Irregular waves- single spectrum

The spectrum is defined following a JONSWAP spectrum (see Irregular waves)

Irregular waves

Input file

The file that is imported to define the sine components of this irregular waves situation. If no file is chosen (i.e. if this parameter field is empty) then there is no irregular waves situation (Note: By default this field is empty).

A currents text input file must have at least one line of data. Each line of data has the three numbers format: wave period, wave height, phase angle, relative angle.

Note: this parameter is only accessible in the batch manager. In order to import an irregular wave realization in time simulation, use the Action buttons at the top of the parameter window.

  • Default value:
  • Unit:

Principal angle

The angle between the principal direction of the waves and the north direction. For example, an angle of 90° leads to the waves coming from the east direction.

  • Default value: 0
  • Unit: $°$
  • Range: -360 — 360

Spectrum

The kind of spectrum that will be used to simulate the irregular waves situation.


Options:

JONSWAP (default):

JONSWAP spectrum is a generalization of the PM spectrum. The peakedness is a variable of the JONSWAP, but not of the PM. JONSWAP applies to seastates that are not fully developed.

PM:

Pierson-Moskowitz spectrum. The PM spectrum is relevant for fully developed seastates. The JONSWAP spectrum is a generalization of the PM spectrum. If the peakedness of a JONSWAP spectrum is 1 (minimum value), then it becomes a PM spectrum.

Peak period

The peak wave period of the wave spectrum.


  • Default value: 10
  • Unit: $\text{s}$
  • Range: 0 — 1e+06

Significant wave height

The significant wave height.


  • Default value: 6
  • Unit: $\text{m}$
  • Range: 0 — 1e+06

Seed scheme

Select if you want to enter the seed of the phase angles manually or if it should be randomly generated by Ashes.


Options:

Random:

The seed is randomly set based on the current time.

User defined (default):

The seed is set explicitly by the user. This is necessary to do in order to reproduce a simulation.

Seed

Set the integer value for the seed to be used when initializing the pseudo random number generator. Random numbers are used (only) for the phase angles of the wave components. This parameter is visible when the Random seed scheme parameter is set to User defined. Minimum value is 0. Default value is 0.


  • Default value: 0
  • Unit:
  • Range: 0 — 1e+06

Simulation scheme

Choose how the frequency band of the spectrum is divided into frequency intervals. Default: Equal energy.


Options:

Equal energy (default):

The wave components have (approx.) the same energy and thus the same height.

Equal frequency:

The wave components have the same delta frequency. Thus, the energy (and amplitudes) of the components are different.

Frequency delta limit

For a simulation based on energy equality, the frequency delta will be different and this parameter is a factor that limits the maximum length of the frequency delta to the average frequency delta multiplied by this factor. This means that you can get more wave components than asked for, e.g. you ask for 100 wave components and get 102.
Min value: 1. Max value: 100. Default value: 10.


  • Default value: 10
  • Unit:
  • Range: 1 — 100

N

The number of wave components in the irregular wave simulation.


  • Default value: 100
  • Unit:
  • Range: 1 — 10000

Error estimate

The simulated spectrum will have an error compared to the theoretical spectrum. This value is the approximate error. Suggested value: 1 %.

This value also determines the cut-off frequency of the spectrum (defined in Irregular waves): the energy in the component at the cut-off frequency will be the error estimate times the energy in the component with the highest energy.

  • Default value: 1
  • Unit:
  • Range: 0.01 — 10

Peakedness scheme

The peakedness can be set by the user (you) or calculated based on DNV-RP-C205.


Options:

User defined (default):

The peakedness factor is set by the user (you).

Calculated:

The peakedness factor is calculated from DNV-RP-C205, sec. 3.5.5.5, p.65 (2017).

Peakedness

The peakedness factor for the JONSWAP spectrum. For a peakedness factor of 1 the JONSWAP spectrum becomes a Pierson-Moskowitz spectrum.


  • Default value: 3.3
  • Unit:
  • Range: 1 — 7

Wave spreading

Irregular waves can be modelled with a spreading function.


Options:

None (default):

No wave spreading. The waves are modeled uni-directional only using the wave spectrum.

cos^n:

Spreading function from DNV-RP-C205, sec. 3.5.8.4

cos^2s:

Spreading function from DNV-RP-C205, sec. 3.5.8.7

n

The cos^n wave spreading distributions is dependent on the parameter n. DNV-RP-C205, sec. 3.5.8.6: Due consideration should be taken to reflect an accurate correlation between the actual sea-state and the constant n. Typical values for wind sea are n = 2 to n = 4. If used for swell, n > 7 is more appropriate.


  • Default value: 2
  • Unit:
  • Range: 0 — 20

s

The cos^2s wave spreading distributions is dependent on the parameter s. DNV-RP-C205, sec. 3.5.8.7: Typical values for wind sea are s = 5 to s = 15. If used for swell s > 15 is more appropriate.


  • Default value: 5
  • Unit:
  • Range: 0 — 20

Extreme wave search

Number of extremes

Set the number of extreme wave occurences that shall be found.
Note: this parameter does NOT affect the actual number of extreme wave occurences. It just determines how many of the extreme occurences that are found, starting with the most extreme.
All the extreme wave occurences are listed in the 'Information' tab.

  • Default value: 10
  • Unit:
  • Range: 0 — 1000

Search duration

The duration (time) in which extremes are searched for. This is not related to the the length of an actual simulation, but it makes sense to look in a relevant duration.

  • Default value: 600
  • Unit: $\text{s}$
  • Range: 10 — 10800

Time step

The time step to be used in the search.

  • Default value: 1
  • Unit: $\text{s}$
  • Range: 0.1 — 10

Time buffer

The time buffer on either side of a extreme wave elevation where there cannot be a higher wave elevation.

  • Default value: 15
  • Unit: $\text{s}$
  • Range: 1 — 60