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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 379 records · Page 21
All sourcesNASA NTRSDOE OSTI
NASA NTRS1989

Velocity space diffusion of pickup ions from the water group at Comet Halley

The diffusion in velocity space of cometary ions was studied using the distributions of ions measured by the implanted ion spectrometer in Giotto during the inbound pass. The measurements were transformed into a frame comoving with the solar wind and oriented with the magnetic field. The observations show the evolution of the pitch angle distribution in the solar wind turbulence to form a shell from the initial ring. Diffusion in energy takes place simultaneously but on a longer time scale. Comparison with theory is inhibited by the lack of a suitable spatial model, but the simple arguments that can be made indicate that pitch angle diffusion and the process of parallel pickup take place more slowly than theory suggests.

Coates, A. J.↗
NASA NTRS2001

Cosmic Rays in Interplanetary Space

The science problem I have tackled in this grant is the derivation of the diffusion tensor of energetic particles in turbulent magnetic fields, with a sensible mean field. The new approach was to use quasi-linear theory with a consistent treatment of those scattering terms leading to diffusion perpendicular to the mean magnetic field; and, to use modern data and formats for three-dimensional turbulence.

Forman, Miriam A.↗
NASA NTRS1956

A Theory for Stability and Buzz Pulsation Amplitude in Ram Jets and an Experimental Investigation Including Scale Effects

From a theory developed on a quasi-one-dimensional-flow basis, it is found that the stability of the ram jet is dependent upon the instantaneous values of mass flow and total pressure recovery of the supersonic diffuser and immediate neighboring subsonic diffuser. Conditions for stable and unstable flow are presented. The theory developed in the report is in agreement with the experimental data of NACA-TN-3506 and NACA-RM-L50K30. A simple theory for predicting the approximate amplitude of small pressure pulsation in terms of mass-flow decrement from minimum-stable mass flow is developed and found to agree with experiments. Cold-flow tests at a Mach number of 1.94 of ram-jet models having scale factors of 3.15:1 and Reynolds number ratios of 4.75:1 with several supersonic diffuser configurations showed only small variations in performance between geometrically similar models. The predominant variation in steady-flow performance resulted from the larger boundary layer in the combustion chamber of the low Reynolds number models. The conditions at which buzz originated were nearly the same for the same supersonic diffuser (cowling-position angle) configurations in both large and small diameter models. There was no appreciable variation in stability limits of any of the models when the combustion-chamber length was increased by a factor of three. The unsteady-flow performance and wave patterns were also similar when considered on a reduced-frequency basis determined from the relative lengths of the model. The negligible effect of Reynolds number on stability of the off-design configurations was not anticipated in view of the importance of boundary layer to stability, and this result should not be construed to be generally applicable. (author)

Trimpi, Robert L↗
NASA NTRS1952

Amplitude of Supersonic Diffuser Flow Pulsations

A theoretical method for evaluating the stability characteristics and the amplitude and the frequency of pulsation of ram-jet engines without heat addition is presented herein. Experimental verification of the theoretical results are included where data were available. Theory and experiment show that the pulsation amplitude of a high mass-flow-ratio diffuser having no cone surface flow separation increases with decreasing mass flow. The theoretical trends for changes in amplitude, frequency, and mean-pressure recovery with changes in plenum-chamber volume were experimentally confirmed. For perforated convergent-divergent-type diffusers, a stability hysteresis loop was predicted on the pressure-recovery mass-flow-ratio curve. At a given mean mass-flow ratio, the higher.value of mean pressure recovery corresponded to oscillatory flow in the diffuser while the lower branch was stable. This hysteresis has been observed experimentally. The theory indicates that for a ram-jet engine of given diameter, the amplitude of pulsation of a supersonic diffuser is increased by decreasing the relative size of the plenum chamber with respect to the diffuser volume down to a critical value at which oscillations cease. In the region of these critical values, the stable mass-flow range of the diffuser may be increased either by decreasing the combustion chamber volume or by increasing the length of the diffuser.

Sterbentz, William H.↗
NASA NTRS1989

Parallel electric fields in a simulation of magnetotail reconnection and plasmoid evolution

Properties of the electric field component parallel to the magnetic field (E sub parallel) in a three-dimensional MHD simulation of plasmoid formation and evolution in the magnetotail in the presence of a net dawn-dusk magnetic field component were observed. Particularly emphasized was the spatial location of E(sub parallel), the concept of a diffusion zone and the role of E(sub parallel) in accelerating electrons. A localization of the region of enhanced E(sub parallel) in all space directions with a strong concentration in the z direction was found. This region was identified as the diffusion zone, which plays a crucial role in reconnection theory through the local break-down of magnetic flux conservation. The presence of B(sub y) implies a north-south asymmetry of the injection of accelerated particles into the near-earth region, if the net B(sub y) field is strong enough to force particles to follow field lines through the diffusion region. It is estimated that for a typical net B(sub y) field this should affect the injection of electrons into the near-earth dawn region, so that precipitation into the Northern (Southern) Hemisphere should dominate for duskward (dawnward) net B(sub y). In addition, a spatial clottiness of the expected injection of adiabatic particles which could be related to the appearance bright spots in auroras was observed.

Hesse, Michael↗
NASA NTRS2013

An Update on the Non-Mass-Dependent Isotope Fractionation under Thermal Gradient

Mass flow and compositional gradient (elemental and isotope separation) occurs when flu-id(s) or gas(es) in an enclosure is subjected to a thermal gradient, and the phenomenon is named thermal diffusion. Gas phase thermal diffusion has been theoretically and experimentally studied for more than a century, although there has not been a satisfactory theory to date. Nevertheless, for isotopic system, the Chapman-Enskog theory predicts that the mass difference is the only term in the thermal diffusion separation factors that differs one isotope pair to another,with the assumptions that the molecules are spherical and systematic (monoatomic-like structure) and the particle collision is elastic. Our previous report indicates factors may be playing a role because the Non-Mass Dependent (NMD) effect is found for both symmetric and asymmetric, linear and spherical polyatomic molecules over a wide range of temperature (-196C to +237C). The observed NMD phenomenon in the simple thermal-diffusion experiments demands quantitative validation and theoretical explanation. Besides the pressure and temperature dependency illustrated in our previous reports, efforts are made in this study to address issues such as the role of convection or molecular structure and whether it is a transient, non-equilibrium effect only.

Sun, Tao↗
NASA NTRS1997

Shapes of Buoyant and Nonbuoyant Methane Laminar Jet Diffusion Flames

Laminar gas jet diffusion flames represent a fundamental combustion configuration. Their study has contributed to numerous advances in combustion, including the development of analytical and computational combustion tools. Laminar jet flames are pertinent also to turbulent flames by use of the laminar flamelet concept. Investigations into the shapes of noncoflowing microgravity laminar jet diffusion flames have primarily been pursued in the NASA Lewis 2.2-second drop tower, by Cochran and coworkers and by Bahadori and coworkers. These studies were generally conducted at atmospheric pressure; they involved soot-containing flames and reported luminosity lengths and widths instead of the flame-sheet dimensions which are of Greater value to theory evaluation and development. The seminal model of laminar diffusion flames is that of Burke and Schumann, who solved the conservation of momentum equation for a jet flame in a coflowing ambient by assuming the velocity of fuel, oxidizer and products to be constant throughout. Roper and coworkers improved upon this model by allowing for axial variations of velocity and found flame shape to be independent of coflow velocity. Roper's suggestion that flame height should be independent of gravity level is not supported by past or present observations. Other models have been presented by Klajn and Oppenheim, Markstein and De Ris, Villermaux and Durox, and Li et al. The common result of all these models (except in the buoyant regime) is that flame height is proportional to fuel mass flowrate, with flame width proving much more difficult to predict. Most existing flame models have been compared with shapes of flames containing soot, which is known to obscure the weak blue emission of flame sheets. The present work involves measurements of laminar gas jet diffusion flame shapes. Flame images have been obtained for buoyant and nonbuoyant methane flames burning in quiescent air at various fuel flow-rates, burner diameters and ambient pressures. Soot concentrations were minimized by selecting conditions at low flowrates and low ambient pressures; this allows identification of actual flame sheets associated with blue emissions of CH and CO2. The present modeling effort follows that of Roper and is useful in explaining many of the trends observed.

Sunderland, Peter B.↗
NASA NTRS1993

Kinetics of diffusional droplet growth in a liquid/liquid two-phase system

This report contains experimental results for the interdiffusion coefficient of the system, succinonitrile plus water, at a number of compositions and temperatures in the single phase region of the phase diagram. The concentration and temperature dependence of the measured diffusion coefficient has been analyzed in terms of Landau - Ginzburg theory, which assumes that the Gibb free energy is an analytic function of its variables, and can be expanded in a Taylor series about any point in the phase diagram. At most points in the single phase region this is adequate. Near the consolute point (critical point of solution), however, the free energy is non-analytic, and the Landau - Ginzburg theory fails. The solution to this problem dictates that the Landau - Ginzburg form of the free energy be replaced by Widom scaling functions with irrational values for the scaling exponents. As our measurements of the diffusion coefficient near the critical point reflect this non-analytic character, we are preparing for publication in a refereed journal a separate analysis of some of the data contained herein as well as some additional measurements we have just completed. When published, reprints of this article will be furnished to NASA.

Baird, James K.↗
NASA NTRS1967

Self-diffusion in simple fluids.

Self-diffusion coefficients of simple liquids as predicted by Rice-Allnatt theory, noting friction coefficient and correlation function

CORRELATION FUNCTION↗
NASA NTRS1978

A measurement of the interplanetary radial gradient of low-energy oxygen nuclei from 1-10 AU and implications of this measurement

Cosmic-ray telescopes on the Pioneer 10 and 11 spacecraft has been used to measure the interplanetary radial gradients of oxygen nuclei between 7.1 and 26.6 MeV/nuc between 1 and 10 AU, in 1972-1976. Between 1 and 5 AU the average gradient for 10.6-26.6-MeV/nuc particles is 25 + or - 5%/AU, while between 5 and 10 AU this gradient is 12 + or - 4%/AU. Conventional cosmic-ray transport theory can be used to determine an average diffusion mean free path from these data. The value of lambda sub r for both energy ranges is deduced to be 0.19 + or - 0.04 AU between 1 and 5 AU and 0.40 + or - 0.13 AU between 5 and 10 AU.

Webber, W. R.↗
NASA NTRS1985

Two-fluid models of turbulence

The defects of turbulence models are summarized and the importance of so-called nongradient diffusion in turbulent fluxes is discussed. The mathematical theory of the flow of two interpenetrating continua is reviewed, and the mathematical formulation of the two fluid model is outlined. Results from plane wake, axisymmetric jet, and combustion studies are shown.

Spalding, D. B.↗
NASA NTRS1986

A three dimensional model of the Venusian thermosphere with superrotation

An improved three dimensional spectral model of the thermosphere of Venus is described. The model solves the Navier-Stokes equations and includes nonlinear effects for an arbitrary number of atmospheric species. A two dimensional axisymmetric model of the superrotation of the thermosphere is also presented. This model addresses the Pioneer-Venus mission finding, which suggested the thermospheric rotation rate to be much higher than that of the planet as seen from the asymmetric distribution of hydrogen and helium. Both models include the effects of an anisotropic eddy diffusion that is consistent with atmospheric mixing length theory.

Stevens-Rayburn, D. R.↗
NASA NTRS1986

Comptonization in supercritical winds. I - Spectral evolution. II - Dynamics and observational diagnostics

The physics governing spectral evolution in steady, fully ionized, radiatively driven winds from compact objects is examined. A largely analytic theory is developed to describe the effects of diffusion, adiabatic cooling, and repeated Compton scattering on the emergent radiation spectrum. Analytic forms for the wind temperature profile are derived, and spectral solutions obtained by considering a modified, time-independent Kompaneets equation are presented subject to a variety of initial conditions. A probabilistic formalism which describes the effect of diffusion photons travelling beyond the trapping radius is developed to allow the calculation of spectra observed at infinity. The second paper focuses on the dynamics of supercritical winds and on observational diagnostics which may be used to constrain model parameters in the contexts of specific astrophysical systems, including X-ray burst sources and mass-losing quasars.

Becker, Peter A.↗
NASA NTRS1988

Vapor concentration measurement with photothermal deflectometry

Theoretical and experimental results for using the photothermal deflection technique to measure vapor species concentration, while minimizing the disturbance of the transport (material) parameters due to vapor heating, are developed and described. In contrast to common practice, the above constraints require using a pump-beam duty cycle of less than 50 percent. The theoretical description of the shortened heating time is based on a step-function formulation of the pumping cycle. The results are obtained as closed-form solutions of the energy equation for many chopping cycles until steady state is reached, by use of a Green's-function method. The Euler formulation of the Fermat principle is used to calculate the deflection angle. The equations are expanded to include the effects of vapor velocity on both the temperature and temperature gradient profiles. The effects of finite (unfocused) pump and probe beams and thermal (Soret) diffusion are also accounted for. Excellent agreement between theory and experiment is obtained.

Banish, R. Michael↗