SEARCH · Search NASA

Results for “INHOMOGENEITY”

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.

SourceResearch connectionFrom yearThrough year

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

At least 271 records · Page 15

All sources NASA NTRS DOE OSTI

NASA NTRS1984

S-P wave travel time residuals and lateral inhomogeneity in the mantle beneath Tibet and the Himalaya

S-P wave travel time residuals were measured in earthquakes in Tibet and the Himalaya in order to study lateral inhomogeneities in the earth's mantle. Average S-P residuals, measured with respect to Jeffrey-Bullen (J-B) tables for 11 earthquakes in the Himalaya are less than +1 second. Average J-B S-P from 10 of 11 earthquakes in Tibet, however, are greater than +1 second even when corrected for local crustal thickness. The largest values, ranging between 2.5 and 4.9 seconds are for five events in central and northern Tibet, and they imply that the average velocities in the crust and upper mantle in this part of Tibet are 4 to 10 percent lower than those beneath the Himalaya. On the basis of the data, it is concluded that it is unlikely that a shield structure lies beneath north central Tibet unless the S-P residuals are due to structural variations occurring deeper than 250 km.

Molnar, P.↗

NASA NTRS1985

Resistive wave dissipation on magnetic inhomogeneities Normal modes and phase mixing

Numerical solutions of the linearized, resistive MHD equations indicate the presence of two characteristic forms of wave decay on magnetic inhomogeneities. Solutions for relatively long-wavelength disturbances and large values of the resistivity have the behavior of decaying normal modes. In the opposite limits, phase mixing becomes important, with the eventual build-up of large spatial gradients in which the resistive terms dominate. This composite behavior is shown to be conceptually consistent with analytic results, which predict that the complete solution consists of the sum of collective and noncollective contributions. The numerical simulations go beyond analytic theory by defining where each contribution prevails and computing wave decay when phase mixing is effective. The dispersion relation in the normal-mode regime is also determined analytically, with some approximations, and is in good agreement with the numerical solution. The decay time of the normal modes varies with Lundquist number at S1/6, while the phase-mixed decay time scales as S1/3.

Steinolfson, R. S.↗

NASA NTRS1985

Ultraviolet DIAL measurements of O3 profiles in regions of spatially inhomogeneous aerosols

The differential absorption lidar (DIAL) technique generally assumes that atmospheric optical scattering is the same at the two laser wavelengths used in the DIAL measurement of a gas concentration profile. Erros can arise in this approach when the wavelengths are significantly separated, and there is a range dependence in the aerosol scattering distribution. This paper discusses the errors introduced by large DIAL wavelength separations and spatial inhomogeneity of aerosols in the atmosphere. A Bernoulli solution for determining the relative distribution of aerosol backscattering in the UV region is presented, and scattering ratio boundary values for these solutions are discussed. The results of this approach are used to derive a backscatter correction to the standard DIAL analysis method. It is shown that for the worst cases of severe range dependence in aerosol backscattering, the residual errors in the corrected DIAL O3 measurements were less than 10 ppbv for DIAL wavelengths at 286 and 300 nm.

Browell, E. V.↗

NASA NTRS1985

A study of backscattering and emission from closely packed inhomogeneous media

The effects of close spacing between small scattering spheres were examined by keeping the distance-dependent terms in the expressions for the transverse scattered fields. The phase matrix was then derived from these fields and was used in the radiative transfer formulation to model scattering and emission from a densely populated, inhomogeneous layer. Computed results were compared with those obtained when the phase matrix was specialized to the far-field condition. It was found that the use of the far-zone condition tended to underestimate both the level of the copolarized backscattering and the cross-polarized backscattering. In emission computations, the use of the far-zone condition overestimated the level of the brightness temperature. These effects decreased with a decrease in the volume fraction or an increase in the exploring frequency, as expected. An improvement on the snow parameter (density and crystal size) estimation was shown to be possible when this new phase matrix was used.

Fung, A. K.↗

NASA NTRS1985

Resonance cone structure in a warm inhomogeneous bounded plasma with lower-hybrid resonance layers

The paper presents a theoretical study of the problem of the wave fields excited by a gap source at the edge of an inhomogeneous magnetized plasma with a pair of lower-hybrid resonance layers present and bounded by conducting walls. The approach used is that of a solution as a sum of multiply reflected extraordinary mode and ion-thermal resonance cones as an alternative to the guided-wave mode approach. A diagrammatic scheme for writing the solution is given which can be used to determine in great detail the structure and properties of the resonance cones and the way they transform across the back-to-back hybrid layers. Evanescent resonance cones are shown to exist in the high-density region between the hybrid resonance layers and to tunnel through to the other side, maintaining this general structure if the layer is relatively thin.

Grabbe, C. L.↗

NASA NTRS1986

Observations of three BL Lacertae objects - Constraints on inhomogeneous relativistically beamed jet models

Multifrequency observations are presented of three BL Lac objects, including the first UV measurements of 0912 + 297 and 1219 + 285. An eight day optical flare was measured from 0422 = 004 in which a doubling in visual flux density was noted; other sources displayed smaller daily fluctuations. The 5-15 GHz radio flux from these three objects remained constant for two months, although the 15-20 GHz emission from 0912 + 297 varied slightly over a period of a month. Constraints which are provided by these and other Bl Lac data are discussed for inhomogeneous relativistically beamed jet models. It is concluded that a tapered jet model is in good agreement with the data for each source and that additional emission regions are not required.

Worrall, D. M.↗

NASA NTRS1987

Estimation of the average surface heat flux over an inhomogeneous terrain from the vertical velocity variance

An indirect method of estimating the surface heat flux from observations of vertical velocity variance at the lower mid-levels of the convective atmospheric boundary layer is described. Comparison of surface heat flux estimates with those from boundary-layer heating rates is good, and this method seems to be especially suitable for inhomogeneous terrain for which the surface-layer profile method cannot be used.

Eilts, M. D.↗

NASA NTRS1987

Forbidden coronal iron-like emission in the Puppis A shock front - The effect of inhomogeneities

CCD images of the shock front at the eastern rim of Pup A, obtained in the forbidden lines of Fe X (6374 A) and Fe XIV (5303 A), are reported and compared to Einstein high-resolution-image soft X-ray data. The observed part of the remnant is complex, containing density irregularities. Optical and X-ray data are consistent in showing a nearly flat gradient of ionization temperature behind the shock. To determine conditions in the shock, scans of surface brightness across it in the optical lines were compared to surface brightnesses predicted by idealized Sedov models. It was not possible to match both the red and green line scans by a simple, single-component model, and the failure is ascribed to the presence of the density inhomogeneities. This result has important implications for the determination of SNR shock-front models by fitting X-ray data with Sedov models.

Teske, Richard G.↗

NASA NTRS1988

Effective-Mass Theory For Inhomogeneous Semiconductors

Five papers present derivations in effective-mass theory for semiconducing materials of gradually varying spatial composition. In paper entitled "Position-Dependent Effective Masses in Semiconductor Theory," concept of spatially-varying effective masses reviewed and shown unworkable. Second paper considers compound semiconductor having gradual spatial variation of composition. Third paper shows effective masses depend on average of chemical composition over entire crystal. Pair of papers entitled "Forces Acting on Free Carriers in Semiconductors of Inhomogeneous Composition" show it is incorrect to apply extended Wannier-Slater theorem using spatially changing effective masses, and quasi-electric fields due to spatial variations in chemical composition do not depend explicitly on temperature.

Von Roos, Oldwig↗

NASA NTRS1987

Characteristics of inhomogeneous coupled cylindrical striplines

Using conformal transformation, closed-form expressions for the even and odd mode impedances of inhomogeneous coupled cylindrical striplines are presented. It is shown that the analysis can be extended for the calculation of characteristics of coupled cylindrical microstrip lines. The analysis can also be extended to reveal the effects of warping due to environmental changes on an otherwise planar structure.

Reddy, C. J.↗

NASA NTRS1988

Light propagation in inhomogeneous universes

Refsdal's (1970) method is generalized to study the propagation of light rays through an inhomogeneous universe. The probability distribution for the linear component of the cumulative shear (CS) along light rays is derived, and it is shown that the CS can be dominated by nonlinear components, espcially for light rays in empty cones. The amplification tail of the amplification probability distribution is compared with analytic results; these linear investigations are shown to underestimate the high-amplification probability and hence the importance of the amplification bias in source counts. The distribution of the ellipticity of images of infinitesimal circular sources is derived, and it is shown that this can be dominated by the nonlinear contributions to the CS.

Schneider, Peter↗

NASA NTRS1988

Inhomogeneous conditions at open boundaries for wave propagation problems

Absorbing boundary conditions contain differential operators even for first-order systems. There is a fundamental difficulty with this, since the conditions are applied on the ingoing variables, and the approximations necessarily become weakly unstable. This difficulty is more pronounced for inhomogeneous boundary conditions which occur if there is a source outside the computational domain D, or if the initial data are nonzero outside D. In this paper this is further investigated and it is shown that reasonable estimates can still be obtained if the solution is smooth. However, it is demonstrated that the approximations are less robust. A cure for this is proposed and an implementation of high-order conditions for first-order systems is described.

Gustafsson, Bertil↗

NASA NTRS1988

Light propagation in inhomogeneous universes - The ray-shooting method

The propagation of light in a clumpy universe is studied using the ray-shooting method. It is found that 'empty cones' in a clumpy universe are rare, which points out the limitation of frequently used distance measures in an inhomogeneous universe. The results show some qualitatively new features of multiple-gravitational-lens geometry, and a likely explanation for these features is presented. A statistical analysis of the simulations give amplification probability distributions for extended sources, as well as the distribution for the ratio of amplifications of compact sources to amplifications of more extended sources. Sample light curves of individual sources are obtained. Finally, the results support the validity of recent investigations on the influence of gravitational light deflection on source counts of compact extragalactic objects.

Schneider, Peter↗

NASA NTRS1986

Forbidden coronal iron line emission in the Puppis A shock front: The effect of inhomogeneities

We have obtained CCD images of the shock front at the eastern rim of Puppis A in (Fe X) lambda 6374 and (Fe XIV) lambda 5303 and have compared the optical data to Einstein HRI soft X-ray data. The observed part of the remnant is complex, containing density irregularities. Optical and X-ray data are consistent in showing a nearly flat gradient of ionization behind the shock. To determine conditions in the shock, scans of surface brightness across it in the optical lines were compared to surface brightnesses predicted by idealized Sedov models. We were unable to match both the red and green line scans by a simple, single-component model, and have ascribed the failure to the presence of the density inhomogeneities. Our result has important implications for the determination of SNR shock front models by means of fitting X-ray data with Sedov models.

Teske, Richard G.↗

NASA NTRS1989

Phase conjugation by four-wave mixing in inhomogeneous plasmas

The effects of density, temperature, and velocity gradients on four-wave mixing (FWM) in a plasma are investigated. A fluid model is used in which the stimulated Brillouin terms are included, but pump depletion is neglected. The steady state phase conjugate reflectivity and signal transmission coefficients are calculated and discussed for both degenerate and resonant FWM. The substantial effects of inhomogeneity on the use of FWM as a plasma diagnostic are discussed.

Williams, Edward A.↗

NASA NTRS1989

AFE dynamic effects in inhomogeneous plasmas

The Microwave Reflectometer Ionization Sensor (MRIS) is an instrument on the Aeroassist Flight Experiment (AFE) satellite which will be deployed from the space shuttle. The flow characteristic around a hypersonic bluff reentry vehicle will be measured by the AFE. The general mission of the MRIS is to measure the electron density within the range from 10(exp12) to 10(exp 15) electrons per cu cm and determine the distance to the location of each measured from the surface of the aerobrake. These measurements will be compared with prior aerothermodynamic computer code predictions. Since a knowledge of plasma dynamic effects is important for MRIS design and post flight analysis, it is of interest to consider any possibility of plasma dynamic effects and especially in inhomogeneous plasmas. Of particular interest is the need to study plasma dynamic effects that may emerge from a flow field stationary state that was determined without regard to electric or magnetic fields.

Juang, Jeng-Nan↗

NASA NTRS1990

Inhomogeneous electric field effects in a linear RF quadruple trap

The exact potential corresponding to confining fields inside a linear rf quadrupole particle trap of finite length is presented. The analytic expressions for the trapping potential is derived by introducing a linear trap employing a relatively simple cylindrical geometry and solving Laplace's equation for the trap electrodes. The finite length of linear traps results in field distortion near the trap ends. An exact analytic determination of the fields is useful because the profile of the trapped ion cloud is highly dependent on the fields confining it. It is shown that near the ends of the trap, the effective potential arising from the rf fields acts to propel particles out of the trap, and further, that the addition of a dc bias generates an inhomogeneous in the trap that influences the particles both perpendicularly to and along the trap's long axis.

Melborne, R. K.↗

NASA NTRS1989

Rapid calculation of radiative heating rates and photodissociation rates in inhomogeneous multiple scattering atmospheres

The solution of the generalized two-stream approximation for radiative transfer in homogeneous multiple scattering atmospheres is extended to vertically inhomogeneous atmospheres in a manner which is numerically stable and computationally efficient. It is shown that solar energy deposition rates, photolysis rates, and infrared cooling rates all may be calculated with the simple modifications of a single algorithm. The accuracy of the algorithm is generally better than 10 percent, so that other uncertainties, such as in absorption coefficients, may often dominate the error in calculation of the quantities of interest to atmospheric studies.

Toon, Owen B.↗