Dear Jason,
Hello，I am a newcomer to OpenFAST and Bmodes so sorry to bother you again.I’m wondering why the output of Bmodes doesn’t have the mode shape and frequency of the torsion mode.In addition，Why is the Yaw Bearing Z rotation（TTDspTwst） always 0 in the output of OpenFast（The correct answer should be non-zero）， but the Tower Base Z rotation（PtfmYaw） is not.Is this because there is no consideration for torsional degrees of freedom in OpenFast？
I would be most grateful to receive your reply.My sincerest thanks in advance.
Best regard，
Monge Law.
Dear Monge.Low,
BModes can calculate the torsion modes…the last column in the mode shape outputs is the twist of the beam.
The ElastoDyn module of OpenFAST does not have a tower-torsion degree of freedom (DOF), so, TTDspTwst, which represents the elastic twist relative to the tower base, is always zero. Often, the tower torsion DOF is approximated in ElastoDyn using the nacelle-yaw DOF instead. Alternatively, if the tower is modeled in SubDyn rather than ElastoDyn, the tower torsion DOF can be modeled within SubDyn (of course, the SubDyn is inherently linear, so, misses the geometric nonlinear terms inherent within ElastoDyn).
Best regards,
Dear Jason,
I’m sorry I didn’t make myself clear.I know that the last column in the Bmodes output is torsion mode.I can get the correct mode shapes and frequencies of SS and FA for the first bending mode and the second bending mode from the Bmodes output results，but there is no correct mode shapes and frequencies of the torsion mode.The mode shapes and frequencies of the torsional modes seem to need to be calculated by other methods.
Please forgive me if I have said something inappropriate.
My sincerest thanks in advance.
Best regards，
Dear Monge.Low,
I’m not sure why you are not getting the torsion response you are expecting from BModes. Obvious things to check would be the boundary condition at the tower base, the distributed rotational inertia and torsional stiffness, and the tower-top yaw inertia.
Best regards,
Dear Jason,
Thank you very much for your reply.I’ll check these things out.
Wish you all the best in your work.
Best regards，