Sea and swell in HydroDyn

Hi, I would like to know what would be the best way to simulate a two-peaks wave spectrum in HydroDyn.
I know that the following options are available in the

  • Irregular (stochastic) waves based on a user-defined frequency spectrum
  • Externally generated wave-elevation time series

I suppose the simplest way is to define a user-defined frequency spectrum, with two-peaks i.e. including a sea and a swell, but how is this done practically? Is there an example somewhere?

Also, woudl it be possible to simulate a sea wave spectrum coming from one direction, and a swell wave spectrum coming from another direction?

I tried to look up for this in the forum, but found nothing relevant.


Dear @Maurizio.Collu,

To generate irregular waves with a wave spectrum that includes two peaks, I agree that you could use WaveMod = 4 (user-defined spectrum) or 5 (user-defined wave elevation time series) as you propose. Using WaveMod = 4 will require that you implement in source code the spectrum within SUBROUTINE UserWaveSpctrm(), a template of which is provided in the Waves.f90 source file of HydroDyn, and recompile OpenFAST.

HydroDyn does not currently make it easy to define irregular waves with a wind waves and a swell waves coming from different directions. You could always use WaveMod = 6 (user-defined full wave kinematics), but that is a bit of a pain. OpenFAST v4.0 will include a major upgrade to HydroDyn, including the inclusion of a WaveMod = 7 option (user-defined waves based on frequency components) that will make it easier to define direction-dependent wave data. That said, we should add to the HydroDyn to-do list even easier approaches to define irregular waves with wind waves and a swell waves coming from different directions. Would you like to add a feature request to the OpenFAST issues page (Issues · OpenFAST/openfast · GitHub) requesting this HydroDyn enhancement?

Best regards,

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Thanks @Jason.Jonkman , I’ll try and let you know.

I’ve also followed your suggestion and atted the issue in the OpenFAST Github.

Kind Regards

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Dear @Jason.Jonkman,

I want to simulate the spar OC3 Hywind in OpenFAST in the case of user-defined wave elevation time series. I read in HydroDyn module that is possible to do this but one should set WaveMod=5 in HydroDyn input file. According to HydroDyn Manual, the externally generated wave data is provided through input files, all of which have the root name given by the WvKinFile.
I searched a lot but i did not succeed to find an example of the input file. Even in the r-test, none of them contain externally generated wave data.

Could you please share with the community an example of such input file ?

Bets Regards,


Dear @Jason.Jonkman ,

I solved the problem. In fact, one should paste the time and the wave elevation in a text file (.txt). Then, go to the properties of the file and change the extension from (.txt) to (.Elev)

The following link shows how to change the extension of a file in Windows 10:

Best Regards,


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Dear @Riad.Elhamoud,

I’m not aware of any OpenFAST test cases that support WaveMod = 5 or 6. There were some tests in the old HydroDyn driver of FAST v8 that supported WaveMod = 5 or 6, but it doesn’t appear that these have been ported over to the OpenFAST r-test per OpenFAST issue #179. You can find the old HydroDyn driver tests from FAST v8 here: HydroDyn | Wind Research | NREL, but these will need to be upgraded to work with OpenFAST. Regardless, the WvKinFile example files should provide you the needed guidance.

Best regards,

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