Dear Alessandro,

How are you obtaining these natural frequencies out of FAST e.g. through a FAST linearization analysis or by post-processing time series?

FAST does not directly compute the total tower-top inertia i.e. with the rotor, drivetrain, and nacelle treated as a single rigid body, but the effect of all the inertias and distributed masses are included in the dynamic response. Other than a laborious hand calculation, we’ve used two methods to derive the total tower-top mass and inertia by calculating 6x6 rigid-body mass matrix of the tower-top. You’ll find several examples on the forum where this has been done using either (1) the linearization of FAST v7 with only the six platform DOFs enabled (setting the tower and platform masses and inertias to zero to leave only the tower-top mass and inertias remaining) or (2) the FAST-to-ADAMS preprocessor of FAST v7 to generate an ADAMS model, from which the aggregate mass can be calculated within MSC.ADAMS. However, without modification of the source code, it is not possible to directly output the 6x6 mass matrix from FAST v8.

Best regards,