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At least 631 records · Page 35
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NASA NTRS1983

Legendre-Tau approximations for functional differential equations

The numerical approximation of solutions to linear functional differential equations are considered using the so called Legendre tau method. The functional differential equation is first reformulated as a partial differential equation with a nonlocal boundary condition involving time differentiation. The approximate solution is then represented as a truncated Legendre series with time varying coefficients which satisfy a certain system of ordinary differential equations. The method is very easy to code and yields very accurate approximations. Convergence is established, various numerical examples are presented, and comparison between the latter and cubic spline approximations is made.

Ito, K.↗
NASA NTRS1984

Boundary-layer model of pattern formation in solidification

A model of pattern formation in crystal growth is proposed, and its analytic properties are investigated. The principal dynamical variables in this model are the curvature of the solidification front and the thickness (or heat content) of a thermal boundary layer, both taken to be functions of position along the interface. This model is mathematically much more tractable than the realistic, fully nonlocal version of the free-boundary problem, and still recaptures many of the features that seem essential for studying dendritic behavior, for example. Preliminary numerical solutions produce snowflakelike patterns similar to those seen in nature.

Ben-Jacob, E.↗
NASA NTRS1985

Ultrasonic inspection of carbon fiber reinforced plastic by means of sample-recognition methods

In the case of carbon fiber reinforced plastic (CFRP), it has not yet been possible to detect nonlocal defects and material degradation related to aging with the aid of nondestructive inspection method. An approach for overcoming difficulties regarding such an inspection involves an extension of the ultrasonic inspection procedure on the basis of a use of signal processing and sample recognition methods. The basic concept involved in this approach is related to the realization that the ultrasonic signal contains information regarding the medium which is not utilized in conventional ultrasonic inspection. However, the analytical study of the phyiscal processes involved is very complex. For this reason, an empirical approach is employed to make use of the information which has not been utilized before. This approach uses reference signals which can be obtained with material specimens of different quality. The implementation of these concepts for the supersonic inspection of CFRP laminates is discussed.

Bilgram, R.↗
NASA NTRS1984

The influence of ocean surface temperature gradient and continentality on the Walker circulation. II - Prescribed global changes

The series of experiments presently used to investigate the mechanisms responsible for forcing the global Walker circulation features worldwide changes in ocean surface temperatures (OSTs), topography, and/or continents. The primary factor affecting circulation is noted to be the global distribution of continents and oceans; while OST gradients are also important, topography emerges as comparatively unimportant. Continentality and OST gradients force the model atmosphere through the introduction of zonal variations in surface heating. The vertical motions to which they give rise yield moisture convergence and condensation variations which reinforce vertical motions. The forcing by OST gradients is partly nonlocal, and the atmospheric response is effected by continentality. In all cases, vertical motion zonal variations correlate with precipitation.

Stone, P. H.↗
NASA NTRS1985

Electrostatic wave observation during a space simulation beam-plasma discharge

ELF waves which were observed during beam-plasma discharge in the large vacuum chamber at Johnson Space Center are studied. Phase delays as a function of radius (obtained from cross-correlation measurements of density fluctuations) along with measurements of frequency and plasma potential, density, and temperature have been compared to a zero-order slab model of nonlocal azimuthal drift wave propagation. The inferred wave phase velocity in the plasma frame after Doppler correction is found to be near one half the electron diamagnetic drift velocity. Although the measurements presented do not uniquely define a propagation mode, a model of azimuthal drift wave propagation is found to be consistent with observations.

Walker, D. N.↗
NASA NTRS1984

The evolution of the inner regions of viscous accretion disks surrounding neutron stars

The structure and evolution of the inner regions of an accretion disk around a neutron star have been investigated with respect to variations in the mass flow rate. In order to examine the sensitivity of the general response of the disk to the form of viscous dissipation, several theoretical constraints on disk evolution are proposed on the basis of current accretion disk theory. It is shown through a series of calculations that under certain circumstances nonlocal radial energy transfer can stabilize regions of the disk which appear to be unstable by local analysis. When viscous stress was scaled with the total pressure of the disk, global analysis was found to be consistent with local analysis, and instability was manifested in luminosity fluctuations and in bursts of less than 10 s. Analysis of the bursts in their limit cycles showed that the disk remained optically thick and geometrically thin throughout its evolution. Substantial agreement was found between the theoretical results and the available observational data.

Taam, R. E.↗
NASA NTRS1985

Electrostatic ion-cyclotron instability caused by a nonuniform electric field perpendicular to the external magnetic field

A new mechanism that can destablize kinetic ion-cyclotron waves in the presence of a nonuniform electric field perpendicular to the uniform ambient magnetic field is given. In the absence of the electric field, the mode energy is positive, while in the presence of a uniform electric field the mode energy could be negative. However, when the electric field is nonuniform, it is possible for a finite region to be of negative wave energy surrounded by regions of positive wave energy. A nonlocal wave packet couples the two regions so that a flow of energy from the region of negative wave energy to the region of positive wave energy will cause the mode to grow. This gives rise to the instability.

Ganguli, G.↗
NASA NTRS1985

Pattern selection in a boundary-layer model of dendritic growth in the presence of impurities

Presently analyzed, in the context of a boundary-layer model, is the problem of pattern selection in dendritic growth in a situation where impurities are present in the undercooled liquid. It is found that the tip-velocity selection criterion that has been proposed recently for the geometrical model and the boundary-layer model of a pure substance can be extended, in a nontrivial way, to this more complex situation where two coupled diffusion fields (temperature and solute) determine the interface dynamics. This model predicts a sharp enhancement of tip velocity in good qualitative agreement with experiment. This agreement is consistent with the conjecture that a solvability condition can be used to determine the operating point of the dendrite in the full nonlocal problem.

Karma, A.↗
NASA NTRS1985

Laboratory experiments on reconnection in current sheets

Insights obtained from laboratory experiments concerning the reconnection of current sheets are discussed. Consideration is given to: the nonlocal characteristics of reconnection; the build-up of pinch current sheets; and impulsive flux transfer events. The application of the experimental results to present models of current sheet reconnection during solar flares is also discussed.

Bratenahl, A.↗
NASA NTRS1986

Theoretical Study of the Transverse Dielectric Constant of Superlattices and Their Alloys

The optical properties of III to V binary and ternary compounds and GaAs-Al(x)Ga(1-x)As superlattices are determined by calculating the real and imaginary parts of the transverse dielectric constant. Emphasis is given to determining the influence of different material and superlattice parameters on the values of the index of refraction and absorption coefficient. In order to calculate the optical properties of a material, it is necessary to compute its electronic band structure. This was accomplished by introducing a partition band structure approach based on a combination of the vector k x vector p and nonlocal pseudopotential techniques. The advantages of this approach are that it is accurate, computationally fast, analytical, and flexible. These last two properties enable incorporation of additional effects into the model, such as disorder scattering, which occurs for alloy materials and excitons. Furthermore, the model is easily extended to more complex structures, for example multiple quantum wells and superlattices. The results for the transverse dielectric constant and absorption coefficient of bulk III to V compounds compare well with other one-electron band structure models and the calculations show that for small frequencies, the index of refraction is determined mainly by the contibution of the outer regions of the Brillouin zone.

Kahen, K. B.↗
NASA NTRS1986

Low-frequency electromagnetic waves driven by gyrotropic gyrating ion beams

The origin of left- and right-hand-polarized low-frequency waves in space plasmas is analyzed. It has been shown that a gyrotropic gyrating ion beam, a ring in velocity space, can excite electromagnetic modes in the plasma near the beam gyrofrequency. It excites left-hand-polarized shear Alfven waves and their harmonics via the coupling of Alfven modes with the beam modes. It can also excite right-hand-polarized fast-mode magnetosonic waves and their harmonics as well. The excitation is possible for beam ions heavier than the plasma ions. The growth rate varies as one-third power of the beam density and decreases with the angle of wave propagation with respect to the ambient magnetic field. The nonlocality has a stabilizing effect on the instability. The predicted values of the wave frequencies compare reasonably well with those observed in satellite data.

Sharma, O. P.↗
NASA NTRS1985

Can the master time asymmetry in nature be electrodynamic in origin?

If the electrodynamic measurement process is operationally described by imposing measurement color onto classical electrodynamics, self-energy infinities are eliminated and a retarded electrodynamic time arrow emerges dynamically from internal dynamic-stability conditions. This occurs because the new formalism contains nonlocal radiation fields which carry a negative parity under mathematical time reversal. This result is shown to be compatible with an extended form of T and CPT symmetry which preserves the dynamically chosen retarded electrodynamic arrow of time in the measurement color electrodynamic formalism.

Leiter, D.↗
NASA NTRS1986

Magnetized thermal conduction fronts

The evolution of planar thermal conduction fronts in the presence of a dynamically weak, but otherwise self-consistent, magnetic field is considered. The field is assumed to be connected and untangled. In the diffusion limit for the thermal conductivity, these fronts exhibit self-similar behavior, even in the presence of a field. The role of the field is restricted to channeling the heat flux along its lines of force, and it enters into the problem as a dimensionless angle variable. 'Combing' (or opening) of insulating field lines by the evaporative flow is explicitly demonstrated. Unless the field is nearly perpendicular to the front normal in the hot gas, insulating effects are not profound. Self-similarity breaks down if the front becomes saturated, and under certain conditions magnetized saturated conduction fronts cannot propagate: the solution characteristics of the wave equation form caustics. The physical resolution is the advent of two-fluid (nonlocal) heating. Such Coulomb-heated fronts are expected to be relatively rare in typical astrophysical systems. The large-scale effects of a magnetic field on cloud evaporation in the interstellar medium are briefly discussed, and it is suggested that these fields preclude the presence of time-independent evaporative solutions. Thermal interfaces may then continue to evolve until radiative cooling halts their development; large tracts of warm 10,000 K gas may result.

Balbus, S. A.↗
NASA NTRS1986

The generation of kilometer scale irregularities in equatorial spread F

Two rockets launched into spread F during March 1983 show that the intermediate-scale spectrum of density irregularities sometimes exhibits a knee near 1 km scale length. This result confirms previous reports which are reviewed. Two different nonlinear theories of spread F turbulence are studied here and shown to be consistent with the experimental spectra when the knee is observed. Also, the existence or nonexistence of the knee may be effected by two linear mechanisms: the injection of kilometer scale turbulence at the steepened walls of spread F bubbles and the nonlocal role of the compressible E region in the development of spread F. This spectral feature, which seems to be associated with the most turbulent state of equatorial spread F, has not yet been identified in computer simulations.

Labelle, J.↗
NASA NTRS1986

Legendre-tau approximations for functional differential equations

The numerical approximation of solutions to linear retarded functional differential equations are considered using the so-called Legendre-tau method. The functional differential equation is first reformulated as a partial differential equation with a nonlocal boundary condition involving time-differentiation. The approximate solution is then represented as a truncated Legendre series with time-varying coefficients which satisfy a certain system of ordinary differential equations. The method is very easy to code and yields very accurate approximations. Convergence is established, various numerical examples are presented, and comparison between the latter and cubic spline approximation is made.

Ito, K.↗
NASA NTRS1985

General theory of the transverse dielectric constant of III-V semiconducting compounds

A general model of the transverse dielectric constant of III-V compounds is developed using a hybrid method which combines the kp method with a nonlocal pseudopotential calculation. In this method the Brillouin zone is partitioned into three regions by expanding the energy bands and matrix elements about the F, X, and L symmetry points. The real and imaginary parts of the dielectric constant are calculated as a sum of the individual contributions of each region. By using this partition method, it is possible to get good insight into the dependence of the dielectric constant on the shape of the band structure.

Kahen, K. B.↗
NASA NTRS1986

Future directions for H sub x O sub y detection, executive summary

New methods for the measurement of OH radicals were assessed as were currently available and possible future methods for the other H sub x O sub y species, HO2 and H2O2. The workshop participants were invited from different groups: modelers of atmospheric photochemistry, experimentalists measuring H sub x O y species with laser and nonlaser methods, and chemists and physicists familiar with such experiments but not involved in atmospheric monitoring. There were three major conclusions from the workshop concerning the OH radical. First, it was felt that local measurements made by laser techniques would be ready within 2 or 3 years to furnish reliable measurements at the level of 1,000,000 cu. cm. Second, measurements at this level of sensitivity and with attainable levels of precision could indeed be used to make useful and interesting tests of the fast photochemistry of the troposphere. It is important, however, that the measurements be carefully designed, with respect to spatial and temporal averaging, if there is to be a meaningful comparison between results from two experimental methods or a measurement and a model. Third, nonlocal measurements using released reactants and tracers would also be very useful. These could be made on a regional or global basis, although they still require experimental design including choice of compounds.

Source record↗
NASA NTRS1986

Temperature-plant pigment-optical relations in a recurrent offshore mesoscale eddy near Point Conception, California

The temperature-plant pigment-optical structure of a mesoscale anticyclonic eddy consistently found in shipboard surveys and satellite-sensed data several hundred kilometers southwest of Point Conception, CA, is described on three different time scales (100-day mesoscale, annual, and several-year). The satellite coastal zone color scanner (CZCS) ocean color imagery detected the near-surface chlorophyll structure of the eddy, but in situ optical and plant pigment data suggest that such imagery does not provide a good estimate of the integrated chlorophyll field of the eddy. The temperature and plant pigment boundaries of the eddy, as determined from two-dimensional gradients of advanced very high resolution radiometer (AVHRR) and CZCS imagery, do not coincide spatially. This and in situ temperature, plant pigment, and optical structure provide additional evidence that some eddy systems in the California Current are not isolated vortex systems but rather continuously entrain waters of nonlocal origin laterally into their upper layers. Within the California Current a ratio of AVHRR/CZCS data is useful for separating inshore from oceanic water masses and following their surface entrainment by offshore vortices. The historical 28-year California Cooperative Oceanic Fisheries Investigations data for the Point Conception region of the California Current and remotely sensed data over this region show that the Point Conception eddy is a recurrent feature in the offshore California Current. Moreover, the available data provide evidence that a large number of warm-core mesoscale eddies occur simultaneously in a transition zone between coastal and oceanic regimes, that these features recur at preferred locations within the transition zone, and that this family of eddies should impose a significant offshore boundary condition on the flow of the California Current.

Simpson, James J.↗