Dear Xing Wei,
Here are a few comments that have been discussed several times on this forum:
- Tower mode shapes are strongly effected by the tower-base boundary condition. That is, the mode shapes of a cantilevered tower will be different from the mode shapes of a tower mounted atop a floating platform. You’ll need to specify the platform mass/inertia, added mass, hydrostatic, and mooring stiffness in BModes if you wish to derive the tower mode shapes for floating wind system–see: BModes : Input parameters about tower support subsystem.
- The tower model of the ElastoDyn module of FAST v8 is simpler than that of BModes, which will effect how the distributed tower properties should be specified in BModes–see: Consistency between Bmodes and MBC - #2 by Jason.Jonkman.
- When the NREL 5-MW models were first developed, we used MSC.ADAMS to derive the tower mode shapes, not BModes, so even if the BModes model is assembled correctly, the results will differ somewhat from what NREL has provided–see: Generate new mode shapes for the 5MW ITIBarge4 wind turbine - #2 by Jason.Jonkman.
From your results, mode 1 is a side-to-side mode, uncoupled from fore-aft motion, and mode 2 is a fore-aft mode, uncoupled from side-to-side motion.
Best regards,