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The Problem of Scattering by a Mixture of Cracks and Obstacles
Boundary Value Problems volume 2009, Article number: 524846 (2009)
Abstract
Consider the scattering of an electromagnetic time-harmonic plane wave by an infinite cylinder having an open crack and a bounded domain
in
as cross section. We assume that the crack
is divided into two parts, and one of the two parts is (possibly) coated on one side by a material with surface impedance
. Different boundary conditions are given on
and
. Applying potential theory, the problem can be reformulated as a boundary integral system. We obtain the existence and uniqueness of a solution to the system by using Fredholm theory.
1. Introduction
Crack detection is a problem in nondestructive testing of materials which has been often addressed in literature and more recently in the context of inverse problems. Early works on the direct and inverse scattering problem for cracks date back to 1995 in [1] by Kress. In that paper, Kress considered the direct and inverse scattering problem for a perfectly conducting crack and used Newton's method to reconstruct the shape of the crack from a knowledge of the far-field pattern. In 1997, Mnch considered the same scattering problem for sound-hard crack [2], and in the same year, Alves and Ha Duong discussed the scattering problem but for flat cracks in [3]. Later in 2000, Kress's work was continued by Kirsch and Ritter in [4] who used the factorization method to reconstruct the shape of the crack from the knowledge of the far-field pattern. In 2003, Cakoni and Colton in [5] considered the direct and inverse scattering problem for cracks which (possibly) coated on one side by a material with surface impedance
. Later in 2008, Lee considered an inverse scattering problem from an impedance crack and tried to recover impedance function from the far field pattern in [6]. However, studying an inverse problem always requires a solid knowledge of the corresponding direct problem. Therefore, in the following we just consider the direct scattering problem for a mixture of a crack
and a bounded domain
, and the corresponding inverse scattering problem can be considered by similar methods in [1, 2, 4–12] and the reference therein.
Briefly speaking, in this paper we consider the scattering of an electromagnetic time-harmonic plane wave by an infinite cylinder having an open crack and a bounded domain
in
as cross section. We assume that the cylinder is (possibly) partially coated on one side by a material with surface impedance
. This corresponds to the situation when the boundary or more generally a portion of the boundary is coated with an unknown material in order to avoid detection. Assuming that the electric field is polarized in the
mode, this leads to a mixed boundary value problem for the Helmholtz equation defined in the exterior of a mixture in
.
Our aim is to establish the existence and uniqueness of a solution to this direct scattering problem. As is known, the method of boundary integral equations has widely applications to various direct and inverse scattering problems (see [13–17] and the reference therein). A few authors have applied such method to study the scattering problem with mixture of cracks and obstacles. In the following, we will use the method of boundary integral equations and Fredholm theory to obtain the existence and uniqueness of a solution. The difficult thing is to prove the corresponding boundary integral operator which is a Fredholm operator with index zero since the boundary is a mixture and we have complicated boundary conditions.
The outline of the paper is as follows. In Section 2, the direct scattering problem is considered, and we will establish uniqueness to the problem and reformulate the problem as a boundary integral system by using single- and double-layer potentials. The existence and uniqueness of a solution to the corresponding boundary integral system will be given in Section 3. The potential theory and Fredholm theory will be used to prove our main results.
2. Boundary Integral Equations of the Direct Scattering Problem
Consider the scattering of time-harmonic electromagnetic plane waves from an infinite cylinder with a mixture of an open crack and a bounded domain
in
as cross section. For further considerations, we suppose that
has smooth boundary
(e.g.,
), and the crack
(smooth) can be extended to an arbitrary smooth, simply connected, closed curve
enclosing a bounded domain
such that the normal vector
on
coincides with the outward normal vector on
which we again denote by
. The bounded domain
is located inside the domain
, and
.
In the whole paper, we assume that and
.
Suppose that

where is an injective piecewise
function. We denote the outside of
with respect to the chosen orientation by
and the inside by
. Here we suppose that the
is divided into two parts
and
and consider the electromagnetic field E-polarized. Different boundary conditions on
,
and
lead to the following problem:

where for
and
for
. The total field
is decomposed into the given incident field
, and the unknown scattered field
which is required to satisfy the Sommerfeld radiation condition

uniformly in with
.
We recall some usual Sobolev spaces and some trace spaces on in the following.
Let be a piece of the boundary. Use
and
to denote the usual Sobolev spaces,
is the trace space, and we define

Just consider the scattered field , then (2.2) and (2.3) are a special case of the following problem.
Given ,
,
and
find
such that

and is required to satisfy the Sommerfeld radiation condition (2.3). For simplicity, we assume that
and
.
Theorem 2.1.
The problems (2.5) and (2.3) have at most one solution.
Proof.
Let be a solution to the problem (2.5) with
, we want to show that
in
.
Suppose that (with boundary
) is a sufficiently large ball which contains the domain
. Obviously, to the Helmholtz equation in (2.5), the solution
satisfies the following transmission boundary conditions on the complementary part
of
:

where "" denote the limit approaching
from outside and inside
, respectively. Applying Green's formula for
and
in
and
, we have

where is directed into the exterior of the corresponding domain.
Using boundary conditions on ,
and the above transmission boundary condition (2.6), we have

Hence

So, from [13, Theorem ] and a unique continuation argument we obtain that
in
.
We use and
to denote the jump of
and
across the crack
, respectively. Then we have the following.
Lemma 2.2.
If is a solution of (2.5) and (2.3), then
and
.
The proof of this lemma can be found in [11].
We are now ready to prove the existence of a solution to the above scattering problem by using an integral equation approaching. For , by Green representation formula

and for

where

is the fundamental solution to the Helmholtz equation in , and
is a Hankel function of the first kind of order zero.
By making use of the known jump relationships of the single- and double-layer potentials across the boundary (see [5, 11]) and approaching the boundary
from inside
, we obtain (for
)


where ,
,
, and
are boundary integral operators:

defined by (for )

Similarly, approaching the boundary from inside
we obtain (for
)


From (2.13)–(2.18), we have


Restrict on
, from (2.19) we have

where means a restriction to
.
Define

Then zero extend ,
, and
to the whole
in the following:

By using the boundary conditions in (2.5), we rewrite (2.21) as

where

Furthermore, we modify (2.24) as

where the operator is the operator applied to a function with
and evaluated on
, with analogous definition for
,
, and
. We have mapping properties (see [5, 11])

Again from (2.13)–(2.18), restricting to boundary
we have

or

Like previous, define

Then we can rewrite (2.29) as

where

Similar to (2.26), we modify (2.31) as

and we have mapping properties:

Combining (2.13) and (2.14),


Using (2.17) and (2.18),

Then using (2.36),

From (2.29), we have

Restricting (2.38) to and using (2.39), we modify (2.38) as

where

for .
Define

and using the notation in previous, we can rewrite (2.40) as

or

where the operators ,
, and
are restriction operators (see (2.29)). As before, we have mapping properties:

By using Green formula and approaching the boundary from inside
we obtain (for
)

The last term in (2.46) can be reformulated as

Since and
in (2.47), we have the following result (see [13]).
Lemma 2.3.
By using Green formula and the Sommerfeld radiation condition (2.3), one obtains

Proof.
Denote by a sufficiently large ball with radius
containing
and use Green formula inside
. Furthermore noticing
,
, and the Sommerfeld radiation condition (2.3), we can prove this lemma.
Combining (2.46), (2.47), and Lemma 2.3 and restricting to
we have

where

Define

and then we can rewrite (2.49) as

Similarly, and
are restriction operators as before, and we have mapping properties:

Combining (2.52), (2.26), (2.33), and (2.44), we have

If we define

then (2.54) can be rewritten as a boundary integral system:

Remark 2.4.
If the above system (2.56) has a unique solution, our problem (2.5) with (2.3) will have a unique solution (see [13, 14]).
3. Existence and Uniqueness
Based on the Fredholm theory, we show the existence and uniqueness of a solution to the integral system (2.56).
Define

and its dual space

Theorem 3.1.
The operator maps
continuously into
and is Fredholm with index zero.
Proof.
As is known, the operator is positive and bounded below up to a compact perturbation (see [18]); that is, there exists a compact operator

such that

where denote the duality between
and
.
For convenience, in the following discussion we define

Similarly, the operators and
are positive and bounded below up to compact perturbations (see [18]), that is, there exist compact operators

such that

Define and
, then
and
are bounded below up and positive.
Take , and let
,
, and
be the extension by zero to
of
,
, and
respectively.
Denote .
It is easy to check that the operators ,
,
,
,
, and
are compact operators, and then we can rewrite
as the following:

with

where is compact and
defines a sesquilinear form, that is,

Here denotes the scalar product on
or
defined by
or
, and
is the scalar product on
(
).
By properties of the operators ,
, and
, we have

Similarly,

So the operator is coercive, that is,

whence the operator is Fredholm with index zero.
Theorem 3.2.
The operator has a trivial kernel if
is not Dirichlet eigenvalue of the Laplace operator in
.
Proof.
In this part, we show that . To this end let
be a solution of the homogeneous system
, and we want to prove that
.
However, means that

Define a potential

where ,
and
have the same meaning as before and

This potential satisfies Helmholtz equation in
and the Sommerfeld radiation condition (see [13, 14]).
Considering the potential inside
and approaching the boundary
(
), we have

and (3.14) implies that

Similarly, considering the potential inside
and approaching the boundary
(
), then restricting
to the partial boundary
:

and restricting to the partial boundary
, we have

Now, we consider the potential in the region
and approach the boundary
(
), and then restricting
to the partial boundary
, similar to (3.19), we have

Refering to (3.20),

Combining (3.22), from (3.14) we have

From (3.18)–(3.23), the potential satisfies the following boundary value problem:

and the Sommerfeld radiation condition:

uniformly in with
.
The uniqueness result Theorem 2.1 in Section 2 implies that

Notice that is not Dirichlet eigenvalue of the Laplace operator in
, and so

Therefore, the well-known jump relationships (see [13, 14]) imply that

So we complete the proof of the theorem.
Combining Theorems 3.1 and 3.2, we have the following
Theorem 3.3.
The boundary integral system (2.56) has a unique solution.
Remark 3.4.
If we remove the condition that " is not Dirichlet eigenvalue of the Laplace operator in
," instead of it by the assumption that Im
, then Theorem 2.1 in Section 2 and Theorem 3.3 in Section 3 are also true.
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Acknowledgment
This research is supported by NSFC Grant no. 10871080, Laboratory of Nonlinear Analysis of CCNU, COCDM of CCNU.
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Yan, G. The Problem of Scattering by a Mixture of Cracks and Obstacles. Bound Value Probl 2009, 524846 (2009). https://doi.org/10.1155/2009/524846
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DOI: https://doi.org/10.1155/2009/524846
Keywords
- Compact Operator
- Boundary Integral Equation
- Scatter Problem
- Partial Boundary
- Inverse Scattering Problem