Definition of the Subject
A wave is usually defined as the propagation of a disturbance in a medium.
The simplest example is the exponential or sinusoidal wave which has the form
where a is called the amplitude, Re stands for the real part, \( \smash{ k \: (= \frac{2\pi}{\lambda}) } \) is called the wavenumber and λ is called the wavelength of the wave, and ω is called the frequency and it is a definite function of the wavenumber k and hence, \( { \omega=\omega(k) } \) is determined by the particular equation of the problem. The quantity \( { \theta=kx-\omega t } \) is called the phase of the wave so that a wave of a constant phase propagate with \( { kx-\omega t=\text{constant} } \).
The mathematical relation
is called the dispersion relation...
Abbreviations
- Axisymmetric (concentric) KdV equation:
-
This is a partial differential equation for the free surface \( { \eta(R,t) } \) in the form \( \smash{ (2\eta_{R}+\frac{1}{R}\,\eta+3\eta\eta_{\xi})+\frac{1}{3}\,\eta_{\xi\xi\xi}=0 } \).
- Benjamin–Feir instability of water waves:
-
This describes the instability of nonlinear water waves.
- Bernoulli's equation:
-
This is a partial differential equation which determines the pressure in terms of the velocity potential in the form \( \phi_{t} + \smash{\frac{1}{2}(\nabla\phi)^{2} + \frac{P}{\rho}} + gz = 0 \), where ϕ is the velocity potential, P is the pressure, ρ is the density and g is the acceleration due to gravity.
- Boussinesq equation:
-
This is a nonlinear partial differential equation in shallow water of depth h given by
$$ u_{tt}-c^{2}u_{xx}+\frac{1}{2}\big(u^{2}\big)_{xx}=\frac{1}{3}\, h^{2}u_{xxtt}\:, \\ \text{where } \; c^{2}=\sqrt{gh}\:.$$ - Cnoidal waves:
-
Waves are represented by the Jacobian elliptic function \( { cn(z,m) } \).
- Continuity equation:
-
This is an equation describing the conservation of mass of a fluid. More precisely, this equation for an incompressible fluid is \( { \mathit{div} \: \mathbf{u}=\frac{\partial u}{\partial x}} \allowbreak {+\frac{\partial v}{\partial y}+\frac{\partial w}{\partial z}=0 } \), where \( { \mathbf{u}=(u,v,w) } \) is the velocity field, and \( { \mathbf{x}=(x,y,z) } \).
- Continuum hypothesis:
-
It requires that the velocity \( { \mathbf{u}=} \allowbreak {(u,v,w) } \), pressure p and density ρ are continuous functions of position \( { \mathbf{x}=(x,y,z) } \) and time t.
- Crapper's nonlinear capillary waves:
-
Pure progressive capillary waves of arbitrary amplitude.
- Dispersion relation:
-
A mathematical relation between the wavenumber, frequency and/or the amplitude of a wave.
- Euler equations:
-
This is a nonlinear partial differential equation for an inviscid incompressible fluid flow governed by the velocity field \( { \mathbf{u}=(u,v,w) } \) and pressure \( { P(\mathbf{x},t) } \) under the external force \( { \mathbf{F} } \). More precisely, \( { \frac{\partial\mathbf{u}}{\partial t}+(\mathbf{u}\cdot\nabla)\mathbf{u}=-\frac{1}{\rho}\,\nabla P+\mathbf{F} } \), where ρ is the constant density of the fluid.
- Group velocity:
-
The velocity defined by the derivative of the frequency with respect to the wavenumber \( { (c_{g}=\mskip2mu\mathrm{d}\omega/\text{d} k) } \).
- Johnson's equation:
-
This is a nearly concentric KdV equation for \( { \eta(R,\xi,\theta) } \) in cylindrical polar coordinates in the form
$$ \left(2\eta_{R}+\frac{1}{R}\,\eta+3\eta\eta_{\xi}+\frac{1}{3}\,\eta_{\xi\xi\xi}+\frac{1}{R^{2}}\,\eta_{\theta\theta}\right)=0\:.$$ - Kadomtsev–Petviashvili (KP) equation:
-
This is a two‐dimensional KdV equation in the form
$$ \left(2\eta_{t}+3\eta\eta_{\xi}+\frac{1}{3}\,\eta_{\xi\xi\xi}\right)_{\xi}+\eta_{yy}=0\:.$$ - KdV–Burgers equation:
-
This is a nonlinear partial differential equation in the form
$$ \eta_{t}+c_{0}\,\eta_{x}+d\,\eta\,\eta_{x}+\mu\,\eta_{xxx}-\nu\,\eta_{xx}=0\:,\quad\\ \text{where }\mu=\frac{1}{6}\, c_{0}h_{0}^{2}\:. $$ - Korteweg–de Vries (KdV) equation:
-
This is a nonlinear partial differential equation for a solitary wave (or soliton). This equation in a shallow water of depth h is governed by the free surface elevation \( { \eta(x,t) } \) in the form \( { \frac{\partial\eta}{\partial t}+c(1+\frac{3}{2h}\,\eta)\eta_{x}+\left(\frac{ch^{2}}{6}\right)\eta_{xxx}=0 } \), where \( \smash{ c} \allowbreak \smash{=\sqrt{gh} } \) is the shallow water speed.
- Laplace equation:
-
This is partial differential equation of the form \( { \nabla^{2}\phi=\phi_{xx}+\phi_{yy}+\phi_{zz} } \) where the \( { \phi=} \allowbreak {\phi(x,y,z)} \) is the potential.
- Linear dispersion relation:
-
A mathematical relation between the wavenumber k and the frequency ω of waves \( { (\omega=\omega(k)) } \).
- Linear dispersive waves:
-
Waves with the given dispersion relation between the wavenumber and the frequency.
- Linear Schrödinger equation:
-
This can be written in the form \( { i\, a_{t}+\frac{1}{2}\,\omega^{\prime\prime}(k)\frac{\partial^{2}a}{\partial x^{2}}=0 } \), where \( { a=a(x,t) } \) is the amplitude and \( { \omega=\omega(k) } \).
- Linear wave equation in Cartesian coordinates:
-
In three dimensions this can be written in the form \( { u_{tt}=c^{2}\,\nabla^{2}u } \), where c is a constant and \( {\nabla^{2}=} \allowbreak {\frac{\partial^{2}}{\partial x^{2}}+\frac{\partial^{2}}{\partial y^{2}}+\frac{\partial}{\partial z^{2}} } \) is the three‐dimensional Laplacian.
- Linear wave equation in cylindrical polar coordinates:
-
This can be written in the form \( { u_{tt}=u_{rr}+\frac{1}{r}\, u_{r}} \allowbreak {+\frac{1}{r^{2}}\, u_{\theta\theta}} \).
- Navier–Stokes equations:
-
This is a nonlinear partial differential equation for an incompressible and viscous fluid flow governed by the velocity field \( { \mathbf{u}=(u,v,w) } \) and pressure \( { P(\mathbf{x},t) } \) under the action of external force \( { \mathbf{F} } \). More precisely, \( { \frac{\partial\mathbf{u}}{\partial t}+(\mathbf{u}\cdot\nabla)\mathbf{u}=-\frac{1}{\rho}\,\nabla P} \allowbreak {+\nu^{2}\mathbf{u}+\mathbf{F}} \), where ρ is the density and \( { \nu=(\mu/\rho) } \) is the kinematic viscosity.
- Nonlinear dispersion relation:
-
A mathematical relation between the wavenumber k, frequency ω and the amplitude a that is \( { D(\omega,k,a)=0 } \).
- Nonlinear dispersive waves:
-
Waves with the given dispersion relation between the wavenumber, frequency and the amplitude.
- Non‐linear Schrödinger (NLS) equation:
-
This is a nonlinear partial differential equation for the nonlinear modulation of a monochromatic wave. The amplitude, \( { a(x,t) } \) of the modulation satisfies the equation \( \smash{ i\big(\frac{\partial a}{\partial t}+\omega^{\prime}_{0}\,\frac{\partial a}{\partial x}\big)+\frac{1}{2}\,\omega^{\prime\prime}_{0}\,\frac{\partial^{2}a}{\partial x^{2}}+\gamma|a|^{2}a=0 } \), where \( { \omega_{0}} \allowbreak {=\omega_{0}(k) } \), and γ is a constant.
- Ocean waves:
-
Waves observed on the surface or inside the ocean.
- Phase velocity:
-
The velocity defined by the ratio of the frequency ω and the wavenumber k, \( \smash{ (c_{p}=\frac{\omega}{k}) } \).
- Resonant or critical phenomenon:
-
Waves with unbounded amplitude.
- Sinusoidal (or exponential) wave:
-
A wave is of the form \( u(x,t)=a\, \operatorname{Re}\exp[i(kx-\omega t)]=a\cos(kx-\omega t) \), where a is amplitude, k is the wavenumber \( \smash{ (k=\frac{2\pi}{\lambda}) } \), λ is the wavelength and ω is the frequency.
- Solitary waves (or soliton):
-
Waves describing a single hump of given height travel in a medium without change of shape.
- Stokes expansion:
-
This is an expansion of the frequency in terms of the wavenumber k and the amplitude a, that is, \( \smash{ \omega(k)=\omega_{0}(k)+\omega_{2}(k)a^{2}+\ldots\, } \).
- Stokes wave:
-
Water waves with dispersion relation involving the wavenumber, frequency and amplitude.
- Surface‐capillary gravity waves:
-
Waves under the joint action of the gravitational field and surface tension.
- Surface gravity waves:
-
Water waves under the action of the gravitational field.
- Variational principle:
-
For three‐dimensional water waves, it is of the form \( \smash{ \delta I=\delta\iint_{D}L\, \mskip2mu\mathrm{d}\mathbf{x}\, \text{d} t=0} \), where L is called the Lagrangian.
- Velocity potential:
-
A single valued function \( { \phi=\phi(\mathbf{x},t) } \) defined by \( { \mathbf{u}=\nabla\phi } \).
- Water waves:
-
Waves observed on the surface or inside of a body of water.
- Waves on a running stream:
-
Waves observed on the surface or inside of a body of fluid which is moving with a given velocity.
- Whitham averaged variational principle:
-
This can be formulated in the form \( \smash{ \delta\iint\mathcal{L}\, \mskip2mu\mathrm{d}\mathbf{x}\, \text{d} t=0 } \), where \( \smash{ \mathcal{L} } \) is called the Whitham average Lagrangian over the phase of the integral of the Lagrangian L defined by \( \smash{ \mathcal{L}(\omega,\mathbf{k},a,\mathbf{x},t)=\frac{1}{2\pi}\int_{0}^{2\pi}L\, \mskip2mu\mathrm{d}\theta } \), where L is the Lagrangian.
- Whitham's conservation equations:
-
These are first order nonlinear partial differential equations in the form \( \frac{\partial k}{\partial t}+\frac{\partial\omega}{\partial x}=0 \), \( \frac{\partial}{\partial t}\left\{ f(k)A^{2}\right\} +\frac{\partial}{\partial x}\left\{ f(k)C(k)A^{2}\right\} =0 \), where \( { k=k(x,t) } \) is the density of waves, \( { \omega=\omega(x,t) } \) is the flux of waves, \( { A=A(x,t) } \) is the amplitude and \( \smash{ f(k) } \) is an arbitrary function.
- Whitham's equation:
-
This first order nonlinear partial differential equation represents the conservation of waves. Mathematically, \( { (\partial k/\partial t)+(\partial\omega/\partial x)=0 } \) where \( { k=k(x,t) } \) is the density of waves and \( { \omega=\omega(x,t) } \) is the flux of waves.
- Whitham's equation for slowly varying wavetrain:
-
This is written in the form \( { \frac{\partial}{\partial t}\,\mathcal{L}_{\omega}-\frac{\partial}{\partial x_{i}}\,\mathcal{L}_{k_{i}}=0 } \), where \( { \mathcal{L} } \) is the Whitham averaged Lagrangian.
- Whitham's nonlinear nonlocal equations:
-
It is in the form \( \smash{u_{t}+ duu_{x}+\!\int_{-\infty}^{\infty} K(x\!-\!s) \, u_{s}(s,t) \, \mathrm{d} s=0} \), where \( { K(x)=\mathcal{F}^{-1}\left\{ c(k)=\omega/k\right\} } \) and \( { \mathcal{F}^{-1} } \) is the inverse Fourier transformation.
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Debnath, L. (2009). Water Waves and the Korteweg–de Vries Equation. In: Meyers, R. (eds) Encyclopedia of Complexity and Systems Science. Springer, New York, NY. https://doi.org/10.1007/978-0-387-30440-3_586
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