Difference between revisions of "Green Function Methods for Floating Elastic Plates"

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 +
{{complete pages}}
 +
 
== Introduction ==
 
== Introduction ==
  
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using a [[Free-Surface Green Function]] by [[Newman 1994]] and [[Meylan and Squire 1994]]. We describe
 
using a [[Free-Surface Green Function]] by [[Newman 1994]] and [[Meylan and Squire 1994]]. We describe
 
here both methods (which are closely related).
 
here both methods (which are closely related).
A related paper was given by [[Hermans 2003]] and we extended to  
+
A related paper was given by [[Hermans 2003]] and this was extended to  
 
multiple plates in [[Hermans 2004]].
 
multiple plates in [[Hermans 2004]].
  
Line 10: Line 12:
 
We begin with the equations. The solution can also be found using  
 
We begin with the equations. The solution can also be found using  
 
[[Eigenfunction Matching for a Finite Floating Elastic Plate using Symmetry]].
 
[[Eigenfunction Matching for a Finite Floating Elastic Plate using Symmetry]].
 +
 +
The simpler problem of dock is treated in [[Green Function Method for Finite Dock]]
  
 
== Equations for a Finite Plate in Frequency Domain ==
 
== Equations for a Finite Plate in Frequency Domain ==
 
{{finite plate frequency domain}}
 
{{finite plate frequency domain}}
  
 +
== Transformation using [[Eigenfunctions for a Uniform Free Beam]] ==
  
 +
{{equations for a eigenfunction of a free beam}}
  
== Transformation using [[Eigenfunctions for a Free Beam]] ==
+
This solution is discussed further in [[Eigenfunctions for a Free Beam]]. <br /><br />
 
+
Using the expression <math>\partial_n \phi =\partial_t w</math>, we can form
{{equations for a free beam}}
 
 
 
This solution is discussed further in [[Eigenfunctions for a Free Beam]].
 
 
 
Expanding
 
 
<center>
 
<center>
 
<math>
 
<math>
\frac{\partial \phi}{\partial z} = i\omega \sum_{n=0}^{\infty} \zeta_n w_n
+
\frac{\partial \phi}{\partial z} = -\mathrm{i}\omega \sum_{n=0}^{\infty} \zeta_n X_n
 
</math>
 
</math>
 
</center>
 
</center>
 +
where <math>\zeta_n \,</math> are coefficients to be evaluated.
  
 
== Equation in Terms of the Modes of the Plate ==
 
== Equation in Terms of the Modes of the Plate ==
Line 42: Line 44:
 
</math></center>
 
</math></center>
 
<center><math>
 
<center><math>
i\omega\sum_{n=0}^{\infty}\xi_{n}w_{n}    =\partial_{z}\phi,\,\,x\in
+
i\omega\sum_{n=0}^{\infty}\zeta_{n}X_{n}    =\partial_{z}\phi,\,\,x\in
 
(-L,L),\,\, z=0,
 
(-L,L),\,\, z=0,
 
</math></center>
 
</math></center>
 
<center><math>
 
<center><math>
\sum_{n=0}^{\infty}\alpha_{n}\left(  1+\beta\lambda_{n}^{4}\right)
+
\sum_{n=0}^{\infty}\zeta_{n}\left(  1+\beta\lambda_{n}^{4}\right)
w_{n}-\alpha\gamma\sum_{n=0}^{\infty}\alpha_{n}w_{n}    = -i\omega
+
X_{n}-\alpha\gamma\sum_{n=0}^{\infty}\zeta_{n}X_{n}    = -i\omega
 
\phi,\,\,x\in(-L,L),\,\, z=0.
 
\phi,\,\,x\in(-L,L),\,\, z=0.
 
</math></center>
 
</math></center>
Line 53: Line 55:
 
the diffracted and radiation potentials in the standard way,
 
the diffracted and radiation potentials in the standard way,
 
as for a rigid body.
 
as for a rigid body.
We begin with the diffraction potential <math>\phi^{(d)}</math> which
 
satisfies the following equations
 
 
<center><math>
 
<center><math>
\Delta\phi^{(d)}   =0,\,\,-h<z<0,
+
\phi=\phi^{\mathrm{D}}+\phi^{\mathrm{R}} ,\,
 
</math></center>
 
</math></center>
 +
We begin with the diffraction potential <math>\phi^{\mathrm{D}}</math> which
 +
satisfies the following equations
 +
{{diffraction potential equations for a dock}}
 +
{{radiation condition for diffracted potential}}
 +
 +
As the plate is floating on the surface, we can denote it as follows:
 
<center><math>
 
<center><math>
\partial_{z}\phi^{(d)}   =0,\,\,z=-h,
+
\phi^{\rm I}|_{z=0= e^{-k_0 x} \,
 
</math></center>
 
</math></center>
 +
where we have set the amplitude to be unity.
 +
 +
We now consider the scattered potentials <math>\phi^{\mathrm{S}}</math>.  The relationship between scattered potentials, diffracted potentials and the incident wave are as follows:
 
<center><math>
 
<center><math>
\partial_{z}\phi^{(d)}   =\alpha \phi^{(d)},\,\,x\notin(-L,L),\,\,
+
\phi^{\mathrm{D}}=\phi^{\mathrm{I}}+\phi^{\mathrm{S}} \,
z=0,
 
 
</math></center>
 
</math></center>
 +
from this, we can construct the following conditions:
 
<center><math>
 
<center><math>
\partial_{z}\phi^{(d)}    =0,\,\,x\in(-L,L),\,\,z=0.
+
\Delta\phi^{\mathrm{S}}    =0,\,\,-h<z<0,
 
</math></center>
 
</math></center>
Furthermore, <math>\phi^{(d)}</math>  satisfies the [[Sommerfeld Radiation Condition]]
 
 
<center><math>
 
<center><math>
\frac{\partial}{\partial x} \left(\phi^{(d)}-\phi_{\kappa}^{\rm
+
\partial_{z}\phi^{\mathrm{S}}   =0,\,\,z=-h,
In} \right) \pm ik\left( \phi^{(d)}-\phi^{\rm In}_{\kappa}\right)
 
= 0
 
,\,\,\mathrm{as}
 
\,\,x\rightarrow\pm\infty,
 
 
</math></center>
 
</math></center>
where
 
<math>k</math> is the wavenumber,
 
which is the positive real solution of the [[Dispersion Relation for a Free Surface]]
 
 
<center><math>
 
<center><math>
k\tanh(kh)=\omega^{2},
+
\partial_{z}\phi^{\mathrm{S}}    =\alpha\phi^{\mathrm{S}},\,\,x\notin(-L,L),\, \,
 +
z=0
 
</math></center>
 
</math></center>
and <math>\phi^{\rm In}</math> is the incident wave given by
 
 
<center><math>
 
<center><math>
\phi^{\rm In} = \frac{i\omega}{k\sinh kh} \cosh k(z+h) e^{-i kx}
+
\partial_{z}\phi^{\mathrm{S}}   = -\partial_{z}\phi^{\mathrm{I}},\,\,x\in(-L,L),\,\,z=0.
</math></center>
+
</math></center> <br /><br />
(which has unit amplitude in displacement) and
 
is travelling towards positive
 
infinity
 
  
We now consider the radiation potentials <math>\phi^{(n)}</math>, which satisfy the
+
We now consider the radiation potentials <math>\phi^{\mathrm{R}}</math>.  We can express the radiation potential as:
following equations
 
 
<center><math>
 
<center><math>
\Delta\phi^{(n)}    =0,\,\,-h<z<0,
+
\phi^{\mathrm{R}}=\sum_{n=0}^{\infty}\zeta_n \phi_n^{\mathrm{R}}
</math></center>
 
<center><math>
 
\partial_{z}\phi^{(n)}   =0,\,\,z=-h,
 
</math></center>
 
<center><math>
 
\partial_{z}\phi^{(n)}    =\alpha\phi^{(n)},\,\,x\notin(-L,L),\, \,
 
z=0
 
</math></center>
 
<center><math>
 
\partial_{z}\phi^{(n)}    = i\omega w_{n},\,\,x\in(-L,L),\,\,z=0.
 
</math></center>
 
The radiation condition for the radiation potential is
 
<center><math>
 
\frac{\partial\phi^{(n)}}{\partial x}\pm ik\phi^{(n)}=0,\,\,\mathrm{as}
 
\,\,x\rightarrow\pm\infty.
 
 
</math></center>
 
</math></center>
 +
which satisfy the following equations
 +
 +
{{radiation potential equations for a dock like structure}}
 +
 
Therefore we find the potential as
 
Therefore we find the potential as
 
<center><math>
 
<center><math>
\phi=\phi^{(s)} +\sum_{n=0}^{\infty}\xi_{n}\phi^{(n)},
+
\phi=\phi^{\mathrm{D}} +\sum_{n=0}^{\infty}\zeta_{n}\phi_n^{\mathrm{R}},
 
</math></center>
 
</math></center>
 
so that
 
so that
 
<center><math>
 
<center><math>
 
\sum_{n=0}^{\infty}\left( 1+\beta\lambda_{n}^{4} - \alpha\gamma\right)
 
\sum_{n=0}^{\infty}\left( 1+\beta\lambda_{n}^{4} - \alpha\gamma\right)
\xi_{n}w_{n}=-i\omega\phi
+
\zeta_{n}X_{n}=-i\omega \left( \phi^{\mathrm{D}}+\sum_{n=0}^{\infty}\zeta_{n}\phi_n^{\mathrm{R}} \right).
^{(s)}_\kappa-i\omega\sum_{n=0}^{\infty}\xi_{n}\phi^{(n)}.
 
 
</math></center>
 
</math></center>
If we multiply by <math>w_m</math> and take an inner product over the plate we obtain
+
If we multiply by <math>X_m</math> and take an inner product over the plate we obtain
 
<center><math>
 
<center><math>
\left(  1+\beta\lambda_{n}^{4} - \alpha\gamma\right)  \xi_{n}=-i\omega
+
\left(  1+\beta\lambda_{n}^{4} - \alpha\gamma\right)  \zeta_{n}=-i\omega
\int_{-b}^{b}\phi^{(s)}_\kappa w_{n}dx +
+
\int_{-L}^{L}\phi^{\mathrm{D}} X_{n}\mathrm{d}x +
 
\sum_{m=0}^{\infty}\left(\omega^2 a_{mn}(\omega) - i\omega b_{mn}(\omega)\right)
 
\sum_{m=0}^{\infty}\left(\omega^2 a_{mn}(\omega) - i\omega b_{mn}(\omega)\right)
\xi_{m},
+
\zeta_{m},
 
</math></center>
 
</math></center>
 
where the functions <math>a_{mn}(\omega)</math> and <math>b_{mn}(\omega)</math> are given by
 
where the functions <math>a_{mn}(\omega)</math> and <math>b_{mn}(\omega)</math> are given by
 
<center><math>
 
<center><math>
\omega^2 a_{mn}(\omega) -i\omega b_{mn}(\omega) = - i\omega\int_{-L}^{L}\phi^{(m)}w_{n}dx,
+
\omega^2 a_{mn}(\omega) -i\omega b_{mn}(\omega) = - i\omega\int_{-L}^{L}\phi_m^{\mathrm{R}}X_{n}\mathrm{d}x,
 
</math></center>
 
</math></center>
and they are referred to as the added mass and damping coefficients (see [[Added-Mass, Damping Coefficients And Exciting Forces]]
+
and they are referred to as the added mass and damping coefficients (see [[Linear Wave-Body Interaction]]
 
for the equivalent definition for a rigid body).
 
for the equivalent definition for a rigid body).
 
respectively.
 
respectively.
Line 138: Line 123:
 
== Solution for the Radiation and Diffracted Potential ==
 
== Solution for the Radiation and Diffracted Potential ==
  
We use the [[Free-Surface Green Function]] for two-dimensional waves, with singularity at
+
{{Green's function equations for the diffracted potential}}
the water surface since we are only
 
interested in its value at <math>z=0.</math> Using this we can transform the  
 
system of equations to
 
  
<center><math>
 
\phi^{(d)}(x) = \phi^{i}(x) + \int_{-L}^{L}G(x,\xi)
 
\alpha\phi^{(d)}(\xi) d \xi
 
</math></center>
 
 
and  
 
and  
<center><math>
 
\phi^{(n)}(x) =  \int_{-L}^{L}G(x,\xi)
 
\left(
 
\alpha\phi^{(n)}(\xi) - i\omega w_n(\xi)
 
\right)d \xi
 
</math></center>
 
  
Details about this method can be found in [[Integral Equation for the Finite Depth Green Function at Surface]].
+
{{Green's function equations for the radiation potential for a dock like structure}}
  
 
== Reflection and Transmission Coefficients ==
 
== Reflection and Transmission Coefficients ==
  
We can calculate the Reflection and Transmission coefficients as follows
+
{{derivation of reflection and transmission in two dimensions}}
 
 
[[Image:energy_schematic.jpg|thumb|right|300px|A diagram depicting the area <math>\mathcal{U}</math> which is bounded by the rectangle <math>\mathcal{S}</math>.]
 
{A diagram depicting the area <math>\mathcal{U}</math> which is bounded by the rectangle <math>\mathcal{S}</math>.
 
The rectangle <math>\mathcal{S}</math> is bounded by <math> -h\leq z \leq0</math> and  <math>-\infty\leq x \leq \infty</math>]]
 
 
 
Applying Green's theorem to <math>\phi</math> and <math>\phi^{i} </math> gives
 
<center><math>
 
0 = \int\int_\mathcal{U}(\phi\nabla^2\phi^{i} - \phi^{i}\nabla^2\phi)dxdz
 
= \int_\mathcal{S}(\phi\frac{\partial\phi^*}{\partial n} - \phi^*\frac{\partial\phi}{\partial n})dl,
 
</math></center>
 
<center><math>
 
= \int_{-L}^{L} e^{k_0 x} \left(\alpha \phi(x) - \partial_n \phi(x)\right)dx
 
  - k_0 R  \int_{-h}^{0} \left(\phi_0(z)\right)^2 dz.
 
</math></center>
 
Therefore
 
<center><math>
 
R = k_0 \frac{\int_{-L}^{L} e^{k_0 x} \left(\alpha \phi(x) - \partial_n \phi(x)\right)dx }
 
{\int_{-h}^{0} \left(\phi_0(z)\right)^2 dz}.
 
</math></center>
 
and using a wave incident from the right we obtain
 
<center><math>
 
1 + T = k_0 \frac{\int_{-L}^{L} e^{-k_0 x} \left(\alpha \phi(x) - \partial_n \phi(x)\right)dx }
 
{\int_{-h}^{0} \left(\phi_0(z)\right)^2 dz}.
 
</math></center>
 
  
 
== Matlab Code ==
 
== Matlab Code ==
  
 +
{{elastic plate modes code}}
  
A program to calculate the solution in elastic modes can be found here
+
== Alternative Solution Method using Green Functions for a Uniform Plate ==
[http://www.math.auckland.ac.nz/~meylan/code/boundary_element/elastic_plate_modes.m elastic_plate_modes.m]
 
 
 
=== Additional code ===
 
 
 
This program requires
 
* {{green function surface code}}
 
* {{free surface dispersion equation code}}
 
 
 
== Alternative Solution Method using Green Function for the Plate ==
 
  
 
We can also solve the equation by a closely related method which was given in  
 
We can also solve the equation by a closely related method which was given in  
Line 203: Line 143:
 
We can transform the equations to
 
We can transform the equations to
 
<center><math>
 
<center><math>
\phi(x) = \phi^{\rm In}(x) + \int_{-L}^{L}G(x,\xi)
+
\phi(x) = \phi^{\rm I}(x) + \int_{-L}^{L}G(x,\xi)
 
\left(
 
\left(
 
\alpha\phi(\xi) - \partial_z\phi(\xi)
 
\alpha\phi(\xi) - \partial_z\phi(\xi)
\right)d \xi
+
\right)\mathrm{d} \xi
 
</math></center>
 
</math></center>
  
Line 212: Line 152:
 
<center>
 
<center>
 
<math>
 
<math>
\partial_z \phi = i\omega \sum \xi_n w_n
+
\partial_z \phi = i\omega \sum \xi_n X_n
 
</math>
 
</math>
 
</center>
 
</center>
 
we obtain
 
we obtain
 
<center><math>
 
<center><math>
-i\omega \phi = \sum \left(\beta\lambda_n^4 - \gamma\alpha + 1\right)\xi_n w_n
+
-i\omega \phi = \sum \left(\beta\lambda_n^4 - \gamma\alpha + 1\right)\xi_n X_n
 
</math>
 
</math>
 
</center>
 
</center>
Line 223: Line 163:
 
<center>
 
<center>
 
<math>
 
<math>
\partial_z\phi(x) = \frac{1}{\alpha} \int_{-L}^{L} \frac{w_n(x)w_n(\xi)}{\beta\lambda_n^4 - \gamma\alpha + 1} \phi(\xi)d\xi
+
\partial_z\phi(x) = \frac{1}{\alpha} \int_{-L}^{L} \frac{X_n(x)X_n(\xi)}{\beta\lambda_n^4 - \gamma\alpha + 1} \phi(\xi)\mathrm{d}\xi
 
</math>
 
</math>
 
</center>
 
</center>
Line 229: Line 169:
 
<center>
 
<center>
 
<math>
 
<math>
\partial_z\phi(x) = \frac{1}{\alpha} \int_{-L}^{L} g(x,\xi) \phi(\xi)d\xi
+
\partial_z\phi(x) = \frac{1}{\alpha} \int_{-L}^{L} g(x,\xi) \phi(\xi)\mathrm{d}\xi
 
</math>
 
</math>
 
</center>
 
</center>
Line 235: Line 175:
 
<center>
 
<center>
 
<math>
 
<math>
g(x,\xi) = \frac{w_n(x)w_n(\xi)}{\beta\lambda_n^4 - \gamma\alpha + 1}
+
g(x,\xi) = \frac{X_n(x)X_n(\xi)}{\beta\lambda_n^4 - \gamma\alpha + 1}
 
</math>
 
</math>
 
</center>
 
</center>

Latest revision as of 10:50, 28 April 2010


Introduction

The problem of a two-dimensional Floating Elastic Plate was solved using a Free-Surface Green Function by Newman 1994 and Meylan and Squire 1994. We describe here both methods (which are closely related). A related paper was given by Hermans 2003 and this was extended to multiple plates in Hermans 2004.

We present here the solution for a floating elastic plate using dry modes. We begin with the equations. The solution can also be found using Eigenfunction Matching for a Finite Floating Elastic Plate using Symmetry.

The simpler problem of dock is treated in Green Function Method for Finite Dock

Equations for a Finite Plate in Frequency Domain

We consider the problem of small-amplitude waves which are incident on finite floating elastic plate occupying water surface for [math]\displaystyle{ -L\lt x\lt L }[/math]. These equations are derived in Floating Elastic Plate The submergence of the plate is considered negligible. We assume that the problem is invariant in the [math]\displaystyle{ y }[/math] direction. We also assume that the plate edges are free to move at each boundary, although other boundary conditions could easily be considered using the methods of solution presented here. We begin with the Frequency Domain Problem for a semi-infinite Floating Elastic Plates in the non-dimensional form of Tayler 1986 (Dispersion Relation for a Floating Elastic Plate). We also assume that the waves are normally incident (incidence at an angle will be discussed later).

[math]\displaystyle{ \Delta \phi = 0, \;\;\; -h \lt z \leq 0, }[/math]
[math]\displaystyle{ \partial_z \phi = 0, \;\;\; z = - h, }[/math]
[math]\displaystyle{ \partial_z\phi=\alpha\phi, \,\, z=0,\,x\lt -L,\,\,{\rm or}\,\,x\gt L }[/math]
[math]\displaystyle{ \partial_x^2\left\{\beta(x) \partial_x^2\partial_z \phi\right\} - \left( \gamma(x)\alpha - 1 \right) \partial_z \phi - \alpha\phi = 0, \;\; z = 0, \;\;\; -L \leq x \leq L, }[/math]

where [math]\displaystyle{ \alpha = \omega^2 }[/math], [math]\displaystyle{ \beta }[/math] and [math]\displaystyle{ \gamma }[/math] are the stiffness and mass constant for the plate respectively. The free edge conditions at the edge of the plate imply

[math]\displaystyle{ \partial_x^3 \partial_z\phi = 0, \;\; z = 0, \;\;\; x = \pm L, }[/math]
[math]\displaystyle{ \partial_x^2 \partial_z\phi = 0, \;\; z = 0, \;\;\; x = \pm L, }[/math]

Transformation using Eigenfunctions for a Uniform Free Beam

We can find eigenfunctions which satisfy

[math]\displaystyle{ \partial_x^4 X_n = \lambda_n^4 X_n \,\,\, -L \leq x \leq L }[/math]

plus the edge conditions of zero bending moment and shear stress

[math]\displaystyle{ \begin{matrix} \partial_x^3 X_n= 0 \;\;\;\; \mbox{ at } z = 0 \;\;\; x = \pm L, \end{matrix} }[/math]
[math]\displaystyle{ \begin{matrix} \partial_x^2 X_n = 0 \;\;\; \mbox{ at } z = 0 \;\;\; x = \pm L. \end{matrix} }[/math]

This solution is discussed further in Eigenfunctions for a Free Beam.

Using the expression [math]\displaystyle{ \partial_n \phi =\partial_t w }[/math], we can form

[math]\displaystyle{ \frac{\partial \phi}{\partial z} = -\mathrm{i}\omega \sum_{n=0}^{\infty} \zeta_n X_n }[/math]

where [math]\displaystyle{ \zeta_n \, }[/math] are coefficients to be evaluated.

Equation in Terms of the Modes of the Plate

Under these assumptions, the equations become

[math]\displaystyle{ \Delta\phi =0,\,\,-h\lt z\lt 0, }[/math]
[math]\displaystyle{ \partial_{z}\phi =0,\,\,z=-h, }[/math]
[math]\displaystyle{ \alpha\phi =\partial_{z}\phi,\,\,x\notin(-L,L),\ \ z=0, }[/math]
[math]\displaystyle{ i\omega\sum_{n=0}^{\infty}\zeta_{n}X_{n} =\partial_{z}\phi,\,\,x\in (-L,L),\,\, z=0, }[/math]
[math]\displaystyle{ \sum_{n=0}^{\infty}\zeta_{n}\left( 1+\beta\lambda_{n}^{4}\right) X_{n}-\alpha\gamma\sum_{n=0}^{\infty}\zeta_{n}X_{n} = -i\omega \phi,\,\,x\in(-L,L),\,\, z=0. }[/math]

We solve for the potential (and displacement) as the sum of the diffracted and radiation potentials in the standard way, as for a rigid body.

[math]\displaystyle{ \phi=\phi^{\mathrm{D}}+\phi^{\mathrm{R}} ,\, }[/math]

We begin with the diffraction potential [math]\displaystyle{ \phi^{\mathrm{D}} }[/math] which satisfies the following equations

[math]\displaystyle{ \Delta\phi^{\mathrm{D}} =0,\,\,-h\lt z\lt 0, }[/math]
[math]\displaystyle{ \partial_{z}\phi^{\mathrm{D}} =0,\,\,z=-h, }[/math]
[math]\displaystyle{ \partial_{z}\phi^{\mathrm{D}} =\alpha \phi^{\mathrm{D}},\,\,x\notin(-L,L),\,\, z=0, }[/math]
[math]\displaystyle{ \partial_{z}\phi^{\mathrm{D}} =0,\,\,x\in(-L,L),\,\,z=0. }[/math]

[math]\displaystyle{ \phi^{\mathrm{D}} }[/math] satisfies the Sommerfeld Radiation Condition

[math]\displaystyle{ \frac{\partial}{\partial x} \left(\phi^{\mathrm{D}}-\phi^{\rm I} \right) \pm k_0\left( \phi^{\mathrm{D}}-\phi^{\rm I}\right) = 0 ,\,\,\mathrm{as} \,\,x\rightarrow\infty. }[/math]

[math]\displaystyle{ \phi^{\mathrm{I}}\, }[/math] is a plane wave travelling in the [math]\displaystyle{ x }[/math] direction,

[math]\displaystyle{ \phi^{\mathrm{I}}(x,z)=A \phi_0(z) e^{\mathrm{i} k x} \, }[/math]

where [math]\displaystyle{ A }[/math] is the wave amplitude (in potential) [math]\displaystyle{ \mathrm{i} k }[/math] is the positive imaginary solution of the Dispersion Relation for a Free Surface (note we are assuming that the time dependence is of the form [math]\displaystyle{ \exp(-\mathrm{i}\omega t) }[/math]) and

[math]\displaystyle{ \phi_0(z) =\frac{\cosh k(z+h)}{\cosh k h} }[/math]

As the plate is floating on the surface, we can denote it as follows:

[math]\displaystyle{ \phi^{\rm I}|_{z=0} = e^{-k_0 x} \, }[/math]

where we have set the amplitude to be unity.

We now consider the scattered potentials [math]\displaystyle{ \phi^{\mathrm{S}} }[/math]. The relationship between scattered potentials, diffracted potentials and the incident wave are as follows:

[math]\displaystyle{ \phi^{\mathrm{D}}=\phi^{\mathrm{I}}+\phi^{\mathrm{S}} \, }[/math]

from this, we can construct the following conditions:

[math]\displaystyle{ \Delta\phi^{\mathrm{S}} =0,\,\,-h\lt z\lt 0, }[/math]
[math]\displaystyle{ \partial_{z}\phi^{\mathrm{S}} =0,\,\,z=-h, }[/math]
[math]\displaystyle{ \partial_{z}\phi^{\mathrm{S}} =\alpha\phi^{\mathrm{S}},\,\,x\notin(-L,L),\, \, z=0 }[/math]
[math]\displaystyle{ \partial_{z}\phi^{\mathrm{S}} = -\partial_{z}\phi^{\mathrm{I}},\,\,x\in(-L,L),\,\,z=0. }[/math]



We now consider the radiation potentials [math]\displaystyle{ \phi^{\mathrm{R}} }[/math]. We can express the radiation potential as:

[math]\displaystyle{ \phi^{\mathrm{R}}=\sum_{n=0}^{\infty}\zeta_n \phi_n^{\mathrm{R}} }[/math]

which satisfy the following equations

[math]\displaystyle{ \Delta\phi_n^{\mathrm{R}} =0,\,\,-h\lt z\lt 0, }[/math]
[math]\displaystyle{ \partial_{z}\phi_n^{\mathrm{R}} =0,\,\,z=-h, }[/math]
[math]\displaystyle{ \partial_{z}\phi_n^{\mathrm{R}} =\alpha\phi_n^{\mathrm{R}},\,\,x\notin(-L,L),\, \, z=0 }[/math]
[math]\displaystyle{ \partial_{z}\phi_n^{\mathrm{R}} = i\omega X_{n},\,\,x\in(-L,L),\,\,z=0. }[/math]

The radiation condition for the radiation potential is

[math]\displaystyle{ \frac{\partial\phi_n^{\mathrm{R}}}{\partial x}\pm ik\phi_n^{\mathrm{R}}=0,\,\,\mathrm{as} \,\,x\rightarrow\pm\infty. }[/math]

Therefore we find the potential as

[math]\displaystyle{ \phi=\phi^{\mathrm{D}} +\sum_{n=0}^{\infty}\zeta_{n}\phi_n^{\mathrm{R}}, }[/math]

so that

[math]\displaystyle{ \sum_{n=0}^{\infty}\left( 1+\beta\lambda_{n}^{4} - \alpha\gamma\right) \zeta_{n}X_{n}=-i\omega \left( \phi^{\mathrm{D}}+\sum_{n=0}^{\infty}\zeta_{n}\phi_n^{\mathrm{R}} \right). }[/math]

If we multiply by [math]\displaystyle{ X_m }[/math] and take an inner product over the plate we obtain

[math]\displaystyle{ \left( 1+\beta\lambda_{n}^{4} - \alpha\gamma\right) \zeta_{n}=-i\omega \int_{-L}^{L}\phi^{\mathrm{D}} X_{n}\mathrm{d}x + \sum_{m=0}^{\infty}\left(\omega^2 a_{mn}(\omega) - i\omega b_{mn}(\omega)\right) \zeta_{m}, }[/math]

where the functions [math]\displaystyle{ a_{mn}(\omega) }[/math] and [math]\displaystyle{ b_{mn}(\omega) }[/math] are given by

[math]\displaystyle{ \omega^2 a_{mn}(\omega) -i\omega b_{mn}(\omega) = - i\omega\int_{-L}^{L}\phi_m^{\mathrm{R}}X_{n}\mathrm{d}x, }[/math]

and they are referred to as the added mass and damping coefficients (see Linear Wave-Body Interaction for the equivalent definition for a rigid body). respectively. This equation is solved by truncating the number of modes.

Solution for the Radiation and Diffracted Potential

We use the Free-Surface Green Function for two-dimensional waves, with singularity at the water surface since we are only interested in its value at [math]\displaystyle{ z=0 }[/math] (details about this method can be found in Integral Equation for the Finite Depth Green Function at Surface). Using this we can transform the system of equations to

[math]\displaystyle{ \phi^{\mathrm{D}}(x) = \phi^{\mathrm{I}}(x) + \int_{-L}^{L}G(x,\xi) \alpha\phi^{\mathrm{D}}(\xi) \mathrm{d} \xi }[/math]

and

[math]\displaystyle{ \phi_n^{\mathrm{R}}(x) = \int_{-L}^{L}G(x,\xi) \left( \alpha\phi_n^{\mathrm{R}}(\xi) - i\omega X_n(\xi) \right)\mathrm{d} \xi }[/math]

Reflection and Transmission Coefficients

The Reflection and Transmission Coefficients represent the ratio of the amplitude of the reflected or transmitted wave to the amplitude of the incident wave. Conservation of energy means that [math]\displaystyle{ |R|^2+|T|^2=1\, }[/math].

A diagram depicting the area [math]\displaystyle{ \Omega\, }[/math] which is bounded by the rectangle [math]\displaystyle{ \partial \Omega \, }[/math]. The rectangle [math]\displaystyle{ \partial \Omega \, }[/math] is bounded by [math]\displaystyle{ -h \leq z \leq 0 \, }[/math] and [math]\displaystyle{ -\infty \leq x \leq \infty \, }[/math] or [math]\displaystyle{ -N \leq x \leq N\, }[/math]

We can calculate the Reflection and Transmission coefficients by applying Green's theorem to [math]\displaystyle{ \phi\, }[/math] and [math]\displaystyle{ \phi^{\mathrm{I}}\, }[/math] [math]\displaystyle{ \phi^{\mathrm{I}}\, }[/math] is a plane wave travelling in the [math]\displaystyle{ x }[/math] direction,

[math]\displaystyle{ \phi^{\mathrm{I}}(x,z)=A \phi_0(z) e^{\mathrm{i} k x} \, }[/math]

where [math]\displaystyle{ A }[/math] is the wave amplitude (in potential) [math]\displaystyle{ \mathrm{i} k }[/math] is the positive imaginary solution of the Dispersion Relation for a Free Surface (note we are assuming that the time dependence is of the form [math]\displaystyle{ \exp(-\mathrm{i}\omega t) }[/math]) and

[math]\displaystyle{ \phi_0(z) =\frac{\cosh k(z+h)}{\cosh k h} }[/math]

We assume that [math]\displaystyle{ A=1 }[/math]. This gives us

[math]\displaystyle{ \iint_{\Omega}(\phi\Delta\phi^{\mathrm{I}} - \phi^{\mathrm{I}}\Delta\phi)\mathrm{d}x\mathrm{d}z = \int_{\partial\Omega}(\phi \partial_n \phi^{\rm I} - \phi^{\rm I}\partial_n\phi)\mathrm{d}s = 0, }[/math]

This means that (using the far field behaviour of the potential [math]\displaystyle{ \phi }[/math])

[math]\displaystyle{ \int_{\partial\Omega_{B}} (\phi \partial_n \phi^{\rm I} - \phi^{\rm I}\partial_n\phi)\mathrm{d}s + 2k_0 R \int_{-h}^{0} \left(\phi_0(z)\right)^2 \mathrm{d}z = 0, }[/math]

For the present case the body is present only on the surface and we therefore have

[math]\displaystyle{ \int_{-L}^{L} e^{-k_0 x} \left(\alpha \phi(x) - \partial_n \phi(x)\right)\mathrm{d}x + 2k_0 R \int_{-h}^{0} \left(\phi_0(z)\right)^2 \mathrm{d}z = 0 }[/math]

Therefore

[math]\displaystyle{ R = -\frac{\int_{-L}^{L} e^{-k_0 x} \left(\alpha \phi(x) - \partial_n \phi(x)\right)\mathrm{d}x } {2 k_0 \int_{-h}^{0} \left(\phi_0(z)\right)^2 \mathrm{d}z}. }[/math]

and using a wave incident from the right we obtain

[math]\displaystyle{ T = 1 - \frac{\int_{-L}^{L} e^{k_0 x} \left(\alpha \phi(x) - \partial_n \phi(x)\right)\mathrm{d}x } {2 k_0 \int_{-h}^{0} \left(\phi_0(z)\right)^2 \mathrm{d}z}. }[/math]

Note that an expression for the integral in the denominator can be found in Eigenfunction Matching for a Semi-Infinite Dock

Matlab Code

A program to calculate the solution in elastic modes can be found here

elastic_plate_modes.m

Additional code

This program requires

Alternative Solution Method using Green Functions for a Uniform Plate

We can also solve the equation by a closely related method which was given in Meylan and Squire 1994. We can transform the equations to

[math]\displaystyle{ \phi(x) = \phi^{\rm I}(x) + \int_{-L}^{L}G(x,\xi) \left( \alpha\phi(\xi) - \partial_z\phi(\xi) \right)\mathrm{d} \xi }[/math]

Expanding as before

[math]\displaystyle{ \partial_z \phi = i\omega \sum \xi_n X_n }[/math]

we obtain

[math]\displaystyle{ -i\omega \phi = \sum \left(\beta\lambda_n^4 - \gamma\alpha + 1\right)\xi_n X_n }[/math]

This leads to the following equation

[math]\displaystyle{ \partial_z\phi(x) = \frac{1}{\alpha} \int_{-L}^{L} \frac{X_n(x)X_n(\xi)}{\beta\lambda_n^4 - \gamma\alpha + 1} \phi(\xi)\mathrm{d}\xi }[/math]

or

[math]\displaystyle{ \partial_z\phi(x) = \frac{1}{\alpha} \int_{-L}^{L} g(x,\xi) \phi(\xi)\mathrm{d}\xi }[/math]

where

[math]\displaystyle{ g(x,\xi) = \frac{X_n(x)X_n(\xi)}{\beta\lambda_n^4 - \gamma\alpha + 1} }[/math]

which is the Green function for the plate.