Radiation forces from WAMIT

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Felipe.Vittori
Posts: 4
Joined: Tue Jul 14, 2020 4:35 am
Organization: CENER
Location: Spain

Radiation forces from WAMIT

Postby Felipe.Vittori » Fri Jul 31, 2020 6:36 am

Dear all,

I have being trying to analyze the response of a floating wind turbine without the effect of the radiation forces. Following recommendations in the forum I enter RdtnMod=0 and RdtnTMax=0. After run the simulation I still have forces in the variables RdtnFxi…and RdtnMxi,…

However, these new forces are lower in magnitude with respect with the case where the radiation is enabled RdtnMod=1 and RdtnTMax=60.

I was wondering if there are other forces considered inside the variables Rdtn… moreover the convolution effect or am I missing something else to remove radiations forces besides setting RdtnMod=0 and RdtnTMax=0.

Thank you

Felipe Vittori

Jason.Jonkman
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Location: Boulder, CO
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Re: Radiation forces from WAMIT

Postby Jason.Jonkman » Fri Jul 31, 2020 10:10 am

Dear Felipe,

I would expect to see zero-valued results for the radiation-load outputs RdtnFxi, etc. when RdtnMod = 0. With RdtnMod = 1 and RdtnTMax = 0, I would expect to see the hydrodynamic added mass show up in the radiation-load outputs, but the wave-radiation damping to be neglected. With RdtnTMod = 1 and RdtnTMax > 0, I would expect the radiation-load outputs to include both hydrodynamic added mass and wave-radiation damping. Is this not what you are seeing?

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

Felipe.Vittori
Posts: 4
Joined: Tue Jul 14, 2020 4:35 am
Organization: CENER
Location: Spain

Re: Radiation forces from WAMIT

Postby Felipe.Vittori » Mon Aug 03, 2020 8:08 am

Dear Jason,

Seem that the hydrodynamic added mass is always taken account in the HydroDyn calculation.

I have ran more tests and when RdtnMod=0 and RdtnTMax=0 the hydrodynamic added mass still appearing in the Rdtn forces outputs.

In order to remove all radiation forces (RdtnFxi,… RdtnMzi=0) I had to modify the “.1” file by replacing all Aij=0.

If RdtnMod=1 and RdtnTMax=0 the radiation damping is neglected and the hydrodynamic added mass is in the radiation-load output.

Best Regards,

Felipe Vittori

Jason.Jonkman
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Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
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Re: Radiation forces from WAMIT

Postby Jason.Jonkman » Mon Aug 03, 2020 8:39 am

Dear Felipe,

The solution with RdtnMod = 1 and RdtnTMax = 0 s is what I expect, but the solution with RdtnMod = 0 is not what I expect. Which version of FAST / OpenFAST are you using?

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

Felipe.Vittori
Posts: 4
Joined: Tue Jul 14, 2020 4:35 am
Organization: CENER
Location: Spain

Re: Radiation forces from WAMIT

Postby Felipe.Vittori » Tue Aug 04, 2020 2:48 am

Dear Jason,

I was using openfast 2.2.

I also tried openfast 2.1 and 2.3, but is happening the same.

Best Regards,
Felipe Vittori

Jason.Jonkman
Posts: 4952
Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
Contact:

Re: Radiation forces from WAMIT

Postby Jason.Jonkman » Tue Aug 04, 2020 12:52 pm

Dear Felipe,

I talked with Greg Hayman and I think I was a bit confused on how RdtnMod is used. RdtnMod only changes how the memory effect (radiation damping) is processed:
  • RdtnMod = 0 = no memory effect; this would be the same as RdtnMod = 1 with TMax = 0;
  • RdntMod = 1 = memory effect via numerical convolution; and
  • RdtnMod = 2 = memory effect via a linear state-space model.
The hydrodynamic added mass is included for all RdtnMod settings (including RdtnMod = 0).

It may make sense to change RdtnMod = 0 to eliminate all wave radiation calculations (both added mass and damping), but that is not how it is implemented now.

Sorry for the confusion.

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

Felipe.Vittori
Posts: 4
Joined: Tue Jul 14, 2020 4:35 am
Organization: CENER
Location: Spain

Re: Radiation forces from WAMIT

Postby Felipe.Vittori » Wed Aug 05, 2020 3:14 am

Dear Jason,

Thank you for your assistance it was helpful to understand more about HydroDyn.

Best Regards,
Felipe Vittori


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