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Environment: Waves preview |
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Environment: Waves preview |
When using a random wave or a time history wave, OrcaFlex provides two preview facilities to help you to select a suitable wave, namely list events and view profile. These are provided on the waves preview page on the environment data form. They enable you to search through a period of simulation time looking for an interesting wave event. You can then set the time origins so that the simulation covers that event.
| Note: | If you are using multiple wave trains then these commands report the combined sea state from all of the wave trains. |
Typically, the wave preview facilities might be used as follows. First, with the list events function, scan over a long period of simulation time (e.g. 10000 seconds or more), looking for waves sufficiently large to be of interest. For each one, use view profile to examine that section in more detail. Having decided which part of the wave train to use, set the wave time origin to just before the period of interest, so that the simulation covers that period. See also finding a suitable design wave, for a more detailed description of the use of the wave preview tools.
The $(X,Y)$ point to which the list events, horizontal velocity and view profile commands apply. Since wave trains vary in space as well as time you should normally set this point to be close to a system point of interest, such as a riser top end position.
The simulation time period, given by starting time and duration, over which the view profile, view autocorrelation, and list events commands apply.
Plots a time history of wave elevation at the given position, over the given simulation time period.
Plots the autocorrelation coefficient function of the discretised total wave process, i.e. the process of all wave components across all wave trains.
This is available for both regular and irregular waves, but is only really useful for irregular waves. The autocorrelation of a regular wave does not decay – it repeats at wave period, which is just a reminder that a repeating wave process is being modelled.
The autocorrelation of irregular waves is a way of judging whether a particular discretisation will give reasonable statistical modelling of the continuous-spectrum process it aims to represent. The real-world continuous-spectrum process autocorrelation will decay to very small values after only a few peak wave periods. The discretised process will also decay initially, but then show more significant correlation for longer time lags, due to the discretisation into a finite number of wave components. You should use a number of wave components that is sufficiently large enough that the autocorrelation function decays to a reasonably small value and stays small throughout the simulation period, otherwise the simulations will contain unrealistic correlation and not be statistically representative of the real-world continuous-spectrum process.
This command searches for individual waves within a random sea which satisfy a given set of criteria. All the matching wave events in the specified simulation time interval, at the given position, are reported.
The criteria may be defined in one of two ways, according to the wave search method:
For each event, the height (total rise or fall) is given. An equivalent period is derived from the time interval between the peak and trough,and from these and the water depth we calculate a corresponding Airy wave of the same height and period and report its properties.
If there is only one wave train then, for comparison purposes, a reference wave is reported at the top of the table. This is the Airy wave whose height and period match the $\Hs$ and $\Tz$ of the random wave train.
Finally, various wave elevation statistics are reported for the position and period of time specified. These include the largest rise and fall, the highest crest and lowest trough, the number of up and down zero-crossings and the sample's estimated $\Hs$ and $\Tz$ values. These statistics enable you to judge how typical this wave elevation sample is, compared to the whole spectrum.
Plots the wave horizontal velocity variation with depth, at the specified (X, Y) position at the given simulation time.