Low‐frequency scattering by a penetrable body with an eccentric soft or hard core

A plane wave is scattered by an acoustically soft or hard sphere, covered by a penetrable non‐concentric spherical lossless shell that disturbs the propagation of the incident wave field. The dimensions of the coated sphere are much smaller than the wavelength of the incident field. Low‐frequency theory reduces this scattering problem to a sequence of potential problems, which can be solved iteratively. Exactly one bispherical coordinate system exists that fits the given geometry of the obstacle. For the case of a soft and hard core, the exact low‐frequency coefficients of the zeroth and the first‐order for the near field as well as the first‐ and second‐order coefficients for the normalized scattering amplitude are obtained and the cross sections are calculated. Discussion of the results and their physical meaning is included. Copyright © 2009 John Wiley & Sons, Ltd.     

Scattering of electromagnetic plane waves by a spheroid uniformly moving in free space

We investigate the scattering process, generated by a plane electromagnetic field that is incident upon a moving perfectly conducting spheroid. An accurate treatment of the electromagnetic waves interaction with scatterers in uniform motion is based on the special relativity principle. In the object's frame the incident wave is assumed to have a wavelength which is much larger than the characteristic dimension of the scatterer and thus the low‐frequency approximation method is applicable to the scattering problem. For the near electromagnetic field we obtain the zeroth‐order low‐frequency coefficients, while in the far field we calculate the leading terms for the scattering amplitude and scattering cross‐section. Finally, using the inverse Lorentz transform, we obtain the same approximations in the observer's frame. Copyright © 2006 John Wiley & Sons, Ltd.     

Point source excitation in direct and inverse scattering: The soft and the hard small sphere

A spherical wave emanating from a point source is scatteredby either a soft or a hard body. The incident spherical wavehas a wavelength which is much larger than the characteristicdimension of the scatterer and it is modified in such a wayas to recover the plane wave incidence when the source pointrecedes to infinity. Using low frequency expansions the scatteringproblem is transformed to a sequence of exterior potential problemsin the presence of a monopole singularity located at the sourceof the incident wave field. Complete expansions for the scatteringamplitude are provided. The method is applied to the cases ofa soft and a hard sphere and the first three approximationsfor the near, as well as the far, field are evaluated. It isobserved that every one, after the first, low frequency approximationof the far field, involves one spherical multipole more thanthe corresponding approximation for the case of an incidentplane wave. As the point singularity tends to infinity, therelative results recover all the known expressions for planeincidence. It is shown that for point excitation the Rayleighapproximation of the scattering amplitude for a hard sphereis of the second order, in contrast to the case of plane excitationwhich is of the third order. Simple algorithms that specifythe radius and the position of a soft and a hard sphere areproposed, which are based on the additional dependence of thescattering amplitude represented by the distance from the pointsource to the centre of the scatterer. The inversion algorithmis shown to be stable whenever the source point is not too faraway from the target sphere. A simple way to decide whetherthe sphere is a soft or a hard body is also provided.     

Uniqueness Theorems for the Inverse Problem of Acoustic Scattering

Uniqueness theorems are obtained for the problem of determiningthe shape of a sound-soft or sound-hard obstacle from a knowledgeof (1) the far-field pattern at a fixed value of the wave numberand a finite number of distinct incident fields, or (2) thetotal scattering cross section for an interval of wave numbersand the incident field propagating in an arbitrary direction.     

The singular sources method for cracks

The singular sources method is given to detect the shape of a thin infinitely cylindrical obstacle from a knowledge of the TM‐polarized scattered electromagnetic field in large distance. The basic idea is based on the singular behaviour of the scattered field of the incident point source on the cross‐section of the cylinder. We assume that the scatterer is a perfect conductor which is possibly coated by a material and investigate two models with different boundary conditions. Also we give a uniqueness proof for the shape reconstruction. Copyright © 2006 John Wiley & Sons, Ltd.     

Scattering Theorems for Acoustic Excitation of a Layered Obstacle by an Interior Point Source

Interesting scientific and technological applications motivate the study of scattering problems, where a layered scatterer is excited by a spherical acoustic wave generated by a point-source located in its interior. The scatterer's core may be acoustically soft, hard, resistive, or penetrable. This paper initiates the investigation of scattering theorems, corresponding to the excitation of a layered scatterer by a point source in its interior. Reciprocity and general scattering theorems are established, relating the total fields and the corresponding far-field patterns. The optical theorem, relating the scattering cross-section with the field in the layer containing the source, is recovered as a corollary of the general scattering theorem. Furthermore, for a scatterer excited by a spherical and a plane wave, mixed scattering theorems are derived. Numerical implementations of the optical theorem in concrete scattering applications are analyzed.     

Electromagnetic scattering theorems for interior dipole excitation of a layered obstacle

A layered scatterer is excited by a time‐harmonic spherical electromagnetic wave, generated by a dipole located either in the interior or in the exterior of the scatterer. The scatterer's core may be perfect conducting, impedance or dielectric. This paper initiates the investigation of scattering theorems corresponding to the excitation of a layered scatterer by a dipole in its interior. We establish reciprocity and general scattering theorems relating the total electric fields with the corresponding far‐field patterns. The optical theorem, relating the scattering cross‐section with the electric field in the layer containing the dipole, is recovered as a corollary of the general scattering theorem. Furthermore, for a scatterer excited by a spherical and a plane wave, mixed scattering theorems are derived. Besides, all the derived theorems recover known results, concerning the excitation of a homogeneous scatterer by an exterior dipole. We also analyze numerical implementations of the optical theorem in certain concrete scattering applications. Copyright © 2007 John Wiley & Sons, Ltd.     

Application of the Factorization Method to Recover Cuts with Oblique Derivative Boundary Condition

Direct and inverse problems for the scattering of cracks with mixed oblique derivative boundary conditions from the incident plane wave are considered, which describe the scattering phenomenons such as the scattering of tidal waves by spits or reefs. The solvability of the direct scattering problem is proven by using the boundary integral equation method. In order to show the equivalent boundary integral system is Fredholm of index zero, some relationships concerning the tangential potential operator is used. Due to the mixed oblique derivative boundary conditions, we cannot employ the factorization method in a usual manner to reconstruct the cracks. An alternative technique is used in the theoretical analysis such that the far field operator can be factorized in an appropriate form and fulfills the range identity theorem. Finally, we present some numerical examples to demonstrate the feasibility and effectiveness of the factorization method.     

Scattering of a spherical wave by a small ellipsoid

A point generated incident field impinges upon a small triaxialellipsoid which is arbitrarily oriented with respect to thepoint source. The point source field is so modified as to beable to recover the corresponding results for plane wave incidencewhen the source recedes to infinity. The main difficulty insolving analytically this low-frequency scattering problem concernsthe fitting of the spherical geometry, which characterizes theincident field, with the ellipsoidal geometry which is naturallyadapted to the scatterer. A series of techniques has been usedwhich lead finally to analytic solutions for the leading twolow-frequency terms of the near as well as the far field. Incontrast to the near-field approximations, which are expressedin terms of ellipsoidal eigenexpansions, the far field is furnishedby a finite number of terms. This is very interesting becausethe constants entering the expressions of the Lamé functionsof degree higher than three are not obtainable analyticallyand therefore, in the near field, not even the Rayleigh approximationcan be completely obtained. On the other hand, since only afew terms survive at the far field, the scattering amplitudeand the scattering cross-section are derived in closed form.It is shown that, in practice, if the source is located a distanceequal to five or six times the biggest semiaxis of the ellipsoidthe Rayleigh term of the approximation behaves almost as theincident field was a plane wave. The special cases of spheroids,needles, discs, spheres as well as plane wave incidence arerecovered. Finally, some theorems concerning monopole and dipolesurface potentials are included.     

Scattering relations for point‐source excitation in chiral media

A spherical electromagnetic wave propagating in a chiral medium is scattered by a bounded chiral obstacle which can have any of the usual properties. Reciprocity and general scattering theorems, relating the scattered fields due to scattering of waves from a point source put in any two different locations are established. Applying the general scattering theorem for appropriate locations and polarizations of the point source we prove an associated forward scattering theorem. Mixed scattering relations, relating the scattered fields due to a plane wave and the far‐field patterns due to a spherical wave, are also established. Copyright © 2005 John Wiley & Sons, Ltd.     

Integral equation methods for scattering by infinite rough surfaces

In this paper, we consider the Dirichlet and impedance boundary value problems for the Helmholtz equation in a non‐locally perturbed half‐plane. These boundary value problems arise in a study of time‐harmonic acoustic scattering of an incident field by a sound‐soft, infinite rough surface where the total field vanishes (the Dirichlet problem) or by an infinite, impedance rough surface where the total field satisfies a homogeneous impedance condition (the impedance problem). We propose a new boundary integral equation formulation for the Dirichlet problem, utilizing a combined double‐ and single‐layer potential and a Dirichlet half‐plane Green's function. For the impedance problem we propose two boundary integral equation formulations, both using a half‐plane impedance Green's function, the first derived from Green's representation theorem, and the second arising from seeking the solution as a single‐layer potential. We show that all the integral equations proposed are uniquely solvable in the space of bounded and continuous functions for all wavenumbers. As an important corollary we prove that, for a variety of incident fields including an incident plane wave, the impedance boundary value problem for the scattered field has a unique solution under certain constraints on the boundary impedance. Copyright © 2003 John Wiley & Sons, Ltd.     

Time‐dependent scattering of generalized plane waves by a wedge

We obtain explicit formulas for the scattering of plane waves with arbitrary profile by a wedge under Dirichlet, Neumann and Dirichlet‐Neumann boundary conditions. The diffracted wave is given by a convolution of the profile function with a suitable kernel corresponding to the boundary conditions. We prove the existence and uniqueness of solutions in appropriate classes of distributions and establish the Sommerfeld type representation for the diffracted wave. As an application, we establish (i) stability of long‐time asymptotic local perturbations of the profile functions and (ii) the limiting amplitude principle in the case of a harmonic incident wave. Copyright © 2015 John Wiley & Sons, Ltd.     

Identification of objects in an acoustic wave guide inversion II: Robin–Dirichlet conditions

In this paper we investigate the unknown body problem in a wave guide where one boundary has a pressure release condition and the other an impedance condition. The method used in the paper for solving the unknown body inverse problem is the intersection canonical body approximation (ICBA). The ICBA is based on the Rayleigh conjecture, which states that every point on an illuminated body radiates sound from that point as if the point lies on its tangent sphere. The ICBA method requires that an analytical solution be known exterior to a canonical body in the wave guide. We use the sphere of arbitrary centre and radius in the wave guide as our canonical body. We are lead then to analytically computing the exterior solution for a sphere between two parallel plates. We use the ICBA to construct solutions at points ranging over the suspected surface of the unknown object to reconstruct the unknown object using a least‐squares matching of computed, acoustic field against the measured, scattered field. Copyright © 2005 John Wiley & Sons, Ltd.     

A complementary theory of light scattering by homogeneous spheres

A theory of the scattering of electromagnetic waves by homogeneous spheres, the so-called Mie theory, is presented in a unique and coherent manner in this paper. We begin with Maxwell's equations, from which the vector wave equations are derived and solved by means of the two orthogonal solutions to the scalar wave equation. The transverse incident electric field is mapped in spherical coordinates and expanded in known mathematical functions satisfying the scalar wave equation. Determination of the unknown coefficients in the scattered and internal fields is achieved by matching the electromagnetic boundary conditions on the surface of a sphere. Far-field solutions for the electric field are then given in terms of the scattering functions. Transformation of the electric field to the reference plane containing incident and scattered waves is carried out. Extinction parameters and the phase matrix are derived from the electric field perpendicular and parallel to the reference plane. On the basis of the independent-scattering assumption, the theory is extended to cases involving a sample of homogeneous spheres.     

Analysis of double surface plasmon resonance by the discrete source method

The discrete source method is modified in order to mathematically simulate and study the scattering properties of nonspherical particles located on the surface of a conducting film deposited on a glass prism. Both differential and integral scattering properties of metal nanoparticles are examined. It is shown that the scattering cross section behind the film can be increased by 10⁷ times by deforming the particle and shifting it with respect to the film. It is also shown that the scattered intensity distribution in the prism is localized in two directions, forming sharp narrow fingers with the intensity exceeding the incident wave amplitude by 15–30 times.     

Aspects of electromagnetic scattering in chiral media

In this work, we study two operators that arise in electromagnetic scattering in chiral media. We first consider electromagnetic scattering by a chiral dielectric with a perfectly conducting core. We define a chiral Calderon‐type surface operator in order to solve the direct electromagnetic scattering problem. For this operator, we state coercivity and prove compactness properties. In order to prove existence and uniqueness of the problem, we define some other operators that are also related to the chiral Calderon‐type operator, and we state some of their properties that they and their linear combinations satisfy. Then we sketch how to use these operators in order to prove the existence of the solution of the direct scattering problem. Furthermore, we focus on the electromagnetic scattering problem by a perfect conductor in a chiral environment. For this problem, we study the chiral far‐field operator that is defined on a unit sphere and contains the far‐field data, and we state and prove some of its properties that are preliminaries properties for solving the inverse scattering problem. Copyright © 2016 John Wiley & Sons, Ltd.     

Inverse scattering from an open arc

A Newton method is presented for the approximate solution of the inverse problem to determine the shape of a sound-soft or perfectly conducting arc from a knowledge of the far-field pattern for the scattering of time-harmonic plane waves. Fréchet differentiability with respect to the boundary is shown for the far-field operator, which for a fixed incident wave maps the boundary arc onto the far-field pattern of the scattered wave. For the sake of completeness, the first part of the paper gives a short outline on the corresponding direct problem via an integral equation method including the numerical solution.     

Low Frequency Scattering by Spheroids and Disks 2. Neumann Problem for a Prolate Spheroid

The problem of scattering of a scalar plane wave by a prolatespheroid is solved for Neumann boundary condition, arbitrarymajor to minor axis ratio, and arbitrary incident direction.The solution is obtained by using an iterative method appliedto solutions of the corresponding potential problem and is expressedas a series of products of Legendre and trigonometric functionsand ascending powers of wave number. A recursion relation forthe coefficients in this series is derived. These results andthe corresponding results for the Dirichlet case are employedto calculate scattering cross-sections for 2: 1, 5: 1 and 10:1 prolate spheroids.     

Dynamical stability of compressible drops and stars

Interest is directed to linearized free boundary motion of a compressible liquid subject to surface tension and self‐gravitation respectively. Linearization relative to an a‐priori given solution to the non‐linear equations leads to a non‐local second order evolution problem to be posed in a space‐time cylinder with variable cross section subject to Fréchet boundary conditions along the lateral boundary part. Well‐posedness of the corresponding initial value problem in a natural weak formulation is proved. Copyright © 2005 John Wiley & Sons, Ltd.     

Elastic Wave Velocities in Two-component Systems

The problem is formulated for the scattering of long wavelengthplane elastic waves by a single homogeneous obstacle in an infiniteelastic medium. The problem differs from earlier studies inthat discrete differences in the physical properties are permittedto exist at the boundary. Earlier treatments of the scatteringproblem in which the Green's function for the scatterer is asimple operation on the incident field are inadequate to accountfor the refraction of the field at the boundary. For the caseof scattering of long waves by a spherical obstacle the scatteredfields are shown to involve the elastic constants in identicallythe same way as does the static field for the same geometryin the presence of a uniform static field. As a special casea new solution is given for the static problem of the inhomogeneousinclusion. A wave equation is derived for the "average" fielddue to multiple scattering by a statistical distribution ofspheres. The macroscopic wave parameters for the long wavelengthapproximation are obtained as a weighted contribution of theproperties of the two components.