Floating Elastic Plate

2006-06-09T03:38:37Z

Hyuck: /* Equations of Motion */

= Introduction =

The floating elastic plate is one of the best studied problems in hydroelasticity. It can be used to model a range of
physical structures such as a floating break water, an ice floe or a [[VLFS]]). The equations of motion were formulated
more than 100 years ago and a discussion of the problem appears in [[Stoker 1957]]. The problem can
be divided into the two and three dimensional formulations which are closely related. The plate is assumed to be isotropic while the water motion is irrotational and inviscid.

The solution methods are divided up into those for [[Two-Dimensional Floating Elastic Plate]] and those for
a [[Three-Dimensional Floating Elastic Plate]]

= Two Dimensional Problem =

== Equations of Motion ==

When considering a two dimensional problem, the <math>y</math> variable is dropped and the plate is regarded as a beam. There are various beam theories that can be used to describe the motion of the beam. The simplest theory is the [[Bernoulli-Euler Beam]] which is commonly used in the two dimensional hydroelastic analysis. Other beam theories include the [[Timoshenko Beam]] theory and [[Reddy-Bickford Beam]] theory where shear deformation of higher order is considered.

For a Bernoulli-Euler beam on the surface of the water, the equation of motion is given
by the following

<math>D\frac{\partial^4 \eta}{\partial x^4} + \rho_i h \frac{\partial^2 \eta}{\partial t^2} = p</math>

where <math>D</math> is the flexural rigidity, <math>\rho_i</math> is the density of the beam,
<math>h</math> is the thickness of the beam (assumed constant), <math> p</math> is the pressure
and <math>\eta</math> is the beam vertical displacement.

The edges of the plate satisfy the natural boundary condition (i.e. free-edge boundary conditions).

<math>\frac{\partial^2 \eta}{\partial x^2} = 0, \,\,\frac{\partial^3 \eta}{\partial x^3} = 0</math>

at the edges of the plate.

The pressure is given by the linearised Bernoulli equation at the wetted surface (assuming zero
pressure at the surface), i.e.

<math>p = \rho g \frac{\partial \phi}{\partial z} + \rho \frac{\partial \phi}{\partial t}</math>

where <math>\rho</math> is the water density and <math>g</math> is gravity, and <math>\phi</math>
is the velocity potential. The velocity potential is governed by Laplace's equation through out
the fluid domain subject to the free surface condition and the condition of no flow through the
bottom surface. If we denote the beam-covered (or possible multiple beams covered) region of the fluid by <math>P</math> and the free surface by <math>F</math> the equations of motion for the
[[Frequency Domain Problem]] with frequency <math>\omega</math> for water of
[[Finite Depth]] are the following. At the surface
we have the dynamic condition

<math>D\frac{\partial^4 \eta}{\partial x^4} +\left(\rho g- \omega^2 \rho_i h \right)\eta =
i\omega \rho \phi, \, z=0, \, x\in P</math>

<math>0=
\rho g \frac{\partial \phi}{\partial z} + i\omega \rho \phi, \, x\in F</math>

and the kinematic condition

<math>\frac{\partial\phi}{\partial z} = i\omega\eta</math>

The equation within the fluid is governed by [[Laplace's Equation]]

<math>\nabla^2\phi =0 </math>

and we have the no-flow condition through the bottom boundary

<math>\frac{\partial \phi}{\partial z} = 0, \, z=-h</math>

(so we have a fluid of constant depth with the bottom surface at <math>z=-h</math> and the
free surface or plate covered surface are at <math>z=0</math>).
<math> g </math> is the acceleration due to gravity, <math> \rho_i </math> and <math> \rho </math>
are the densities of the plate and the water respectively, <math> h </math> and <math> D </math>
the thickness and flexural rigidity of the plate.

== Solution Methods ==

There are many different methods to solve the corresponding equations ranging from highly analytic such
as the [[Wiener-Hopf]] to very numerical based on [[Eigenfunction Matching Method]] which are
applicable and have advantages in different situations. We describe here some of the solutions
which have been developed grouped by problem

=== Single Crack ===

The simplest problem to consider is one where there are only two semi-infinite plates of identical properties separated by a crack. A related problem in acoustics was considered by [[Kouzov 1963]] who used an integral representation of the problem to solve it explicitly using the Riemann-Hilbert technique. Recently the crack problem has been considered by [[Squire and Dixon 2000]] and [[Williams and Squire 2002]] using a Green function method applicable to infinitely deep water and they obtained simple expressions for the reflection and transmission coefficients. [[Squire and Dixon 2001]] extended the single crack problem to a multiple crack problem in which the semi-infinite regions are separated by a region consisting of a finite number of plates of finite size with all plates having identical properties. [[Evans and Porter 2005]] further considered the multiple crack problem for finitely deep water and provided an explicit solution.

We present here the solution of [[Evans and Porter 2005]] for the simple
case of a single crack with waves incident from normal (they also considered multiple cracks
and waves incident from different angles).
The solution of [[Evans and Porter 2005]] expresses the potential
<math>\phi</math> in terms of a linear combination of the incident wave and certain source functions located at the crack.
Along with satisfying the field and boundary conditions, these source functions satisfy the jump conditions in the displacements and gradients across the crack.
They first define <math>\chi(x,z)</math> to be the Two-Dimensional solution to the [[Free-Surface Green Function for a Floating Elastic Plate]]
given by

<math>
\chi(x,z) = -i\sum_{n=-2}^\infty\frac{\sin{(k_n h)}\cos{(k(n)(z-h))}}{2\alpha C_n}e^{-k_n|x|},\,\,\,(1)
</math>

where

<math>
C_n=\frac{1}{2}\left(h + \frac{(5\beta k_n ^4 + 1 - \alpha\gamma)\sin^2{(k_n h)}}{\alpha}\right),
</math>

and <math>k_n</math> are the solutions of the [[Dispersion Relation for a Floating Elastic Plate]].

Consequently, the source functions for a single crack at <math>x=0</math> can be defined as

<math>
\psi_s(x,z)= \beta\chi_{xx}(x,z),\,\,\,
\psi_a(x,z)= \beta\chi_{xxx}(x,z),\,\,\,(2)
</math>

It can easily be shown that <math>\psi_s</math> is symmetric about <math>x = 0</math> and
<math>\psi_a</math> is antisymmetric about <math>x = 0</math>.

Substituting (1) into (2) gives

<math>
\psi_s(x,z)=
{
-\frac{\beta}{\alpha}
\sum_{n=-2}^\infty
\frac{g_n\cos{(k_n(z+h))}}{2k_{xn}C_n}e^{k_n|x|} },
\psi_a(x,z)=
{
{\rm sgn}(x) i\frac{\beta}{\alpha}\sum_{n=-2}^\infty
\frac{g_n'\cos{(k_n(z+h))}}{2k_{xn}C_n}e^{k_n|x|}},
</math>

where

<math>
g_n = ik_n^3 \sin{k_n h},\,\,\,\,
g'_n= -k_n^4 \sin{k_n h}.
</math>

We then express the solution to the problem as a linear combination of the
incident wave and pairs of source functions at each crack,

<math>
\phi(x,z) =
e^{-k_0 x}\frac{\cos(k_0(z+h))}{\cos(k_0h)}
+ (P\psi_s(x,z) + Q\psi_a(x,z))\,\,\,(3)
</math>

where <math>P</math> and <math>Q</math> are coefficients to be solved which represent the jump in the gradient
and elevation respectively of the plates across the crack <math>x = a_j</math>.
The coefficients <math>P</math> and <math>Q</math> are found by applying the edge conditions and to
the <math>z</math> derivative of <math>\phi</math> at <math>z=0</math>,

<math>
\frac{\partial^2}{\partial x^2}\left. \frac{\partial \phi}{\partial z}\right|_{x=0,z=0}=0,\,\,\,
{\rm and}\,\,\,\,
\frac{\partial^3}{\partial x^3}\left. \frac{\partial \phi}{\partial z}\right|_{x=0,z=0}=0.
</math>

The reflection and transmission coefficients, <math>R</math> and <math>T</math> can be found from (3)
by taking the limits as <math>x\rightarrow\pm\infty</math> to obtain

<math>
R = {- \frac{\beta}{\alpha}
(g'_0Q + ig_0P)}
</math>

and

<math>
T= 1 + {\frac{\beta}{\alpha}(g'_0Q - ig_0P)}
</math>

=== Two Semi-Infinite Plates of Different Properties ===

The next most simple problem is two semi-infinite plates of different properties. Often one of
the plates is taken to be open water which makes the problem simpler. In general, the solution method
developed for open water can be extended to two plates of different properties, the exception to
this is the [[Residue Calculus]] solution which applies only when one of the semi-infinite regions
is water.

====[[Wiener-Hopf]]====

The solution to the problem of two semi-infinite plates with different properties can be
solved by the Wiener-Hopf method. The first work on this problem was by [[Evans and Davies 1968]]
but they did not actually develop the method sufficiently to be able to calculate the solution.
The explicit solution was not found until the work of ...

====[[Eigenfunction Matching Method]]====

The eigenfunction matching solution was developed by [[Fox and Squire 1994]].
Essentially the solution is expanded on either side of the crack.

====[[Residue Calculus]]====

= Three Dimensional Problem =

== Equations of Motion ==

For a classical thin plate, the equation of motion is given by
<center><math>
D\nabla ^4 w + \rho _i h w = p
</math></center>
Equation ((plate)) is subject to the free edge boundary
conditions for a thin plate
<center><math>
\frac{\partial ^{2}w}{\partial n^{2}}+\nu \frac{\partial ^{2}w}{\partial
s^{2}}=0,\;\;\;=\textrm{and= }\mathrm{\;\;\;}\frac{\partial ^{3}w}{
\partial n^{3}}+\left( 2-\nu \right) \frac{\partial ^{3}w}{\partial
n\partial s^{2}}=0,
</math></center>
[[Hildebrand65]] where <math>n</math> and <math>s</math> denote the normal and tangential
directions respectively.

The pressure, <math>p</math>, is given by the linearized Bernoulli's equation at the
water surface,
<center><math>
p=-\rho \frac{\partial \phi }{\partial t}-\rho gW
</math></center>
where <math>\Phi </math> is the velocity potential of the water, <math>\rho </math> is the density
of the water, and <math>g</math> is the acceleration due to gravity.

We now introduce non-dimensional variables. We non-dimensionalise the length
variables with respect to <math>a</math> where the surface area of the floe is <math>4a^{2}.</math>
We non-dimensionalise the time variables with respect to <math>\sqrt{g/a}</math> and
the mass variables with respect to <math>\rho a^{3}</math>.
In the non-dimensional variables equations ((plate)) and ((pressure)
) become
<center><math>
\beta \nabla ^{4}\bar{w}+\gamma \frac{\partial ^{2}\bar{w}}{\partial \bar{t}
^{2}}=\frac{\partial \bar{\Phi}}{\partial \bar{t}}-\bar{w}, (n-d_ice)
</math></center>
where
<center><math>
\beta =\frac{D}{g\rho a^{4}}, \; \mathbf{and} \; \gamma =\frac{\rho _{i}h}{\rho
a}.
</math></center>
We shall refer to <math>\beta </math> and <math>\gamma </math> as the stiffness and mass
respectively.

We will determine the response of the ice floe to wave forcing of a single
frequency (the response for more complex wave forcing can be found by
superposition of the single frequency solutions). Since the equations of
motion are linear the displacement and potential must have the same single
frequency dependence. Therefore they can be expressed as the real part of a
complex quantity whose time dependence is <math>e^{-i\sqrt{\alpha }t}</math> where <math>
\alpha </math> is the non-dimensional wavenumber and we write <math>\bar{W}(\bar{x},
\bar{y},\bar{t})={Re}\left[ w\left( \bar{x},\bar{y}\right) e^{-i\sqrt{
\alpha }\bar{t}}\right] \ </math> and <math>\;\Phi (\bar{x},\bar{y},\bar{z},\bar{t})=
{Re}\left[ \phi \left( \bar{x},\bar{y},\bar{z}\right) e^{-i\sqrt{\alpha
}\bar{t}}\right] .</math> In the complex variables the equation of motion of the
ice floe ((n-d_ice)) is
<center><math>
\beta \nabla ^{4}w+\alpha \gamma w=\sqrt{\alpha }\phi -w. (plate2)
</math></center>
From now on we will drop the overbar and assume all variables are
non-dimensional.

==Equations of Motion for the Water==

We require the equation of motion for the water to solve equation (\ref
{plate2}). We begin with the non-dimensional equations of potential theory
which describe linear surface gravity waves
<center><math> (bvp)
\left.
\begin{matrix}{rr}
\nabla ^{2}\phi =0, & -\infty <z<0, \\
{\frac{\partial \phi }{\partial z}=0}, & z\rightarrow -\infty , \\
{\frac{\partial \phi }{\partial z}=}-i\sqrt{\alpha }w, & z\;=\;0,\;\;
\mathbf{x}\in \Delta , \\
{\frac{\partial \phi }{\partial z}-}\alpha \phi {=}p, & z\;=\;0,\;\;\mathbf{
x}\notin \Delta ,
\end{matrix}
\right\} (bvp_nond)
</math></center>
([[Weh_Lait]]). As before, <math>w</math> is the displacement of the floe and <math>p</math>
is the pressure at the water surface. The vector <math>\mathbf{x=(}x,y)</math> is a
point on the water surface and <math>\Delta </math> is the region of the water surface
occupied by the floe. The water is assumed infinitely deep. A schematic
diagram of this problem is shown in Figure (vibration).
\begin{figure}[tbp]
\begin{center}
\epsfbox{vibration.eps}
\end{center}
\caption{{The schematic diagram of the boundary value problem and the
coordinate system used in the solution.}}
(vibration)
\end{figure}

The boundary value problem ((bvp)) is subject to an incident wave which
is imposed through a boundary condition as <math>\left| \mathbf{x}\right|
\rightarrow \infty </math>. This boundary condition, which is called the
Sommerfeld radiation condition, is essentially that at large distances the
potential consists of a radial outgoing wave (the wave generated by the ice
floe motion) and the incident wave. It is expressed mathematically as
<center><math>
\lim_{\left| \mathbf{x}\right| \rightarrow \infty }\sqrt{|\mathbf{x}|}\left(
\frac{\partial }{\partial |\mathbf{x}|}-i\alpha \right) (\phi -\phi ^{
\mathrm{In}})=0, (summerfield)
</math></center>
[[Weh_Lait]]. The incident potential (i.e. the incoming wave) <math>\phi ^{
\mathrm{In}}</math> is
<center><math>
\phi ^{\mathrm{In}}(x,y,z)=\frac{A}{\sqrt{\alpha }}e^{i\alpha (x\cos \theta
+y\sin \theta )}e^{\alpha z}, (input)
</math></center>
where <math>A</math> is the non-dimensional wave amplitude.

The standard solution method to the linear wave problem is to transform the
boundary value problem into an integral equation using a Green function
\citep{john1,
john2,Sarp_Isa,jgrfloecirc}. Performing such a transformation, the boundary
value problem ((bvp)) and ((summerfield)) becomes
<center><math>
\phi (\mathbf{x})=\phi ^{i}(\mathbf{x})+\iint_{\Delta }G_{\alpha }(\mathbf{x}
;\mathbf{y})\left( \alpha \phi (\mathbf{x})+i\sqrt{\alpha }w(\mathbf{x}
)\right) dS_{\mathbf{y}}. (water)
</math></center>
The Green function <math>G_{\alpha }</math> is
<center><math>
G_{\alpha }(\mathbf{x};\mathbf{y)}=\frac{1}{4\pi }\left( \frac{2}{|\mathbf{x}
-\mathbf{y}|}-\pi \alpha \left( \mathbf{H_{0}}(\alpha |\mathbf{x}-\mathbf{y}
|)+Y_{0}(\alpha |\mathbf{x}-\mathbf{y}|)\right) +2\pi i\alpha J_{0}(\alpha |
\mathbf{x}-\mathbf{y}|)\right) ,
</math></center>
[[Weh_Lait,jgrfloecirc]], where <math>J_{0}</math> and <math>Y_{0}</math> are respectively
Bessel functions of the first and second kind of order zero, and <math>\mathbf{
H_{0}}</math> is the Struve function of order zero [[abr_ste]]. A solution for
water of finite depth could be found by simply using the depth dependent
Green function [[Weh_Lait]].

The integral equation ((water)) will be solved using numerical
integration. The only difficulty arises from the non-trivial nature of the
kernel of the integral equation (the Green function). However, the Green
function has no <math>z</math> dependence due to the shallow draft approximation and
depends only on <math>|\mathbf{x}-\mathbf{y}|.</math> This means that the Green
function is one dimensional and the values which are required for a given
calculation can be looked up in a previously computed table.

==Solving for the Wave Induced Ice Floe Motion==

To determine the ice floe motion we must solve equations ((plate2)) and (
(water)) simultaneously. We do this by expanding the floe motion in the
free modes of vibration of a thin plate. The major difficulty with this
method is that the free modes of vibration can be determined analytically
only for very restrictive geometries, e.g. a circular thin plate. Even the
free modes of vibration of a square plate with free edges must be determined
numerically. This is the reason why the solution of [[jgrfloecirc]] was
only for a circular floe.

Since the operator <math>\nabla ^{4},</math> subject to the free edge boundary
conditions, is self adjoint a thin plate must possess a set of modes <math>w_{i}</math>
which satisfy the free boundary conditions and the following eigenvalue
equation
<center><math>
\nabla ^{4}w_{i}=\lambda _{i}w_{i}.
</math></center>
The modes which correspond to different eigenvalues <math>\lambda _{i}</math> are
orthogonal and the eigenvalues are positive and real. While the plate will
always have repeated eigenvalues, orthogonal modes can still be found and
the modes can be normalized. We therefore assume that the modes are
orthonormal, i.e.
<center><math>
\iint_{\Delta }w_{i}\left( \mathbf{Q}\right) w_{j}\left( \mathbf{Q}\right)
dS_{\mathbf{Q}}=\delta _{ij}
</math></center>
where <math>\delta _{ij}</math> is the Kronecker delta. The eigenvalues <math>\lambda _{i}</math>
have the property that <math>\lambda _{i}\rightarrow \infty </math> as <math>i\rightarrow
\infty </math> and we order the modes by increasing eigenvalue. These modes can be
used to expand any function over the wetted surface of the ice floe <math>\Delta </math>
.

We expand the displacement of the floe in a finite number of modes <math>N,</math> i.e.
<center><math>
w\left( \mathbf{x}\right) =\sum_{i=1}^{N}c_{i}w_{i}\left( \mathbf{x}\right) .
(expansion)
</math></center>
From the linearity of ((water)) the potential can be written in the
following form
<center><math>
\phi =\phi _{0}+\sum_{i=1}^{N}c_{i}\phi _{i} (expansionphi)
</math></center>
where <math>\phi _{0}</math> and <math>\phi _{i}</math> satisfy the integral equations
<center><math>
\phi _{0}(\mathbf{x})=\phi ^{\mathrm{In}}(\mathbf{x})+\iint_{\Delta }\alpha
G_{\alpha }(\mathbf{x};\mathbf{y})\phi (\mathbf{y})dS_{\mathbf{y}}
(phi0)
</math></center>
and
<center><math>
\phi _{i}(\mathbf{x})=\iint_{\Delta }G_{\alpha }(\mathbf{x};\mathbf{y}
)\left( \alpha \phi _{i}(\mathbf{x})+i\sqrt{\alpha }w_{i}(\mathbf{y})\right)
dS_{\mathbf{y}}. (phii)
</math></center>
The potential <math>\phi _{0}</math> represents the potential due the incoming wave
assuming that the displacement of the ice floe is zero. The potentials <math>\phi
_{i}</math> represent the potential which is generated by the plate vibrating with
the <math>i</math>th mode in the absence of any input wave forcing.

We substitute equations ((expansion)) and ((expansionphi)) into
equation ((plate2)) to obtain
<center><math>
\beta \sum_{i=1}^{N}\lambda _{i}c_{i}w_{i}-\alpha \gamma
\sum_{i=1}^{N}c_{i}w_{i}=i\sqrt{\alpha }\left( \phi
_{0}+\sum_{i=1}^{N}c_{i}\phi _{i}\right) -\sum_{i=1}^{N}c_{i}w_{i}.
(expanded)
</math></center>
To solve equation ((expanded)) we multiply by <math>w_{j}</math> and integrate over
the plate (i.e. we take the inner product with respect to <math>w_{j})</math> taking
into account the orthogonality of the modes <math>w_{i}</math>, and obtain
<center><math>
\beta \lambda _{j}c_{j}+\left( 1-\alpha \gamma \right) c_{j}=\iint_{\Delta }i
\sqrt{\alpha }\left( \phi _{0}\left( \mathbf{Q}\right)
+\sum_{i=1}^{N}c_{i}\phi _{i}\left( \mathbf{Q}\right) \right) w_{j}\left(
\mathbf{Q}\right) dS_{\mathbf{Q}} (final)
</math></center>
which is a matrix equation in <math>c_{i}.</math>

We cannot solve equation ((final)) without determining the modes of
vibration of the thin plate <math>w_{i}</math> (along with the associated eigenvalues <math>
\lambda _{i})</math> and solving the integral equations ((phi0)) and (\ref
{phii}). We use the finite element method to determine the modes of
vibration [[Zienkiewicz]] and the integral equations ((phi0)) and (
(phii)) are solved by a constant panel method [[Sarp_Isa]]. The same
set of nodes is used for the finite element method and to define the panels
for the integral equation.

[[Category:Linear Hydroelasticity]]

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Floating Elastic Plate