The steadily-progressing wave problem

Solution using programs FOURIER, CNOIDAL and STOKES

The idealisation of a wave

One wave of a steady wave train,
                showing dimensions, co-ordinates and velocities

Update Notes - Reverse chronological order

July 2015 - I have added some notes and comments to the file Instructions.pdf. Errors had crept into some of the figures, including the incorrect dimensioning of wave height H on Figure 4-1. The corrected version of the figure is as shown above. I have taken the opportunity to redefine the free surface elevation eta with origin on the bed, as the output from the program uses that. This required changes to the presentation of some of the theory in the document, but is unimportant as far as program operation is concerned. I added a little section on the value of gravitational acceleration g to use. It is more whimsical than anything else, but it does point out that the often-used g = 9.81 m/s^2 is actually not correct to that accuracy for most of the world's population. The changes are described in the comments which are included in that file.

March 2014 - I have made cosmetic input/output changes to all three programs. The screen output for all three programs, Fourier, Stokes, and Cnoidal, has been simplified and made consistent with each other. Similarly the output files have had all headers made consistent. It should now be easier to compare results between programs.

I have made a slight change to the Stokes program so as to use the secant method to solve for wavelength if period is known. The previous use of the bisection method could fail in extreme cases. Also, one can now specify the order of the Stokes theory without restriction, from 1 to 5.

May 2013 - The header files of the source code for the program have been simplified and a number of little incompatibilities with correct C++ eliminated. It has now been compiled without any warnings.

The introduction to the explanatory document Instructions.pdf has been re-written, putting the Fourier approach in context, and introducing the Stokes-Ursell number for the boundary between Stokes and cnoidal theories, even if that is incidental to the present work.

29 May 2013 - Much more importantly, I have discovered that there has been an error in the program for some years. It used a value of current that was roughly half of the value specified. If the current specified was zero the program was correct, also using zero. In general, there was an error. I have corrected it in the current download version. This is described rather more fully in the instructions file.

3 June 2013 - To help guard against such errors, I have upgraded my cnoidal theory and Stokes theory packages, and now distribute them with Fourier, to provide something of a check on each. The two programs based on the complementary theories uses the same data files as Fourier, but each should be installed in a subdirectory of the Fourier one. The two programs agree well with Fourier for short waves in the case of Stokes and long waves in the case of Cnoidal. In general, however, Fourier is to be preferred.

Download programs and instruction manual

There are now threeprograms that can be used. The instructions file for all is Instructions.pdf, which is also included in Fourier.zip.

  1. A Fourier approximation method whose only approximation lies in truncating the number of terms in the approximating series: Fourier.zip - current version, 20 March 2015.
  2. An implementation of cnoidal theory, which is based on series expansions in shallowness, requiring that the waves be long relative to the water depth : Cnoidal.zip - current version, 20 March 2015 (to be unpacked in a sub-directory of the Fourier one). This is an approximation, and not as applicable to higher waves as the Fourier method. It can be used as a check on that method - for waves that are not high, both should give the same results.
  3. An implementation of Stokes theory, requiring that the waves be not too long relative to the water depth: Stokes.zip - current version 20 March 2015 (to be unpacked in a sub-directory of the Fourier one). This is also an approximation, and not as applicable to higher waves as the Fourier method. It can be used as a check on that method - for waves that are not high, both should give the same results.

Questions, problems: please e-mail me here: John Fenton

Description

The main theories and methods for the steady wave problem which have been used are: Stokes theory, an explicit theory based on an assumption that the waves are not very steep and which is best suited to waves in deeper water; and cnoidal theory, an explicit theory for waves in shallower water. The accuracy of both depends on the waves not being too high. In addition, both have a similar problem, that in the inappropriate limits of shallower water for Stokes theory and deeper water for cnoidal theory, the series become slowly convergent and ultimately do not converge.

An approach which overcomes this is the Fourier approximation method, which does not use series expansions based on a small parameter, but obtains the solution numerically. It could be described as a nonlinear spectral approach, where a series is assumed, each term of which satisfies the field equation, and then the coefficients are found by solving a system of nonlinear equations. This, originated by Chappelear in 1961, is the basis of the computer program FOURIER. It has been widely used to provide solutions in a number of practical and theoretical applications, providing solutions for engineering and geophysics. The method provides accurate solutions for waves up to very close to the highest. Below is shown the surface profile for a wave of length L/d=50 and a height 98% of the maximum for that length - for such a long and high wave (approaching a sharp crest), the method is close to its limits.
Extreme
                wave

The original theory and Fortran code can be found here, or a more recent presentation here. The current download version Fourier also contains the code in the language C++. A description of the theory and program inputs and outputs is given in Instructions.
A package with programs for cnoidal theory is also available: Cnoidal.zip, which is also described in Instructions, as is a package for Stokes theory: Stokes.zip.

There is also a numerical cnoidal theory where long waves can be treated without difficulty, however for wavelengths at least as long as 50 times the depth, the Fourier method provides good solutions.

Results from Fourier program shown plotted with program Gnuplot

The output from the program is three text files which are machine readable and can be used for program development by others. The package also contains plotting files written in the Gnuplot language (http://www.gnuplot.info/) which uses the output to produce figures such as these:

Surface profileVelocity
                fieldAcceleration field

Maintained and authorised by John Fenton; Last modified:  Thursday 23 July 2015
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