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At least 109 records · Page 6
All sourcesNASA NTRSDOE OSTI
NASA NTRS1982

Fundamental aspects in quantitative ultrasonic determination of fracture toughness: The scattering of a single ellipsoidal inhomogeneity

The scattering of a single ellipsoidal inhomogeneity is studied via an eigenstrain approach. The displacement field is given in terms of volume integrals that involve eigenstrains that are related to mismatch in mass density and that in elastic moduli. The governing equations for these unknown eigenstrains are derived. Agreement with other approaches for the scattering problem is shown. The formulation is general and both the inhomogeneity and the host medium can be anisotrophic. The axisymmetric scattering of an ellipsoidal inhomogeneity in a linear elastic isotropic medium is given as an example. The angular and frequency dependence of the scattered displacement field, the differential and total cross sections are formally given in series expansions for the case of uniformly distributed eigenstrains.

Fu, L. S. W.↗
NASA NTRS1991

Method for calculating and observing microwave absorption by a sphere in a single mode rectangular cavity

A new theory of microwave absorption by a lossy dielectric sphere in a single mode rectangular cavity has been recently developed. The absorption was treated in the framework of an electromagnetic scattering problem. That theory is summarized here and calculated results that bear on optimizing the processing of materials are illustrated. Methods for observing power absorption and other results predicted by the scattering model are discussed. Cavity perturbation theory provides a bridge between theoretical calculations and experimental observations, and a special problem that arises when an established version of cavity perturbation theory is applied to spheres is identified, analyzed, and resolved. The direct problem of predicting shifts in frequency and Q from model calculations is discussed for a sphere in a cavity when the sphere's complex dielectric constant is known. Also, the inverse problem of determining the complex dielectric constants from measured values of those shifts is considered. The small sphere limit, where an electrostatic or quasistatic model is valid, is treated in detail, and planned work on parallel problems for larger spheres is described.

Jackson, H. W.↗
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.↗
NASA NTRS1975

Nucleon and heavy-ion total and absorption cross section for selected nuclei

Approximate solutions of the coupled-channel equations for high-energy composite particle scattering are obtained and are applied to the nuclear scattering problem. Relationships between several approximation procedures are established and discussed. The eikonal formalism is used with a small-angle approximation to calculate the coherent elastic scattered amplitude from which total and absorption cross sections are derived. Detailed comparisons with nucleon-nucleus experiments show agreement within 5 percent except at lower energies where the eikonal approximation is of questionable accuracy. Even at these lower energies, agreement is within 15 percent. Tables of cross sections required for cosmic heavy-ion transport and shielding studies are presented.

Wilson, J. W.↗
NASA NTRS1996

Classical problems in computational aero-acoustics

In relation to the expected problems in the development of computational aeroacoustics (CAA), the preliminary applications were to classical problems where the known analytical solutions could be used to validate the numerical results. Such comparisons were used to overcome the numerical problems inherent in these calculations. Comparisons were made between the various numerical approaches to the problems such as direct simulations, acoustic analogies and acoustic/viscous splitting techniques. The aim was to demonstrate the applicability of CAA as a tool in the same class as computational fluid dynamics. The scattering problems that occur are considered and simple sources are discussed.

Hardin, Jay C.↗
NASA NTRS1991

User's manual for three dimensional FDTD version A code for scattering from frequency-independent dielectric materials

The Finite Difference Time Domain Electromagnetic Scattering Code Version A is a three dimensional numerical electromagnetic scattering code based upon the Finite Difference Time Domain Technique (FDTD). This manual provides a description of the code and corresponding results for the default scattering problem. In addition to the description, the operation, resource requirements, version A code capabilities, a description of each subroutine, a brief discussion of the radar cross section computations, and a discussion of the scattering results.

Beggs, John H.↗
NASA NTRS1988

Theory and application of radiation boundary operators

A succinct unified review is provided of the theory of radiation boundary operators. With the recent introduction of the on-surface radiation condition (OSRC) method and the continued growth of finite-difference and finite-element techniques for modeling electromagnetic wave scattering problems, the understanding and use of radiation boundary operators has become increasingly important. Results are presented to illustrate the application of radiation boundary operators in both these areas. Recent OSRC results include analysis of the scattering behavior of both electrically small and large cylinders, a reactively loaded acoustic sphere, and a simple reentrant duct. Radiation boundary operator results include the demonstration of the effectiveness of higher-order operators in truncating finite-difference time-domain grids.

Moore, Thomas G.↗
NASA NTRS1991

Chaotic jumps in the generalized first adiabatic invariant in current sheets

The present study examines how the changes in the generalized first adiabatic invariant J derived from the separatrix crossing theory can be incorporated into the drift variable approach to generating distribution functions. A method is proposed for determining distribution functions for an ensemble of particles following interaction with the tail current sheet by treating the interaction as a scattering problem characterized by changes in the invariant. Generalized drift velocities are obtained for a 1D tail configuration by using the generalized first invariant. The invariant remained constant except for the discrete changes caused by chaotic scattering as the particles cross the separatrix.

Brittnacher, M. J.↗
NASA NTRS1991

Electromagnetic scattering from arbitrary shaped conducting bodies coated with lossy materials of arbitrary thickness

A simple and efficient numerical technique is presented to solve the electromagnetic scattering problem of coated conducting bodies of arbitrary shape. The surface equivalence principle is used to formulate the problem in terms of a set of coupled integral equations involving equivalent electric and magnetic surface currents which represent boundary fields. The conducting structures and the dielectric materials are modeled by planar triangular patches, and the method of moments is used to solve the integral equations. Numerical results for scattering cross sections are given for various structures and compared with other available data. These results are proved accurate by a number of representative examples.

Rao, Sadasiva M.↗
NASA NTRS1981

Scattering of Pruppacher-Pitter raindrops at 30 GHz

Optimum design of modern ground-satellite communication systems requires the knowledge of rain-induced differential attenuation, differential phase shift, and cross polarization factors. Different available analytical techniques for raindrop scattering problems were assessed. These include: (1) geometrical theory of diffraction; (2) method of moment; (3) perturbation method; (4) point matching methods; (5) extended boundary condition method; and (6) global-local finite element method. The advantages and disadvantages of each are listed. The extended boundary condition method, which was determined to yield the most scattering results, is summarized. The scattered fields for Pruppacher-Pitter raindrops with sizes ranging from 0.5 mm to 3.5 mm at 20 C and at 30 GHz for several incidence angles are tabulated.

Yeh, C.↗
NASA NTRS1990

Boundary element analysis of sound scattered by a moving surface

A solution for the acoustic field scattered from a uniformly moving rigid body in the presence of a harmonic incident source has been obtained using a boundary integral method. A derivation of the Kirchhoff formula given by Farassat and Myers (1988) for moving surfaces forms the basis for the analysis, and the development of a boundary integral method for the solution of scattering problems from moving rigid bodies is described. Finite elements are used in conjunction with the Galerkin method in order to solve the integral equation that results from the Kirchhoff formula when the observer point is placed on the moving body surface. Once appropriate surface field values are known they are inserted back into the formula in order to predict the field scattered off the body. Tests, including the so called superposition method, are carried out in order to validate the technique and to establish some confidence in its accuracy. Application of the superposition method to moving bodies is presented, and results of the two approaches are discussed. Sample calculations of scattering from a simple body are presented to illustrate the effects of variations in relevant parameters.

Myers, M. K.↗
NASA NTRS1994

Investigation of finite element: ABC methods for electromagnetic field simulation

The mechanics of wave propagation in the presence of obstacles is of great interest in many branches of engineering and applied mathematics like electromagnetics, fluid dynamics, geophysics, seismology, etc. Such problems can be broadly classified into two categories: the bounded domain or the closed problem and the unbounded domain or the open problem. Analytical techniques have been derived for the simpler problems; however, the need to model complicated geometrical features, complex material coatings and fillings, and to adapt the model to changing design parameters have inevitably tilted the balance in favor of numerical techniques. The modeling of closed problems presents difficulties primarily in proper meshing of the interior region. However, problems in unbounded domains pose a unique challenge to computation, since the exterior region is inappropriate for direct implementation of numerical techniques. A large number of solutions have been proposed but only a few have stood the test of time and experiment. The goal of this thesis is to develop an efficient and reliable partial differential equation technique to model large three dimensional scattering problems in electromagnetics.

Chatterjee, A.↗
NASA NTRS1978

Variational principle for scattering of light by dielectric particles

Consideration is given to the work of Purcell and Pennypacker (1973) where a dielectric particle is taken to be an aggregate of N polarizable elements mounted on a cubic lattice. The simultaneous equations which result from the scattering problem are presented. This theory has been discussed in the case of nonspherical and inhomogeneous objects whose dimensions are smaller than or comparable to the wavelength of incident light. A more precise numerical treatment is derived for further progress. The variational principle is invoked and the practical limit for the current version of the scheme is a dipole array on the order of 10,000 atoms. Limits to the scattering parameter due to the phase difference between neighboring atoms are discussed.

Yung, Y. L.↗
NASA NTRS1978

Backscattering from a Gaussian-distributed perfectly conducting rough surface

The original intent of the reported investigation was to find a solution to the composite surface scattering problem which provided for a continuous transition between the near-specular physical optics and wide-angle tilted plane Bragg solutions. The employed approach is based on analytical techniques rather than physical considerations. The analysis employs a perturbation technique developed by Burrows (1967, 1968, 1973), which because of its simplicity, is the key to the entire solution. The analysis is restricted to the case of backscattering from a perfectly conducting surface. General expressions are presented for the scattering from a Gaussian perfectly conducting surface characterized by an anisotropic spectrum and possessing many scales of roughness. The accuracy of the result is dependent upon the large-scale structure having large height excursions and small curvature.

Brown, G. S.↗
NASA NTRS1987

Application of numerical methods to planetary radiowave scattering

Existing numerical techniques for the solution of scattering problems were investigated to determine those which might be applicable to planetary surface studies, with the goal of improving the interpretation of radar data from Venus, Mars, the Moon, and icy satellites. The general characteristics of the models are described along with computational concerns. In particular, the Numerical Electrogmatics Code (NEC) developed at the Lawrence Livermore Laboratory is discussed. Though not developed for random rough surfaces, the NEC contains elements which may be generalized and which could be valuable in the study of scattering by planetary surfaces.

Simpson, Richard A.↗
NASA NTRS2020

INVESTIGATING THE FEASIBILITY AND STABILITY FOR MODELING ACOUSTIC WAVE SCATTERING USING A TIME-DOMAIN BOUNDARY INTEGRAL EQUATION WITH IMPEDANCE BOUNDARY CONDITION

Reducing aircraft noise is a major objective in the field of computational aeroacoustics. When designing next generation quiet and environmentally friendly aircraft, it is important to be able to accurately and efficiently predict the acoustic scattering by an aircraft body from a given noise source. Acoustic liners are an effective tool for aircraft noise reduction and are characterized by a frequency-dependent impedance. Converted into the time-domain using Fourier transforms, an impedance boundary condition can be used to simulate the acoustic wave scattering by geometric bodies treated with acoustic liners. This work considers using either an impedance or an admittance (inverse of impedance) boundary condition to allow for acoustic scattering problems to be modeled with geometries consisting of both un-lined and lined surfaces. Three acoustic liner models are discussed: the Extended Helmholtz Resonator Model, the Three-Parameter Impedance Model, and the Broadband Impedance Model. In both the Helmholtz and Three-Parameter models, liner impedance is specified at a given frequency, whereas the Broadband model allows for the investigation of multiple frequencies simultaneously. The impedance and admittance boundary conditions for acoustic liners are derived for each model and coupled with a time-domain boundary integral equation. The scattering solution is obtained iteratively using a boundary element method with constant spatial and third-order temporal basis functions. Time-domain boundary integral equations are unfortunately prone to numerical instabilities due to resonant frequencies resulting from non-trivial solutions in the interior domain. When reformulated with the Burton-Miller method, the instabilities are eliminated. Using a Burton-Miller reformulation, the stability of the boundary element method assuming a liner boundary condition is assessed using eigenvalue analysis. The stability of each liner model is discussed, and it is shown that the Three-Parameter and Broadband models are sufficient for modeling an acoustic liner on the surface of scattering bodies. The Helmholtz model demonstrates strict limitations for stability, whereas the Three-Parameter and Broadband models are stable for most cases. Also included in this work is an assessment of the spatial accuracy of the time-domain boundary element method with respect to the surface element basis functions, as well as a performance study of the numerical algorithm.

computational mathematics↗
NASA NTRS1987

Complex space multipole theory for scattering and diffraction problems

Classical multipole theory can be extended to multipoles located in complex space and applied in scattering and diffraction problems with the advantage that, if the point of the multipole is correctly chosen, the first term may give an order of magnitude better approximation to the source than when the multipole is in real space. The basic theory, given elsewhere, is presented here in a more straightforward manner and the improvement in radiation pattern is demonstrated for sources of constant polarization. Applications on scattering by spheroidal dielectric bodies and diffraction by a dielectric half-space are discussed.

Lindell, Ismo V.↗