Radiation forces from WAMIT

Wind & Water Computer-Aided Engineering Software Tools
1

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

2

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,

3

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

4

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,

5

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

6

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,

7

Dear Jason,

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

Best Regards,
Felipe Vittori