Time History of Water Surface Elevation |
 
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Time History of Water Surface Elevation |
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You may also define a random seastate in terms of a time history of water surface elevation. When this option is invoked, Flexcom first calculates a wave spectrum from the time history. A record with a fixed time step is required for this; if your record has a variable time step, then Flexcom synthesises a record with a fixed step by interpolation. You have the option of specifying the fixed time step to use in this process, or of letting Flexcom choose a suitable value. The output from this process is then treated as a series of regular waves, each with its individual frequency, amplitude and phase.
In order for the realised wave elevation history to be in exact agreement with the input wave history defined in the data file, a very large number of harmonics are required (n/2 in fact, where n is the number of data lines in the wave elevation time history). Naturally the subsequent dynamic analysis can be quite time consuming if it contains thousands of harmonics. A more efficient approach is to divide the input time history into a number of shorter time histories using the Ensembles entry. For example, using ten ensembles rather than one will result in approximately a 90% reduction in the number of harmonics used, and this has obvious benefits in terms of the overall run time. Where the number of ensembles is greater than one, the Fourier Transform process is repeated at regular intervals over the course of the dynamic analysis, with each region having its own unique combination of regular wave harmonics.
•*WAVE-TIME-HISTORY is used to specify a random seastate in terms of a time history of water surface elevation.