# Linearization with FAST v8, obtain mass matrix

Dear Jason,

I know that the topic was discussed in this forum often but I have still questions about it.

As I read in one of your posts it is not possible to get directly the linearized 6x6 mass matrix by linearization with FAST v8.
Can I obtain the mass matrix out of the linearized state matrix A from the result of the linearization with FAST v8?
I need the matrix for the input data for calculating the tower modes with BModes.

Thank you.

Best regards,
René

Dear René,

No, it is not possible to derive the mass matrix from the linearized state matrix “A” obtained through a FAST v8 linearization analysis.

For calculating tower mode shapes with BModes, you’ll need the rigid-body mass/inertia properties of the rotor-nacelle assembly (RNA). These will have to be calculated by hand or through another tool. For calculating tower mode shapes for floating systems with BModes, you’ll also need the rigid-body mass/inertia properties of the floating platform, including hydrodynamic added mass. The structural mass, center of mass, and inertias are already set within the ElastoDyn module and can be entered directly within BModes. The 6x6 hydrodynamic added mass can be obtained from the WAMIT output and then entered within BModes.

Best regards,

Dear Jason,

So I have two options for calculating the mass/inertia properties of the rotor-nacelle assembly: Either I use ADAMS as described in this forum or I create a FAST v7-Model of the Wind Turbine and calculate the linearization analysis with the mass matrix as result (then using MBC and so on).

Can you recommend me another option for calculating the necessary mass/inertia properties?

Thank you very much.

Best regards,
René

Dear René,

Those are two options. As I said in my prior post, you could also calculate the mass/inertia by hand. You could use the information from the ElastoDyn summary file to aid in this effort–the ElastoDyn summary file includes the total integrated mass, center of mass, and moment of inertia of each blade about its root.

Best regards,

Dear Dr. Jonkman,

Is it possible to obtain an output of the rotor inertia per timestep? In the Test*.ED.sum file I get an overall rotor inertia, but I would like to have a time-series of it.

Best regards,
Koen

Dear Koen,

It is not possible to output the instantaneous rotor inertia per time step without your own customization of the FAST source code.

Best regards,

Dear Jason,

Thanks for the prompt response. That is fine with me to code and compile this bit. Could you tell me which variable and in which module could I find the rotor inertia?

Kind regards.
Koen

Dear Koen,

The undeflected rotor inertia that is written to the FAST summary file is calculated within SUBROUTINE Coeff() of ElastoDyn.f90. However, the instantaneous rotor inertia reflecting the deflected state of the rotor is never actually computed within the ElastoDyn module of FAST presently.

Best regards,

Alright Jason, that’s exactly what I was thinking, that the rotor inertia were adapted according to the blade deformations, and I was curious to check it out for various wind cases. Thanks for the clear replies.