# Eigenfunction Matching for a Semi-Infinite Change in Depth

## Introduction

The problem consists of a region to the left with a free water surface with depth $\displaystyle{ d }$ and to the right a region of depth $\displaystyle{ h }$. The problem with a finite change in depth is treated in Eigenfunction Matching for a Finite Change in Depth

## Governing Equations

We begin with the Frequency Domain Problem for a submerged dock which occupies the region $\displaystyle{ x\gt 0 }$ (we assume $\displaystyle{ e^{-\mathrm{i}\omega t} }$ time dependence). The depth of is constant $\displaystyle{ h }$ for $\displaystyle{ x\lt 0 }$ and constant $\displaystyle{ d }$ for $\displaystyle{ x\gt 0 }$. The $\displaystyle{ z }$-direction points vertically upward with the water surface at $\displaystyle{ z=0 }$. The boundary value problem can therefore be expressed as

$\displaystyle{ \Delta\phi=0, \,\, -h\lt z \lt 0, \,\, x\lt 0 }$
$\displaystyle{ \Delta\phi=0, \,\, -d\lt z \lt 0, \,\, x\gt 0 }$
$\displaystyle{ \partial_z\phi=\alpha\phi, \,\, z=0, }$
$\displaystyle{ \partial_x\phi=0, \,\, -d\lt z \lt -h,\,x=0, }$
$\displaystyle{ \partial_z\phi=0, \,\, z=-h,\, x\lt 0 }$
$\displaystyle{ \partial_z\phi=0, \,\, z=-d,\, x\gt 0 }$

We must also apply the Sommerfeld Radiation Condition as $\displaystyle{ |x|\rightarrow\infty }$. This essentially implies that the only wave at infinity is propagating away and at negative infinity there is a unit incident wave and a wave propagating away.

## Solution Method

We use separation of variables in the two regions, $\displaystyle{ x\lt 0 }$ and $\displaystyle{ x\gt 0 }$.

We express the potential as

$\displaystyle{ \phi(x,z) = X(x)Z(z)\, }$

and then Laplace's equation becomes

$\displaystyle{ \frac{X^{\prime\prime}}{X} = - \frac{Z^{\prime\prime}}{Z} = k^2 }$

### Separation of variables for a free surface

We use separation of variables

We express the potential as

$\displaystyle{ \phi(x,z) = X(x)Z(z)\, }$

and then Laplace's equation becomes

$\displaystyle{ \frac{X^{\prime\prime}}{X} = - \frac{Z^{\prime\prime}}{Z} = k^2 }$

The separation of variables equation for deriving free surface eigenfunctions is as follows:

$\displaystyle{ Z^{\prime\prime} + k^2 Z =0. }$

subject to the boundary conditions

$\displaystyle{ Z^{\prime}(-h) = 0 }$

and

$\displaystyle{ Z^{\prime}(0) = \alpha Z(0) }$

We can then use the boundary condition at $\displaystyle{ z=-h \, }$ to write

$\displaystyle{ Z = \frac{\cos k(z+h)}{\cos kh} }$

where we have chosen the value of the coefficent so we have unit value at $\displaystyle{ z=0 }$. The boundary condition at the free surface ($\displaystyle{ z=0 \, }$) gives rise to:

$\displaystyle{ k\tan\left( kh\right) =-\alpha \, }$

which is the Dispersion Relation for a Free Surface

The above equation is a transcendental equation. If we solve for all roots in the complex plane we find that the first root is a pair of imaginary roots. We denote the imaginary solutions of this equation by $\displaystyle{ k_{0}=\pm ik \, }$ and the positive real solutions by $\displaystyle{ k_{m} \, }$, $\displaystyle{ m\geq1 }$. The $\displaystyle{ k \, }$ of the imaginary solution is the wavenumber. We put the imaginary roots back into the equation above and use the hyperbolic relations

$\displaystyle{ \cos ix = \cosh x, \quad \sin ix = i\sinh x, }$

to arrive at the dispersion relation

$\displaystyle{ \alpha = k\tanh kh. }$

We note that for a specified frequency $\displaystyle{ \omega \, }$ the equation determines the wavenumber $\displaystyle{ k \, }$.

Finally we define the function $\displaystyle{ Z(z) \, }$ as

$\displaystyle{ \chi_{m}\left( z\right) =\frac{\cos k_{m}(z+h)}{\cos k_{m}h},\quad m\geq0 }$

as the vertical eigenfunction of the potential in the open water region. From Sturm-Liouville theory the vertical eigenfunctions are orthogonal. They can be normalised to be orthonormal, but this has no advantages for a numerical implementation. It can be shown that

$\displaystyle{ \int\nolimits_{-h}^{0}\chi_{m}(z)\chi_{n}(z) \mathrm{d} z=A_{n}\delta_{mn} }$

where

$\displaystyle{ A_{n}=\frac{1}{2}\left( \frac{\cos k_{n}h\sin k_{n}h+k_{n}h}{k_{n}\cos ^{2}k_{n}h}\right). }$

### Expansion of the potential

We need to apply some boundary conditions at plus and minus infinity, where are essentially the the solution cannot grow. This means that we only have the positive (or negative) roots of the dispersion equation. However, it does not help us with the purely imaginary root. Here we must use a different condition, essentially identifying one solution as the incoming wave and the other as the outgoing wave.

Therefore the scattered potential (without the incident wave, which will be added later) can be expanded as

$\displaystyle{ \phi(x,z)=\sum_{m=0}^{\infty}a_{m}e^{k_{m}^{h} x}\phi_{m}^{h}(z), \;\;x\lt 0 }$

and

$\displaystyle{ \phi(x,z)=\sum_{m=0}^{\infty}b_{m} e^{-k_{m}^{d}x}\phi_{m}^{d}(z), \;\;x\gt 0 }$

where $\displaystyle{ a_{m} }$ and $\displaystyle{ b_{m} }$ are the coefficients of the potential in the left and right respectively and $\displaystyle{ k_{m}^{h} }$ denotes the solution for depth $\displaystyle{ h }$ etc.

### Incident potential

To create meaningful solutions of the velocity potential $\displaystyle{ \phi }$ in the specified domains we add an incident wave term to the expansion for the domain of $\displaystyle{ x \lt 0 }$ above. The incident potential is a wave of amplitude $\displaystyle{ A }$ in displacement travelling in the positive $\displaystyle{ x }$-direction. We would only see this in the time domain $\displaystyle{ \Phi(x,z,t) }$ however, in the frequency domain the incident potential can be written as

$\displaystyle{ \phi_{\mathrm{I}}(x,z) =e^{-k_{0}x}\chi_{0}\left( z\right). }$

The total velocity (scattered) potential now becomes $\displaystyle{ \phi = \phi_{\mathrm{I}} + \phi_{\mathrm{D}} }$ for the domain of $\displaystyle{ x \lt 0 }$.

The first term in the expansion of the diffracted potential for the domain $\displaystyle{ x \lt 0 }$ is given by

$\displaystyle{ a_{0}e^{k_{0}x}\chi_{0}\left( z\right) }$

which represents the reflected wave.

In any scattering problem $\displaystyle{ |R|^2 + |T|^2 = 1 }$ where $\displaystyle{ R }$ and $\displaystyle{ T }$ are the reflection and transmission coefficients respectively. In our case of the semi-infinite dock $\displaystyle{ |a_{0}| = |R| = 1 }$ and $\displaystyle{ |T| = 0 }$ as there are no transmitted waves in the region under the dock.

### An infinite dimensional system of equations

The potential and its derivative must be continuous across $\displaystyle{ x=0 }$. Therefore, the potentials and their derivatives at $\displaystyle{ x=0 }$ have to be equal or equal to zero as appropriate. We obtain:

\displaystyle{ \begin{align} \phi_{0}^{h}\left( z\right) + \sum_{m=0}^{\infty}a_{m} \phi_{m}^{h}\left( z\right) &=\sum_{m=0}^{\infty}b_{m}\phi_{m}^{d}(z), \quad -h\lt z \lt 0 \\ -\sum_{m=0}^{\infty}b_{m}k_{m}^{d}\phi_{m}^{d}(z)= & \begin{cases} -k_{0}^{h}\phi_{0}^{h}\left( z\right) +\sum_{m=0}^{\infty} a_{m}k_{m}^{h}\phi_{m}^{h}\left( z\right),\quad -h\lt z \lt 0 \\ 0,\quad -d\lt z \lt -h \end{cases} \end{align} }

For the first equation we multiply both sides by $\displaystyle{ \phi_{n}^{h}(z) \, }$ and integrating from $\displaystyle{ -h }$ to $\displaystyle{ 0 }$ to obtain:

$\displaystyle{ A_{0}^{h}\delta_{0n} + a_{n}A_{n}^{h} = \sum^{\infty}_{m=0}b_{m}B_{mn}, n\in\mathbb{N}\cup\left\{0\right\} }$

and for the second equation we multiply both sides by $\displaystyle{ \phi_{n}^{d}(z) \, }$ and integrating from $\displaystyle{ -d }$ to $\displaystyle{ 0 }$ to obtain:

$\displaystyle{ -k_{0}^{h}B_{n0} + \sum^{\infty}_{m=0}a_{m}k_{m}^{h}B_{nm} = -b_{n}^{d}k_{n}^{d}A_{n}^{d}, n\in\mathbb{N}\cup\left\{0\right\} }$

Solving the equations above will yield the coefficients of the water velocity potential in the dock covered region.

### Inner product between free surface and dock modes

$\displaystyle{ B_{mn} = \int\nolimits_{-h}^{0}\phi_{m}^{d}(z)\phi_{n}^{h}(z) \mathrm{d} z }$

where

$\displaystyle{ B_{mn}= \int\nolimits_{-h}^{0} \frac{\cos(k_{m}^{h}(z+h))\cos(k_{n}^{d}(z+d))}{\cos(k_{m}h)\cos(k_{n}d)} \mathrm{d} z =\frac{k_{m}^{h}\sin(k_{m}^{d} h)\cos(k_{n}^{d}h)-k_{n}^{d}\cos(k_{m}^{h} h)\sin(k_{n}^{d}h)} {\cos(k_{m}h)\cos(k_{n}d)({k_{m}^{d}}^{2}-{k_{n}^{d}}^{2})} }$

## Matlab Code

A program to calculate the coefficients for the semi-infinite change in depth can be found here semi_infinite_change_in_depth.m