. The vestibular models are used to evaluate the motion fidelity of transport category aircraft (Boeing 727) simulation in a pilot performance and simulator acceptability study at the Man-Vehicle Systems Research Facility at NASA Ames Research Center. Eighteen airline pilots, currently flying B-727, were given a series of flight scenarios in the simulator under various conditions of simulator motion. The scenarios were chosen to reflect the flight maneuvers that these pilots might expect to be given during a routine pilot proficiency check. Pilot performance and subjective rating of simulator fidelity was relatively insensitive to the motion condition, despite large differences in the amplitude of motion provided. This lack of sensitivity may be explained by means of the vestibular models, which predict little difference in the modeled motion sensations of the pilots when different motion conditions are imposed.

Bussolari, Steven R.↗
NASA NTRS1972

Dayside auroral-oval plasma density and conductivity enhancements due to magnetosheath electron precipitation.

Demonstration that magnetosheath electrons precipitating into the dayside auroral oval are a significant source of ionization and consequently will lead to electrical conductivity enhancements within the oval. By assuming that the electrons are maintained isotropic by strong pitch-angle diffusion as they precipitate into the ionosphere, the precipitation heat flux can be simply related to solar-wind energy density and consequently to the level of magnetic activity. For quiet solar-wind conditions, the heat fluxes of 1 to 10 ergs/sq cm/sec expected and observed lead to height-integrated Pedersen conductivity enhancements of 4 to 15 mhos. During magnetic storms the conductivity enhancements could increase by a factor of 3 to 5. Since the precipitating electrons are soft, the Hall conductivity enhancements are smaller than the Pedersen conductivity enhancements. For typical electric fields the computed conductivity enhancements lead to field-aligned currents bounding the enhancements in order-of-magnitude agreement with observation. The topside ionosphere should also have a density enhancement over the auroral oval on the dayside.

Kennel, C. F.↗
NASA NTRS1972

Auroral emissions and particle precipitation in the noon sector.

Comparison of airborne optical measurements during December 1969 with satellite particle measurements made earlier in 1969. The latitudinal distributions of H beta, 4278-A and 6300-A emissions during quiet periods are observed to agree with what would be expected from the latitudinal distribution of particle precipitation during similar conditions. In particular, there is a belt 200 to 500 km wide (at ionospheric heights) in which the magnetosheath plasma penetrates down to low altitudes through the cusps in the dayside magnetosphere and in which both H beta and 6300-A emissions are observed. There are brighter red arcs with green lower borders within this belt. It is suggested that these are produced by narrow regions of precipitation of slightly more energetic electrons embedded in the cusp fluxes. Poleward of this belt the flux of particles is low, and the atmospheric emissions are very weak, except for occasional narrow regions of electron precipitation (without measurable protons) and the corresponding polar-cap aurora (without the H beta emission).

Heikkila, W. J.↗
NASA NTRS1973

The influence of laser ranging on selenodetic control.

A mathematical model for performing an adjustment, using laser distances to improve coordinates of points on the earth and on the moon, as well as the orientation of these two bodies in space, is presented. The observation equations are given, and the orientation of the earth and the moon are defined in terms of three Eulerian angles. Numerical experiments were performed to investigate the expected accuracies of parameters in the adjustment model under differing conditions. The results indicate that the statistics both for the coordinates of points on earth and on the moon, and for the orientation parameters of the two bodies are favorable. However, very high correlations exist between some of the parameters; therefore, the recovery of these parameters from a simultaneous adjustment is questionable.

Mueller, I. I.↗
NASA NTRS1981

A variational approach to the question of temporal stability of equilibrium models of solar prominences. I - The formal theory

A Lagrangian approach to the equations describing small amplitude departures from equilibrium of solar prominences is presented. Seven quantities are derived which, by analogy to the concepts of energy, momentum, and angular momentum, are conserved under conditions corresponding to ignorable coordinates of classical mechanics. It is expected that these conservation laws will be useful as criteria of accuracy in obtaining eigenfrequencies for the perturbation equations when numerical techniques are used. The method is capable of handling complicated spatial dependences and has internal criteria for assessing the accuracy of any given numerical convergence scheme.

Lerche, I.↗
NASA NTRS1982

The need for an optical transition detection system in the NTF

Methods for transition detection were studied. The very high test and model costs necessitate testing of any new configuration over a range of Reynolds numbers for correlation with data obtained in conventional tunnels. The Reynolds number variations will be carried out at constant dynamic pressure, i.e., constant aeroelastic condition, by varying test temperature. Boundary layer transition cannot be expected to occur at the full scale location at significantly less than full-scale Reynolds numbers, and transition patterns will change with varying Reynolds number. Knowledge of the location of transition on model surfaces is essential for correct interpretation of drag data. The importance of transition, the limitations of artificial transition fixing, and their relation to the NTF are reviewed. The calculated minimum roughness height to instantaneously trip transition on a model transport wing over a range of Reynolds numbers of interest is shown in the NTF.

Fancher, M. F.↗
NASA NTRS1988

Metamorphic P-T paths and Precambrian crustal growth in East Antarctica

The metamorphic constraints on crustal thicknesses in Archean and post-Archean terranes are summarized along with possible implications for tectonic processes. It is important to recognize that P-T estimates represent perturbed conditions and should not be used to estimate steady state geothermal gradients or crustal thicknesses. The example is cited of the Dora Maira complex in the French Alps, where crustal rocks record conditions of 35 kbar and 800 C, implying their subduction to depths of 100 km or more, followed by subsequent uplift to the surface. Therefore such P-T estimates tell more about processes than crustal thicknesses. More importantly, according to the author, are determinations of P-T paths, particularly coupled with age measurements, because these may provide constraints on how and when perturbed conditions relax back to steady state conditions. P-T paths are illustrated that should be expected from specific tectonic processes, including Tibetan style collision, with and without subsequent extension, rifting of thin or thickened crust, and magmatic accretion. Growth of new crust, associated with magmatic accretion, for example, could possibly be monitored with these P-T paths.

Harley, S. L.↗