Directional wave spectrum verification using WAFO

Wind & Water Computer-Aided Engineering Software Tools
1

Hi everyone,

I hope you can help me with the problem I’m currently facing.
As part of my masters’ thesis, I am simulating the response of a FOWT with a TLP foundation. I am interested in using response data, which has been generated by a directional wave spectrum ie. response in a multidirectional sea.

My problem arises when comparing the wave elevation time series obtained from FAST, with a theoretically calculated direction wave spectrum.

I use the MatLab toolbox WAFO, to analyse and obtain the directional wave spectrum, based on 5 measurement point described in the hydrodyn file, this data is then used as an input in the WAFO function dat2dspec, along with additional required information.

I also use the WAFO toolbox to generate a theoretical directional wave spectrum based on the coefficients used in the hydrodyn file. When comparing these spectra, I am not able to obtain the same spreading function and therefore not able to validate the directional wave spectrum used in FAST.

In the figure below I have compared to two resultant spectra with their spreading functions as well, as you can see, the spreading functions do not match.

I have tried to change the spreading coefficient, but this does not seem to make a difference in the result calculated by WAFO, neither does changing WaveNDir, which in the above case is set to 125.
I have also tried to change the distance between the wave gauges, but this didn’t seem to make a difference either.

I have tried to utilize some of the other methods for determining the directional wave spectrum from data, such as BDM, MLM, and MEM, but all gave similar results.

I have attached a compressed file containing the hydrodyn file used in FAST and the MatLab script used to plot the above figure.
If wanted, the data from FAST used in the MatLab script can be downloaded here.

Best Regards
Thomas Hansen
Aalborg University
Dir_spectrum.rar (5.54 KB)

2

Hi Thomas,

I’m not familiar with WAFO, but the what you’ve plotted as “Direction spectrum - FAST” makes sense to me given your related HydroDyn input settings:

-10 WaveDir
1 WaveDirMod
10 WaveDirSpread
125 WaveNDir
60 WaveDirRange

The directional spreading function is in the range +/-30 deg centered around -10 deg, and with S (WaveDirSpread) = 10, the function is very narrow and peaky (reducing S would broaden the peak).

The directional spreading function implemented in the HydroDyn module of OpenFAST is documented in our OMAE 2014 paper by Duarte et al: nrel.gov/docs/fy14osti/61161.pdf.

Best regards,

3

Hi Jason, thank you for such a quick reply!

Thank you for the reference, I have also been using this exact paper in my research.
I believe that is it the WAFO package, which does not support the definition of WaveDirRange. I have not been able to find other references, than the one provided by you, which presents a cos2S spreading function with the added capability to specify the outer limits of the spreading function.

From the articles I have read, all cos2S functions is defined to extend the whole range from ±90 degrees and solely rely on the spreading coefficients to define the shape of the function, do you have any thoughts on why this is? And why did you choose to include the capability of specifying WaveDirRange?

For reference I will run a simulation with WaveDirRange = 180, and compare it with WAFO’ calculated spreading function, using the same spreading coefficient, and post my findings here when available.

4

Dear Thomas,

I believe the use of WaveDirRange, which equals 2*theta_max in our OMAE paper, came from reference [1]. That paper states that theta_max is often set to pi/2, which is equivalent to WaveDirRange = 180 deg. Including WaveDirRange is a nice feature to have to better tailor the wave directional spreading function with fewer discretized wave directions (WaveNDir), i.e., always requiring WaveDirRange = 180 deg means a higher value of WaveNDir is required to discretize a narrow-banded spreading function when compared to the case where you can set WaveDirRange < 180.

Best regards,