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At least 1,783 records · Page 99
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NASA NTRS1992

Estimates of tropical analysis differences in daily values produced by two operational centers

To assess the uncertainty of daily synoptic analyses for the atmospheric state, the intercomparison of three First GARP Global Experiment level IIIb datasets is performed. Daily values of divergence, vorticity, temperature, static stability, vertical motion, mixing ratio, and diagnosed diabatic heating rate are compared for the period of 26 January-11 February 1979. The spatial variance and mean, temporal mean and variance, 2D wavenumber power spectrum, anomaly correlation, and normalized square difference are employed for comparison.

Kasahara, Akira↗
NASA NTRS1992

Vegetation canopy PAR absorptance and the normalized difference vegetation index - An assessment using the SAIL model

Relationships are studied between the normalized-difference vegetation index (NDVI) and absorbed photosynthetically active radiation (APAR) in a vegetation canopy. The SAIL model of bidirectional canopy radiative transfer is employed to compare NDVI measurements that are instantaneous with diurnally integrated canopy APAR capacity. The NDVI measurements - taken at solar-zenith angles of more than 60 deg and sensor views of less than 40 deg from nadir - give stable near-linear estimates of diurnal APAR capacity. Discrepancies in the relations between APAR and NDVI are associated with variations in the optical properties of the canopy and with background spectral reflectance. The results are significant for the practical use of these remote sensing techniques but suggest that instantaneous observations can be used to characterize the diurnally integrated APAR in vegetation canopies.

Goward, Samuel N.↗
NASA NTRS1991

Resolution of differences between collision number definitions in particle and continuum simulations

The use of the same symbol, Z, representing a 'collision number' for thermal relaxation, has led to confusion regarding its definition in the context of both continuum and particle simulations. Examination of the relaxation mechanics employed in particle simulations demonstrates that these definitions differ by a numerical factor that depends upon the intermolecular potential. Particle and continuum simulations employing appropriate definitions of Z lead to identical results during isothermal and adiabatic stationary relaxation.

Lumpkin, Forrest E., III↗
NASA NTRS1992

Calculation of compressible boundary layer flow about airfoils by a finite element/finite difference method

Preliminary results are presented of a finite element/finite difference method (semidiscrete Galerkin method) used to calculate compressible boundary layer flow about airfoils, in which the group finite element scheme is applied to the Dorodnitsyn formulation of the boundary layer equations. The semidiscrete Galerkin (SDG) method promises to be fast, accurate and computationally efficient. The SDG method can also be applied to any smoothly connected airfoil shape without modification and possesses the potential capability of calculating boundary layer solutions beyond flow separation. Results are presented for low speed laminar flow past a circular cylinder and past a NACA 0012 airfoil at zero angle of attack at a Mach number of 0.5. Also shown are results for compressible flow past a flat plate for a Mach number range of 0 to 10 and results for incompressible turbulent flow past a flat plate. All numerical solutions assume an attached boundary layer.

Strong, Stuart L.↗
NASA NTRS1992

Gibbs free energy difference between the undercooled liquid and the beta phase of a Ti-Cr alloy

The heat of fusion and the specific heats of the solid and liquid have been experimentally determined for a Ti60Cr40 alloy. The data are used to evaluate the Gibbs free energy difference, delta-G, between the liquid and the beta phase as a function of temperature to verify a reported spontaneous vitrification (SV) of the beta phase in Ti-Cr alloys. The results show that SV of an undistorted beta phase in the Ti60Cr40 alloy at 873 K is not feasible because delta-G is positive at the temperature. However, delta-G may become negative with additional excess free energy to the beta phase in the form of defects.

Ohsaka, K.↗
NASA NTRS1992

A staggered mesh finite difference scheme for the computation of compressible flows

A simple high resolution finite difference technique is presented to approximate weak solutions to hyperbolic systems of conservation laws. The method does not rely on Riemann problem solvers and is therefore easy to extend to a wide variety of problems. The overall performance (resolution and CPU requirements) is competitive, with other state-of-the-art techniques offering sharp nonoscillatory shocks and contacts. Theoretical results confirm the reliability of the approach for linear systems and nonlinear scalar equations.

Sanders, Richard↗
NASA NTRS1991

Low-dispersion finite difference methods for acoustic waves in a pipe

A new algorithm for computing one-dimensional acoustic waves in a pipe is demonstrated by solving the acoustic equations as an initial-boundary-value problem. Conventional dissipation-free second-order finite difference methods suffer severe phase distortion for grids with less that about ten mesh points per wavelength. Using the signal generation by a piston in a duct as an example, transient acoustic computations are presented using a new compact three-point algorithm which allows about 60 percent fewer mesh points per wavelength. Both pulse and harmonic excitation are considered. Coupling of the acoustic signal with the pipe resonant modes is shown to generate a complex transient wave with rich harmonic content.

Davis, Sanford↗
NASA NTRS1992

Upscale integration of normalized difference vegetation index - The problem of spatial heterogeneity

An analysis is conducted of the correspondence between the normalized difference vegetation index (NDVI) calculated from average reflectances, or M(NDVI), and the I(NDVI) that is integrated from individual NDVIs, by simulating AVHRR data from high spatial resolution SPOT 1 radiometer and Landsat TM data. The West African and French sites analyzed at 300-1000 m scale show a strong correlation between the two types of index; the relationship is almost perfectly linear, with a slope that is somewhat dependent on vegetation cover. Effecting the scale change using M(NDVI) instead of I(NDVI) does not introduce significant errors.

Aman, Angora↗
NASA NTRS1992

On the differences between plage and quiet sun in the solar photosphere

Time sequences of interleaved observations of the continuum intensity, longitudinal magnetic field, vertical velocity in the midphotosphere, and the line-center intensity in Ni I 6768 A were obtained in an active-region plage and the surrounding relatively field-free area near disk center. Spacetime Fourier filtering techniques are used to separate the convective and oscillatory components of the solar atmosphere. The properties of the photosphere are found to differ qualitatively and quantitatively between the plage, where the field is 150 G or more, and its quiet surroundings. The scale of granulation is smaller, the contrast lower, and the temporal evolution slower in the plage than the quiet sun. In the plage, the vertical velocity is reduced in amplitude compared to the quiet sun, and there is little evidence of a granulation pattern, while in the quiet sun the vertical flow pattern is similar in size and shape to the underlying granulation pattern in the continuum.

Title, Alan M.↗
NASA NTRS1992

Finite-difference algorithms for the time-domain Maxwell's equations - A numerical approach to RCS analysis

The applications of two CFD-based finite-difference methods to computational electromagnetics are investigated. In the first method, the time-domain Maxwell's equations are solved using the explicit Lax-Wendroff scheme and in the second method, the second-order wave equations satisfying the Maxwell's equations are solved using the implicit Crank-Nicolson scheme. The governing equations are transformed to a generalized curvilinear coordinate system and solved on a body-conforming mesh using the scattered-field formulation. The induced surface current and the bistatic radar cross section are computed and the results are validated for several two-dimensional test cases involving perfectly-conducting scatterers submerged in transverse-magnetic plane waves.

Vinh, Hoang↗
NASA NTRS1992

Measured rotational and vibrational temperature differences in arc jet shock layers

Shock-layer-radiation emission spectra from the first negative system of the nitrogen molecular ion N2(+) are analyzed over the wavelength region between 370 and 430 nm. The shock layer is produced by a blunt body placed in the expanded flow from an arc heater employing either pure nitrogen or a nitrogen-oxygen air mixture. A rotational temperature is obtained from analysis of (0,1) band emission from N2(+). Vibrational temperatures are extracted from population distributions obtained through spectral fitting of the N2(+) (B-X) band sequences. Rotational temperatures up to 9500 K near the front of the shocks are determined for different energy and flow conditions. The rotational temperatures are found to be lower than those obtained for vibration confirming previous measurements and contrary to expectation based on theory.

Blackwell, Harvel E.↗
NASA NTRS1992

A fully nonlinear, mixed spectral and finite difference model for thermally driven, rotating flows

Finite difference in time and the meridional plane, in conjunction with a spectral technique in the azimuthal direction, are used to approximate the Navier-Stokes equations in a model that can simulate a variety of thermally driven rotating flows in cylindrical and spherical geometries. Axisymmetric flow, linearized waves relative to a fixed or changing axisymmetric flow, nonlinear waves without wave-wave interaction, and fully nonlinear 3D flow, can in this way be calculated. A reexamination is conducted of the steady baroclinic wave case previously treated by Williams (1971) and Quon (1976).

Miller, Timothy L.↗
NASA NTRS1992

Runge-Kutta methods combined with compact difference schemes for the unsteady Euler equations

An investigation of the Runge-Kutta time-stepping, combined with compact difference schemes to solve the unsteady Euler equations, is presented. Initially, a generalized form of a N-step Runge-Kutta technique is derived. By comparing this generalized form with its Taylor's series counterpart, the criteria for the three-step and four-step schemes to be of third- and fourth-order accurate are obtained.

Yu, S. T.↗
NASA NTRS1992

One-dimensional transient finite difference model of an operational salinity gradient solar pond

This paper describes the modeling approach used to simulate the transient behavior of a salinity gradient solar pond. A system of finite difference equations are used to generate the time dependent temperature and salinity profiles within the pond. The stability of the pond, as determined by the capacity of the resulting salinity profile to suppress thermal convection within the primary gradient region of the pond, is continually monitored and when necessary adjustments are made to the thickness of the gradient zone. Results of the model are then compared to measurements taken during two representative seasonal periods at the University of Texas at El Paso's (UTEP's) research solar pond.

Hicks, Michael C.↗
NASA NTRS1991

The 19 mm date recorders: Similarities and differences

Confusion over the use of non-video 19 mm data recorders is becoming more pronounced in the world of high performance computing. The following issues are addressed: (1) the difference between ID-1, ID-2, MIL-STD-2179, and DD-2; (2) the proper machine for the necessary application; and (3) integrating the machine into an existing environment. Also, an attempt is made to clear up any misconceptions there might be about 19 mm tape recorders.

Atkinson, Steve↗
NASA NTRS1992

Finite-Difference Solution for Laminar or Turbulent Boundary Layer Flow over Axisymmetric Bodies with Ideal Gas, CF4, or Equilibrium Air Chemistry

A computer code was developed that uses an implicit finite-difference technique to solve nonsimilar, axisymmetric boundary layer equations for both laminar and turbulent flow. The code can treat ideal gases, air in chemical equilibrium, and carbon tetrafluoride (CF4), which is a useful gas for hypersonic blunt-body simulations. This is the only known boundary layer code that can treat CF4. Comparisons with experimental data have demonstrated that accurate solutions are obtained. The method should prove useful as an analysis tool for comparing calculations with wind tunnel experiments and for making calculations about flight vehicles where equilibrium air chemistry assumptions are valid.

Hamilton, H. Harris, II↗
NASA NTRS1992

Finite difference time domain modeling of spiral antennas

The objectives outlined in the original proposal for this project were to create a well-documented computer analysis model based on the finite-difference, time-domain (FDTD) method that would be capable of computing antenna impedance, far-zone radiation patterns, and radar cross-section (RCS). The ability to model a variety of penetrable materials in addition to conductors is also desired. The spiral antennas under study by this project meet these requirements since they are constructed of slots cut into conducting surfaces which are backed by dielectric materials.

Penney, Christopher W.↗