Dear Jason,
I am currently looking at the IEA 15 MW turbine, I am calculating the natural frequency using BModes. Could I know how can I find the cm_loc, cm_axial, ixx_tip, iyy_tip, izz_tip, ixy_tip, izx_tip, iyz_tip" which are input for the .bmi file.
Thanks,
Satish J
You can find those quantities in Table 5-1 in the IEA Wind 15-MW report. However, there is a known issue with the summation of the moments of inertia (see Github Issue here). I am working on a more reliable estimate, but it will be a bit before it is ready to go.
Dear Jason,
I am trying to better understand the body-weight restoring into elements (4,4) and (5,5) of hydro_k in Bmodes.
I see from WAMIT’s manual that C(4,4) and C(5, 5) contain -mgz_g where m is supposedly the platform mass, and z_g its CM z coordinate.
So if we modify these terms by adding -MtotgZ_Gtot, (Mtot=mass of the entire system except for mooring), wouldn’t it double count the contribution from the platform?
Thank you in advance.
Dear Rick,
You should use the full-system mass and center of mass to derive the hydro_k in BModes (what you refer to as Mtot and Z_Gtot). The WAMIT documentation implies this as well, as it is generally referring to oil and gas platforms, whose platform mass is effectively the full-system mass.
Best regards,
Dear Jason,
I have modelled both tower and flexible foundation of an Onshore WT in SubDyn. And I got relevent mass and stiffness matrices. Now I am trying to find tower Modes using BModes.
Tower height= 136.8 m
SubDyn’s Total Mass (structural and non-structural)= 0.121917E+07
SubDyn’s Total Mass CM coordinates (Xcm,Ycm,Zcm) = (0, 0, 40.5)
What should be my draft, cm_pform, mass_pform, ref_msl in BModes?
I have given some lumped masses when I modelled the Tower in SubDyn. Also should I give the whole tower again (along with lumped masses at different sections) in Tower section properties when running BModes?
Thank you in advance.
Dear @Tom.Jose,
I’m not sure I fully understand your question. BModes is most useful when calculating tower mode shapes for ElastoDyn. But it sounds like you are modeling the entire support structure (tower + foundation) in SubDyn, so, presumably the tower degrees of freedom of ElastoDyn are disabled. What are you trying to use BModes for?
Best regards,