# BModes - hydrodynamic matrix

Dear Jason,

I want to use Bmodes to get the mode shapes for an elliptical tower cross section mounted on a floating structure. I have obtained the hydrodynamic 6x6 matrix from NEMOH and rewritten the results to the WAMIT-format (.1, .3 and .hst). My plan for getting the mooring stiffness matrix is to follow the approach described by you, where the WAMIT-inputs are all set to zero.
My question is now related to which frequency one would use for the hydrodynamic matrix in BModes?
I will do the linearization for different values of the wind speed (and thus rotor speed) and in this way get a Campbell diagram, but I am not sure about which wave frequency to use for the hydrodynamic matrix in the linearization.

Dear Jacob,

You cannot define a frequency-dependent hydrodynamic added mass in BModes, so, you, must pick a frequency at which to characterize the added mass. Typically, the zero- or infinite-frequency limit of added mass is chosen, but you could also choose the added mass closest to the frequency of most interest to your structural response (a frequency close to zero, rather than infinite, is likely the most representative for the modes of interest). You could always derive the mode shapes for a range of frequencies for the added mass matrix to see how sensitive the results are to the selection.

Best regards,

Dear Jason,

Thank you. So, if I want to see the influence of the hydrodynamic terms when using BModes, I should use sets of A_11, A_15, A_22, A_24, A_33, A_42, A_44, A_51, A_55 and A_66 from the WAMIT .1-file at different wave frequencies to see the effect on natural frequencies on the total system?

Dear Jacob,

Yes, except that the frequency-dependence of the hydrodynamic added mass relates to the frequency of body motion rather than the frequency of wave excitation.

Best regards,

Dear Jason,

OK, I see. Thanks a lot again.

Dear Jason,

I followed your post from tue oct 09, 2012 on how to get the mooring stiffness matrix for the hywind spar buoy. By comparing the values from the OC3hywind.bmi file from BModesJJ with the DspCMat (multiplied with -1000), I can see that the last 6x6 part of this 118x6 matrix corresponds to the mooring stiffness matrix. Is there some overview, which shows where each of the DOFs of the wind turbine is situated in this matrix? Thanks again for all your help.

Dear Jacob,

The orders of the inputs, outputs, and states in the linearized matrices output by FAST through a FAST linearization analysis are listed at the top of the linearization output (*.lin) file.

Best regards,