Dear Alessandro,
Just a few comments:
If you are interesting in the dynamic response of the tower, without specific interest in blade dynamics/loading, I suppose it would be reasonable to model a cantilevered beam with a lumped mass and inertia at the tower top. Make sure that you include the rotational inertia of the tower top in addition to the mass, as this effect tends to be large for most wind turbines. For a 3-bladed rigid rotor, this inertia would be fixed, but for a 2-bladed rotor, this inertia would be periodic. Depending on the tower-top weight and/or deflection of the tower, geometric nonlinearities (e.g. the axial shortening and p-delta effect) may be important, as are included in the structural model of FAST v7 and ElastoDyn module of FAST v8. For a rigid rotor, the centrifugal forces will balance out with respect to the tower-top loading, but gyroscopic terms may be important. Aerodynamically, you’d want to capture ideally all six components of the rotor load (3 forces including thrust, 3 moments include torque).
Information on the theory basis of the structural model of FAST v7 and the ElastoDyn module of FAST v8 is provided in the following forum topic: Coupled blade modes in FAST.
Best regards,