Second-order waves

Dear Jason,

I’ve set the WaveMod in HydroDyn to 5 and I’m using an externally generated irregular wave elevation time series. Despite setting the 2ND-order waves Flag to true for both mean and difference-frequencies, I’m getting linear waves in the output channel. Could you tell me what I could do to get the exact wave elevations as the data from the wave gauge?
I’m attaching a plot from gage and output channel data.

Best regards
Masih

wave.JPG

Dear Masih,

A similar question has been asked and answered on our forum here: http://forums.nrel.gov/t/user-input-kinematics-for-floater-in-fast/1472/8. My guess is that you are applying a first-order low- and/or high-frequency cut-offs.

Please note the WaveMod = 5 option assumes that the user-specified wave elevation is first order. Adding second-order terms along with WaveMod = 5 will not allow you to get the exact wave-elevation time series back, because additional energy will be added at the difference and sum frequencies.

Best regards,

.