We'd like to add a tower torsion degree of freedom (DOF) to the ElastoDyn module of OpenFAST at some point, but we are not currently funded to do that. Until then, there are two workarounds to capture the effect:
- 1) Model the tower torsion DOF through the nacelle-yaw DOF, i.e., by setting appropriate values for the nacelle inertia and yaw spring that mimic the effect of tower torsion.
2) Model the entire tower using the SubDyn module rather using ElastoDyn (essentially locating the platform reference point in ElastoDyn at the yaw bearing). SubDyn captures not only bending, but also extension, shear, and torsion effects for circular members. The big limitation of SubDyn is that it is entirely linear, not including geometric nonlinearities that ElastoDyn considers for the tower (and blades). Modeling the entire tower / support structure in SubDyn is discussed in the SubDyn User's Guide and Theory Manual.