Hi Jason,
I'm using the HydroDyn module used by FASTv8 for the offshore hydrodynamics calculations applies to fixedbottom 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
Thank you ! I appreciate your help.
Best regards
Zhou.Lin
FASTV8HydroDynWavemode5
Moderators: Bonnie.Jonkman, Jason.Jonkman

 Posts: 4750
 Joined: Thu Nov 03, 2005 4:38 pm
 Location: Boulder, CO
 Contact:
Re: FASTV8HydroDynWavemode5
Dear Zhou.Lin,
It looks like the output waveelevation time series is shifted from the input waveelevation time series in only the mean value. What is the mean of each waveelevation signal?
The userspecified waveelevation time series option of HydroDyn (WaveMod=5) will derive the amplitudes and phases of each frequency component from the userspecified wave elevation to calculate the wave kinematics throughout the domain, but it will ensure that the waveelevation time series has zero mean, i.e., HydroDyn will not use a nonzero mean value (amplitude at zero frequency) from the userspecified waveelevation time series.
Best regards,
It looks like the output waveelevation time series is shifted from the input waveelevation time series in only the mean value. What is the mean of each waveelevation signal?
The userspecified waveelevation time series option of HydroDyn (WaveMod=5) will derive the amplitudes and phases of each frequency component from the userspecified wave elevation to calculate the wave kinematics throughout the domain, but it will ensure that the waveelevation time series has zero mean, i.e., HydroDyn will not use a nonzero mean value (amplitude at zero frequency) from the userspecified waveelevation time series.
Best regards,
Jason Jonkman, Ph.D.
Senior Engineer  National Wind Technology Center (NWTC)
National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway  Golden, CO 80401
+1 (303) 384 – 7026  Fax: +1 (303) 384 – 6901
nwtc.nrel.gov
Senior Engineer  National Wind Technology Center (NWTC)
National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway  Golden, CO 80401
+1 (303) 384 – 7026  Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

 Posts: 27
 Joined: Tue May 15, 2018 6:00 am
 Organization: Dalian University of Techology
 Location: China
Re: FASTV8HydroDynWavemode5
Dear Jason,
Thank you for you reply.
If I want to obtain the same wave height between input and output(wave1Elev), the nonzero mean value (amplitude at zero frequency) from the userspecified waveelevation 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 fixedbottom model. If I want to calculate the secondorder 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
Thank you for you reply.
If I want to obtain the same wave height between input and output(wave1Elev), the nonzero mean value (amplitude at zero frequency) from the userspecified waveelevation 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 fixedbottom model. If I want to calculate the secondorder 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

 Posts: 4750
 Joined: Thu Nov 03, 2005 4:38 pm
 Location: Boulder, CO
 Contact:
Re: FASTV8HydroDynWavemode5
Dear Zhou.Lin,
Simply remove the timeseries mean from the waveelevation time series before using that data in HydroDyn.
For a fixedbottom offshore wind turbine, presumably you are using the striptheory solution of HydroDyn (as opposed to the potentialflow (WAMIT) solution). In this case, yes, to enable secondorder hydrodynamic loads, all you have to do is enable WvDiffQTF for differencefrequency terms and WvSumQTF for sumfrequency terms. You may also need to set the low and high cutoff frequencies appropriately for each sea state you are modeling (see the draft HydroDyn User's Guide and Theory Manuel for guidance: https://wind.nrel.gov/nwtc/docs/HydroDyn_Manual.pdf).
Best regards,
Simply remove the timeseries mean from the waveelevation time series before using that data in HydroDyn.
For a fixedbottom offshore wind turbine, presumably you are using the striptheory solution of HydroDyn (as opposed to the potentialflow (WAMIT) solution). In this case, yes, to enable secondorder hydrodynamic loads, all you have to do is enable WvDiffQTF for differencefrequency terms and WvSumQTF for sumfrequency terms. You may also need to set the low and high cutoff frequencies appropriately for each sea state you are modeling (see the draft HydroDyn User's Guide and Theory Manuel for guidance: https://wind.nrel.gov/nwtc/docs/HydroDyn_Manual.pdf).
Best regards,
Jason Jonkman, Ph.D.
Senior Engineer  National Wind Technology Center (NWTC)
National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway  Golden, CO 80401
+1 (303) 384 – 7026  Fax: +1 (303) 384 – 6901
nwtc.nrel.gov
Senior Engineer  National Wind Technology Center (NWTC)
National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway  Golden, CO 80401
+1 (303) 384 – 7026  Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

 Posts: 27
 Joined: Tue May 15, 2018 6:00 am
 Organization: Dalian University of Techology
 Location: China
Re: FASTV8HydroDynWavemode5
Dear Jason,
I'm using the HydroDyn module used by FASTv8 for the offshore hydrodynamics calculations applies to fixedbottom 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
I'm using the HydroDyn module used by FASTv8 for the offshore hydrodynamics calculations applies to fixedbottom 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

 Posts: 4750
 Joined: Thu Nov 03, 2005 4:38 pm
 Location: Boulder, CO
 Contact:
Re: FASTV8HydroDynWavemode5
Dear Zhou.Lin,
The WaveMod=5 option of HydroDyn uses the userspecified waveelevation 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,
The WaveMod=5 option of HydroDyn uses the userspecified waveelevation 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,
Jason Jonkman, Ph.D.
Senior Engineer  National Wind Technology Center (NWTC)
National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway  Golden, CO 80401
+1 (303) 384 – 7026  Fax: +1 (303) 384 – 6901
nwtc.nrel.gov
Senior Engineer  National Wind Technology Center (NWTC)
National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway  Golden, CO 80401
+1 (303) 384 – 7026  Fax: +1 (303) 384 – 6901
nwtc.nrel.gov
Return to “ComputerAided Engineering Software Tools”
Who is online
Users browsing this forum: No registered users and 1 guest