Hi Jason,

I’m using the HydroDyn module used by FASTv8 for the offshore hydrodynamics calculations applies to fixed-bottom monopilesI (Test19),I enable tower and platform of DOF, the rest of the DOF are closed, My model only considered wave load, so I disabled comAero ,CompInflow and CompSero modules d, enable only the CompElast and ComHydro and CompSub module, use the external wave import way namely Wavemod5 calculating wave .

So,I have one questions.

1.The input wave height is inconsistent with the output wave height, The output wave height is higher than the input wave, As shown in figure.

I think may be due to WaveStmod choice, but now can only choose Wavestmode = 0, Wavestmod = 3 is not applied, so my question is caused by the difference of input and output to because this problem or other problems. attachments is my HydroDyn

NRELOffshrBsline5MW_OC3Monopile_HydroDyn.rar (4.62 KB)

Thank you ! I appreciate your help.

Best regards

Zhou.Lin

Dear Zhou.Lin,

It looks like the output wave-elevation time series is shifted from the input wave-elevation time series in only the mean value. What is the mean of each wave-elevation signal?

The user-specified wave-elevation time series option of HydroDyn (WaveMod=5) will derive the amplitudes and phases of each frequency component from the user-specified wave elevation to calculate the wave kinematics throughout the domain, but it will ensure that the wave-elevation time series has zero mean, i.e., HydroDyn will not use a nonzero mean value (amplitude at zero frequency) from the user-specified wave-elevation time series.

Best regards,

Dear Jason,

Thank you for you reply.

If I want to obtain the same wave height between input and outputb[/b], the nonzero mean value (amplitude at zero frequency) from the user-specified wave-elevation time series must be removed. If so, how do I get rid of the nonzero mean value (amplitude at zero frequency).

For the hydrodynamic module, I have another question to ask you. My model is based on a fixed-bottom model. If I want to calculate the second-order wave force, do you only need to turn on 2nd ORDER WAVES and WvDiffQTF and WvSumQTF. other Flags need to be opened?

best regards

Zhou.Lin

Dear Zhou.Lin,

Simply remove the time-series mean from the wave-elevation time series before using that data in HydroDyn.

For a fixed-bottom offshore wind turbine, presumably you are using the strip-theory solution of HydroDyn (as opposed to the potential-flow (WAMIT) solution). In this case, yes, to enable second-order hydrodynamic loads, all you have to do is enable WvDiffQTF for difference-frequency terms and WvSumQTF for sum-frequency terms. You may also need to set the low- and high- cut-off frequencies appropriately for each sea state you are modeling (see the draft HydroDyn User’s Guide and Theory Manuel for guidance: wind.nrel.gov/nwtc/docs/HydroDyn_Manual.pdf).

Best regards,

Dear Jason,

I’m using the HydroDyn module used by FASTv8 for the offshore hydrodynamics calculations applies to fixed-bottom monopiles, use the external wave import way namely Wavemod5 calculating wave kinematics, How do we figure out the hydrodynamic load from the input of the wave height, was it calculated directly by Morison equation, and if you did it directly what theory was applied, was there a formula in HydroDyn.

Thank you ! I appreciate your help.

Best regards,

Zhou.Lin

Dear Zhou.Lin,

The WaveMod=5 option of HydroDyn uses the user-specified wave-elevation time history to calculate the wave kinematics throughout the fluid domain. Any of the hydrodynamic load models of HydroDyn (strip theory or potential flow) can be enabled. Presumably you’ve enabled strip theory (Morison’s equation) for a monopile, but you haven’t stated you’ve enabled.

Best regards,