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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 325 records · Page 18
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NASA NTRS1986

Computation of nonstationary strong shock diffraction by curved surfaces

A two-dimensional, high resolution shock-capturing algorithm was used on a supercomputer to solve Eulerian gasdynamic equations in order to simulate nonstationary strong shock diffraction by a circular arc model in a shock tube. The hypersonic Mach shock wave was assumed to arrive at a high angle of incidence, and attention was given to the effect of varying values of the ratio of specific heats on the shock diffraction process. Details of the conservation equations of the numerical algorithm, written in curvilinear coordinates, are provided, and model output is illustrated with the results generated for a Mach shock encountering a 15 deg circular arc. The sample graphics include isopycnics, a shock surface density profile, and pressure and Mach number contours.

Yang, J. Y.↗
NASA NTRS1986

Rapid pickup of cometary ions due to strong magnetic turbulence

Magnetic turbulence observed with the ICE spacecraft near Giacobini-Zinner indicates that the energy density of the fluctuating field is higher than the thermal energy density of the solar wind plasma. It is shown that in the presence of such strong turbulence the newly created ions are assimilated into the solar wind very rapidly. The time scale for the assimilation process is estimated and shown to be on the order of 100 sec which is consistent with the ICE results for the H2O(+) cometary ions.

Wu, C. S.↗
NASA NTRS1986

Plasma wave turbulence in the strong coupling region at comet Giacobini-Zinner

Within 100,000 km of comet Giacobini-Zinner's nucleus, strong plasma wave turbulence was detected by the ICE electric and magnetic field wave instruments. The spatial profiles of the wave amplitudes are compared with measurements of the heavy ion fluxes of cometary origin, the plasma electron density, and the magnetic field strength. The general similarity of the wave and heavy ion profiles suggest that the waves might be generated by free energy in the pick-up ion distribution function. However, the expected parallel streaming instability of electrostatic modes generates waves with frequencies that are too low to explain the observations. The observed low frequency magnetic turbulence is plausibly explained by the lower hybrid loss-cone instability of heavy ions.

Coroniti, F. V.↗
NASA NTRS1986

One- and two-photon Compton scattering in strong magnetic fields

Calculations are made of the Compton scattering cross section in a very strong magnetic field, such as encountered in pulsars, for arbitrary photon and electron energies. The effect of the vacuum polarization in the weak-field limit is included as well as the plasma polarizability. Radiative transitions between any pair of Landau levels are included as well as two-photon scattering. The latter process, due to the presence of the cyclotron resonance, becomes comparable to nonresonant one-photon scattering when excited final states are allowed, and acts as a source of photons which is more important than bremsstrahlung at low plasma densities.

Bussard, R. W.↗
NASA NTRS1986

Compton scattering in strong magnetic fields

The relativistic cross section for Compton scattering by electrons in strong magnetic fields is derived. The results confirm and extend earlier work which has treated only transitions to the lowest or first excited Landau levels. For the teragauss field strengths expected in neutron star magnetospheres, the relative rates for excited state transitions are found to be significant, especially for incident photon energies several times the cyclotron frequency. Since these transitions must result in the rapid emission of one or more cyclotron photons as well as the Compton-scattered photon, the scattering process actually becomes a photon 'splitting' mechanism which acts to soften hard photon spectra, and also provides a specific mechanism for populating higher Landau levels in the electron distribution function. The results should be significant for models of gamma-ray bursters and pulsating X-ray sources.

Daugherty, Joseph K.↗
NASA NTRS1987

Application of the strongly coupled-mode theory to integrated optical devices

A theory for strongly coupled waveguides is discussed and applied to two- and three-waveguide couplers and optical wavelength filters. This theory makes use of an exact analytical relation governing the coupling coefficients and the overlap integrals. It removes almost all of the constraints imposed by a simpler and approximate coupled-mode theory by Marcatili (1986). It also satisfies the energy conservation and the reciprocity theorem self-consistently. Very good numerical results with the overlap integral as large as 49 percent are shown. The applications to electrooptical modulators, power dividers, power transfer devices, and optical filters are all presented with numerical results.

Chuang, Shun-Lien↗
NASA NTRS1987

The influence of strong hydromagnetic turbulence on newborn cometary ions

By means of a test particle model, the time evolution of the velocity distribution function of newborn cometary ions in the presence of strong hydromagnetic turbulence is studied. The test particle model employs a realistic spectrum of hydromagnetic turbulence corresponding to the observations of the International Cometary Explorer spacecraft at Comet Giacobini-Zinner. It is found that pitch-angle scattering processes can rapidly result in the formation of the shell distribution which has been observed near Comets Giacobini-Zinner and Halley. The model also indicates that radial diffusion in velocity space develops on a much longer time scale.

Price, C. P.↗
NASA NTRS1987

On the solution of integral equations with strongly singular kernels

Some useful formulas are developed to evaluate integrals having a singularity of the form (t-x) sup-m, m greater than or equal 1. Interpreting the integrals with strong singularities in Hadamard sense, the results are used to obtain approximate solutions of singular integral equations. A mixed boundary value problem from the theory of elasticity is considered as an example. Particularly for integral equations where the kernel contains, in addition to the dominant term (t-x) sup-m, terms which become unbounded at the end points, the present technique appears to be extremely effective to obtain rapidly converging numerical results.

Kaya, A. C.↗
NASA NTRS1987

Electric field and plasma density measurements in the strongly driven daytime equatorial electrojet. I - The unstable layer and gradient drift waves. II - Two-stream waves

The results of electric field and plasma density measurements in the strongly driven daytime equatorial electrojet over Peru, made during the March 1983 Condor electrojet experiment from Punta Lobos, Peru, are discussed together with the rocket instrumentation used for the measurements and the pertinent payload dynamics. The overall characteristics of the irregularity layer observed in situ in the electrojet are described. Special consideration is given to the waves generated by the gradient drift instability (observed between 90 and 106.5 km) and to primary and secondary two-stream waves detected by the two probes on the topside between 103 and 111 km, where the electron current was considered to be strongest.

Pfaff, R. F.↗
NASA NTRS1988

Strong north/south asymmetry in the Jovian stratosphere

Trailed spectra of standard B stars have been used as the 'flat' field light source for spatial calibration of IUE large-aperture observations of the north and south polar regions of Jupiter. Longward of 2000 A, the south polar spectrum is lower than the north polar one by a factor of 2.5; shortward of 2000 A, strong C2H2 absorption appears in both north polar and midlatitude spectra, while neither continuum nor band absorption features of C2H2 appear in the south polar spectrum. The very low reflectivity of the south polar region calls for the presence of a stratospheric absorber at pressures lower than 10 mbar.

Wagener, Richard↗
NASA NTRS1988

Observations of a gradual transition between Ps 6 activity with auroral torches and surgelike pulsations during strong geomagnetic disturbances

This paper describes a long-lasting large-amplitude pulsation event, which occurred on January 10, 1983 in the ionosphere and magnetosphere and was characterized by Steen and Rees (1983). Over the 4-h period (0200-0600 UT), the characteristics of the pulsations in the ionosphere changed from being Ps 6 auroral torches toward substorms and back to Ps 6. At GEO, the corresponding characteristics were a modulation of the high-energy particle intensity and plasma dropouts. Based on the ideas presented by Rostoker and Samson (1984), an interpretation of the event is offered, according to which the pulsations are caused by the Kelvin-Helmholtz instability during an interval of strong magnetospheric convection. On the basis of this explanation, a new interpretation of the substorm time sequence is proposed.

Steen, A.↗
NASA NTRS1988

Three-dimensional strong Langmuir turbulence and wave collapse

Results from the first fully three-dimensional simulations of driven damped strong Langmuir turbulence and wave collapse are presented. Key results are that turbulence is maintained at least in part by nucleation, the cores of most collapsing objects are pancake shaped in form, and the power spectrum falls off approximately as the product of a power law and an exponential at large wave number.

Robinson, P. A.↗
NASA NTRS1989

Strong, Low-Resistance Bonds For AMTEC Electrodes

Heat of operating cell helps form contacts. Strong, low-electrical-resistance contacts between elements of current-collection grid in alkali-metal thermoelectric converter (AMTEC) cell obtained by simple in-place thermocompression bonding. Bonds formed between porous electrode of molybdenum film, nickel or cobalt strips overlying film, and molybdenum tie wires running at right angles to strips. Bonding method also works on films of tungsten or tungsten/platinum.

Williams, Roger M.↗
NASA NTRS1989

Processing of strong flux trapping high T(subc) oxide superconductors: Center director's discretionary fund

Magnetic suspension effect was first observed in samples of YBa2Cu3O7/AgO(Y-123/AgO) composites. Magnetization measurements of these samples show a much larger hysteresis which corresponds to a large critical current density. In addition to the Y-123AgO composites, recently similar suspension effects in other RE-123/AgO, where RE stands for rare-Earth elements, were also observed. Some samples exhibit even stronger flux pinning than that of the Y-123/AgO sample. An interesting observation was that in order to form the composite which exhibits strong flux trapping effect the sintering temperature depends on the particular RE-123 compound used. The paper presents the detailed processing conditions for the formation of these RE-123/AgO composites, as well as the magnetization and critical field data.

Wu, M. K.↗
NASA NTRS1989

Acoustic waves in gases with strong pressure gradients

The effect of strong pressure gradients on the acoustic modes (standing waves) of a rectangular cavity is investigated analytically. When the cavity response is represented by a sum of modes, each mode is found to have two resonant frequencies. The lower frequency is near the Viaesaela-Brundt frequency, which characterizes the buoyant effect, and the higher frequency is above the ordinary acoustic resonance frequency. This finding shows that the propagation velocity of the acoustic waves is increased due to the pressure gradient effect.

Zorumski, William E.↗
NASA NTRS1989

Field structure of collapsing wave packets in 3D strong Langmuir turbulence

A simple model is constructed for the electric fields in the collapsing wave packets found in 3D simulations of driven and damped isotropic strong Langmuir turbulence. This model, based on a spherical-harmonic decomposition of the electrostatic potential, accounts for the distribution of wave-packet shapes observed in the simulations, particularly the predominance of oblate wave packets. In contrast with predictions for undamped and undriven subsonic collapse of scalar fields, oblate vector-field wave packets do not flatten during collapse but, instead, remain approximately self-similar and rigid.

Newman, D. L.↗
NASA NTRS1989

Soliton and strong Langmuir turbulence in solar flare processes

The occurrence of modulational instability in the current sheet of a solar flare is investigated. Special attention is given to the plasma microinstability in this sheet and its relation to the flare process. It is found that solitons or strong Langmuir turbulence are likely to occur in the diffusion region under solar flare conditions in which the electric resistivity could be enhanced by several orders of magnitude in the region, resulting in significant heating and stochastic acceleration of particles. A numerical example is used to demonstrate the transition of the magnetic field velocity and plasma density from the outer MHD region into the diffusive region and then back out again with the completion of the energy conversion process. This is all made possible by an increase in resistivity of four to five orders of magnitude over the classical value.

Song, M. T.↗
NASA NTRS1990

Creep and creep rupture of strongly reinforced metallic composites

A creep and creep damage theory is presented for metallic composites with strong fibers. Application is to reinforced structures in which the fiber orientation may vary throughout but a distinct fiber direction can be identified locally (local transverse isotropy). The creep deformation model follows earlier work and is based on a flow potential function that depends on invariants reflecting stress and the material symmetry. As the focus is on the interaction of creep and damage, primary creep is ignored. The creep rupture model is an extension of continuum damage mechanics and includes an isochronous damage function that depends on invariants specifying the local maximum transverse tension and the maximum longitudinal shear stress. It is posited that at high temperature and low stress, appropriate to engineering practice, these stress components damage the fiber/matrix interface through diffusion controlled void growth, eventually causing creep rupture. Experiments are outlined for characterizing a composite through creep rupture tests under transverse tension and longitudinal shear. Application is made to a thin-walled pressure vessel with reinforcing fibers at an arbitrary helical angle. The results illustrate the usefulness of the model as a means of achieving optimal designs of composite structures where creep and creep rupture are life limiting.

Robinson, D. N.↗