Category:Multipole Methods for Linear Water Waves
Multipole expansions are a technique used in linear water wave theory where the potential is represented as a sum of singularities (multipoles) placed within any structures that are present. Multipoles can be constructed with their singularity submerged or on the free surface and these are treated separately.
(note that the last expression can be obtained from combining the expressions:
In two-dimensions the Sommerfeld Radiation Condition is
where is the incident potential.
We consider here the theory only in two dimensions. The multipoles are singular at a point and they have various orders of singularity. For
where are the polar coordinates centered at A linear combination of these multipoles can then be made to satisfy the body boundary conditions.
Motivation for Multipoles
We present here the motivation for multipoles. For the case of Laplace's equation in an infinite region surrounding a disk we may construct the solution very simply using a separation of variables solution.
Consider Laplace's equation for a disk of radius (refer to Figure 1) centered at the origin in an infinite medium. The Laplace's equation in polar coordinates is
We also have boundary conditions at the disk which we assume are Neumann given by
and we have a decay condition at infinity
Using separation of variables we write Substituting into Laplace's equations gives
The equation for is
When the solution is
The equation for is
This is a standard differential equation, to solve we substitute giving:
This gives two independent solutions for all .
So the solution for is
The derivatives of the radial eigenfunctions must go to zero indicating zero fluid flow at infinity:
Hence the general solution can be expressed as:
The solution can also be expressed simply in terms of complex exponentials.
With the Free Surface Boundary Condition
When we introduce the free surface boundary condition we need to find a new set of eigenfunctions. We can use eigenfunctions from the infinite domain problem to construct them.
Here and are polar coordinates defined by , , note also that the origin is in the free surface directly above the center of the disc for cartesian coordinates. However the origin for the polar coordinates is still at the center of the disk, as shown in Figure 1.
We add a solution of Laplace's equation to the eigenfunctions for the infinite domain problem and require that these new functions (multipoles) satisfy the free surface condition.
We want the multipoles to satisfy the free surface condition
For the free surface condition gives our boundary condition for :
where we used the following integral representation of the singularity (see Ref.)
The singularity at (refer to Figure 1) is not of the form since this would give a potential that corresponds to a pulsating source of fluid. From this point on Multipoles with higher order with a submerged singularity are only considered for this problem
For the free surface condition yields the boundary condition for :
where we used the following integral representation of the singularity (see Ref.) (valid for )
The functions now satisfy the following problem:
We solve for by taking a Fourier transform in to simplify Laplace's equation
After applying the Fourier transform we have
The solution of this equation is
After applying the boundary condition at
We apply the Fourier transform to the surface boundary condition
we can obtain by the inverse Fourier transform
For the form of can be obtained from by rewriting it as a Fourier transform. Refer to Appendix A2 for confirmation that changing the limits of integration with different constant outside of integration for gives the same result. Now substituting the result into , easily simplifies to:
Note: the integral is singular at .
can be expanded in a power series:
Note: the integral is singular at . Refer to Appendix A1 on steps required to numerically integrate this singular integral.
This result is arrived at by expanding the exponential part of the integral of . So we have
This identity is then substituted into the expression for to complete the power series.
In order to have a complete set to expand our fluid potential we need to include for . This accounts for the second linearly independent solution for .
The combination , corresponds to a wave free singularity i.e. no waves are radiated to infinity so the potential dies off in the far field.
where is defined by .
The above relationship can be simplified by using the coordinate relationships defined below. The coordinates that are use are illustrated in Figure 2.
Resulting in the expression:
Once again to obtain a complete set we also use the complex conjugates of these potentials.
Example: 2D Wave Scattering for a Submerged Horizontal Cylinder (in Infinite Depth)
- Picture to show problem set up***
The potential for the scattering problem splits as incident and diffracted potentials and also into symmetric and antisymmetric parts.
and are the symmetric and antisymmetric parts of respectively.
For a wave incident from the left we have where
The above power series converge for where and are both defined by .
Applying the body boundary condition and noting that the cosines are orthogonal over gives the results
From which we can see that .
The above sums can be truncated at the Nth term yielding NxN system of linear equations that can be solved for the coefficients
Hence we can calculate from:
This expression also gives the values of the reflection and transmission coefficients when we substitute the far field expressions for the symmetric and antisymmetric multipoles.
In the far field:
Soon will write something here
The coefficients are evaluated by the following singular integral:
To evaluate this singular integral numerically we separate into less onerous components. If we let
and if we let it follows that , then
The first integral is no longer singular and can be numerically evaluated using an adaptive Gauss-Kronrod quadrature. The second integral remains singular and can be transformed into a series of known functions as follows:
now if we let it follows that . The limits of integration follow suit, at and at .
We can write:
We can rewrite as an inverse Fourier transform by using the Euler identity and the odd and even properties of sine and cosine. Allowing us to obtain :
Since is even, its integral against sine over all is zero so we can replace the cosine with a complex exponential in the first integral (similarly we replace (-i sine) for the second).
Finally, combining the integrals, emerges:
and so these two integrals are equivalent
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