FASTV8-HydroDyn-Wavemode5

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Zhou.Lin
Posts: 22
Joined: Tue May 15, 2018 6:00 am
Organization: Dalian University of Techology
Location: China

FASTV8-HydroDyn-Wavemode5

Postby Zhou.Lin » Sat Sep 21, 2019 8:40 pm

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.
微信截图_20190922103417.png
微信截图_20190922103417.png (39.06 KiB) Viewed 76 times

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 KiB) Downloaded 2 times


Thank you ! I appreciate your help.

Best regards
Zhou.Lin

Jason.Jonkman
Posts: 4280
Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
Contact:

Re: FASTV8-HydroDyn-Wavemode5

Postby Jason.Jonkman » Mon Sep 23, 2019 8:29 am

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,
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

Zhou.Lin
Posts: 22
Joined: Tue May 15, 2018 6:00 am
Organization: Dalian University of Techology
Location: China

Re: FASTV8-HydroDyn-Wavemode5

Postby Zhou.Lin » Wed Sep 25, 2019 2:35 am

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 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

Jason.Jonkman
Posts: 4280
Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
Contact:

Re: FASTV8-HydroDyn-Wavemode5

Postby Jason.Jonkman » Wed Sep 25, 2019 6:55 am

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: 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


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