Search NASA⌕ Search

SEARCH · Search NASA

Results for “Diffusion coefficient”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 37 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1975

Calculated diffusion coefficients and the growth rate of olivine in a basalt magma

Concentration gradients in glass adjacent to skeletal olivines in a basalt have been examined by electron probe. The glass is depleted in Mg, Fe, and Cr and enriched in Si, Al, Na, and Ca relative to that far from olivine. Ionic diffusion coefficients for the glass compositions are calculated from temperature, ionic radius and melt viscosity, using the Stokes-Einstein relation. At 1170 C, the diffusion coefficient of Mg(2+) ions in the basalt is 4.5 billionths sq cm per sec. Comparison with measured diffusion coefficients in a mugearite suggests this value may be 16 times too small. The concentration gradient data and the diffusion coefficients are used to calculate instantaneous olivine growth rates. Growth necessarily preceded emplacement such that the composition of the crystals plus the enclosing glass need not be that of a melt. The computed olivine growth rates are compatible with the rate of crystallization deduced for the Skaegaard intrusion.

Donaldson, C. H.↗
NASA NTRS1982

On the energy dependence of the radial diffusion coefficient and spectra of inner radiation belt particles - Analytic solutions and comparison with numerical results

A theoretical method by which the energy dependence of the radial diffusion coefficient may be deduced from spectral observations of the particle population at the inner edge of the earth's radiation belts is presented. This region has previously been analyzed with numerical techniques; in this report an analytical treatment that illustrates characteristic limiting cases in the L shell range where the time scale of Coulomb losses is substantially shorter than that of radial diffusion (L approximately 1-2) is given. It is demonstrated both analytically and numerically that the particle spectra there are shaped by the energy dependence of the radial diffusion coefficient regardless of the spectral shapes of the particle populations diffusing inward from the outer radiation zone, so that from observed spectra the energy dependence of the diffusion coefficient can be determined. To insure realistic simulations, inner zone data obtained from experiments on the DIAL, AZUR, and ESRO 2 spacecraft have been used as boundary conditions. Excellent agreement between analytic and numerical results is reported.

Westphalen, H.↗
NASA NTRS1974

The electron diffusion coefficient in Jupiter's magnetosphere

A steady-state model of Jupiter's electron radiation belt is developed. The model includes injection from the solar wind, radial diffusion, energy degradation by synchrotron radiation, and absorption at Jupiter's surface. A diffusion coefficient of the form D sub RR/R sub J squared = k times R to the m-th power is assumed, and then observed data on synchrotron radiation are used to fit the model. The free parameters determined from this fit are m = 1.95 plus or minus 0.5, k = 1.7 plus or minus 0.5 x 10 to the 9th power per sec, and the magnetic moment of injected particles equals 770 plus or minus 300 MeV/G. The value of m shows quite clearly that the diffusion is not caused by magnetic pumping by a variable solar wind or by a fluctuating convection electric field. The process might be field line exchange driven by atmospheric-ionospheric winds; our diffusion coefficient has roughly the same radial dependence but is considerably smaller in magnitude than the upper bound diffusion coefficients recently suggested for this process by Brice and McDonough (1973) and Jacques and Davis (1972).

Birmingham, T.↗
NASA NTRS1988

Effect of computed horizontal diffusion coefficients on two-dimensional N2O model distributions

The effects of horizontal diffusion coefficients K(yy) and K(yz), computed directly from the residual circulation, on the N2O distribution in a photochemical model were investigated, using a modified version of the two-dimensional model of Guthrie et al. (1984). The residual circulation was computed using the NMC's temperature data and the heating rates reported by Rosenfield et al. (1987). As compared with the effect of the residual circulation alone, the use of horizontal diffusion coefficients produced substantial changes in the N2O distribution and increased the N2O's lifetime values by a few percent. It is suggested that trace gases, such as CH4, CFCl3, CF2Cl2, CH3Cl, and CCl4, which impact the NO(x), HO(x), and Cl(x) radical distributions and therefore ozone, will be influenced in a similar manner by the addition of more realistic diffusion fields.

Jackman, Charles H.↗
NASA NTRS1972

Cosmic-ray diffusion coefficient in interplanetary space.

The authors of three recent papers reporting cosmic-ray electron differential intensities near the earth during 1966 and 1968 in the rigidity range above 500 MV have concluded that the observations are not compatible with a diffusion coefficient that can be written as a product of a rigidity-dependent part and a part that is a function of heliocentric distance. It is shown in this paper that, with an interstellar electron spectrum and a near-earth spectrum given, a diffusion coefficient of the above form can always be determinedand the conclusion noted above cannot be sustained. Diffusion coefficients appropriate to the observations are given.

Gleeson, L. J.↗
NASA NTRS1986

Limits on ion radial diffusion coefficients in Saturn's inner magnetosphere

The development of upper and lower limits for the rate of radial diffusion of energetic ions in Saturn's inner magnetosphere is discussed. Improved calculations of the satellite-sweeping rate and phase space density profiles for a wide range of ion invariants are utilized to determine the limits. The lower limit for the radial diffusion coefficient is established by requiring the rate of inward diffusion to be large enough to balance satellite sweeping losses; the upper limit is obtained by requiring the rate of inward diffusion to be less than the observable ultraviolet aurora on plasma torus L shell. It is concluded that the radial diffusion coefficient for ions in Saturn's inner magnetosphere is calculated to about two orders of magnitude.

Paonessa, M.↗
NASA NTRS1976

The absorption of trapped particles by the inner satellites of Jupiter and the radial diffusion coefficient of particle transport

The process of trapped particle absorption by the inner Jovian satellites is considered in detail taking into account both the particle and satellite motions in a magnetic dipole field which is displaced from the center of the planet and tilted with respect to the planetary rotation axis. An expression is derived for computing the sweeping time at a given satellite, defined as the time required for the satellite to sweep up a given fraction of the trapped particles within its sweeping region. By making use of the sweeping time and the radial diffusion equation of particle transport approximate expressions for the diffusion coefficient are derived. Measurements obtained by Pioneer 10 are then used to obtain estimates of the diffusion coefficient at the orbits of Io and Europa. We find that the diffusion coefficient is a function of energy and magnetic latitude for electrons in the energy range 0.7-14 MeV.

Mogro-Campero, A.↗
NASA NTRS1977

Consequences of using nonlinear particle trajectories to compute spatial diffusion coefficients

In a study of cosmic ray propagation in interstellar and interplanetary space, a perturbed orbit resonant scattering theory for pitch angle diffusion in a slab model of magnetostatic turbulence is slightly generalized and used to compute the diffusion coefficient for spatial propagation parallel to the mean magnetic field. This diffusion coefficient has been useful for describing the solar modulation of the galactic cosmic rays, and for explaining the diffusive phase in solar flares in which the initial anisotropy of the particle distribution decays to isotropy.

Goldstein, M. L.↗
NASA NTRS1981

Eddy diffusion coefficient for the atmosphere of Venus from radio scintillation measurements

Estimates are obtained of the vertical mass eddy diffusion coefficient of the Venus atmosphere in the region of turbulence near 60 km on the basis of radio scintillations observed during radio occultation by the atmosphere. The structure constant estimated from Pioneer Venus orbit 18 entrance radio occultation measurements is used, under the assumption that the turbulence is generated by wind-shear, to derive a value of 40,000 sq cm/sec for the vertical mass eddy diffusion coefficient, together with an energy dissipation rate of 20 sq cm/sec and a temperature fluctuation dissipation rate of 0.001 K-squared/sec. Results are noted to fall within the range measured for the earth's troposphere, however, indicate that small-scale turbulence is probably the dominant mechanism for vertical transport near the tropopause in the Venus atmosphere.

Woo, R.↗
NASA NTRS1987

Determination of diffusion coefficients in polypyrrole thin films using a current pulse relaxation method

The current pulse E sub oc relaxation method and its application to the determination of diffusion coefficients in electrochemically synthesized polypyrrole thin films is described. Diffusion coefficients for such films in Et4NBF4 and MeCN are determined for a series of submicron film thicknesses. Measurement of the double-layer capacitance, C sub dl, and the resistance, R sub u, of polypyrrole thin films as a function of potential obtained with the galvanostatic pulse method is reported. Measurements of the electrolyte concentration in reduced polypyrrole films are also presented to aid in the interpretation of the data.

Penner, Reginald M.↗
NASA NTRS1977

On determining magnetospheric diffusion coefficients from the observed effects of Jupiter's satellite Io

A method is derived for determining the radial diffusion coefficient from observed satellite effects of the inner Jovian satellites on the energetic particle fluxes. The method is based on data from L values which are significantly removed from the actual sweeping region. With regard to the large losses to the protons at Io's L shell, it is suggested that in addition to satellite sweepup, the losses may be associated with an enhanced precipitation due to resonant interaction with ion cyclotron waves near Io's orbit. It is noted that such additional loss mechanisms may also apply to electrons, and that such losses may significantly affect the estimated diffusion coefficient.

Thomsen, M. F.↗
NASA NTRS1973

Correlation and prediction of gaseous diffusion coefficients.

A new correlation method for binary gaseous diffusion coefficients from very low temperatures to 10,000 K is proposed based on an extended principle of corresponding states, and having greater range and accuracy than previous correlations. There are two correlation parameters that are related to other physical quantities and that are predictable in the absence of diffusion measurements. Quantum effects and composition dependence are included, but high-pressure effects are not. The results are directly applicable to multicomponent mixtures.

Marrero, T. R.↗
NASA NTRS1980

Iterative solutions for one-dimensional diffusion with time varying surface composition and composition-dependent diffusion coefficient

Solutions are given for one-dimensional diffusion problems with a time varying surface composition and also a composition dependent diffusion coefficient. The most general solution does not require special mathematical functions to fit the variation in surface composition or D(C). In another solution, a series expansion may be used to fit the time dependent surface concentration. These solutions make use of iterative calculations that converge rapidly and are highly stable. Computer times are much shorter than that required for finite difference calculations and can efficiently make use of interactive graphics terminals. Existing gas carburization data were used to provide an illustration of an iterative approach with a time varying carbon composition at the free surface.

Chow, M.↗