Difference between revisions of "Wavemaker Theory"

From WikiWaves
Jump to navigationJump to search
 
(54 intermediate revisions by 3 users not shown)
Line 1: Line 1:
= Introduction =
+
{{Ocean Wave Interaction with Ships and Offshore Structures
 +
| chapter title = Wavemaker Theory
 +
| next chapter = [[Ship Kelvin Wake]]
 +
| previous chapter =  [[Wave Momentum Flux]]
 +
}}
  
[[Image:Wavemaker.jpg|thumb|right|600px|Wavemaker]]
+
{{complete pages}}
 +
 
 +
== Introduction ==
 +
 
 +
[[Image:Wave_maker.png|600px|right|thumb|Wavemaker]]
  
 
We will derive the potential in a two-dimensional wavetank due the motion of the wavemaker. The method is based on the [[:Category:Eigenfunction Matching Method|Eigenfunction Matching Method]].
 
We will derive the potential in a two-dimensional wavetank due the motion of the wavemaker. The method is based on the [[:Category:Eigenfunction Matching Method|Eigenfunction Matching Method]].
A paddle with draft <math> D\, </math> is undergoing small amplitude horizontal oscillations with displacement
+
A paddle is undergoing small amplitude horizontal oscillations with displacement
<center><math> \xi (t) = \mathfrak{Re} \left \{ f(z) e^{i\omega t} \right \} </math></center>
+
<center><math> \zeta (z,t) = \mathrm{Re} \left \{\frac{1}{-\mathrm{i}\omega} f(z) e^{-i\omega t} \right \} </math></center>
where <math> f(z) </math> is assumed known. Since the time <math>t=0</math> is arbitrary we can assume that
+
where <math> f(z) \, </math> is assumed known. Since the time <math>t=0 \,</math> is arbitrary we can assume that
<math>f(z)</math> is real but this is not necessary.  
+
<math>f(z)\,</math> is real but this is not necessary.  
 
Because the oscillations are small the [[Linear and Second-Order Wave Theory| linear equations]] apply (which will be given formally below).  
 
Because the oscillations are small the [[Linear and Second-Order Wave Theory| linear equations]] apply (which will be given formally below).  
 
This excitation creates plane progressive waves with amplitude <math> A \, </math> down the tank. The principal objective of wavemaker theory is to determine <math> A \, </math>  
 
This excitation creates plane progressive waves with amplitude <math> A \, </math> down the tank. The principal objective of wavemaker theory is to determine <math> A \, </math>  
as a function of <math> \omega, f(z) \, </math> and <math> H \, </math>. Time-dependent wavemaker theories can also be developed.
+
as a function of <math> \omega, f(z) \, </math> and <math> h \, </math>. Time-dependent wavemaker theories can also be developed.
  
= Expansion of the solution =  
+
== Expansion of the solution ==  
  
In general, the wavemaker displacement at <math> X=0\, </math> may be written in the form
+
{{frequency definition}}
<center><math> \xi(t) = \mathfrak{Re} \left \{ \Pi (Z) e^{i\omega t} \right \} </math></center>
 
where <math> \Pi(Z) \, </math> is a known function of <math> Z \, </math>. The standard
 
[[Linear and Second-Order Wave Theory| linear equations]] apply.
 
Let the total velocity potential be
 
<center><math> \Phi = \mathfrak{Re} \left \{ \phi e^{i\omega t} \right \} </math></center>
 
where
 
<center><math> \phi = \phi_\omega \ + \psi </math></center>
 
  
The first term is a velocity potential that represents a [[Linear Plane Progressive Regular Waves|Linear Plane Progressive Regular Wave]] down the tank with amplitude <math> A \, </math>, yet unknown. Thus
 
<center><math> \phi_\omega = \frac{igA}{\omega} \frac{\cosh K (Z+H)}{\cosh KH} e^{-iKX + i\omega t} </math></center>
 
with <math> \omega^2 = gK \tanh KH. \,</math> (the [[Dispersion Relation for a Free Surface]]).
 
  
== Evanescent modes and separation of variables ==
+
{{velocity potential in frequency domain}}
  
The second component potential <math>\psi\,</math> is by definition a decaying disturbance as <math> X \to \infty \, </math> and otherwise satisfies the following boundary value problem
+
The equations therefore become
<center><math> \begin{cases}
+
{{standard linear wave scattering equations without body condition}}
  \nabla^2 \psi = \psi_{XX} + \psi_{ZZ} = 0, -H < Z < 0 \\
+
The boundary condition at the wavemaker is  
  \psi_Z - \frac{\omega^2}{g} \psi = 0, Z=0 \\
+
<center>
  \psi_Z = 0, Z=-H \\
+
<math>
  \psi \to 0, X \to \infty
+
\left. \partial_x\phi \right|_{x=0} = \partial_t \xi = f(z).
\end{cases}
+
</math>
</math></center>
+
</center>
The condition on the wavemaker <math> (X=0) \, </math> is yet to be enforced.
+
We must also apply the [[Sommerfeld Radiation Condition]]
Note that unlike <math> \phi_\omega,\, \psi \, </math> does not representing a propagating wave down the tank so it is called a non-wavelike (evanescent) mode. Such modes do exist as will be shown below. On the wavemaker <math> (X=0) \, </math> the horizontal velocity due to <math> \phi_\omega\, </math> and that due to <math> \psi\,</math> must sum to the forcing velocity due to <math> \xi(t) \, </math>.
+
as <math>x\rightarrow\infty</math>. This essentially implies
 +
that the only wave at infinity is propagating away.
  
When we solve Laplace's equation by [http://en.wikipedia.org/wiki/Separation_of_Variables separation of variables] and apply the bottom boundary conditions we obtain solutions of the form  <math> e^{-iKX} \cosh K(Z+H) </math> (which represent propagating waves and of the form <math> \cos \lambda (Z+H) \,</math>. Not that this solution satisfies the condition of vanishing value as <math> X \to \infty </math> provided that <math> \lambda > 0 \,</math>.
+
{{separation of variables for a free surface}}
These solutions satisfy Laplace's equation  <math> \psi_{XX} + \psi_{ZZ} = 0, \, </math> for all <math> \lambda</math> and the seafloor condition </u>: <math> \psi_Z = 0, Z=-H.</math>
 
The free-surface condition implies that
 
<center><math> \psi_Z - \frac{\omega^2}{g} \psi = 0 \qquad \qquad \Longrightarrow \quad - \lambda \sin \lambda H - \frac{\omega^2}{g} \cos \lambda H = 0 </math></center>
 
<center><math> \Longrightarrow \quad \lambda \tan \lambda H = - \nu \equiv \frac{\omega^2}{g} </math></center>
 
So for the non-wavelike modes <math> \psi, \lambda \,</math> must satisfy the [[Dispersion Relation for a Free Surface]],
 
<center><math> \lambda \tan \lambda H = - \nu = - \frac{\omega^2}{g} < 0 </math></center>
 
  
For positive values of <math> \lambda \, </math> so that <math> e^{-\lambda X} \to 0, X \to + \infty \, </math>.
+
== Expansion in Eigenfunctions ==
Values of <math>\lambda_i \, </math> satisfying the dispersion relation follow from the solution of the non-dimensional nolinear equation
 
<center><math> \tan \omega = - \frac{\nu}{\omega}, \omega = \lambda H \, </math></center>
 
Solutions <math> \omega_i, i = 1, 2, \cdots \, </math> exist as shown above with <math> \omega_i \sim i \pi \, </math> for large <math> i \, </math>. These values are known as the eigenvalues or eigen-wavenumbers of the non-wavelike modes. The eigen-wavenumber of the wavelike solution <math> K\, </math> is given by the dispersion relation:
 
<center><math> \frac{\omega^2 H}{g} = KH \tan KH. \, </math></center>
 
It can easily be shown that setting <math> K = i \lambda \, </math>, the dispersion relation of the non-wavelike nodes follows. In summary the purely imaginary roots of the surface wave dispersion relation and its single real positive root enter the solution of the wavemaker problem.
 
  
== Orthogonal eigenfunctions==
+
The wavemaker velocity potential <math> \phi \,</math> can be expressed simply in terms of eigenfunctions
  
The solution of the for [[Dispersion Relation for a Free Surface]] leads to an infinite series of vertical eigenfunctions from [http://en.wikipedia.org/wiki/Sturm-Liouville_theory Sturm-Liouville theory]. This theory also shows that the eigenfunctions are orthogonal and we may define the following orthogonal eigenfunctions in the vertical direction <math> Z \, </math>:
+
<center><math> \phi = \sum_{n=0}^{\infty} a_n \phi_n (z) e^{-k_n x} </math></center>
  
<center><math> f_0 (Z) = \frac{\sqrt{2} \cosh K ( Z + H )}{{ (H + \frac{1}{v} \sinh^2 KH )}^{1/2}} </math></center>
+
and we can solve for the coefficients by matching at <math>x=0 \,</math>
  
<center><math> f_n (Z) = \frac{\sqrt{2} \cosh \lambda_n ( Z + H )}{(H + \frac{1}{v} \sinh^2 \lambda_n H )}, \qquad n = 1, 2, \cdots </math></center>
+
<center><math> \left. \phi_x \right|_{x=0} = \sum_{n=0}^{\infty} -k_n a_n  \phi_n (z) = f(z)
 +
</math></center>
  
Selected to satisfy:
+
It follows that
  
<center><math> \begin{cases}
+
<center><math> a_n = -\frac{1}{k_n A_n} \int_{-h}^0 \phi_n(z) f(z)\mathrm{d}</math></center>
  \int_{-H}^0 f_0^2 (Z) dZ = \int_{-H}^0 f_n^2 (Z) dZ = 1 \\
 
  \int_{-H}^0 f_m^2 (Z) f_n (Z) dZ = 0, \quad m \ne n
 
\end{cases} </math></center>
 
  
So the wavemaker velocity potentials <math> \phi_w \, </math> and <math> \psi\, </math> can be expressed simply in terms of their respective eigen modes:
+
=== Far Field Wave ===
 
+
One of the primary objecives of wavemaker theory is to determine the far-field wave amplitude <math> A \, </math> in terms of <math> f(z) \, </math>.  
<center><math> \phi_w = a_0 f_0 (Z) e^{-iKX} </math></center>
+
The far-field wave component representing progagating waves is given by:
 
 
<center><math> \psi = \sum_{n=1}^{\infty} a_n f_n (Z) e^{-\lambda_n X} </math></center>
 
 
 
and:
 
 
 
<center><math> \Phi = \mathfrak{Re} \left \{ ( \phi_w + \psi)_ e^{i\omega t} \right \} </math></center>
 
 
 
On <math> X=0 \, </math>:
 
 
 
<center><math> \Phi_X = \mathfrak{Re} \left \{ \partial_X( \phi_W + \psi){X=0} e^{i\omega t} \right \}</math></center>
 
and
 
<center><math> \frac{d\xi}{dt} = \mathfrak{Re} \left \{ \Pi (Z) i \omega e^{i\omega t} \right \} </math></center>
 
 
 
Or:
 
 
 
<center><math> \frac{\partial}{\partial X} (\phi_W + \psi)_{X=0} = \Pi (Z) i \omega </math></center>
 
 
 
<center><math> \left. \frac{\partial\phi_W}{\partial X} \right |_{X=0} = a_0 ( -iK) f_0 (Z) </math></center>
 
 
 
<center><math> \left. \frac{\partial\psi}{\partial X} \right |_{X=0} = \sum_{n=1}^{\infty} a_n ( -\lambda_n) f_n (Z) </math></center>
 
 
 
It follows that:
 
  
<center><math> - i K a_0 f_0 (Z) + \sum_{n=1}^{\infty} a_n (- \lambda_n) f_n (Z) = i \omega \Pi (Z) </math></center>
+
<center><math> \lim_{x\to\infty} \phi = a_0 \phi_0(z) e^{-k_0 x} =
 +
a_0 \frac{\cos k_0(z+h)}{\cos k_0 h } e^{-k_0 x} </math></center>
  
== Far Field Wave ==
+
Note that  <math> k_0 \, </math> is imaginery. We therefore obtain the complex amplitude of the propagating wave at infinity, namely modulus and phase, in terms of the wave maker displacement <math> f(z) \, </math>.
One of the primary objecives of wavemaker theory is to determine <math> a_0 \, </math> (or the far-field wave amplitude <math> A \, </math> ) in terms of <math> \Pi (Z) \, </math>. Multiplying both sides by <math> f_0 (Z) \, </math>, integrating from <math> - H \to 0 \, </math> and using orthogonality we obtain:
 
 
 
<center><math> - i K a_0 = i \omega \int_{-H}^0 dZ f_0 (Z) \Pi (Z) </math></center>
 
 
 
<center><math> \Rightarrow \quad a_0 = - \frac{\omega}{K} \int_{-H}^0 dZ f_0 (Z) \Pi (Z) </math></center>
 
 
 
The far-field wave component representing progagating waves is given by:
 
  
<center><math> \phi_w = a_0 \frac{\sqrt{2} \cosh K (Z+H)}{{\left( H+\frac{1}{v} \sinh^2 KH \right)}^{1/2}} e^{-iKX} </math></center>
+
For what type of <math> f(z) \, </math> are the non-wavelike modes zero? It is easy to verify by virtue of orthogonality that
  
<center><math> \equiv \frac{igA}{\omega} \frac{\cosh K (Z +H)}{\cosh KH} e^{-iKX} </math></center>
+
<center><math> f(z) \ \sim \ \phi_0 (z) </math></center>
  
Plugging in <math> a_0\, </math> and solving for <math> A \, </math> we obtain the complex amplitude of the propagating wave at infinity, namely modulus and phase, in terms of the wave maker displacement <math> \Pi (Z) \, </math> and the other flow parameters.
+
Unfortunately this is not a "practical" displacement since <math> \phi_0 (z) \, </math> depends on <math> \omega\, </math>, so one would need to build a flexible paddle.
  
For what type of <math> \Pi(Z) \,</math> are the non-wavelike modes <math> \psi \equiv 0 \, </math>? It is easy to verify by virtue of orthogonality that:
+
== Matlab Code ==
  
<center><math> \Pi(Z) \ \sim \ f_0 (Z) </math></center>
+
A program to calculate the coefficients for the wave maker problems can be found here
 +
[http://www.math.auckland.ac.nz/~meylan/code/eigenfunction_matching/wavemaker.m wavemaker.m]
  
Unfortunately this is not a "practical" displacement since <math> f_0 (Z,K) \, </math> depends on <math> K\,</math>, thus on <math> \omega\, </math>. So one would need to build a flexible paddle!
+
=== Additional code ===
  
 +
This program requires
 +
[http://www.math.auckland.ac.nz/~meylan/code/dispersion/dispersion_free_surface.m dispersion_free_surface.m]
 +
to run
  
 
-----
 
-----
  
This article is based on the MIT open course notes and the original article can be found
+
This article is based in part on the MIT open course notes and the original article can be found
 
[http://ocw.mit.edu/NR/rdonlyres/Mechanical-Engineering/2-24Spring-2002/737E217E-0582-45D5-B1F9-B2ECF977C66E/0/lecture6.pdf here]
 
[http://ocw.mit.edu/NR/rdonlyres/Mechanical-Engineering/2-24Spring-2002/737E217E-0582-45D5-B1F9-B2ECF977C66E/0/lecture6.pdf here]
  
Line 128: Line 90:
  
 
[[Category:Eigenfunction Matching Method]]
 
[[Category:Eigenfunction Matching Method]]
 +
[[Category:Pages with Matlab Code]]
 +
[[Category:Complete Pages]]

Latest revision as of 01:13, 5 May 2023

Wave and Wave Body Interactions
Current Chapter Wavemaker Theory
Next Chapter Ship Kelvin Wake
Previous Chapter Wave Momentum Flux



Introduction

Wavemaker

We will derive the potential in a two-dimensional wavetank due the motion of the wavemaker. The method is based on the Eigenfunction Matching Method. A paddle is undergoing small amplitude horizontal oscillations with displacement

[math]\displaystyle{ \zeta (z,t) = \mathrm{Re} \left \{\frac{1}{-\mathrm{i}\omega} f(z) e^{-i\omega t} \right \} }[/math]

where [math]\displaystyle{ f(z) \, }[/math] is assumed known. Since the time [math]\displaystyle{ t=0 \, }[/math] is arbitrary we can assume that [math]\displaystyle{ f(z)\, }[/math] is real but this is not necessary. Because the oscillations are small the linear equations apply (which will be given formally below). This excitation creates plane progressive waves with amplitude [math]\displaystyle{ A \, }[/math] down the tank. The principal objective of wavemaker theory is to determine [math]\displaystyle{ A \, }[/math] as a function of [math]\displaystyle{ \omega, f(z) \, }[/math] and [math]\displaystyle{ h \, }[/math]. Time-dependent wavemaker theories can also be developed.

Expansion of the solution

We also assume that Frequency Domain Problem with frequency [math]\displaystyle{ \omega }[/math] and we assume that all variables are proportional to [math]\displaystyle{ \exp(-\mathrm{i}\omega t)\, }[/math]


The water motion is represented by a velocity potential which is denoted by [math]\displaystyle{ \phi\, }[/math] so that

[math]\displaystyle{ \Phi(\mathbf{x},t) = \mathrm{Re} \left\{\phi(\mathbf{x},\omega)e^{-\mathrm{i} \omega t}\right\}. }[/math]

The equations therefore become

[math]\displaystyle{ \begin{align} \Delta\phi &=0, &-h\lt z\lt 0,\,\,\mathbf{x} \in \Omega \\ \partial_z\phi &= 0, &z=-h, \\ \partial_z \phi &= \alpha \phi, &z=0,\,\,\mathbf{x} \in \partial \Omega_{\mathrm{F}}, \end{align} }[/math]


(note that the last expression can be obtained from combining the expressions:

[math]\displaystyle{ \begin{align} \partial_z \phi &= -\mathrm{i} \omega \zeta, &z=0,\,\,\mathbf{x} \in \partial \Omega_{\mathrm{F}}, \\ \mathrm{i} \omega \phi &= g\zeta, &z=0,\,\,\mathbf{x} \in \partial \Omega_{\mathrm{F}}, \end{align} }[/math]

where [math]\displaystyle{ \alpha = \omega^2/g \, }[/math]) The boundary condition at the wavemaker is

[math]\displaystyle{ \left. \partial_x\phi \right|_{x=0} = \partial_t \xi = f(z). }[/math]

We must also apply the Sommerfeld Radiation Condition as [math]\displaystyle{ x\rightarrow\infty }[/math]. This essentially implies that the only wave at infinity is propagating away.

Separation of variables for a free surface

We use separation of variables

We express the potential as

[math]\displaystyle{ \phi(x,z) = X(x)Z(z)\, }[/math]

and then Laplace's equation becomes

[math]\displaystyle{ \frac{X^{\prime\prime}}{X} = - \frac{Z^{\prime\prime}}{Z} = k^2 }[/math]

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

[math]\displaystyle{ Z^{\prime\prime} + k^2 Z =0. }[/math]

subject to the boundary conditions

[math]\displaystyle{ Z^{\prime}(-h) = 0 }[/math]

and

[math]\displaystyle{ Z^{\prime}(0) = \alpha Z(0) }[/math]

We can then use the boundary condition at [math]\displaystyle{ z=-h \, }[/math] to write

[math]\displaystyle{ Z = \frac{\cos k(z+h)}{\cos kh} }[/math]

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

[math]\displaystyle{ k\tan\left( kh\right) =-\alpha \, }[/math]

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 [math]\displaystyle{ k_{0}=\pm ik \, }[/math] and the positive real solutions by [math]\displaystyle{ k_{m} \, }[/math], [math]\displaystyle{ m\geq1 }[/math]. The [math]\displaystyle{ k \, }[/math] of the imaginary solution is the wavenumber. We put the imaginary roots back into the equation above and use the hyperbolic relations

[math]\displaystyle{ \cos ix = \cosh x, \quad \sin ix = i\sinh x, }[/math]

to arrive at the dispersion relation

[math]\displaystyle{ \alpha = k\tanh kh. }[/math]

We note that for a specified frequency [math]\displaystyle{ \omega \, }[/math] the equation determines the wavenumber [math]\displaystyle{ k \, }[/math].

Finally we define the function [math]\displaystyle{ Z(z) \, }[/math] as

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

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

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

where

[math]\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). }[/math]

Expansion in Eigenfunctions

The wavemaker velocity potential [math]\displaystyle{ \phi \, }[/math] can be expressed simply in terms of eigenfunctions

[math]\displaystyle{ \phi = \sum_{n=0}^{\infty} a_n \phi_n (z) e^{-k_n x} }[/math]

and we can solve for the coefficients by matching at [math]\displaystyle{ x=0 \, }[/math]

[math]\displaystyle{ \left. \phi_x \right|_{x=0} = \sum_{n=0}^{\infty} -k_n a_n \phi_n (z) = f(z) }[/math]

It follows that

[math]\displaystyle{ a_n = -\frac{1}{k_n A_n} \int_{-h}^0 \phi_n(z) f(z)\mathrm{d}z }[/math]

Far Field Wave

One of the primary objecives of wavemaker theory is to determine the far-field wave amplitude [math]\displaystyle{ A \, }[/math] in terms of [math]\displaystyle{ f(z) \, }[/math]. The far-field wave component representing progagating waves is given by:

[math]\displaystyle{ \lim_{x\to\infty} \phi = a_0 \phi_0(z) e^{-k_0 x} = a_0 \frac{\cos k_0(z+h)}{\cos k_0 h } e^{-k_0 x} }[/math]

Note that [math]\displaystyle{ k_0 \, }[/math] is imaginery. We therefore obtain the complex amplitude of the propagating wave at infinity, namely modulus and phase, in terms of the wave maker displacement [math]\displaystyle{ f(z) \, }[/math].

For what type of [math]\displaystyle{ f(z) \, }[/math] are the non-wavelike modes zero? It is easy to verify by virtue of orthogonality that

[math]\displaystyle{ f(z) \ \sim \ \phi_0 (z) }[/math]

Unfortunately this is not a "practical" displacement since [math]\displaystyle{ \phi_0 (z) \, }[/math] depends on [math]\displaystyle{ \omega\, }[/math], so one would need to build a flexible paddle.

Matlab Code

A program to calculate the coefficients for the wave maker problems can be found here wavemaker.m

Additional code

This program requires dispersion_free_surface.m to run


This article is based in part on the MIT open course notes and the original article can be found here

Ocean Wave Interaction with Ships and Offshore Energy Systems