Dear Jason,

I would like to ask some quesions about how to get mass matrix from FAST v.7 linearization.

I am calculating the tower top mass properties for the input in BModes. I did try to calculate it by hand, but I doubt that the tower top mass properties in BModes (ixx_tip, iyy_tip, izz_tip) are different from FAST. So, I would like to perform FAST linearization in order to get mass matrix and obtain the tower top mass inertias from that matrix. However, after I did linearization and post-processing by using MBC. I obtained MBC_M (which I believe that it is the mass matrix) with the size of 14x14. However, I am not sure about the definition of each elements in matrix. For example, I am not sure from column and row to which column and row contains the mass matrix for tower part, and what elements to look for the tower top mass properties. In FAST v.7, I enabled all Tower DoFs, blade DoFs, and Drivetrain DoF.

Also, from this forum (http://forums.nrel.gov/t/linearization-with-fast-v8-obtain-mass-matrix/1626/1) you mentioned about using the rotor inertia from ElastroDyn Summary file. From what I understand that inertia is about x axis which corresponds to ixx in BModes. What about other inertia (iyy_tip and izz_tip)? How to calculate them?

Thank you very much

Best Regards,

Danupon Subanapong

Dear Danupon,

Using FAST v7 to obtain the mass, center of mass, and inertias of the rotor-nacelle assembly (RNA) for use as an input to BModes has been discussed on this forum before e.g. see my responses in the following forum topic: http://forums.nrel.gov/t/ansys-model-tower/1722/4.

Best regards,

Dear Jason,

Thank you so much for your quick reply.