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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 739 records · Page 41
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

Fundamental and harmonic electron cyclotron maser emission

The plasma conditions and features of the energetic electron distribution in electron cyclotron maser emission for which growth in a particular mode is favored when the ratio of the plasma frequency omega(p) to the electron cyclotron frequency Omega(e) is greater than about 0.3 are determined. It is shown that growth at the fundamental is suppressed as omega(p)/Omega(e) increases and emission at harmonics of Omega(e) dominates. Growth at harmonics of Omega(e) is not restricted to the O and X modes, but can also occur for the Z mode. Whether or not growth in a particular mode dominates depends both on omega(p)/Omega(e) and on the form of the distribution. If the density of the energetic electrons is sufficiently large, the dispersion relations of the O and X modes are modified so that the group velocities of the growing O and X mode waves can be comparable to that of the growing Z mode waves.

Winglee, R. M.↗
NASA NTRS1987

Low-altitude ISIS 1 observations of auroral radio emissions and their significance to the cyclotron maser instability

Low-altitude ISIS 1 observations are presented which support the cyclotron maser instability as the generation mechanism of most, but not all, auroral radio emissions. The observations confirm the ability of the mechanism to generate different wave modes and harmonics as the ratio of the plasma frequency to gyrofrequency f(N)/f(H) increases depending on the form of the energetic electron distribution function. The present observations correspond to conditions where f(N)/F(H) is always larger than 0.2 and is typically about 1.0. The observations and instability growth rate calculations indicate that second harmonic radiation in the extraordinary and the Z modes is directly generated at low altitudes. By far the most common signals observed near ISIS 1 perigee, however, are in the fundamental Z and whistler modes. The observations and calculations in the case of the Z mode suggest that the pertinent process involves a maser that does not saturate, i.e., that group velocity effects are important. The whistler mode is the most intense signal observed at low altitudes. Since the cyclotron maser generated whistler mode waves have peak growth rates for downward field-aligned propagation, they should be able to penetrate the ionosphere. This process may provide the explanation of occasional reports, dating back to the International Geophysical Year time period, of ground level detection of radio noise associated with the aurora.

Benson, Robert F.↗
NASA NTRS1989

The evolution of cooling flows - Self-similar cooling waves

Similarity solutions are presented for the time-dependent evolution of cooling flows. A cooling flow expands when the cooling time computed from the initial gas distribution increases with radius. If certain simplifying conditions are met, the evolution of the resulting cooling wave becomes self-similar. The similarity solutions obtained here assumed subsonic flow, which is valid in the outer parts of observed cooling flows; they are matched to transonic accretion solutions valid in the central parts. The models are applied to the cooling flow onto M87, with unsatisfactory agreement at small radius. A local linear stability analysis shows that the similarity solution suffers isobaric thermal instability, suggesting that the neglect of star formation may be responsible for the poor agreement. The similarity solution suggests that the cooling flow around M87 was more vigorous in the past.

Bertschinger, Edmund↗
NASA NTRS1989

Analysis of organic compounds in returned comet nucleus samples

Techniques for analysis of organic compounds in returned comet nucleus samples are described. Interstellar, chondritic and transitional organic components are discussed. Appropriate sampling procedures will be essential to the success of these analyses. It will be necessary to return samples that represent all the various regimes found in the nucleus, e.g., a complete core, volatile components (deep interior), and crustal components (surface minerals, rocks, processed organics such as macromolecular carbon and polymers). Furthermore, sampling, storage, return, and distribution of samples must be done under conditions that preclude contamination of the samples by terrestrial matter.

Cronin, J. R.↗
NASA NTRS1990

Magnetotail acceleration using generalized drift theory - A kinetic merging scenario

It is possible to describe particle behavior in the magnetotail, including particle energization, by means of generalized drift theory. Generalized drift velocities are obtained by using the generalized first invariant which has been shown to be useful in such current sheet configurations. Particles whose generalized invariant is preserved gain energy entirely in the field-aligned direction. The form of the accelerated particle velocity distribution is obtained and self-consistency conditions are derived.

Whipple, E. C.↗
NASA NTRS1990

The incident solar irradiance at the sea surface

Computations have been performed of the incident spectral irradiance at the sea surface using LOWTRAN-7 as the basis to describe the incident scalar and vector irradiance in terms of the true solar zenith angle and the nominal visibility in the atmosphere. These computations have been used to describe the contributions to the incident irradiance from the direct and the sky components of the total irradiance and the average cosine of the sky component as a measure of the radiance distribution of the sky for varying atmospheric conditions. Comparisons of the computations from LOWTRAN-7 have been made with the results from other models, and with data obtained from field measurements, and excellent agreement has been obtained for the daily profiles of the vector and scalar irradiance at the surface. These computations have been used to provide a description of the irradiance at the sea surface for use in the analysis of remotely sensed data based on information on the radiative transfer through the atmosphere above the sea surface.

Van Tran, AN↗
NASA NTRS1991

Transport of N2O in the stratosphere related to the equatorial semiannual oscillation

The double-peak structure observed in the distribution of N2O during the spring equinoctial condition is studied by means of a 2D time-dependent tracer model. The meridional transport circulation is determined from computed diabatic heating rates, and the horizontal eddy diffusion coefficients are computed as residuals by potential vorticity conservation, using observed mean winds to compute the budget of zonal mean potential vorticity. This model is used to predict the 30-d evolution of the observed N2O field. The integrated results are generally in good agreement with observed tracer fields up to the midstratosphere. It is proposed that standard climatologies underestimate the strength of the equatorial semiannual oscillation in the zonal mean wind and temperature fields.

Choi, Woo Kap↗
NASA NTRS1992

Particle simulation of rarefied aeropass maneuvers of the Magellan spacecraft

Rarefied and free molecular flowfields corresponding to proposed aeropass maneuvers through the atmosphere of Venus by the Magellan spacecraft are computed with a vectorized particle simulation method. The significance of rarefaction and reaction effects are assessed and surface heat flux, drag coefficients, and flowfield properties are computed. A simple surface heat transfer model is coupled directly to the simulation and assumes that each surface element is in radiative equilibrium with deep space. This allows direct computation of surface temperature distributions rather than requiring prescribed isothermal boundary conditions. Uncoupled dual-node heat transfer models, which account for heat capacity and thermal conductivity within each structural component, permit more accurate determination of surface temperatures. Such temperatures restrict the allowable altitudes and entry speeds of the aeropass maneuvers, and must therefore be estimated accurately. Simulations with an entry velocity of 8600 m/s at altitudes between 125 and 140 km reveal that allowable surface temperatures occur only at the highest altitudes where the flowfield is appropriately modeled as free-molecular. Excessive surface heat flux occurs at lower altitudes where molecular collisions are significant.

Haas, Brian L.↗
NASA NTRS1992

Synthesis of actively adjustable springs by antagonistic redundant actuation

A methodology for active spring generation is presented based on antagonistic redundant actuation. Antagonistic properties are characterized using an effective system stiffness. 'Antagonistic stiffness' is generated by preloading a closed-chain (parallel) linkage system. Internal load distribution is investigated along with the necessary conditions for spring synthesis. The performance and stability of a proposed active spring are shown by simulation, and applications are discussed.

Yi, Byung-Ju↗
NASA NTRS1993

Acceleration of convergence and spectrum transformation of implicit finite difference operators associated with Navier-Stokes equations

Implicit difference operator spectra are presently computed by applying eigensystem analysis techniques to finite-difference formulations of 2D Euler and Navier-Stokes equations, and attention is given to these iterative methods' convergence and stability characteristics by taking into account the effects of grid geometry, time-step, numerical viscosity, and boundary conditions. On the basis of the eigenvalue distributions for various flow configurations, the feasibility of applying such convergence-acceleration techniques as eigenvalue annihilation and relaxation is discussed. Spectrum-shifting is applied to NASA-Ames' ARC2D flow code, achieving a 20-33 percent efficiency.

Saleem, M.↗
NASA NTRS1957

Comparison of NACA 65-series compressor-blade pressure distributions and performance in a rotor and in cascade

An investigation has been conducted to compare the performance of NACA 65-series compressor blades in two-dimensional cascade with that in an axial flow compressor. Blade pressure distributions were obtained by the use of a mercury-seal pressure-transfer device. The comparison indicated that cascade data accurately predicted the turning angle and blade pressure distribution obtained in the compressor at design conditions.

CASCADES↗
NASA NTRS1945

Pressure Lag in Tubing Used in Flight Research

Tests described in this report were undertaken to obtain a quantitative measure of the pressure lag in typical pressure-tubing systems used by the Ames Aeronautical Laboratory in flight research investigations. Lag measurements were made with both single-direction and oscillating pressure changes. Single-direction pressure changes were investigated to determine if the lag in orifice-pressure lines and in the research airspeed and altitude measuring systems of pursuit-type airplane undergoing flight tests was sufficient to cause an appreciable error in the record of a sudden pressure change. Oscillating pressure changes were investigated with particular reference to the accuracy of pressure peaks in pressure-distribution measurements during the time of buffeting conditions as found in stalls. (author)

Turner, Howard L↗
NASA NTRS1994

Computational Methods for HSCT-Inlet Controls/CFD Interdisciplinary Research

A program aimed at facilitating the use of computational fluid dynamics (CFD) simulations by the controls discipline is presented. The objective is to reduce the development time and cost for propulsion system controls by using CFD simulations to obtain high-fidelity system models for control design and as numerical test beds for control system testing and validation. An interdisciplinary team has been formed to develop analytical and computational tools in three discipline areas: controls, CFD, and computational technology. The controls effort has focused on specifying requirements for an interface between the controls specialist and CFD simulations and a new method for extracting linear, reduced-order control models from CFD simulations. Existing CFD codes are being modified to permit time accurate execution and provide realistic boundary conditions for controls studies. Parallel processing and distributed computing techniques, along with existing system integration software, are being used to reduce CFD execution times and to support the development of an integrated analysis/design system. This paper describes: the initial application for the technology being developed, the high speed civil transport (HSCT) inlet control problem; activities being pursued in each discipline area; and a prototype analysis/design system in place for interactive operation and visualization of a time-accurate HSCT-inlet simulation.

Cole, Gary L.↗
NASA NTRS1994

Early stellar evolution

Research into the formation and early evolution of stars is currently an area of great interest and activity. The theoretical and observational foundations for this development are reviewed in this paper. By now, the basic physics governing cloud collapse is well understood, as is the structure of the resulting protostars. However, the theory predicts protostellar luminosities that are greater than those of most infrared sources. Observationally, it is thought that protostars emit powerful winds that push away remnant cloud gas, but both the origin of these winds and the nature of their interaction with ambient gas are controversial. Finally, the theory of pre-main-sequence stars has been modified to incorporate more realistic initial conditions. This improvement helps to explain the distribution of such stars in the H-R diagram. Many important issues, such as the origin of binary stars and stellar clusters, remain as challenges for future research.

Stahler, Steven W.↗
NASA NTRS1994

Effect of magnetic field geometry on the wave signature of the pickup of interstellar neutrals

By using the technique of Johnstone et al. (1991), the asymptotic wave spectrum induced by the pickup of interstellar ions in the heliosphere is derived. This approach is independent of quasi-linear theory. The asymptotic wave spectrum is calculated from the standard resonance condition by assuming that the asymptotic ion distribution is bispherical and by using conservation of energy. The calculation places no restriction on either the ratio of Alfven speed to solar wind flow speed or the particle pitch angle. Spectra for various geometries (relative orientations of the magnetic field and solar wind velocity) are calculated. Absolute upper limits to the expected wave enhancements are calculated. These upper limits are compared to the background fluctuation spectrum extrapolated from 0.87 AU, assuming a spectral index of -5/3 and radial dependences of -2 and -3. The heliocentric radial distances at which the peaks in the various geometry-dependent wave spectra are expected to be observable above the background are identified. Peaks due to pickup hydrogen at the proton gyrofrequency are found to be always stronger relative to the background in parallel than in perpendicular geometries. The parallel-geometry excitations consist only of sunward propagating waves. In perpendicular geometries a peak also occurs, this time near 0.1 of the proton gyrofrequency (corresponding to the peak calculated by Lee and Ip (1987)). However, the steep spectral dependence of the ambient wave field makes it unlikely that this peak will be observed inside 20 AU. A similar result holds for the case of pickup-helium induced excitation at the helium gyrofrequency. Inside 10 AU, peaks at these lower frequencies tend to be obscured by the steep frequency dependence (proportional to -5/3) of the ambient spectrum and then are only observable in parallel geometries.

Williams, L. L.↗
NASA NTRS1997

Effects of Fuel Preheat on Soot Formation in Microgravity Laminar Diffusion Flames

Nonbuoyant flames offer themselves as an attractive and promising platform to gain a better understanding of soot mechanisms. The effects of buoyancy can be eliminated temporarily in drop towers which sustain brief intervals of reduced gravity-typically lower than 10(exp -3)g- extending up to several seconds at a time. Microgravity facilities have been employed to show that nonbuoyant flames are longer, wider and sootier than their normal-gravity counterparts. Sunderland et al. recently verified the existence of smoke point in laminar nonbuoyant flames. As reported, microgravity flames operating above their smoke point displayed a blunt tip and much broader soot-containing regimes in comparison to their buoyant counterparts. Mortazavi et al. established that residence times in microgravity laminar jet diffusion flames with Re=0(100) tend to be proportional to burner diameter and inversely proportional to burner exit velocity. This offers the capability to alter residence times in nonbuoyant laminar jet diffusion flames when varying the burner exit diameters and velocities. Megaridis et al. presented a quantitative definition of the soot-field structure within laminar microgravity jet diffusion flames which operated well above their smoke point. The experimental methodology involved a full-field laser-light extinction technique and jet diffusion flames of nitrogen-diluted (50% vol.) acetylene fuel burning in quiescent air at atmospheric pressure. The work was conducted at the 2.2s drop tower of the NASA Lewis Research Center (NASA-LeRC). Parallel work on 1-g flames was also presented in (6) to facilitate comparisons on the effect of gravity on the soot fields. As reported, the soot spatial distributions in 0-g flames did not change in a detectable manner after 1s within a typical 2.2s experiment. During that period, the soot field was shown to sustain a pronounced annular structure throughout the luminous nonbuoyant-flame zone. The maximum soot volume fraction measured at 0-g was nearly a factor of two higher than that at 1-g, thus confirming the enhanced sooting tendency of nonbuoyant flames. Greenberg and Ku presented a similar study and reported trends that matched those of for the 50% (vol.) nitrogen-diluted acetylene fuel. Furthermore, they examined pure acetylene flames and reported similar trends with respect to the influence of gravity on maximum soot volume fractions and flame cross-section-averaged soot loadings. Both studies clearly demonstrated the improved spatial resolution of microgravity flames compared to their normal-gravity counterparts. The current study evaluates the influence of moderate fuel preheat on soot formation within 0-g laminar gas jet diffusion flames. While fuel temperature variations have little influence on residence times in 1-g, they have a much more significant effect in 0-g. The primary objective of this program is to quantify this effect and its consequences on sooting by comparing soot volume fraction distributions under preheated and unpreheated-fuel conditions. Furthermore, the current work aims at expanding the limited soot database available for nonbuoyant flames. Soot fields in such flames can be used to perform additional tests of recently developed soot sub-models which have the potential to become powerful predicting tools in combustion design.

Konsur, Bogdan↗
NASA NTRS2000

Assessment of Low Cycle Fatigue Behavior of Powder Metallurgy Alloy U720

The fatigue lives of modem powder metallurgy disk alloys are influenced by variabilities in alloy microstructure and mechanical properties. These properties can vary as functions of variables the different steps of materials/component processing: powder atomization, consolidation, extrusion, forging, heat treating, and machining. It is important to understand the relationship between the statistical variations in life and these variables, as well as the change in life distribution due to changes in fatigue loading conditions. The objective of this study was to investigate these relationships in a nickel-base disk superalloy, U720, produced using powder metallurgy processing. Multiple strain-controlled fatigue tests were performed at 538 C (1000 F) at limited sets of test conditions. Analyses were performed to: (1) assess variations of microstructure, mechanical properties, and LCF failure initiation sites as functions of disk processing and loading conditions; and (2) compare mean and minimum fatigue life predictions using different approaches for modeling the data from assorted test conditions. Significant variations in life were observed as functions of the disk processing variables evaluated. However, the lives of all specimens could still be combined and modeled together. The failure initiation sites for tests performed at a strain ratio R(sub epsilon) = epsilon(sub min)/epsilon(sub max) of 0 were different from those in tests at a strain ratio of -1. An approach could still be applied to account for the differences in mean and maximum stresses and strains. This allowed the data in tests of various conditions to be combined for more robust statistical estimates of mean and minimum lives.

Gabb, Tomothy P.↗
NASA NTRS2001

An Experimental Study of the Operating Temperature in a Loop Heat Pipe with Two Evaporators and Two Condensers

This paper presents a comprehensive experimental study of the loop operating temperature in a loop heat pipe (LHP) which has two parallel evaporators and two parallel condensers. In a single evaporator LHP, it is well known that the loop operating temperature is a function of the heat load, the sink temperature and the ambient temperature. The objective of the present study emphasizes on the stability of the loop operating temperature and parameters that affects the loop operation. Tests results show that the loop operating temperature is a function of the total system heat load, sink temperature, ambient temperature, and beat load distribution between the two evaporators. Under most conditions, only one compensation chamber (CC) contains two-phase fluid and controls the loop operating temperature, and the other CC is completely filled with liquid. Moreover, as the test condition changes, control of the loop operating temperature often shifted from one CC to another. In spite of complex interactions between various components, the test loop has demonstrated very robust operation even during fast transients.

Ku, Jentung↗