Search NASA⌕ Search

SEARCH · Search NASA

Results for “Numerical diffusion”

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 271 records · Page 15
All sourcesNASA NTRSDOE OSTI
NASA NTRS1979

Interfacial geometry and D-variation effects in two-phase systems

Numerical solutions of the governing diffusion equation for two-phase concentration dependent diffusion coefficients are examined. Solutions were also calculated for planar, cylindrical, and spherical geometries to compare the effect of interface geometries with those caused by concentration-dependent diffusion coefficients, and two methods of averaging D were considered to determine the best averaging method for different types of D-variations. The effects of interface-location criteria on mass conservation and convergence of interface location, diffusion coefficient variation in the alpha and beta-phases of a two-phase binary alloy system, effect of D(alpha) variation in a cylindrical couple on beta-phase thickness, and geometry and D-variation effects on the degree of homogenization were determined. It is concluded that typical D(alpha)-variations can have a greater influence on the kinetics of interdiffusion than the geometry.

Tenney, D. R.↗
NASA NTRS1981

Visible light scatter measurements of the Advanced X-ray Astronomical Facility /AXAF/ mirror samples

NASA is studying the properties of mirror surfaces for X-ray telescopes, the data of which will be used to develop the telescope system for the Advanced X-ray Astronomical Facility. Visible light scatter measurements, using a computer controlled scanner, are made of various mirror samples to determine surface roughness. Total diffuse scatter is calculated using numerical integration techniques and used to estimate the rms surface roughness. The data measurements are then compared with X-ray scatter measurements of the same samples. A summary of the data generated is presented, along with graphs showing changes in scatter on samples before and after cleaning. Results show that very smooth surfaces can be polished on the common substrate materials (from 2 to 10 Angstroms), and nickel appears to give the lowest visible light scatter.

Griner, D. B.↗
NASA NTRS1985

The expected cosmic ray density and stream distributions at the heliolatitudinal asymmetry of solar wind

The results of the spatial distribution of cosmic ray density, gradients, and anisotropy obtained on the basis of the numerical solution of the anisotropic diffusion equation with an account of solar wind velocity change depending on the latitudinal angle theta of the form U=u sub oe sup alpha theta and the diffusion coefficient depending on the spatial coordinates and the particle rigidity are presented. It is shown that the increase of the solar wind velocity and the diffusion coefficient with heliolatitude leads to gradient distributions that are in accord with experimental data observed in space. The results of the energetic spectrum of 11 and 22-year cosmic ray variations obtained with an account of direction of the general magnetic field of the Sun are presented are given.

Alania, M. V.↗
NASA NTRS1985

The effect of the neutral sheet structure of the interplanetary magnetic field on cosmic ray distribution in space

Results of the numerical solution of the anistoropic diffusion equation are presented. The modulation depth of galactic cosmic rays is defined by the degree of curvature of the neutral current sheet in the heliosphere. The effect of the regular interplanetary magnetic field (IMF) on cosmic ray anisotropy in the period of solar activity minimum (in 1976) is analyzed by the data of the neutron super-monitors of the world network, and the heliolatitudinal gradient and cosmic ray diffusion coefficient are defined.

Alania, M. V.↗
NASA NTRS1984

Effect of growth parameters on compositional variations in directionally solidified HgCdTe alloys

A series of Hg(1-x)Cd(x)Te alloy crystals was grown by directional solidification with compositions ranging from x = 0.2 to x = 0.4. The measured axial compositional profiles were interpreted in terms of an exact numerical solution of the appropriate diffusion equation that takes into account both the variations of the segregation coefficient and solidification rate with composition. The solutions for all growth rates agree generally well with the experimental data, and confirm earlier observations that employed approximate analytical solutions. The effective mass diffusion coefficients showed no significant composition or melt temperature dependence. Slightly higher diffusion coefficients were obtained, however, for the highest growth rates.

Szofran, F. R.↗
NASA NTRS1990

Dynamical effects of annihilation in pair-dominated winds

The steady, spherically symmetric flow of an ideal fluid dominated by photons and ultrarelativistic electron-positron pairs is analyzed. A new wind equation and a set of critical point conditions are obtained which describe the relativistic flow of an annihilation gas in which the flow velocity exceeds the diffusion velocity of the photons. Numerical results are reported which suggest the possible existence of trapped, pure-pair winds driven by the combined pressure of the pairs and the photons. Most of the annihilation occurs below the critical radius in trapped flows, and a substantial fraction of the total energy of the injected pairs is converted into kinetic energy and radiation. Accurate numerical solutions for the flow velocity and the positron loss rate in optically thin, Newtonian winds are obtained, and a useful approximate analytic relation between the positron loss rate and the flow velocity is derived which suggests that a large number of pairs may survive the annihilation region, ultimately escaping the potential well.

Becker, Peter A.↗
NASA NTRS1990

Effects of radiation of InP cells epitaxially grown on Si and GaAs substrates

The properties of heteroepitaxial InP cells were determined both before and after 10-MeV proton irradiations. Numerical values, obtained for the diffusion and recombination components of the reverse saturation currents, were found to be consistent with the distribution of dislocations. The radiation resistance of the heteroepitaxial cells was significantly greater than that observed for n/p homoepitaxial InP cells. The carrier removal rate, obtained by C-V measurements, was 1800/cm for 10-MeV protons compared with 2.2/cm for 1-MeV electrons. The high carrier removal rate was found to have no significant effect on the cell's series resistance. It was concluded that the heteroepitaxial cell performance is dominated by the high dislocation density attributable to lattice constant mismatch. Although the efficiencies of the present cells are low, the recent achievement of 13.7 percent AM0 efficiencies using a GaAs substrate demonstrates the marked improvement that can be attained using more appropriate transition layers.

Weinberg, I.↗
NASA NTRS1992

Upper bounds for convergence rates of vector extrapolation methods on linear systems with initial iterations

The application of the minimal polynomial extrapolation (MPE) and the reduced rank extrapolation (RRE) to a vector sequence obtained by the linear iterative technique x(sub j) + 1 = Ax(sub j) = b,j = 1,2,..., is considered. Both methods produce a two dimensional array of approximations s(sub n,k) to the solution of the system (I - A)x = b. Here, s(sub n,k) is obtained from the vectors x(sub j), n is less than or equal to j is less than or equal to n + k + 1. It was observed in an earlier publication by the first author that the sequence s(sub n,k), k = 1,2,..., for n greater than 0, but fixed, possesses better convergence properties than the sequence s(sub 0,k), k = 1,2,.... A detailed theoretical explanation for this phenomenon is provided in the present work. This explanation is heavily based on approximations by incomplete polynomials. It is demonstrated by numerical examples when the matrix A is sparse that cycling with s(sub n,k) for n greater than 0, but fixed, produces better convergence rates and costs less computationally than cycling with s(sub 0,k). It is also illustrated numerically with a convection-diffusion problem that the former may produce excellent results where the latter may fail completely. As has been shown in an earlier publication, the results produced by s(sub 0,k) are identical to the corresponding results obtained by applying the Arnoldi method or generalized minimal residual scheme (GMRES) to the system (I - A)x = b.

Sidi, Avram↗
NASA NTRS1991

Characterization of directionally solidified Hg(1-x)Zn(x)Se semiconducting alloys

Hg(1-x)Zn(x)Se alloys with composition between x = 0.08 and 0.115 were synthesized from elemental constituents and were resolidified using a Bridgman-Stockbarger growth technique. By performing precision mass-density measurements on selected wafers cut perpendicular to the growth axis, it was shown that the axial compositional profiles fit a numerical solution to the 1D diffusion equation which takes into account the variation of interface velocity with time. Infrared transmission-edge measurements performed on selected transverse slices from each ingot showed that the relative radial variations in composition decreased with decreasing growth rate. Van der Pauw measurements on selected wafers showed that the 10 exp 18/cu cm electron concentration, typical of as-grown crystals, could be reduced by approximately an order of magnitude by annealing in Se vapor.

Cobb, S. D.↗
NASA NTRS1994

On the dynamics of some grid adaption schemes

The dynamics of a one-parameter family of mesh equidistribution schemes coupled with finite difference discretisations of linear and nonlinear convection-diffusion model equations is studied numerically. It is shown that, when time marched to steady state, the grid adaption not only influences the stability and convergence rate of the overall scheme, but can also introduce spurious dynamics to the numerical solution procedure.

Sweby, Peter K.↗
NASA NTRS2002

Numerical Simulations of Vortex Generator Vanes and Jets on a Flat Plate

Numerical simulations of a single low-profile vortex generator vane, which is only a small fraction of the boundary-layer thickness, and a vortex generating jet have been performed for flows over a flat plate. The numerical simulations were computed by solving the steady-state solution to the Reynolds-averaged Navier-Stokes equations. The vortex generating vane results were evaluated by comparing the strength and trajectory of the streamwise vortex to experimental particle image velocimetry measurements. From the numerical simulations of the vane case, it was observed that the Shear-Stress Transport (SST) turbulence model resulted in a better prediction of the streamwise peak vorticity and trajectory when compared to the Spalart-Allmaras (SA) turbulence model. It is shown in this investigation that the estimation of the turbulent eddy viscosity near the vortex core, for both the vane and jet simulations, was higher for the SA model when compared to the SST model. Even though the numerical simulations of the vortex generating vane were able to predict the trajectory of the stream-wise vortex, the initial magnitude and decay of the peak streamwise vorticity were significantly under predicted. A comparison of the positive circulation associated with the streamwise vortex showed that while the numerical simulations produced a more diffused vortex, the vortex strength compared very well to the experimental observations. A grid resolution study for the vortex generating vane was also performed showing that the diffusion of the vortex was not a result of insufficient grid resolution. Comparisons were also made between a fully modeled trapezoidal vane with finite thickness to a simply modeled rectangular thin vane. The comparisons showed that the simply modeled rectangular vane produced a streamwise vortex which had a strength and trajectory very similar to the fully modeled trapezoidal vane.

Allan, Brian G.↗
NASA NTRS2003

Protein crystals and their growth

Recent results on the associations between protein molecules in crystal lattices, crystal-solution surface energy, elastic properties, strength, and spontaneous crystal cracking are reviewed and discussed. In addition, some basic approaches to understanding the solubility of proteins are followed by an overview of crystal nucleation and growth. It is argued that variability of mixing in batch crystallization may be a source of the variation in the number of crystals ultimately appearing in the sample. The frequency at which new molecules join a crystal lattice is measured by the kinetic coefficient and is related to the observed crystal growth rate. Numerical criteria used to discriminate diffusion- and kinetic-limited growth are discussed on this basis. Finally, the creation of defects is discussed with an emphasis on the role of impurities and convection on macromolecular crystal perfection.

Review↗
NASA NTRS2004

Experimental and Computational Study of Trapped Vortex Combustor Sector Rig With Tri-Pass Diffuser

The Trapped Vortex Combustor (TVC) potentially offers numerous operational advantages over current production gas turbine engine combustors. These include lower weight, lower pollutant emissions, effective flame stabilization, high combustion efficiency, excellent high altitude relight capability, and operation in the lean burn or RQL modes of combustion. The present work describes the operational principles of the TVC, and extends diffuser velocities toward choked flow and provides system performance data. Performance data include EINOx results for various fuel-air ratios and combustor residence times, combustion efficiency as a function of combustor residence time, and combustor lean blow-out (LBO) performance. Computational fluid dynamics (CFD) simulations using liquid spray droplet evaporation and combustion modeling are performed and related to flow structures observed in photographs of the combustor. The CFD results are used to understand the aerodynamics and combustion features under different fueling conditions. Performance data acquired to date are favorable compared to conventional gas turbine combustors. Further testing over a wider range of fuel-air ratios, fuel flow splits, and pressure ratios is in progress to explore the TVC performance. In addition, alternate configurations for the upstream pressure feed, including bi-pass diffusion schemes, as well as variations on the fuel injection patterns, are currently in test and evaluation phases.

Hendricks, R. C.↗
NASA NTRS2001

Experimental and Computational Study of Trapped Vortex Combustor Sector Rig with Tri-pass Diffuser

The Trapped Vortex Combustor (TVC) potentially offers numerous operational advantages over current production gas turbine engine combustors. These include lower weight, lower pollutant emissions, effective flame stabilization, high combustion efficiency, excellent high altitude relight capability, and operation in the lean burn or RQL (Rich burn/Quick mix/Lean burn) modes of combustion. The present work describes the operational principles of the TVC, and provides detailed performance data on a configuration featuring a tri-pass diffusion system. Performance data include EINOx (NO(sub x) emission index) results for various fuel-air ratios and combustor residence times, combustion efficiency as a function of combustor residence time, and combustor lean blow-out (LBO) performance. Computational fluid dynamics (CFD) simulations using liquid spray droplet evaporation and combustion modeling are performed and related to flow structures observed in photographs of the combustor. The CFD results are used to understand the aerodynamics and combustion features under different fueling conditions. Performance data acquired to date are favorable in comparison to conventional gas turbine combustors. Further testing over a wider range of fuel-air ratios, fuel flow splits, and pressure ratios is in progress to explore the TVC performance. In addition, alternate configurations for the upstream pressure feed, including bi-pass diffusion schemes, as well as variations on the fuel injection patterns, are currently in test and evaluation phases.

Hendricks, Robert C.↗
NASA NTRS1976

A two-dimensional model for the transport of pollutants in an urban basin

The distribution of an inert chemical species, carbon monoxide (CO), emitted mainly from automobile exhaust is investigated. The transport model, in analogy with the shallow water theory in fluid dynamics, considers variation of all physical quantities in the horizontal direction below the temperature inversion layer. Pollutants are found to be carried primarily by the wind; turbulent diffusion in a normal day plays only a minor role. The concentration of CO predicted by the present model for the entire Los Angeles basin is compared with observed data at nine stations. Accuracy of four different numerical schemes, the effect of turbulent diffusivity, and the source strengths are examined.

Liu, C. Y.↗
NASA NTRS1977

A two-dimensional model for the transport of pollutants in an urban basin

The distribution of carbon monoxide emitted mainly from an automobile exhaust is investigated. Carbon monoxide is assumed to be chemically inert. The transport model, in analogy with the shallow-water theory in fluid dynamics, considers variation of all physical quantities in the horizontal direction below the temperature inversion layer. Pollutants are found to be carried primarily by the wind; turbulent diffusion in a normal day plays only a minor role. The concentration of CO predicted by the present model for the entire Los Angeles basin is compared with observed data at nine stations. Accuracy of four different numerical schemes, the effect of turbulent diffusivity, and the source strengths are examined.

Liu, C. Y.↗