# Linear Plane Progressive Regular Waves

## 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

$\displaystyle{ \Delta\Phi\left( \mathbf{x,}t\right) =0,\ \ \mathbf{x}\in\Omega. }$

At the bottom surface we have no flow

$\displaystyle{ \partial_{n}\Phi=0,\ \ z=-h. }$

At the free surface we have the kinematic condition

$\displaystyle{ \partial_{t}\zeta=\partial_{n}\Phi,\ \ z=0,\ x\in \partial\Omega_{F}, }$

and the dynamic condition (the linearized Bernoulli equation)

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

Note that we are assuming here no body is present so that $\displaystyle{ F }$ 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 $\displaystyle{ \omega }$ and we assume that all variables are proportional to $\displaystyle{ \exp(-\mathrm{i}\omega t)\, }$

The water motion is represented by a velocity potential which is denoted by $\displaystyle{ \phi\, }$ so that

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

The equations therefore become

\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} }

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

\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} }

where $\displaystyle{ \alpha = \omega^2/g \, }$)

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

$\displaystyle{ Z^{\prime\prime} + k^2 Z =0. }$

subject to the boundary conditions

$\displaystyle{ Z^{\prime}(-h) = 0 }$

and

$\displaystyle{ Z^{\prime}(0) = \alpha Z(0) }$

We can then use the boundary condition at $\displaystyle{ z=-h \, }$ to write

$\displaystyle{ Z = \frac{\cos k(z+h)}{\cos kh} }$

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

$\displaystyle{ k\tan\left( kh\right) =-\alpha \, }$

which is the Dispersion Relation for a Free Surface

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

The resulting plane progressive wave velocity potential takes the form:

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

where $\displaystyle{ A }$ is the amplitude in displacement.

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

## Propagating Wave

The free surface elevation is

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

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

$\displaystyle{ T = \frac{2\pi}{\omega} = \mbox{Wave Period} }$
$\displaystyle{ \lambda = \frac{2\pi}{k} = \mbox{Wave Length} }$
$\displaystyle{ c= \frac{\omega}{k} = \mbox{Wave Phase Velocity} }$

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

$\displaystyle{ \omega = f (k) \,. }$

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

## Flow Velocity and Pressure

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

$\displaystyle{ \mathbf{v} = \nabla \phi }$

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

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

## Plane progressive waves in general directions

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

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

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

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

where

$\displaystyle{ \omega^2 = g k \tanh k h \, }$

from the Dispersion Relation for a Free Surface as before

## Dispersion Relation for a Free Surface in Deep and Shallow Waters

$\displaystyle{ \omega^2 = gk\tanh kh \, }$

which is a nonlinear algebraic equation for $\displaystyle{ \omega\, }$ as a function of $\displaystyle{ k\, }$ 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 $\displaystyle{ k \, }$ can only be found numerically. Yet it always exists and the iterative methods that may be implemented always converge rapidly. In deep water, $\displaystyle{ h \to \infty }$ and therefore

$\displaystyle{ \tanh kh \to 1 }$

which in turn implies the deep water dispersion relation

$\displaystyle{ \omega^2 = g k \,. }$

The phase speed is given by

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

So the speed of the crest of a wave with period $\displaystyle{ \,T = 10 \mathrm{secs} }$ is approximately $\displaystyle{ 15.6 \,\mathrm{m/s} }$ 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:

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

(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 $\displaystyle{ T=10 }$ secs is about $\displaystyle{ 150 m }$ long.

In the limit of Shallow Depth $\displaystyle{ kh \to 0 }$ which in turn implies that

$\displaystyle{ \tanh kh \simeq kh }$

It therefore follows that

$\displaystyle{ \omega^2 = gk (kh) \ \Longrightarrow \ \frac{\omega^2}{k^2} = gh }$

or

$\displaystyle{ \frac{\omega}{k} = c = \sqrt{gh} }$

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 $\displaystyle{ kh \le \pi }$. This is because

$\displaystyle{ \tanh \pi \simeq 1 }$

and for $\displaystyle{ kh = \pi \ \Longrightarrow \ \frac{2\pi h}{\lambda} = \pi \ \Longrightarrow \ \frac{h}{\lambda} = \frac{1}{2} }$ so for $\displaystyle{ \frac{h}{\lambda} \gt \frac{1}{2} }$ or $\displaystyle{ kh \gt \pi }$ 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.