Dear @Jason.Jonkman ,
Thanks for you paper entitled “Dynamics of Offshore Floating Wind Turbines—Model Development and Verification” https://onlinelibrary.wiley.com/doi/10.1002/we.347. The detailed content improves my understanding of theory implemented in Hydrodyn.
I want to share my understanding here. Could you please correct if there are something misundertood?
In the following, omega denoted angular frequency of incident wave, and beta denotes incident wave direction.

Hydrodyn adopted true linear hydrodynamic model, rather than frequencydomain hydrodynamic model. But the excitation force Xi(omega, beta) used in frequencydomain hydrodynamic model is also used in true linear hydrodynamic model (diffraction term).

For added mass matrix Aij(omega) and damping Bij(omega), true linear hydrodynamic model only uses infinitefrequency limit of Aij(omega). Bij(omega) is actually used to integrate wave radiationretardation kernel Kij(t).

For Hydrodyn input files, .1 file contains Aij(omega), Bij(omega). Based on above understanding, I would guess only Aij(infinity) is directly used by Hydrodyn, and other Aij(omega) will be ignored. A range of Bij(omega) are used to derive wave radiationretardation kernel Kij(t) through FFT.
I still have a question irrelevant to Hydrodyn:
When I use BEMRosetta (an opensource software to animate hydrodynamic coefficients) to animate WAMIT results of OC4 platform offered in OpenFAST baseline folder. I found a variable called “Kirf Impulse Response Function”. I checked the WAMIT documentation, .hst, .1 and .3 files do not contain this variable.
4. Do you know what “Kirf Impulse Response Function” is used for? It seems that Hydrodyn does not use this variable.
Regards,
Ran