Difference between revisions of "Linear Plane Progressive Regular Waves"

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}}
 
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= Introduction =  
+
{{incomplete pages}}
 +
 
 +
== Introduction ==  
  
 
Regular time-harmonic linear plane progressive waves are the fundamental building block in describing the propagation of surface wave disturbances in a deterministic and stochastic setting and in predicting the response of floating structures in a seastate.
 
Regular time-harmonic linear plane progressive waves are the fundamental building block in describing the propagation of surface wave disturbances in a deterministic and stochastic setting and in predicting the response of floating structures in a seastate.
  
= Equations =
+
== Equations ==
 
 
Regular time-harmonic linear plane progressive waves satisfy the boundary-value problem (in water of constant [[Finite Depth]])
 
<center><math>
 
\frac{\partial^2\Phi_1}{\partial t^2} + g \frac{\partial\Phi_1}{\partial z} = 0,\,\,\, z=0
 
</math></center>
 
<center><math>
 
\nabla^2\Phi_1 = 0,\,\,\, -h < z < 0
 
</math></center>
 
<center><math>
 
\frac{\partial\Phi_1}{\partial z} = 0,\,\,\, z=-h
 
</math></center>
 
The last condition imposes a zero normal flux condition across a sea floor of constant depth, <math>h</math>. It turns out that if the water is sufficiently deep the wave does not feel any effect from the sea floor and we can make the [[Infinite Depth]] assumption.
 
 
 
= Propagating Wave =
 
 
 
[[Image:Phase_speed.jpg|thumb|right|600px|Propagating Wave]]
 
 
 
There are a number of ways to derive the propagating wave solution, but the simplest is to define the free surface elevation <u>apriori</u> as
 
<center><math> \xi (x,t) = A \cos ( \omega t - k x) = A \mathbf{Re} \{ e^{-ikx+i\omega t} \} </math></center>
 
where A is an apriori known wave amplitude and the wave frequency and wave number <math> (\omega, k) \,</math> pair have the same definitions as in all wave propagation problems, namely:
 
 
 
<center><math> T = \frac{2\pi}{\omega} = \mbox{Wave Period} </math></center>
 
<center><math> \lambda = \frac{2\pi}{k} = \mbox{Wave Length} </math></center>
 
<center><math> c= \frac{\omega}{k} = \mbox{Wave Phase Velocity} </math></center>
 
The relation between <math> \omega \,</math> and <math> k \,</math> is known as the dispersion relation often written in the form
 
<center><math> \omega = f (k) \,. </math></center>
 
<math> f(k) \,</math> depends on the physics of the wave propagation problem under study,
 
surface ocean waves are dispersive since <math> f(k)\, </math> is a non linear function of <math> k \,</math> as we will shortly show.
 
 
 
= Expression in terms of velocity potential =
 
 
 
By virtue of the definition of the wave elevation of a plane progressive regular wave, we seek a compatible definition of the respective velocity potential. Let:
 
<center><math> \Phi_1 = A \mathbf{Re} \{ \phi (x,z) e^{i\omega t} \} </math></center>
 
The problem reduces to the definition of <math> \phi(x,z) \,</math> and the derivation of the appropriate dispersion relation between <math> \omega \,</math> and <math> k \,</math> so that the linear boundary value problem is satisfied. Remember that the equation requires that we consider the fluid potential rather than simply the free surface.
 
 
 
Before proceeding with the algebra, certain underlying principles are always at work:
 
  
* Linear system theory states that when the input signal is <math> e^{i \omega t} \,</math> the output signal must also be harmonic and with the same frequency.
+
We derive the solution for regular time-harmonic linear plane progressive waves
 +
for the first order potential in water of constant [[Finite Depth]].
 +
{{equations of motion time domain without body condition}}
 +
Note that we are assuming here no body is present so that <math>F</math> is the entire domain.
 +
It turns out that if the water is sufficiently deep the wave does not feel any effect from the sea floor and we can make the [[Infinite Depth]] assumption.
  
* We assume that a solution always exists, otherwise the statement of the physical and/or mathematical boundary value problem is flawed. If we can find a solution in most cases it is the solution. So simply try out solutions that may make sense from the physical point of view.
 
  
* If the boundary value problem is satisfied by a complex velocity potential then it is also satisfied by its real and imaginary parts.
 
  
In our case we will first derive the boundary value problem satisfied by the complex potential <math> \phi(x,z) \,</math> and then we will try the plausible representation:
+
== Expression in terms of velocity potential ==
  
<center><math> \phi (x,z) = \psi (z) e^{- i k x} \,</math></center>
+
{{frequency definition}}
  
It follows upon substitution in the boundary value problem satisfied by <math> \Phi_1 (x,z,t) \,</math> that <math> \phi (x,z) \,</math> is subject to:
 
  
<center><math> \begin{cases}
+
{{velocity potential in frequency domain}}
- \omega^2 \phi + g \phi_z = 0,& z=0 \\
 
\nabla^2\phi = \phi_{xx} + \phi_{zz} = 0,& -h<z<0 \\
 
\phi_z = 0,& z=-h
 
\end{cases} </math></center>
 
  
Allowing for:
+
The equations therefore become
 +
{{standard linear wave scattering equations without body condition}}
  
<center><math> \phi (x,z) = \psi(z) e^{-ikx} \,</math></center>
+
{{Separation of variable for a free surface first part}}
  
It follows that <math> \psi(z) \,</math> is subject to:
+
We denote the
 
+
positive imaginary solution of this equation by <math>k_{0} = \mathrm{i} k \,</math>
<center><math> \begin{cases}
 
- \omega^2 \psi + g \psi_z = 0,& z=0 \\
 
\psi_{zz} - k^2\psi = 0,& -h<z<0 \\
 
\psi_z = 0,& z=-h
 
\end{cases} </math></center>
 
 
 
We can verify by simple substitution that
 
<center><math> \psi(z) = \frac{ig}{\omega} \frac{\cosh k(z+h)}{\cosh k h} </math></center>
 
satisfies the field equation <math> \psi_{zz} - k^2 \psi = 0 \,</math>, the seafloor condition <math> \psi_z=0, \, z=-h </math>, and the free surface condition, only when
 
<center><math> \omega^2 = g k \tanh kh \, </math></center>
 
or
 
<center><math> \omega = \{ gk \tanh k h \}^{1/2} \, </math></center>
 
So by enforcing the free surface condition we have derived the [[Dispersion Relation for a Free Surface]] in [[Finite Depth]].
 
  
 
The resulting plane progressive wave velocity potential takes the form:
 
The resulting plane progressive wave velocity potential takes the form:
<center><math> \phi_1 (x,z,t) = A \ \mathbf{Re} \left\{ \frac{ig}{\omega} \frac{\cosh k (z+h)}{\cosh k h} e^{ - i k x + i \omega t} \right\} </math></center>
+
<center><math> \phi(x,z,t) = A \ \mathrm{Re} \left\{ \frac{ig}{\omega} \frac{\cosh k (z+h)}{\cosh k h}  
 +
e^{ \mathrm{i} k x - \mathrm{i} \omega t} \right\} </math></center>
 +
where <math>A</math> is the amplitude in displacement.
  
 
This derivation is discussed in more mathematical depth in [[:Category:Eigenfunction Matching Method|Eigenfunction Matching Method]]
 
This derivation is discussed in more mathematical depth in [[:Category:Eigenfunction Matching Method|Eigenfunction Matching Method]]
  
= Displacement and Pressure =  
+
== Propagating Wave ==
  
Upon substitution we find
+
[[Image:plane_wave.png|thumb|right|600px|Propagating Wave]]
<center><math> \zeta = A \ \mathbf{Re} \{ e^{-ikx+i\omega t} \} = - \frac{1}{g} \left.\frac{\partial\phi_1}{\partial t} \right|_{z=0} </math></center>
 
  
The corresponding flow velocity at some point <math> \mathbf{x} = (x,z) </math> in the fluid domain or on <math> z=0, z=-h \,</math> is simply given by
+
The free surface elevation is
<center><math> \vec V_1 = \nabla \phi_1 </math></center>
+
<center><math> \zeta (x,t) = A \mathrm{Re} \{ e^{\mathrm{i} kx - \mathrm{i} \omega t} \} \,</math></center>
 +
where A is an apriori known wave amplitude and the wave frequency and wave number <math> (\omega, k) \,</math>  
 +
pair have the same definitions as in all wave propagation problems, namely:
  
The linear hydrodynamic pressure due to the plane progressive wave, which must be added to the hydrostatic, is
+
<center><math> T = \frac{2\pi}{\omega} = \mbox{Wave Period} </math></center>
<center><math> P_1 = \rho \frac{\partial \phi_1}{\partial t} = \mathbf{Re} \left\{ \rho g A \frac{\cosh k (z+h)}{\cosh k h } e^{-ikx + i\omega t} \right\} </math></center>
+
<center><math> \lambda = \frac{2\pi}{k} = \mbox{Wave Length} </math></center>
 +
<center><math> c= \frac{\omega}{k} = \mbox{Wave Phase Velocity} </math></center>
 +
The relation between <math> \omega \,</math> and <math> k \,</math> is known as the dispersion relation often written in the form
 +
<center><math> \omega = f (k) \,. </math></center>
 +
<math> f(k) \,</math> depends on the physics of the wave propagation problem under study,
 +
surface ocean waves are dispersive since <math> f(k)\, </math> is a non linear function of <math> k \,</math> as we will shortly show.
  
From the Lagrangian kinematic relation
+
== Flow Velocity and Pressure ==
<center><math> \frac{d \vec \xi_1}{d t} = \vec V ( \vec \xi_1, t) = \nabla \phi_1 ( \vec xi_1, t) </math></center>
 
  
We may obtain ordinary differential equations governing <math> \vec \xi_1 (t) </math>. Marking a paticular particle with the fluid at rest, so that <math> \vec \xi_1 (0) = \vec x</math>, we may write:
+
The corresponding flow velocity at some point <math> \mathbf{x} = (x,z) </math> in the fluid domain or on <math> z=0, z=-h \,</math> is simply given by
 +
<center><math> \mathbf{v} = \nabla \phi </math></center>
 +
This equation leads to a harmonic solution for the particle trajectories which are ellipses (becoming circles as the depth becomes [[Infinite Depth|infinite]].
 +
If second-order effects are included, the particles under a plane progressive waves also undergo a steady-state drift known as the stokes drift.
  
<center><math> \vec \xi_1 (t) = \vec x + \vec {\Delta \xi (t)} </math></center>
+
The linear hydrodynamic pressure due to the plane progressive wave, which must be added to the hydrostatic pressure, is
 +
<center><math> P = \rho \frac{\partial \phi}{\partial t} = \mathrm{Re} \left\{ \rho g A \frac{\cosh k (z+h)}{\cosh k h }
 +
e^{\mathrm{i} kx - \mathrm{i} \omega t} \right\} </math></center>
  
Where <math> \vec x </math> is the particle position at rest and <math> \vec {\Delta\xi} </math> is its displacement due to the "arrival" of a plane progressive wave. Upon substitution in the equation of motion
+
== Plane progressive waves in general directions ==
  
<center><math> \frac{d \vec {\Delta\xi}}{dt} = \nabla \phi_1 (\vec x + \vec{\Delta\xi}, t) </math></center>
+
[[Image:Plane_wave.jpg|thumb|right|400px|Plane wave]]
  
Since <math> \frac{d \vec x}{dt} = 0 </math>. Keeping terms of <math> \,O(\epsilon) </math> on both sides, it follows that
+
For a linear plane progressive regular waves in a general direction the free surface elevation is given by
 +
<center><math> \zeta = \bar{A} \cos \left( \omega t - k x \cos \beta -k y \sin \beta +\chi \right) =
 +
\mathrm{Re} \left \{ A e^{- i k x \cos \theta - i k y \sin \theta + i \omega t} \right \}  
 +
</math></center>
 +
where <math>A</math> is the complex amplitude which includes the phase factor <math>\chi</math>.
 +
<math>\theta</math> is the angle the wave makes to the <math>x</math> axis.  
 +
and the velocity potential in finite depth is given by
 +
<center><math> \Phi = \mathrm{Re} \left \{ \frac{\mathrm{i}gA}{\omega} \frac{\cosh k (z+h)}{\cosh k h} \
 +
e^{ -\mathrm{i} k x \cos \beta - \mathrm{i} k y \sin \beta + \mathrm{i} \omega t} \right \} </math></center>
 +
where
 +
<center><math> \omega^2 = g k \tanh k h \,</math></center>
 +
from the [[Dispersion Relation for a Free Surface]] as before
  
<center><math> \frac{d \vec{\Delta\xi}}{dt} = \nabla \phi_1 ( \vec x, t) + O (\epsilon^2) </math></center>
+
== [[Dispersion Relation for a Free Surface]] in [[Infinite Depth| Deep]] and [[:Category:Shallow Depth| Shallow Waters]] ==
 
 
This equation when forced by the velocity vector that corresponds to the plane progressive wave solution derived above, leads to a harmonic solution for the particle displaced trajectories <math> \vec{\Delta\xi(t)} = (\Delta\xi_1, \Delta\xi_3) </math> which are circular.
 
 
 
If second-order effects are included, the particles under a plane progressive waves also undergo a steady-state drift known as the stokes drift. It can be easily modeled based on the approach described above by substituting second-order effects consistently into the right-hand side of the equation of motion (see Mh).
 
 
 
= [[Dispersion Relation for a Free Surface]] in [[Infinite Depth| deep]] and [[Shallow Depth| shallow waters]] =
 
  
 
In [[Finite Depth]]
 
In [[Finite Depth]]
Line 132: Line 100:
 
<center><math> \omega^2 = g k \,.</math></center>
 
<center><math> \omega^2 = g k \,.</math></center>
 
The phase speed is given by  
 
The phase speed is given by  
<center><math> C = \frac{\omega}{k} = \frac{\omega}{\omega^2/g} = \frac{g}{\omega}  = \frac{g}{2\pi/T} = \frac{gT}{2\pi} </math></center>
+
<center><math> c = \frac{\omega}{k} = \frac{\omega}{\omega^2/g} = \frac{g}{\omega}  = \frac{g}{2\pi/T} = \frac{gT}{2\pi} </math></center>
 
So the speed of the crest of a wave with period <math> \,T = 10 \mathrm{secs} </math> is approximately <math> 15.6 \,\mathrm{m/s} </math> or about 30 knots!
 
So the speed of the crest of a wave with period <math> \,T = 10 \mathrm{secs} </math> is approximately <math> 15.6 \,\mathrm{m/s} </math> or about 30 knots!
  
Often we need a quick estimate of the length of a deed water wave the period of which we can measure accurately with a stop watch. We proceed as follows:
+
Often we need a quick estimate of the wavelength of a water wave the period of which we can measure accurately with a stop watch. We proceed as follows:
  
<center><math> C = \frac{\omega}{k} = \frac{\lambda}{T} \ \Longrightarrow \ \frac{\lambda}{T} = \frac{g}{\omega} = \frac{g}{2\pi/T} \ \Longrightarrow \ \lambda = \frac{gT^2}{2\pi} \simeq T^2 + \frac{1}{2} T^2 </math></center>
+
<center><math> c = \frac{\omega}{k} = \frac{\lambda}{T} \ \Longrightarrow \ \frac{\lambda}{T} = \frac{g}{\omega} = \frac{g}{2\pi/T} \ \Longrightarrow \ \lambda = \frac{gT^2}{2\pi} \simeq T^2 + \frac{1}{2} T^2 </math></center>
 
(by definition the phase speed is the ration of the wave length over the period, or the time it takes for a crest to travel that distance). So the wave length of a deep water wave in m is approximately the square of its period is seconds plus half that amount. So a wave with period <math>T=10</math> secs is about <math>150 m</math> long.
 
(by definition the phase speed is the ration of the wave length over the period, or the time it takes for a crest to travel that distance). So the wave length of a deep water wave in m is approximately the square of its period is seconds plus half that amount. So a wave with period <math>T=10</math> secs is about <math>150 m</math> long.
  
In the limit of [[Shallow Depth]] <math> kh \to 0</math>
+
In the limit of [[:Category:Shallow Depth|Shallow Depth]] <math> kh \to 0</math>
 
which in turn implies that
 
which in turn implies that
 
<center><math> \tanh kh \simeq kh </math></center>
 
<center><math> \tanh kh \simeq kh </math></center>
Line 146: Line 114:
 
<center><math> \omega^2 = gk (kh) \ \Longrightarrow \ \frac{\omega^2}{k^2} = gh </math></center>
 
<center><math> \omega^2 = gk (kh) \ \Longrightarrow \ \frac{\omega^2}{k^2} = gh </math></center>
 
or
 
or
<center><math> \frac{\omega}{k} = C = \sqrt{gh} </math></center>
+
<center><math> \frac{\omega}{k} = c = \sqrt{gh} </math></center>
  
 
Thus, according to linear theory shallow water waves become non dispersive as is the case with acoustic waves. Unfortunately, nonlinear effects become more important as waves propagate from deep to shallow water (because the wave amplidute rises). Solitons and wave breaking are some manifestations of nonlinearity.
 
Thus, according to linear theory shallow water waves become non dispersive as is the case with acoustic waves. Unfortunately, nonlinear effects become more important as waves propagate from deep to shallow water (because the wave amplidute rises). Solitons and wave breaking are some manifestations of nonlinearity.
Line 158: Line 126:
 
This article is based on the MIT open course notes and the original article can be found
 
This article is based on the MIT open course notes and the original article can be found
 
[http://ocw.mit.edu/NR/rdonlyres/Mechanical-Engineering/2-24Spring-2002/C88824A3-CBFC-4857-A3DD-D463461C8B97/0/lecture3.pdf here]
 
[http://ocw.mit.edu/NR/rdonlyres/Mechanical-Engineering/2-24Spring-2002/C88824A3-CBFC-4857-A3DD-D463461C8B97/0/lecture3.pdf here]
 
[[Ocean Wave Interaction with Ships and Offshore Energy Systems]]
 
  
 
[[Category:Linear Water-Wave Theory]]
 
[[Category:Linear Water-Wave Theory]]

Latest revision as of 10:28, 2 May 2010



Introduction

Regular time-harmonic linear plane progressive waves are the fundamental building block in describing the propagation of surface wave disturbances in a deterministic and stochastic setting and in predicting the response of floating structures in a seastate.

Equations

We derive the solution for regular time-harmonic linear plane progressive waves for the first order potential in water of constant Finite Depth. The equations of motion in the time domain are Laplace's equation through out the fluid

[math]\displaystyle{ \Delta\Phi\left( \mathbf{x,}t\right) =0,\ \ \mathbf{x}\in\Omega. }[/math]

At the bottom surface we have no flow

[math]\displaystyle{ \partial_{n}\Phi=0,\ \ z=-h. }[/math]

At the free surface we have the kinematic condition

[math]\displaystyle{ \partial_{t}\zeta=\partial_{n}\Phi,\ \ z=0,\ x\in \partial\Omega_{F}, }[/math]

and the dynamic condition (the linearized Bernoulli equation)

[math]\displaystyle{ \partial_{t}\Phi = -g\zeta ,\ \ z=0,\ x\in \partial\Omega_{F}. }[/math]

Note that we are assuming here no body is present so that [math]\displaystyle{ F }[/math] is the entire domain. It turns out that if the water is sufficiently deep the wave does not feel any effect from the sea floor and we can make the Infinite Depth assumption.


Expression in terms of velocity potential

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 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

We denote the positive imaginary solution of this equation by [math]\displaystyle{ k_{0} = \mathrm{i} k \, }[/math]

The resulting plane progressive wave velocity potential takes the form:

[math]\displaystyle{ \phi(x,z,t) = A \ \mathrm{Re} \left\{ \frac{ig}{\omega} \frac{\cosh k (z+h)}{\cosh k h} e^{ \mathrm{i} k x - \mathrm{i} \omega t} \right\} }[/math]

where [math]\displaystyle{ A }[/math] is the amplitude in displacement.

This derivation is discussed in more mathematical depth in Eigenfunction Matching Method

Propagating Wave

Propagating Wave

The free surface elevation is

[math]\displaystyle{ \zeta (x,t) = A \mathrm{Re} \{ e^{\mathrm{i} kx - \mathrm{i} \omega t} \} \, }[/math]

where A is an apriori known wave amplitude and the wave frequency and wave number [math]\displaystyle{ (\omega, k) \, }[/math] pair have the same definitions as in all wave propagation problems, namely:

[math]\displaystyle{ T = \frac{2\pi}{\omega} = \mbox{Wave Period} }[/math]
[math]\displaystyle{ \lambda = \frac{2\pi}{k} = \mbox{Wave Length} }[/math]
[math]\displaystyle{ c= \frac{\omega}{k} = \mbox{Wave Phase Velocity} }[/math]

The relation between [math]\displaystyle{ \omega \, }[/math] and [math]\displaystyle{ k \, }[/math] is known as the dispersion relation often written in the form

[math]\displaystyle{ \omega = f (k) \,. }[/math]

[math]\displaystyle{ f(k) \, }[/math] depends on the physics of the wave propagation problem under study, surface ocean waves are dispersive since [math]\displaystyle{ f(k)\, }[/math] is a non linear function of [math]\displaystyle{ k \, }[/math] as we will shortly show.

Flow Velocity and Pressure

The corresponding flow velocity at some point [math]\displaystyle{ \mathbf{x} = (x,z) }[/math] in the fluid domain or on [math]\displaystyle{ z=0, z=-h \, }[/math] is simply given by

[math]\displaystyle{ \mathbf{v} = \nabla \phi }[/math]

This equation leads to a harmonic solution for the particle trajectories which are ellipses (becoming circles as the depth becomes infinite. If second-order effects are included, the particles under a plane progressive waves also undergo a steady-state drift known as the stokes drift.

The linear hydrodynamic pressure due to the plane progressive wave, which must be added to the hydrostatic pressure, is

[math]\displaystyle{ P = \rho \frac{\partial \phi}{\partial t} = \mathrm{Re} \left\{ \rho g A \frac{\cosh k (z+h)}{\cosh k h } e^{\mathrm{i} kx - \mathrm{i} \omega t} \right\} }[/math]

Plane progressive waves in general directions

Plane wave

For a linear plane progressive regular waves in a general direction the free surface elevation is given by

[math]\displaystyle{ \zeta = \bar{A} \cos \left( \omega t - k x \cos \beta -k y \sin \beta +\chi \right) = \mathrm{Re} \left \{ A e^{- i k x \cos \theta - i k y \sin \theta + i \omega t} \right \} }[/math]

where [math]\displaystyle{ A }[/math] is the complex amplitude which includes the phase factor [math]\displaystyle{ \chi }[/math]. [math]\displaystyle{ \theta }[/math] is the angle the wave makes to the [math]\displaystyle{ x }[/math] axis. and the velocity potential in finite depth is given by

[math]\displaystyle{ \Phi = \mathrm{Re} \left \{ \frac{\mathrm{i}gA}{\omega} \frac{\cosh k (z+h)}{\cosh k h} \ e^{ -\mathrm{i} k x \cos \beta - \mathrm{i} k y \sin \beta + \mathrm{i} \omega t} \right \} }[/math]

where

[math]\displaystyle{ \omega^2 = g k \tanh k h \, }[/math]

from the Dispersion Relation for a Free Surface as before

Dispersion Relation for a Free Surface in Deep and Shallow Waters

In Finite Depth

[math]\displaystyle{ \omega^2 = gk\tanh kh \, }[/math]

which is a nonlinear algebraic equation for [math]\displaystyle{ \omega\, }[/math] as a function of [math]\displaystyle{ k\, }[/math] which has a unique positive real solution as can be shown graphically. It also has imaginary roots which are important in many application (see Dispersion Relation for a Free Surface and Eigenfunction Matching Method) The unique positive real root [math]\displaystyle{ k \, }[/math] can only be found numerically. Yet it always exists and the iterative methods that may be implemented always converge rapidly. In deep water, [math]\displaystyle{ h \to \infty }[/math] and therefore

[math]\displaystyle{ \tanh kh \to 1 }[/math]

which in turn implies the deep water dispersion relation

[math]\displaystyle{ \omega^2 = g k \,. }[/math]

The phase speed is given by

[math]\displaystyle{ c = \frac{\omega}{k} = \frac{\omega}{\omega^2/g} = \frac{g}{\omega} = \frac{g}{2\pi/T} = \frac{gT}{2\pi} }[/math]

So the speed of the crest of a wave with period [math]\displaystyle{ \,T = 10 \mathrm{secs} }[/math] is approximately [math]\displaystyle{ 15.6 \,\mathrm{m/s} }[/math] or about 30 knots!

Often we need a quick estimate of the wavelength of a water wave the period of which we can measure accurately with a stop watch. We proceed as follows:

[math]\displaystyle{ c = \frac{\omega}{k} = \frac{\lambda}{T} \ \Longrightarrow \ \frac{\lambda}{T} = \frac{g}{\omega} = \frac{g}{2\pi/T} \ \Longrightarrow \ \lambda = \frac{gT^2}{2\pi} \simeq T^2 + \frac{1}{2} T^2 }[/math]

(by definition the phase speed is the ration of the wave length over the period, or the time it takes for a crest to travel that distance). So the wave length of a deep water wave in m is approximately the square of its period is seconds plus half that amount. So a wave with period [math]\displaystyle{ T=10 }[/math] secs is about [math]\displaystyle{ 150 m }[/math] long.

In the limit of Shallow Depth [math]\displaystyle{ kh \to 0 }[/math] which in turn implies that

[math]\displaystyle{ \tanh kh \simeq kh }[/math]

It therefore follows that

[math]\displaystyle{ \omega^2 = gk (kh) \ \Longrightarrow \ \frac{\omega^2}{k^2} = gh }[/math]

or

[math]\displaystyle{ \frac{\omega}{k} = c = \sqrt{gh} }[/math]

Thus, according to linear theory shallow water waves become non dispersive as is the case with acoustic waves. Unfortunately, nonlinear effects become more important as waves propagate from deep to shallow water (because the wave amplidute rises). Solitons and wave breaking are some manifestations of nonlinearity.

The transition from deep to finite depth wave effects occurs for values of [math]\displaystyle{ kh \le \pi }[/math]. This is because

[math]\displaystyle{ \tanh \pi \simeq 1 }[/math]

and for [math]\displaystyle{ kh = \pi \ \Longrightarrow \ \frac{2\pi h}{\lambda} = \pi \ \Longrightarrow \ \frac{h}{\lambda} = \frac{1}{2} }[/math] so for [math]\displaystyle{ \frac{h}{\lambda} \gt \frac{1}{2} }[/math] or [math]\displaystyle{ kh \gt \pi }[/math] we are effectively dealing with Infinite Depth. This means that for most of the world ocean and wave conditions the water depth may be approximate as infinite.


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