Dear Marcos,
Without customization of the FAST source code, the structural model of FAST v7 and the ElastoDyn module of FAST v8 are limited to two fore-aft and two side-to-side bending-mode DOFs in the tower and to two flapwise and one edgewise bending-mode DOFs per blade. If you’ve looked in the source code, you’ve likely noticed that most things are parameterized into loops through enabled DOFs. As such, it would not be too difficult to customize FAST to allow for additional bending-mode DOFs in the tower and blades. However, we’ve not done this ourselves because as you introduce higher-frequency modes/DOFs, likely modes other than bending will become important (e.g., torsion). To capture effects other than bending requires a change to the beam formulation used for the tower and blades.
For the blades, we’ve introduced a more advanced (nonlinear finite-element-based) beam formulation through the BeamDyn module in the latest release of FAST (v8.12), which among other things, includes virtually unlimited DOFs and torsion. We recommend enabling BeamDyn for blades that require models of higher-fidelity than possible with ElastoDyn.
Best regards,