Dear Shuo-Ting Chiang,
Just a few comments:
- Overall, it looks like you’ve specified these BModes input files correctly, except that you’ve specified a substructure mass and stiffness matrix for the tower while using hub_conn = 1, which means that the specified substructure mass and stiffness will not be used.
- However, I’m not an expert on the IEA Wind 15-MW reference wind turbine and can’t confirm your tower-top mass, center of mass, and inertia values, or your substructure mass and stiffness values.
- The tower-top mass, center of mass, and inertias specified in BModes for the tower should represent the properties of the complete RNA as a lumped rigid body. This mass is written to the ElastoDyn summary file (when the blades are modeled in ElastoDyn, not BeamDyn), but the center of mass and inertias are not. These, hoewever, can be obtained through a FAST / OpenFAST linearization analysis, e.g., as discussed in the following forum topic: OpenFast 2nd order Linearization - #2 by Jason.Jonkman. How did you obtain the RNA properties you’ve set in this BModes input file?
- The substructure mass and stiffness should be used if you are trying to derive the tower mode shapes for a system where the tower is connected to fixed-bottom offshore substructure (such as the monopile) or a floating offshore substructure (such as the UMaine semi). In this case, hub_conn should be set different from 1 and you should use the version of BModes referred to as BModes_JJ on this forum. Which system are you considering and how did you obtain the associated substructure mass and stiffness matrices?
- Your BModes inputs look correct for the blade; the tip mass can be zero and hub_conn = 1 as you’ve specified them is OK. The natural frequencies and mode shapes of the blade are output from BModes after execution.
- BModes can only model individual components like a blade or tower of a wind turbine. To calculate the full-system natural frequencies, including coupling between the blades and tower, the linearization functionality of OpenFAST can be used, as discussed many times on this forum.
Best regards,