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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 289 records · Page 16
All sourcesNASA NTRSDOE OSTI
NASA NTRS1992

Damping and scattering of electromagnetic waves by small ferrite spheres suspended in an insulator

The intentional degradation of electromagnetic waves by their penetration into a media comprised of somewhat sparsely distributed energy absorbing ferrite spheres suspended in an electrical insulator is investigated. Results are presented in terms of generalized parameters involving wave length and sphere size, sphere resistivity, permeability, and spacing; their influence on dissipation of wave power by eddy currents, magnetic hysteresis, and scattering is shown.

Englert, Gerald W.↗
NASA NTRS1983

Coherent scattering of a spherical wave from an irregular surface

The scattering of a spherical wave from a rough surface using the Kirchhoff approximation is considered. An expression representing the measured coherent scattering coefficient is derived. It is shown that the sphericity of the wavefront and the antenna pattern can become an important factor in the interpretation of ground-based measurements. The condition under which the coherent scattering-coefficient expression reduces to that corresponding to a plane wave incidence is given. The condition under which the result reduces to the standard image solution is also derived. In general, the consideration of antenna pattern and sphericity is unimportant unless the surface-height standard deviation is small, i.e., unless the coherent scattering component is significant. An application of the derived coherent backscattering coefficient together with the existing incoherent scattering coefficient to interpret measurements from concrete and asphalt surfaces is shown.

Fung, A. K.↗
NASA NTRS1995

Scattering Of Nonplanar Acoustic Waves

Report presents theoretical study of scattering of nonplanar acoustic waves by rigid bodies. Study performed as part of effort to develop means of predicting scattering, from aircraft fuselages, of noise made by rotating blades. Basic approach was to model acoustic scattering by use of boundary integral equation to solve equation by the Galerkin method.

Gillman, Judith M.↗
NASA NTRS2001

Direct Calculation of the Scattering Amplitude Without Partial Wave Analysis

Two new developments in scattering theory are reported. We show, in a practical way, how one can calculate the full scattering amplitude without invoking a partial wave expansion. First, the integral expression for the scattering amplitude f(theta) is simplified by an analytic integration over the azimuthal angle. Second, the full scattering wavefunction which appears in the integral expression for f(theta) is obtained by solving the Schrodinger equation with the finite element method (FEM). As an example, we calculate electron scattering from the Hartree potential. With minimal computational effort, we obtain accurate and stable results for the scattering amplitude.

Shertzer, J.↗
NASA NTRS1990

A boundary integral approach to the scattering of nonplanar acoustic waves by rigid bodies

The acoustic scattering of an incident wave by a rigid body can be described by a singular Fredholm integral equation of the second kind. This equation is derived by solving the wave equation using generalized function theory, Green's function for the wave equation in unbounded space, and the acoustic boundary condition for a perfectly rigid body. This paper will discuss the derivation of the wave equation, its reformulation as a boundary integral equation, and the solution of the integral equation by the Galerkin method. The accuracy of the Galerkin method can be assessed by applying the technique outlined in the paper to reproduce the known pressure fields that are due to various point sources. From the analysis of these simpler cases, the accuracy of the Galerkin solution can be inferred for the scattered pressure field caused by the incidence of a dipole field on a rigid sphere. The solution by the Galerkin technique can then be applied to such problems as a dipole model of a propeller whose pressure field is incident on a rigid cylinder. This is the groundwork for modeling the scattering of rotating blade noise by airplane fuselages.

Gallman, Judith M.↗
NASA NTRS1987

The theory of ionospheric focused heating

Ionospheric modification by high power radio waves and by chemical releases are combined in a theoretical study of ionospheric focused heating. The release of materials which promote electron-ion recombination creates a hole in the bottomside ionosphere. The ionospheric hole focuses high power radio waves from a ground-based transmitter to give a 20 dB or greater enhancement in power density. The intense radio beam excites atomic oxygen by collisions with accelerated electrons. Airglow from the excited oxygen provides a visible trace of the focused beam. The large increase in the intensity of the radio beam stimulates new wave-plasma interactions. Numerical simulations show that the threshold for the two-plasmon decay instability is exceeded. The interaction of the pump electromagnetic wave with the backward plasmon produces a scattered electromagnetic wave at 3/2 the pump frequency. The scattered wave provides a unique signature of the two-plasmon decay process for ground-based detection.

Bernhardt, P. A.↗
NASA NTRS1977

Elastic wave propagation in a highly scattering medium - A diffusion approach

The propagation of elastic waves in the moon, where the first seismograms were characterized by the presence of a long coda attributed to strongly scattered waves, is modeled with the aid of the time-dependent equation of radiative transfer. The average energy density as a function of time and space is described by the diffusion equation with linear dissipation on the assumption that all the energy present has been scattered many times and the time and distance scales of the problem are long compared to the scales of the scattering process. Ultrasonic experiments in the laboratory confirm the applicability of the formalism.

Dainty, A. M.↗
NASA NTRS1991

Edge wave visualization

Scattering mechanisms that involve edge waves are addressed. The behavior of edge waves and their interaction with flat, perfectly conducting plates are depicted in the time domain through a visualization of surface currents that flow on the surface, as an incident Gaussian pulse of energy washes over the surface. Viewing these surface currents allows a very clear physical interpretation and appreciation of the scattering process.

Dominek, Allen K.↗
NASA NTRS2004

Calculation of the Full Scattering Amplitude without Partial Wave Decomposition: Inclusion of Exchange - 2

The development of a practical method of accurately calculating the full scattering amplitude, without making a partial wave decomposition is continued. The method is developed in the context of electron-hydrogen scattering, and here exchange is dealt with by considering e-H scattering in the static exchange approximation. The Schroedinger equation in this approximation can be simplified to a set of coupled integro-differential equations. The equations are solved numerically for the full scattering wave function. The scattering amplitude can most accurately be calculated from an integral expression for the amplitude; that integral can be formally simplified, and then evaluated using the numerically determined wave function. The results are essentially identical to converged partial wave results.

Shertzer, Janine↗
NASA NTRS1971

Wave-wave interactions due to scattering by electrons.

The kinetic wave equation which describes the nonlinear wave-particle interaction in a plasma is considered, and a method which uses the picture of quantized plasmons interacting with particles for the description of nonlinear wave-particle interactions is briefly described. In this method an assumption is made of the Markoffian character of the equation. It is shown that the Markoffian assumption can be justified at least for the case when the plasma is close to a stable stationary state. The diagram method developed by Nishikawa (1966) is used to derive an explicit expression for the kinetic equation. The application of the result to the case of interaction between an electron-wave and an ion-wave is discussed.

Nishikawa, K.↗
NASA NTRS1982

Application of strong fluctuation random medium theory to scattering of electromagnetic waves from a half-space of dielectric mixture

The strong fluctuation random medium theory is applied to calculate scattering from a half-space of dielectric mixture. The first and second moments of the fields are calculated, respectively, by using the bilocal and the distorted Born approximations, and the low frequency limit is taken. The singularity of the dyadic Green's function is taken into account. Expressions for the effective permittivity for the full space case are derived. It is shown that the derived result of the effect permittivity is identical to that of the Polder and van Santern mixing formula. The correlation function of the random medium is obtained by using simple physical arguments and is expressed in terms of the fractional volumes and particle sizes of the constituents of the mixture. Backscattering coefficients of a half-space dielectric mixture are also calculated. Numerical results of the effective permittivity and backscattering coefficients are illustrated using typical parameters encountered in microwave remote sensing of dry and wet snow. It is also shown that experimental data can be matched with the theory by using physical parameters of the medium as obtained from ground truth measurements.

Tsang, L.↗
NASA NTRS1989

Propagation and scattering of elastic waves in fiber-reinforced plates

The objectives of this program are as follows: modelling of guided waves in fiber-reinforced plates in terms of different modes; and analysis of scattering by transverse cracks using modal representation. A hybrid numerical method combining the finite element representation of a region around the crack with the modal representation in the exterior region will be used in this program. Modes will be obtained using the through-the-thickness discretization of the displacement field.

Bratton, Robert L.↗
NASA NTRS1982

Low-level VLF and LR radio emissions observed at earth and Jupiter

Major observational features of magnetospheric VLF-LF radio noise that have been uncovered by the IMP, Hawkeye and ISEE satellites, are examined with emphasis on the capabilities of the noise to diagnose local and remote plasma parameters. The relationship of the radiation to not less than 1-mV/m electrostatic upper hybrid emissions is assessed, and indirect observational evidence suggests that upper noise is associated with the generation of the VLF-LF radiation. Theoretical luminosities of mechanisms including synchrotron radiation, linear mode conversion of upper hybrid waves and nonlinear scattering of upper hybrid waves off plasma density irregularities are estimated, and the wave-wave scattering hypothesis is considered to be a viable mechanism if the presence of low-frequency waves as a scattering agent can be established.

Barbosa, D. D.↗
NASA NTRS1995

Wave propagation and scattering in computational aeroacoustics

This paper describes predictions of model problems in computational aeroacoustics. Two problem classes are considered: The first (Workshop Category 3, Problem 1 and 2) considers two-dimensional wave propagation and non-reflecting boundary conditions in the presence of a mean flow. The second (Workshop Category 4, Problem 1) examines wall boundary conditions. For the last problem we introduce the Impedance Mismatch Method (IMM) to treat the solid wall boundaries. In this method the solid wall is simulated using a wall region in which the characteristic impedance is set to a different value to that in the fluid region. This method has advantages over traditional solid wall boundary conditions including simplicity of coding, speed of computations, and the ability to treat curved boundaries efficiently. Several numerical examples are given in addition to the Workshop Problems. The discretization of the Euler equations is performed in all cases with a Dispersion-Relation-Preserving (DRP) algorithm. The numerical results are compared with either analytical solutions or solutions obtained using traditional solid wall boundary conditions.

Chung, Cathy↗
NASA NTRS1981

Scattering of electromagnetic waves from random media with strong permittivity fluctuations

By taking into account the singularity of the dyadic Green's function in the renormalization method, a theory is derived for vector electromagnetic wave propagation in a random medium with large permittivity fluctuations and with anisotropic correlation function. The strong fluctuation theory is then applied to a discrete scatterer problem in which the permittivity can assume only two values. The results are found to be consistent with those derived from discrete scatterer theory for all values of dielectric constants of the scatterers.

Tsang, L.↗