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In fluid dynamics, a trochoidal wave or Gerstner wave is an exact solution of the Euler equations for periodic surface gravity waves. It describes a progressive wave of permanent form on the surface of an incompressible fluid of infinite depth. The free surface of this wave solution is an inverted (upside-down) trochoid – with sharper crests and flat troughs. This wave solution was discovered by Gerstner in 1802, and rediscovered independently by Rankine in 1863. The flow field associated with the trochoidal wave is not irrotational: it has vorticity. The vorticity is of such a specific strength and vertical distribution that the trajectories of the fluid parcels are closed circles. This is in contrast with the usual experimental observation of Stokes drift associated with the wave motion. Also the phase speed is independent of the trochoidal wave's amplitude, unlike other nonlinear wave-theories (like those of the Stokes wave and cnoidal wave) and observations. For these reasons – as well as for the fact that solutions for finite fluid depth are lacking – trochoidal waves are of limited use for engineering applications. In computer graphics, the rendering of realistic-looking ocean waves can be done by use of so-called Gerstner waves. This is a multi-component and multi-directional extension of the traditional Gerstner wave, often using fast Fourier transforms to make (real-time) animation feasible. ==Description of classical trochoidal wave== Using a Lagrangian specification of the flow field, the motion of fluid parcels is – for a periodic wave on the surface of a fluid layer of infinite depth: : where and are the positions of the fluid parcels in the plane at time , with the horizontal coordinate and the vertical coordinate (positive upward, in the direction opposing gravity). The Lagrangian coordinates label the fluid parcels, with the centres of the circular orbits – around which the corresponding fluid parcel moves with constant speed Further is the wavenumber (and the wavelength), while is the phase speed with which the wave propagates in the -direction. The phase speed satisfies the dispersion relation: : which is independent of the wave nonlinearity (i.e. does not depend on the wave height ), and this phase speed the same as for Airy's linear waves in deep water. The free surface is a line of constant pressure, and is found to correspond with a line , where is a (nonpositive) constant. For the highest waves occur, with a cusp-shaped crest. Note that the highest (irrotational) Stokes wave has a crest angle of 120°, instead of the 0° for the rotational trochoidal wave. The wave height of the trochoidal wave is The wave is periodic in the -direction, with wavelength and also periodic in time with period 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Trochoidal wave」の詳細全文を読む スポンサード リンク
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