Spectra and Ensembles |
 
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Spectra and Ensembles |
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The procedure used by Flexcom in calculating spectra is as follows. Firstly, the output timetrace is divided equally into a number of smaller timetraces or ensembles. A spectrum for each ensemble is then calculated using the Fast Fourier Transform (FFT) algorithm. Finally, the actual spectrum to be output is found as an average of the spectra calculated for each ensemble. This standard procedure minimises the statistical error associated with the FFT process. You specify the number of ensembles to be used in this process, and the specified value should always be greater than 1.
Note also that the FFT algorithm requires a record with a fixed time step. When you perform a variable step analysis, obviously such a record is not available, and so Flexcom must synthesise one by interpolating from the variable step record. So you must inform Flexcom what time step to use in the synthesised record.
It is difficult to provide specific guidance on a suitable number of ensembles, as an appropriate selection is case specific. Generally speaking, if you have performed a lengthy simulation (e.g. 3 hour), you should use a larger number of ensembles than if you have performed a relatively short one (e.g. 0.5 hours). Using a relatively small number of ensembles means that the FFT algorithm uses relatively long individual records, and this tends to ensure good frequency resolution between the realised and target spectra. Conversely, using a larger number of ensembles means that the FFT algorithm uses shorter individual records, and this tends to ensure better amplitude resolution. A useful exercise is to compare the realized wave spectrum against the theoretical or target wave spectrum - Flexcom provides a specific option under the *SPECTRUM keyword in the Database Postprocessor to readily achieve this. If you plot the realized wave spectrum for different numbers of ensembles, the one which produces the closest agreement with the target spectrum should be most suitable.
•*SPECTRUM is used to request the creation of spectrum plots.
•Note also that the TYPE=ELEVATION option under the *SPECTRUM keyword allows you to request a spectrum of water surface elevation. It also allows you to plot the target wave spectrum, in addition to the realised spectrum, which is useful for comparison purposes.