Wave elevation time series

Wind & Water Computer-Aided Engineering Software Tools
1

Dear all,

I’m trying to run a simulation with Fast for a triple spar offshore wind turbine. I’ve set the WaveMod in HydroDyn to 5 and I’m using an externally generated wave elevation time series. The problem is that after the simulation, when I compare the Wave1Elev channel results with the input wave elevation time series file, they don’t completely match. There are some dissimilarities in the wave height and period at the beginning of the simulation and after some time the picks are not matching but the periods match. That means I’m getting smaller wave heights in output than what I have given as Input. In the HydroDyn .sum data the wave elevations are zero.
Could you help me on this? Are there any other output channels which could solve my problems or am I getting it wrong and they shouldn’t be equal?

Best regards
Masih

2

Dear Masih,

With WaveMod = 5, I would expect the wave elevation output by HydroDyn to match the input wave elevation only if:

  • Second-order wave kinematics are disabled i.e. WvDiffQTF = WvSumQTF = False. With WaveMod = 5, the input wave elevation is assumed to be first-order such that the second-order terms are added to it, which would have some influence on the wave elevation.
  • The lower and upper cut-off frequencies of the first-order wave spectrum are disabled i.e. WvLowCOff = 0 and WvHiCOff > WaveOmegaMax.

These issues have been discussed a bit in the following forum topic: User input kinematics for floater in FAST - #5 by Jason.Jonkman.

Best regards,

3

Dear Jason,

thank you for your prompt answer. I guess I know what the problem is now.

regards
Masih