Difference between revisions of "Dispersion Relation for a Free Surface"

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The dispersion equation for a free surface is one of the most important equations
+
{{complete pages}}
in linear water wave theory.
 
  
= Separation of Variables =
+
== Introduction ==
  
The dispersion equation arises when separating
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The dispersion equation for a free surface is one of the most important equations in linear water wave theory. It arises when [http://en.wikipedia.org/wiki/Separation_of_Variables separating variables] subject to the boundary conditions for a free surface.
variables subject to the boundary conditions for a free surface.
 
The same equation arises when separating variables in two or three dimensions
 
and we present here the two-dimensional version. We denote the vertical coordinate
 
by <math>z</math> which is point vertically up and the free surface is at
 
<math>z=0.</math>.
 
The equations for the
 
[[Frequency Domain Problem]] with radial frequency <math>\,\omega</math> in terms of
 
the potential alone which are
 
  
<math>\frac{\partial \phi}{\partial z} - k_{\infty} \phi, \, z=0</math>
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The same equation arises when separating variables in two or three dimensions and we present here the two-dimensional version. We denote the vertical coordinate by <math>z\,,</math> which points vertically upwards, and the free surface is at <math>z=0\,.</math>
  
where <math>k_{\infty}</math> is the wavenumber in [[Infinite Depth]] which is given by
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{{frequency definition}}
<math>k_{\infty}=\omega^2/g</math> where <math>g</math> is gravity. We also have
 
the equations within  the fluid
 
  
<math>\nabla^2\phi =0 </math>
 
  
<math>\frac{\partial \phi}{\partial z} = 0, \, z=-H.</math>
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{{velocity potential in frequency domain}}
  
We then find a separation of variables solution to [[Laplace's Equation]] and
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The equations therefore become
apply the boundary condition at <math>z=-H</math> and we obtain the
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{{standard linear wave scattering equations without body condition}}
following expression for the velocity potential
 
  
<math>\phi(x,z) = e^{ikx} \cosh k(z+H) \,</math>
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We use [http://en.wikipedia.org/wiki/Separation_of_Variables separation of variables]
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{{separation of variables in two dimensions}}
  
If we then apply the condition at <math>z=0</math> we see that the constant <math>\,k</math>
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{{separation of variables for a free surface}}
(which corresponds to the wavenumber) is given by
 
  
<math>  k \sinh(kH) = k_{\infty} \cosh(kH)  </math>
 
  
or
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== See Also ==
 +
* [http://en.wikipedia.org/wiki/Dispersion_(water_waves) Dispersion (water waves)]
  
<math>  k \tanh(kH) = k_{\infty}\,\,\,(1)</math>
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== Matlab Code ==
 
 
 
 
This is the dispersion equation for a free surface.
 
 
 
We can also write the separation of variables as
 
 
 
<math>\phi(x,z) = e^{kx} \cos k(z+H) \,</math> 
 
 
 
in which case the dispersion equation becomes
 
 
 
<math>  k \tan(kH) = -k_{\infty}\,\,\,(2)</math>
 
 
 
Equation (1) has one real positive solution (plus imaginary solutions)
 
and equation (2) has an infinite number of positive real solutions (plus
 
imaginary solutions).  Sometimes (especially in older works) both equations
 
are used so that only real solutions need to be considered. This separation
 
certainly makes sense in numerical solutions but does adds unnecessarily
 
to the notation.
 
 
 
= Solution of the dispersion equation =
 
 
 
The solution of equation (1) consists of one real and infinite
 
number of imaginary roots with positive part plus their negatives.
 
These solutions multiplied by <math>i</math> are the solutions to
 
equation (2). We denote the solutions to (2) by <math>k_n</math>
 
where <math>k_0</math> is the imaginary solution with
 
positive imaginary part and <math>k_n</math>
 
are the real solutions positive solutions ordered so that they are
 
increasing.
 
 
 
The dispersion equation is a classical Sturm-Liouville equation.
 
The vertical eigenfunctions <math>\cos k_n (z-H)</math>
 
form complete set for <math>L_2[-H,0]\,</math> and they are orthogonal.
 
Also, as <math>n\to\infty</math> <math>k_n \to \ n\pi/H</math>
 
so that in the limit the vertical eigenfunctions become the same as
 
the Fourier cosine series for <math>L_2[-H,0]\,</math> (remembering
 
that the eigenfunctions satisfy the boundary conditions of zero
 
normal derivative at <math>z=H</math> which is why we have the
 
cosine series).
 
  
 +
A program to calculate solutions to the dispersion relation for a free surface
 +
[http://www.math.auckland.ac.nz/~meylan/code/dispersion/dispersion_free_surface.m dispersion_free_surface.m]
 
[[Category:Linear Water-Wave Theory]]
 
[[Category:Linear Water-Wave Theory]]
 +
[[Category:Pages with Matlab Code]]

Latest revision as of 09:18, 20 October 2009


Introduction

The dispersion equation for a free surface is one of the most important equations in linear water wave theory. It arises when separating variables subject to the boundary conditions for a free surface.

The same equation arises when separating variables in two or three dimensions and we present here the two-dimensional version. We denote the vertical coordinate by [math]\displaystyle{ z\,, }[/math] which points vertically upwards, and the free surface is at [math]\displaystyle{ z=0\,. }[/math]

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])

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]

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]


See Also

Matlab Code

A program to calculate solutions to the dispersion relation for a free surface dispersion_free_surface.m